Switch-Mode Power Supplies

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Blaise Booker
5 years ago
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1 Switch-Mode Power Supplie Switch-Mode Power Supplie (SMPS) are a family of power circuit deigned to deliver power to a load by caling voltage level from input to put. Thee circuit come in different verion and they can be ued in a variety of device ranging from computer to car and aircraft to cellphone. SMPS can be DC/DC if they cale DC voltage to another DC voltage. Alternately, they can be called AC/DC if an additional firt tage in the circuit rectifie AC voltage to DC voltage. They can be non-iolated or iolated, depending on whether galvanic iolation i ued or not. Galvanic iolation i provided by a tranformer which i an excellent olution for electrical eparation ince it avoid hort circuit between input and put. The mot important non-iolated SMPS are:. Buck. Boot 3. Buck-Boot 4. Ćuk 5. SEPC The mot important iolated SMPS are:. Flyback. Forward 3. Puh-pull 4. Half-bridge 5. Full-bridge SMPS are deigned to operate above the audible frequency range (5kHz and above). They maintain a contant put voltage by mean of a pule width modulator (PWM) which provide a time-varying voltage ignal to a witch that i typically implemented with a tranitor (almot alway a MOSFET). The put voltage of an SMPS i ampled, caled down and compared to a reference voltage. n the implet term, the reulting ignal i then compared to a ramp. The produced pule i fed into the gate of the tranitor and increae or decreae in width in order to maintain the put voltage contant under a pecific put current load condition. SMPS can operate in the Continuou Conduction Mode (CCM) if the current through the inductor never goe to zero. They operate in the Dicontinuou Conduction Mode (DCM) if the current through the inductor reache zero.

oltage-mode control Current-mode control oltage-mode control compare the ignal produced by an error amplifier to a ramp and feed the put of the comparator into a PWM that modulate the gate of one or

2 oltage and current mode control The feedback circuit form a cloed loop and variou control-loop approache are ued to regulate the put voltage. oltage-mode and current-mode control are the mot common cheme although other variation exit and they are ued in variou C by different companie. oltage-mode control Current-mode control oltage-mode control compare the ignal produced by an error amplifier to a ramp and feed the put of the comparator into a PWM that modulate the gate of one or two MOSFET. Current-mode control work in a imilar way but it alo feed a ampled voltage acro the ene reitor of the put inductor, acro the inductor itelf or acro the low-ide MOSFET into the aforementioned comparator. The two control mode have a variety of advantage and diadvantage. oltage-mode control ue more component which make compenation more challenging and it lower in term of load tranient recovery. Current-mode control ue fewer component which make compenation le challenging and it fater in term of load tranient recovery. Eentially, current-mode control i an evolution of voltage-mode control. oltage-mode control ue one loop. Current-mode ue two loop, the econd being voltage ened acro a ene reitor near the put inductor or acro the put inductor itelf.

OA and OTA compenation Mot SMPS require compenation becaue the put C filter caue an abrupt -8 phae hift at the reonating frequency. Some C are internally compenated.

3 OA and OTA compenation Mot SMPS require compenation becaue the put C filter caue an abrupt -8 phae hift at the reonating frequency. Some C are internally compenated. Other require external compenation which can be of type or type. Compenation require a et of carefully elected reitor and capacitor which are placed near operational amplifier (OA) and operational tranconductance amplifier (OTA). The addition of thee component introduce zero and pole to provide a boot in phae which improve tability and regulation of the control-loop over frequency. Type compenation with OA (left) and with OTA (right) pole zero Type compenation zero and pole over frequency 3

4 Type and type compenation Type compenation with OA and OTA i hown below: Note: the harp drop in phae at the C put filter reonating frequency i -8 o type compenation cannot be ued for voltage-mode control in CCM becaue, even with a theoretical phae boot of 9, it till doe not produce a ufficient amount of boot. Therefore, type compenation i only ued for current-mode control (CMC) or for SMPS in DCM. Type compenation with OA and OTA i hown below: Note: the harp drop in phae at the C put filter reonating frequency i -8 and type compenation provide a theoretical phae boot of 8 o thi i the common cheme ued for voltage-mode control (MC). The deign of SMPS involve tradeoff between component ize, power, efficiency, component election, noie, loop control and compenation. Therefore, a pecific application often depend on a pecific topology. 4

5 Texa ntrument ummarize type compenation with OTA a follow: f p i the low-frequency pole frequency f z i the zero frequency (placed before the C reonating frequency) f p 3 i the high-frequency pole frequency (placed at the ESR C frequency) 5

6 Texa ntrument ummarize type compenation with OTA a follow: f p i the low-frequency pole frequency f z i the firt zero frequency (placed before the C reonating frequency) f z 3 i the econd zero frequency (placed after the C reonating frequency) f p 4 i the firt high-frequency pole frequency (placed at the ESR C frequency) f p 5 i the econd high-frequency pole frequency (placed at ½ the witching frequency) 6

7 inear Technology ummarize type compenation with OA a follow: ω ω p ω p ω z ω z ω i the low-frequency pole angular frequency ω z i the firt zero angular frequency (placed before the C reonating frequency) ω z i the econd zero angular frequency (placed after the C reonating frequency) ω p i the firt high-frequency pole angular frequency (placed at the ESR C frequency) ω p i the econd high-frequency pole angular frequency (placed at ½ the witching frequency) Note: recall that ω=πf. Type compenation with OA i imilar to the above but, ince C and R 3 are not ued, ω z and ω p are no longer part of the compenation. 7

Output ripple voltage Typically, a ytem i table if the load tranient produce a fat recovery of the put ripple voltage with ringing (more

8 Stability meaurement The tability of a ytem can be inferred by looking at it put ripple voltage on an ocillocope while load tranient are applied and by examining Bode plot by injecting an AC ignal while operating with a DC load. Output ripple voltage Typically, a ytem i table if the load tranient produce a fat recovery of the put ripple voltage with ringing (more than 45 of phae margin). PM=9 Typically, a ytem i marginally table if the load tranient produce a recovery of the put ripple voltage but ome ringing i noticeable (around 45 of phae margin). PM=46 8

Typically, a ytem i table if the load tranient produce a low recovery of the

of ab ±.5% of the put DC voltage i deirable.

9 Typically, a ytem i table if the load tranient produce a low recovery of the put ripple voltage and a coniderable amount of ringing i clearly viible (le than 45 of phae margin). PM=33 For a converter, a 5% load tranient i typical and an put ripple voltage of ab ±.5% of the put DC voltage i deirable. Bode plot Bode plot are a way to check for tability over frequency. The Bode plot etup look like thi: Bode plot etup for a Buck converter with a Type compenation cheme with OTA 9

A -5Ω injection reitor (Rt) i placed in erie with the upper feedback reitor (R) and a mall AC inuoidal ignal (-% of the put DC voltage) i injected in the ytem through a tranformer.

The AC ignal i wept over frequency at different DC load condition to generate the Bode plot which conit of gain and phae plot. A typical Bode plot provide bandwidth, gain and phae margin.

10 A -5Ω injection reitor (Rt) i placed in erie with the upper feedback reitor (R) and a mall AC inuoidal ignal (-% of the put DC voltage) i injected in the ytem through a tranformer. At the ame time both ide of the injection reitor are monitored on two eparate channel ( and ) by a network analyzer or a frequency repone analyzer. The AC ignal i wept over frequency at different DC load condition to generate the Bode plot which conit of gain and phae plot. A typical Bode plot provide bandwidth, gain and phae margin. The above image are an example of a table ytem with a 5kHz witching frequency with a bandwidth/croover frequency of 8kHz (the point where the gain i ), a gain margin of -db at khz (the point where the phae i ) and a phae margin of 6 at 8kHz (the point where the gain i db). Bode plot can be hard to interpret but an eay rule i that the GM i calculated at the frequency where the phae i by going from db to the blue graph (downward in thi cae) and the PM i calculated at the frequency where the gain i db by going from to the red graph (upward in thi cae). When put ripple meaurement or Bode plot how intability, the location of the aforementioned zero and pole are changed by uing a proper combination of reitor and capacitor. For a converter a bandwidth (BW) of / to /6 of the witching frequency, a gain margin (GM) of at leat -8dB and a phae margin (PM) of at leat 45 are typically deirable.

Controller, converter and module SMPS are made of a variety of component and emiconductor companie ell product that allow the deigner to cutomize with different degree of

Controller Converter Module Companie uch a Texa ntrument, Analog Device, Maxim ntegrated, Monolithic Power Sytem, Microchip Technology or ROHM Semiconductor ue a variety

The ection of a circuit that hot the PWM that drive the MOSFET i typically called a controller. When controller and MOSFET are combined they are called a converter.

11 Controller, converter and module SMPS are made of a variety of component and emiconductor companie ell product that allow the deigner to cutomize with different degree of integration or flexibility. Controller Converter Module Companie uch a Texa ntrument, Analog Device, Maxim ntegrated, Monolithic Power Sytem, Microchip Technology or ROHM Semiconductor ue a variety of term to refer to imilar circuit, primarily for marketing and ale purpoe. However, they more or le converge to the ame terminology. The ection of a circuit that hot the PWM that drive the MOSFET i typically called a controller. When controller and MOSFET are combined they are called a converter. When a converter and inductor are in the ame package, they are called a module (mot companie call them power module, except for Analog Device which ue the term μmodule ). Note: mot of what follow i labeled converter but the term i ued in a general ene a a ynonym of circuit, whether it a proper converter or a module, and it include input/put capacitor.

12 Non-iolated topologie Below are ome of the mot common non-iolated topologie: Buck (non-ynchronou) Buck (ynchronou)

13 Boot Buck-Boot 3

14 Ćuk The imulation for the circuit preented below imply teady condition and introduce oft-tart for a mooth tart-up. SEPC The efficiency of each circuit depend primarily on MOSFET, diode and inductor which lower overall efficiency o a proper choice of component i crucial. The current through the inductor ha a DC component and an AC component. The DC component will go through the load and the AC component will go through the capacitor. 4

15 Buck converter (CCM) Thi circuit convert a DC voltage from a higher level to a lower level. t i generally ued for application that require up to W. Thi pecific example how an aynchronou Buck converter in CCM operation. M MN6796 R - + 6uH - 5m C + C = = TD = n TR = m TF = n PW = m PER = m R uf m = = TD = n TR = n TF = n PW =.76u PER = 5u D MBR35 R3 3.9uF m 3Adc Buck converter (CCM) deally, D in 3 5 m m 3m 4m 5m 6m 7m 8m 9m m (C:) (C:) nput and put waveform at tart-up - 4.A (C:) (:+) (M:g,:) (:) (:,:).A SE>> -.A 9.99m 9.99m 9.99m 9.993m 9.994m 9.995m 9.996m 9.997m 9.998m 9.999m.m D(M) (D) () -(C) Thi converter ha an input of and an put of 5. The put current i 3A o the put power i 5W. Tranient waveform for CCM The MOSFET i modeled with the following parameter: W=.9 =μ. 5

16 Buck converter in CCM tate M MN6796 6uH R 5m C C = = TD = n TR = m TF = n PW = m PER = m R uf m = = TD = n TR = n TF = n PW =.76u PER = 5u R3 3.9uF m 3Adc On-time tate During the on-time the MOSFET i on and the diode i not conducting. 6uH R 5m C C = = TD = n TR = m TF = n PW = m PER = m R uf m = = TD = n TR = n TF = n PW =.76u PER = 5u D MBR35 R3 3.9uF m 3Adc During the off-time the MOSFET i off and the diode i conducting. Off-time tate Note: the diode hown in the circuit could be replaced by a MOSFET (n-type) which would make the circuit ynchronou and, therefore, more efficient. n that cae, the low-ide MOSFET hould have a lower on-reitance compared to the high-ide MOSFET becaue the low-ide MOSFET i on the majority of the time (the off-time i typically longer). 6

17 The pecification for the Buck converter in CCM are the following: in 4 5 3A D. 4 f khz % 5m % 3mA R DS ( ON ) m The N6796 MOSFET ha an R DS(ON) of mω and 3A will flow through it during the on-time o the voltage drop acro the device i: FET RDS ( ON ) 3Am 66m The duty cycle i: D in FET The period i: 5 66m.44 D T f khz 5 On-time and off-time are: t on DT t DT off The corner frequency of the C put filter at in = i: f f khz 5m. 4kHz D.5595 The inductance i calculated at high line which i when the voltage acro the MOSFET i larget, the duty cycle i mallet and the off-time i larget: in(max) FET 4 66m D min in(max) FET D min t off (max) D T min t D toff (max) H 3mA 6H 7

18 The capacitance i given by: C.9F 6H f.4khz o 3.9F The actual corner frequency of the C put filter now i: f o. 44kHz C 6H 3.9F The maximum ripple current through the inductor i: D toff (max) 5.4 t 3.6 (max) 8mA 6H The peak current through MOSFET, inductor and diode i given by: peak (max) 3A 8mA 3. 4A The RMS ripple current through the capacitor ha an inductive hape: 8mA C ( rm) 8mA 3 Therefore, a capacitor with an RMS current rating of at leat 8mA mut be elected. 8

19 The input ripple voltage i a combination of two term: The firt term i produced by the capacitive component of the input capacitor: Note: thi mean that increaing capacitance will reduce the input ripple voltage. The econd term i produced by the reitive component of the input capacitor (ESR): 3 Ω 3 Combining the term give the total input ripple voltage:

20 The put ripple voltage i a combination of two term: The firt term i produced by the put C filter: Note: thi mean that increaing inductance or capacitance will reduce the put ripple voltage. The econd term i produced by the reitive component of the put capacitor (ESR): 8 Ω 8 Combining the term give the total put ripple voltage: Either one of the two term above can dominate over the other. f C dominate over ESR the put will look inuoidal. f ESR dominate over C the put will look triangular m 9.99m 9.99m 9.993m 9.994m 9.995m 9.996m 9.997m 9.998m 9.999m.m (:+) Output ripple voltage Note: in thi cae C dominate over ESR o the put look inuoidal. The implementation of the Buck converter hown above feature a MOSFET and a diode. A more commonly ued implementation i the o-called ynchronou Buck converter topology which replace the diode with an n-type MOSFET. MOSFET make the circuit ynchronou.

21 An example of a Buck circuit i the Texa ntrument MR366, a 4.-6 input -8/6mA put ynchronou Buck converter that operate at or.mhz. Typical application Block diagram Note: the diode i replaced by a MOSFET and both MOSFET are integrated in the converter.

22 The tranfer function for the Buck circuit i C d C r RC rc F i o ~ where r i the equivalent erie reitance (ESR) of the capacitor and ~ d i the mall-ignal duty cycle perturbation [ t d D t d ~ ]. The equation that decribe the operation of the Buck converter in CCM are: FET in Hi D o o o o o P R t off D i o C i i where i the inductor ripple current. The equation for the Buck converter at the edge of CCM and DCM are: off c t R D T R D f R off c t D T D f R where (c) tand for critical.

23 nverting Buck converter (CCM) The put of a Buck converter in CCM can be inverted to produce a negative voltage. Thi i done by wapping the put with the ground reference. M MN6796 R uH - 5m = = TD = n TR = m TF = n PW = m PER = m C R uf m = = TD = n TR = n TF = n PW =.639u PER = 5u + D MBR35 C R3 uf m 3Adc nverting Buck converter (CCM) 4-8 m m 3m 4m 5m 6m 7m 8m 9m m (C:) (:-) nput and put waveform at tart-up SE>> - (C:) (:-) (M:g,:-) (:-) (:-,:) 5.A A -5.A 9.988m 9.989m 9.99m 9.99m 9.99m 9.993m 9.994m 9.995m 9.996m 9.997m 9.998m D(M) (D) () -(C) Thi converter ha an input of and an put of -5. The put current i 3A o the put power i 5W. Tranient waveform for CCM The MOSFET i modeled with the following parameter: W=.9 =μ. 3

24 4.7A 4.6A 4.5A 4.4A 4.3A 9.984m 9.985m 9.986m 9.987m 9.988m 9.989m 9.99m 9.99m 9.99m 9.993m 9.994m 9.995m 9.996m 9.997m 9.998m 9.999m () nductor ripple current Note: ince the right ide of the inductor i now grounded, the inductor ha a voltage of le than acro it (unlike in the Buck converter previouly dicued where the voltage acro the component wa le than 7). Depite the on-time i horter than before, the additional voltage differential increae the ripple current through the inductor o the inductor i a little larger m 9.985m 9.986m 9.987m 9.988m 9.989m 9.99m 9.99m 9.99m 9.993m 9.994m 9.995m 9.996m 9.997m 9.998m 9.999m (:-) Output ripple voltage Note: an average current of ab 4.5A will go through the MOSFET during the on-time and through the diode during the off-time (unlike in the Buck converter previouly dicued where the current wa 3A). Thi increae the put voltage ripple o the put capacitor i larger. 4

25 Buck converter (DCM) Thi pecific example how a non-ynchronou Buck converter in DCM operation. The average current going to the load i ma. The maximum ripple current through the inductor, which occur at high line, i 8mA. Since half of the ripple or 4mA i bigger than the average current which i ma, the current will eventually reach zero. Thi i an intance of light load condition. M MN6796 R - + 6uH - 5m = = TD = n TR = m TF = n PW = m PER = m C R uf m = = TD = n TR = n TF = n PW =.78u PER = 5u + D MBR35 C R3 3.9uF m madc Buck converter (DCM) 3 5.m.4m.6m.8m.m.m.4m.6m.8m.m.m.4m.6m.8m 3.m (:+) (C:) nput and put waveform at tart-up - (:+) (C:) (M:g,:) (:) (:,:) ma A SE>> -ma.99m.99m.99m.993m.994m.995m.996m.997m.998m.999m 3.m D(M) (D) () -(C) () Tranient waveform for DCM Thi converter ha an input of and an put of 5 at 5mA. The MOSFET i modeled with the following parameter: W=.9 =μ. 5

26 Note: decreaing the value of the load current will decreae the inductor current and if the ripple current through the inductor i large enough, during the off-time, the inductor current will reach zero and the converter will operate in DCM. Since the circuit operate in DCM, the current through the inductor will go to zero during the off-time a hown below: 3mA ma ma A -ma.99m.99m.99m.993m.994m.995m.996m.997m.998m.999m 3.m () nductor ripple current m.99m.99m.993m.994m.995m.996m.997m.998m.999m 3.m (:+) Output ripple voltage 6

27 Buck converter in DCM tate M MN6796 6uH R 5m = = TD = n TR = m TF = n PW = m PER = m C R uf m = = TD = n TR = n TF = n PW =.78u PER = 5u C R3 3.9uF m madc On-time tate During the on-time the MOSFET i on and the diode i off. 6uH R 5m = = TD = n TR = m TF = n PW = m PER = m C R uf m = = TD = n TR = n TF = n PW =.78u PER = 5u D MBR35 C R3 3.9uF m madc Off-time tate with inductor current During the off-time the MOSFET i off and the diode i on. R 5m = = TD = n TR = m TF = n PW = m PER = m C R uf m = = TD = n TR = n TF = n PW =.78u PER = 5u C R3 3.9uF m madc Off-time tate with no inductor current During the off-time the current in the inductor will reach zero and the circuit will reduce to an RC circuit. 7

28 Boot converter Thi circuit convert a DC voltage from a lower level to a higher level. t i generally ued for application that require up to 5W. R D + 6uH - 5m MBR45 C M C = = TD = n TR = m TF = n PW = m PER = m R uf m = = TD = n TR = n TF = n PW =.76u PER = 5u RF R3 47uF m 3Adc Boot converter deally, D in 3 m m 3m 4m 5m 6m 7m 8m 9m m (C:) (D:) nput and put waveform at tart-up SE>> - (:+) (C:) (M:g) (:+,R:) (M:d) 5.A A -5.A 9.99m 9.99m 9.99m 9.993m 9.994m 9.995m 9.996m 9.997m 9.998m 9.999m.m () D(M) (D) -(C) Thi converter ha an input of and an put of 4. The put current i 3A o the put power i 7W. Tranient waveform for CCM The MOSFET i modeled with the following parameter: W=.7 =µ. 8

29 Boot converter tate 6uH R 5m C M C = = TD = n TR = m TF = n PW = m PER = m R uf m = = TD = n TR = n TF = n PW =.76u PER = 5u RF R3 47uF m 3Adc On-time tate During the on-time the MOSFET i on and the diode i off. 6uH R 5m C C = = TD = n TR = m TF = n PW = m PER = m R uf m = = TD = n TR = n TF = n PW =.76u PER = 5u R3 47uF m 3Adc Off-time tate During the off-time the MOSFET i off and the diode i on. 9

30 The pecification for the Boot converter in CCM are the following: in 4 4 3A D. 44 f khz % 4m 89% The put power i: % R DS ON 4m The current through the inductor i the um of the input current through the MOSFET and the put current through the diode. The average current through MOSFET and inductor during the on-time i: Therefore, the ripple current through the inductor will be a follow: % The RF MOSFET ha an R DS(ON) of 4mΩ and an average of 6.74A will flow through it during the on-time o the voltage drop acro the device i: The duty cycle i: The period i: ( ) T f khz 5 On-time and off- time are:

31 The inductance i calculated at high line which i when the voltage acro the MOSFET i larget, the duty cycle i mallet and the on-time i mallet: %. 7 The load reitance i: P 7W R 8. or 3A The capacitance i calculated at low line: R 4 8 P 7W The corner frequency of the C put filter i given by: The maximum ripple current through the inductor i: The peak current through MOSFET, inductor and diode i given by:

32 The normalized inductor time contant i: The RMS ripple current through the capacitor ha a trapezoidal hape: Therefore, a capacitor with an RMS current rating of at leat 3.44A mut be elected (the high current figure may imply that two or more parallel capacitor could be ued). 3

33 The put ripple voltage i a combination of two term: C ESR The firt term i produced by the capacitive component of the put C filter: Note: thi mean that increaing capacitance will reduce the put ripple voltage. The econd term i produced by the reitive component of the put capacitor (ESR): Combining the term give the total put ripple voltage: Either one of the two term above can dominate over the other. f C dominate over ESR the put will look triangular. f ESR dominate over C the put will look trapezoidal m 9.99m 9.99m 9.993m 9.994m 9.995m 9.996m 9.997m 9.998m 9.999m.m (D:) Output ripple voltage Note: in thi cae C dominate over ESR o the put look triangular. The implementation of the Boot converter hown above feature a MOSFET and a diode. A more commonly ued implementation i the o-called ynchronou Boot converter topology which replace the diode with an n-type MOSFET. MOSFET make the circuit ynchronou. 33

34 An example of a Boot circuit i the Monolithic Power Sytem MP348, a.6-4 input.8-4/4ma put ynchronou Boot converter that operate at.mhz. Typical application Block diagram Note: the diode i replaced by a MOSFET and both MOSFET are integrated in the converter. 34

35 35 The tranfer function for the Boot circuit i ~ d D R C C r RC rc F e e e e i o where D e The equation that decribe the operation of the Boot converter in CCM are: FET in Hi D in o o o o o P R t off in D i o CHi i i o Co i The equation for the Boot converter at the edge of CCM and DCM are: on c t D R D DT R f D D R on c t D D DT D D f R

36 Buck-Boot converter Thi circuit convert a DC voltage to either a lower level or a higher level. t i generally ued for application that require up to 5W. Thi pecific example how CCM operation in Buck mode (put voltage i lower than input voltage). M RF3 D = = 4 TD = n TR = m TF = n PW = 5m PER = 3m C R uf m = = 4 TD = n TR = n TF = n PW =.44u PER = 5u + - R m 6uH MBR35 C R3 68uF m 3Adc Buck-Boot converter in Buck mode deally, D in m m 3m 4m 5m 6m 7m 8m 9m m m m 3m 4m 5m (:+) (C:) nput and put waveform at tart-up - A (:+) (C:) (M:g,M:) (M:) 5A A SE>> -5A 4.99m 4.99m 4.99m 4.993m 4.994m 4.995m 4.996m 4.997m 4.998m 4.999m 5.m D(M) () (D) (C) Tranient waveform for CCM in Buck mode Thi converter ha an input of 4 and an put of -. The put current i 3A o the put power i 36W. Note: the polarity at the put i oppoite to the one at the input.the MOSFET i modeled with the following parameter: W=.97 =μ. 36

37 Buck-Boot converter tate M RF3 = = 4 TD = n TR = m TF = n PW = 5m PER = 3m C R uf m = = 4 TD = n TR = n TF = n PW =.44u PER = 5u R m 6uH C R3 68uF m 3Adc On-time tate During the on-time the MOSFET i on and the diode i off. D = = 4 TD = n TR = m TF = n PW = 5m PER = 3m C R uf m = = 4 TD = n TR = n TF = n PW =.44u PER = 5u R m 6uH MBR35 C R3 68uF m 3Adc Off-time tate During the off-time the MOSFET i off and the diode i on. 37

38 The pecification for the Buck-Boot converter in CCM in Buck mode are the following: in (max) 4 3A D. 4 f khz % m % R DS ( ON ) 6m 96% The put power i: P 3A 36W The current through the inductor i the um of the input current through the MOSFET and the put current through the diode. The average current through MOSFET and inductor during the on-time i: P 36W ave in 3A A 4.96 in(max) The RF3 MOSFET ha an R DS(ON) of 6mΩ and an average of 5.679A will flow through it during the on-time o the voltage drop acro the device i: FET ave RDS ON 5.679A6m 99m ( ) ( ) The minimum duty cycle i: D min 4 99m in(max) FET.478 D min The period i: T f khz 5 On-time and off-time are: t on(min) D T t D T min off (max) min 38

39 The minimum inductance i calculated at high line which i when the voltage acro the MOSFET i larget, the duty cycle i mallet and the on-time i mallet: % The load reitance i: P 36W R 4 or 3A R 4 P 36W The minimum capacitance at high line i given by: The corner frequency of the C put filter i given by:

40 The maximum ripple current through the inductor i: The current through the inductor i the um of the input current through the MOSFET and the put current through the diode. 6.A 6.A 5.8A 5.6A 4.99m 4.99m 4.99m 4.993m 4.994m 4.995m 4.996m 4.997m 4.998m 4.999m 5.m () The peak current through MOSFET, inductor and diode i given by: The normalized inductor time contant i: nductor ripple current The RMS ripple current through the capacitor ha a trapezoidal hape: Therefore, a capacitor with an RMS current rating of at leat.874a mut be elected (the high current figure may imply that two or more parallel capacitor could be ued). 4

41 The put ripple voltage i a combination of two term: C ESR The firt term i produced by the capacitive component of the put C filter: C DminTS 3A m C 68F Note: thi mean that increaing capacitance will reduce the put ripple voltage. The econd term i produced by the reitive component of the put capacitor (ESR): Combining the term give the total put ripple voltage: Either one of the two term above can dominate over the other. f C dominate over ESR the put will look triangular. f ESR dominate over C the put will look trapezoidal m 4.99m 4.99m 4.993m 4.994m 4.995m 4.996m 4.997m 4.998m 4.999m 5.m (C:) Output ripple voltage Note: in thi cae C dominate over ESR o the put look triangular. 4

42 Thi pecific example how CCM operation in Boot mode (put voltage i higher than input voltage). M RF3 D = = TD = n TR = m TF = n PW = 5m PER = 3m 3 C3 R4 uf m = = TD = n TR = n TF = n PW =.946u PER = 5u R5 m 45uH MBR35 C4 R6 8uF m 3Adc Buck-Boot converter in Boot mode deally, D in m m 3m 4m 5m 6m 7m 8m 9m m m m 3m 4m 5m (3:+) (C4:) nput and put waveform at tart-up - A (3:+) (C4:) (M:g,M:) (M:) 5A A SE>> -5A 4.99m 4.99m 4.99m 4.993m 4.994m 4.995m 4.996m 4.997m 4.998m 4.999m 5.m D(M) () (D) (C4) Thi converter ha an input of and an put of -. The put current i 3A o the put power i 36W. Tranient waveform for CCM in Boot Mode Note: the polarity at the put i oppoite to the one at the input. The MOSFET i modeled with the following parameter: W=.97 =μ. 4

43 The pecification for the Buck-Boot converter in CCM in Boot mode are the following: in (min) 3A D. 4 f khz % m % R DS ( ON ) 6m 96% The put power i: P 3A 36W The current through the inductor i the um of the input current through the MOSFET and the put current through the diode. The average current through MOSFET and inductor during the on-time i: P 36W ave in 3A 6. 75A.96 in(min) The RF3 MOSFET ha an R DS(ON) of 6mΩ and an average of 6.75A will flow through it during the on-time o the voltage drop acro the device i: FET ( ave) RDS ( ON ) 6.75A6m. 8 The maximum duty cycle i: D max.8 in(min) FET.574 D max The period i: T f khz 5 On-time and off-time are: t on(max) D T t D T max off (min) max 43

44 The minimum inductance at low line i given by: t.4 D off (min) H 3mA 95H % The load reitance i: P 36W R 4 or 3A R 4 P 36W The minimum capacitance i calculated at low line which i when the voltage acro the MOSFET i mallet, the duty cycle i greatet and the on-time i larget: The corner frequency of the C put filter now i: Dmax.46 f o 778Hz C 95H 8F

45 The maximum ripple current through the inductor i: The current through the inductor i the um of the input current through the MOSFET and the put current through the diode. 7.8A 7.6A 7.4A 7.A 7.A 4.99m 4.99m 4.99m 4.993m 4.994m 4.995m 4.996m 4.997m 4.998m 4.999m 5.m () nductor ripple current The peak current through MOSFET, inductor and diode i given by: P 36W peak in 3A 79mA A.96 in The normalized inductor time contant i: The RMS ripple current through the capacitor ha a trapezoidal hape: Therefore, a capacitor with an RMS current rating of at leat 3.48A mut be elected (the high current figure may imply that two or more parallel capacitor could be ued). 45

46 The put ripple voltage i a combination of two term: C ESR The firt term i produced by the capacitive component of the put C filter: C(max) DmaxTS 3A m C 95F Note: thi mean that increaing capacitance will reduce the put ripple voltage. The econd term i produced by the reitive component of the put capacitor (ESR): Combining the term give the total put ripple voltage: Either one of the two term above can dominate over the other. f C dominate over ESR the put will look triangular. f ESR dominate over C the put will look trapezoidal m 4.99m 4.99m 4.993m 4.994m 4.995m 4.996m 4.997m 4.998m 4.999m 5.m (C:) Output ripple voltage Note: in thi cae C dominate over ESR o the put look triangular. Note: the inductance i calculated at high line wherea the capacitance at the put i calculated at low line. 46

47 Note: the maximum ripple current through the inductor occur at high line wherea the maximum ripple voltage at the put occur at low line. Note: therefore, the circuit hould ue a 6µH inductor and 8µF of capacitance at the put to meet the requirement. Thi will reduce the maximum ripple current through the inductor from 588mA to 44mA at low line and reduce the maximum ripple voltage at the put at high line from.5m to 95.7m. The implementation of the Buck-Boot converter hown above feature a polarity at the put that i oppoite to the one at the input. The fact the MOSFET doe not have a terminal ground complicate the PWM circuit that turn them on and off. Therefore, a more commonly ued implementation i the o-called 4-witch Buck-Boot converter topology which repectively replace MOSFET and diode by MOSFET each. 4 MOSFET make the circuit ynchronou. 47

48 An example of a Buck-Boot circuit i the Maxim ntegrated MAX865A, a input.5-4/8ma put ynchronou Buck-Boot converter that operate at MHz. Typical application Block diagram Note: the MOSFET and the diode are replaced by 4 MOSFET which are integrated in the converter. 48

49 49 The tranfer function for the Buck-Boot circuit i ~ d D R D C C r RC rc F e e e e i o where D e The equation for the Buck-Boot converter in CCM are a follow: FET in Hi D o o o o o P R t off D i o CHi i i o Co i The equation for the Buck-Boot converter at the edge of CCM and DCM are: ) ( off c t R D R D T f R D off c D t D T D f R

50 Ćuk converter The Ćuk converter i named after Slobodan Ćuk of the California ntitute of Technology who firt preented the circuit in 976. Thi converter ha the ame tranfer function of the Buck-Boot converter a well a oppoite polarity for input and put. Thi circuit can alo provide an put voltage higher or lower than the input. Unlike the previou converter, thi one feature two inductor and an additional capacitor between them. The inductor are not coupled in thi example. R C R3 R4 + 83uH - 5m 4.7uF m + 35uH - m = = TD = n TR = m TF = n PW = m PER = m C uf R m = = TD = n TR = n TF = n PW =.35u PER = 5u M RFR D SD4 C3 uf R5 m 3Adc Ćuk converter deally, D in 5 5 (:+) -.5 SE>> -6..m.4m.6m.8m.m.m.4m.6m.8m.m (R4:) nput and put waveform at tart-up - (:+) (:-) (:+) (:+,R:) (M:d) (D:) (D:,:) 5.A A SE>> -5.A 9.99m 9.99m 9.99m 9.993m 9.994m 9.995m 9.996m 9.997m 9.998m 9.999m.m -(C) () D(M) -(C) (D) -() (C3) Tranient waveform for CCM 5

51 Unlike the Buck converter, the Ćuk converter i lighter becaue intead of having one bulky inductor, it ha two maller inductor. f the inductor are coupled, the amount of inductance can be halved. The ripple voltage acro the coupling capacitor i proportional to the input voltage. A capacitance that yield ab a % ripple voltage i typical. Thi converter ha an input of and an put of -5 (inverted Buck mode). The put current i 3A o the put power i 5W. Note: the polarity at the put i oppoite to the one at the input. The MOSFET i modeled with the following parameter: W=9µ =45n. 3.5A 3.A.5A.A 9.98m 9.98m 9.984m 9.986m 9.988m 9.99m 9.99m 9.994m 9.996m 9.998m.m () () nductor ripple current m 9.98m 9.984m 9.986m 9.988m 9.99m 9.99m 9.994m 9.996m 9.998m.m (:-) Output ripple voltage 5

52 Ćuk converter tate R C R3 R4 83uH 5m 4.7uF m 35uH m = = TD = n TR = m TF = n PW = m PER = m C uf R m = = TD = n TR = n TF = n PW =.35u PER = 5u M RFR C3 uf R5 m 3Adc During the on-time the MOSFET i on and the diode i off. The current through the MOSFET i the um of the current through the inductor. On-time tate R C R3 R4 83uH 5m 4.7uF m 35uH m = = TD = n TR = m TF = n PW = m PER = m C uf R m = = TD = n TR = n TF = n PW =.35u PER = 5u D SD4 C3 uf R5 m 3Adc During the off-time the MOSFET i off and the diode i on. The current through the diode i the um of the current through the inductor. Off-time tate 5

53 An example of a Ćuk circuit i the inear Technology TM845, a.8-8 input -.5 to -5/7mA put Ćuk micromodule that operate between khz and MHz. Typical application Block diagram Note: mot of the component are inide the micromodule which can be configured to work like a SEPC circuit. 53

54 SEPC SEPC tand for Single-Ended Primary nductor Converter. Thi circuit look very much like the Ćuk converter becaue it derived from it. The main difference between the two circuit i that D and are wapped at their repective poition o that the put for the SEPC i no longer negative. Thi circuit can alo provide an put voltage higher or lower than the input. The inductor are not coupled in thi example. R C R3 D + 83uH 5m m 4.7uF SD4 - = = TD = n TR = m TF = n PW = m PER = m C uf R m = = TD = n TR = n TF = n PW =.3u PER = 5u M RFR 35uH R4 m C3 uf R5 m 3Adc SEPC deally, D in 5 5 (:+) 5..5 SE>> -..m.4m.6m.8m.m.m.4m.6m.8m.m (:+) nput and put waveform at tart-up - (:+) (:+) (M:g) (:+,R:) (M:d) (:) 5.A A SE>> -6.A.99m.99m.99m.993m.994m.995m.996m.997m.998m.999m.m -(C) () D(M) -(C) -() (D) -(C3) Tranient waveform for CCM 54

55 The poitive polarity of the voltage at the put i generally deired o thi circuit i more popular than the Ćuk converter. Unlike the Buck converter, the SEPC i lighter becaue intead of having one bulky inductor, it ha two maller inductor. f the inductor are coupled, the amount of inductance can be halved. The ripple voltage acro the coupling capacitor i proportional to the input voltage. A capacitance that yield ab % ripple voltage i typical. Thi converter ha an input of and an put of +5 (Buck mode). The put current i 3A o the put power i 5W. Note: the polarity at the put i the ame a the one at the input. The MOSFET i modeled with the following parameter: W=9µ =45n. 3.5A 3.A.5A.A.98m.98m.984m.986m.988m.99m.99m.994m.996m.998m.m () () nductor ripple current m.98m.984m.986m.988m.99m.99m.994m.996m.998m.m (D:) Output ripple voltage 55

56 SEPC converter tate R C R3 83uH 5m 4.7uF m = = TD = n TR = m TF = n PW = m PER = m C uf R m = = TD = n TR = n TF = n PW =.3u PER = 5u M RFR 35uH R4 m C3 uf R5 m 3Adc During the on-time the MOSFET i on and the diode i off. The current through the MOSFET i the um of the current through the inductor. On-time tate R C R3 D 83uH 5m 4.7uF m SD4 = = TD = n TR = m TF = n PW = m PER = m C uf R m = = TD = n TR = n TF = n PW =.3u PER = 5u 35uH R4 m C3 uf R5 m 3Adc During the off-time the MOSFET i off and the diode i on. The current through the diode i the um of the current through the inductor. Off-time tate 56

57 An example of a SEPC circuit i the inear Technology TM849, a dual.6- input +.5 to +4/A put SEPC micromodule that operate between khz and.5mhz. Typical application Block diagram Note: mot of the component are inide the micromodule which can be configured to work like a Ćuk circuit (a hown above where one put i + (SEPC) and the other i - (Ćuk). 57

58 olated topologie Below are ome of the mot common iolated topologie: Flyback Forward (ingle witch) 58

59 Puh-pull Half-bridge 59

60 The imulation for the circuit preented below imply teady condition and introduce oft-tart for a mooth tart-up. Full-bridge The efficiency of each circuit depend primarily on MOSFET, diode and inductor which lower overall efficiency o a proper choice of component i crucial. The current through the inductor ha a DC component and an AC component. The DC component will go through the load and the AC component will go through the capacitor. 6

61 Except for the Flyback converter, all the other converter are imulated with a Ferroxcube TN33// toroidal core made of P9 material with an inductance index of 87nH/T. Coupling i non-linear. 6

62 Flyback converter Thi circuit i a variation of the Buck-Boot converter. The input inductor i coupled with another inductor to form a power tranformer which provide iolation and voltage level hift. At the ame time the power tranformer tore energy. The duty cycle i typically limited to le than 5%. Thi i done to enure the energy tored on the primary ide of the power tranformer i dicharged during the off-time. Thi circuit typically operate in DCM and it ued for application that require up to 5W. K K K_inear COUPNG =.99 D DMBRB645T4G = = 4 TD = n TR = m TF = n PW = m PER = m C R uf m R3 4uH m 3 : 5 59nH R4 5m C R uf m 3Adc M = = TD = n TR = n TF = n PW = 7n PER = u RF4 Flyback converter deally, D in p f 5.m.4m.6m.8m.m.m.4m.6m.8m.m (:+) (D:) nput and put waveform at tart-up 6

63 5 5 SE>> A (:+) (D:) (:+) (:) (D:) A A.996m.9964m.9968m.997m.9976m.998m.9984m.9988m.999m.9996m.m D(M) (D) -(C) Thi converter ha an input of 4 and an put of 5. The put current i 3A o the put power i 5W. Tranient waveform for DCM The MOSFET i modeled with the following parameter: W=.97 =µ. 63

64 Flyback converter tate K K K_inear COUPNG =.99 = = 4 TD = n TR = m TF = n PW = m PER = m C R uf m R3 4uH m 3 : 5 59nH R4 5m C R uf m 3Adc M = = TD = n TR = n TF = n PW = 7n PER = u RF4 On-time tate During the on-time the MOSFET i on and the diode i off. K K K_inear COUPNG =.99 D DMBRB645T4G = = 4 TD = n TR = m TF = n PW = m PER = m C R uf m R3 4uH 59nH 3 : 5 R4 m 5m C R uf m 3Adc = = TD = n TR = n TF = n PW = 7n PER = u Off-time tate During the off-time the MOSFET i off and the diode i on. 64

65 The pecification for the Flyback converter in DCM are the following: in 6 5 3A D. 4 % 5m f 5kHz 78% The put power i: P 5 3A 5W The input power i: P 5W Pin 9. 3W.78 R m DS ( ON ) 7 D max. 4 The inductance of the primary ide of the tranformer i calculated at low line: p in(min) Dmax.4 f P in 5kHz 9.3W 4.7H 4H The turn ratio i defined a follow: A choice of N p =3 and N =5 i acceptable: N N p N N p.384 The inductance of the econdary ide of the tranformer i given by: N p 4 H nH 59nH N p The peak current on the primary ide of the tranformer i: D.4 4H 5kHz in(min) max p(max) 4. 4 p f A 65

66 The average current through the MOSFET during the on-time ha a triangular hape and it average value i: 4.4. The RF4 MOSFET ha an R DS(ON) of 7mΩ and an average of.a will flow through it during the on-time o the voltage drop acro the device i: The voltage acro the primary ide of the tranformer at in =4 i: The duty cycle i: The period i: T f 5 khz On-time and off-time are: The peak current on the econdary ide of the tranformer i: N p 3 (max) p(max) 4.4A. 44A N 5 66

67 The voltage acro the primary ide of the tranformer at in = i: The duty cycle i: On-time and off-time are: The time it take for the econdary ide of the tranformer to dicharge i: The um of the maximum on-time and the time it take for the tranformer to dicharge mut be le than the period becaue the tranformer mut have time to fully dicharge:

68 m.9964m.9968m.997m.9976m.998m.9984m.9988m.999m.9996m.m (:+) (D:) (:+) (:) (D:) oltage pike and reonance at the drain of the MOSFET At the beginning of the off-time the drain of the MOSFET ee a voltage pike of 63 and experience reonance caued by the combination of leakage inductance of the tranformer and the put capacitance of the MOSFET (red box). f the voltage pike at the drain of the MOSFET i near or above the drain-toource rating of the MOSFET, an RC or RDC nubber can be placed acro the MOSFET. At the end of the off-time the drain of the MOSFET experience reonance caued by the combination of the magnetizing inductance of the tranformer and the put capacitance of the MOSFET (blue box). The current in the MOSFET increae linearly during the on-time. The current in the diode decreae linearly during the off-time. f the current pike are not conidered, the peak in current in the MOSFET and the diode are directly related by the turn ratio of the tranformer. The peak occur when energy i tranferred from the primary ide to the econdary ide of the tranformer. Since the circuit operate in DCM, the current through the diode will go to zero during the off-time a hown below: A A A -5A.996m.9964m.9968m.997m.9976m.998m.9984m.9988m.999m.9996m.m D(M) (D) -(C) The current through the diode reache zero during the off-time 68

69 n order to maintain a relatively low put ripple voltage the ESR of the put capacitor hould be very low m.9964m.9968m.997m.9976m.998m.9984m.9988m.999m.9996m.m (D:) Output ripple voltage with ESR=mΩ i 33.8m m.9964m.9968m.997m.9976m.998m.9984m.9988m.999m.9996m.m (D:) Output ripple voltage with ESR=3mΩ i 48.9m The ESR need to be mall otherwie the voltage pike in the blue box will be large becaue that voltage pike i given by the product of the ESR of the put capacitor and the current pike on the econdary ide of the tranformer. The voltage pike on the bottom i clearly very cloe to the maximum put ripple voltage required by the pecification. 69

70 An example of a Flyback circuit i the inear Technology T3748, a 5- input Flyback controller. Typical application Block diagram 7

71 Forward converter Thi circuit i a variation of the Buck converter. The introduction of a power tranformer provide iolation and voltage level hift. The major difference between the Forward converter and the Flyback converter i that energy i not tored in the tranformer but it paed on directly to the load (the word forward derive from thi behavior). Another difference between the Forward and Flyback converter i one additional inductor and one additional diode. A few variant of the Forward converter exit. The one preented here i the o-called ingle-witch with reonant reet verion which allow the power tranformer to reet a the energy tored in the magnetizing inductance during the on-time diipate during the off-time through the combination of the put capacitance of the MOSFET, the capacitance of the forward diode and any nubber capacitance in the circuit (ometime added acro MOSFET and econdary ide of tranformer). The advantage of the reonant reet variant of the Forward converter i that an additional dedicated reet winding for the power tranformer and a diode on the primary ide can be eliminated. Thi reduce the complexity of the power tranformer, it lower cot of the converter and the duty cycle for the circuit can exceed 5%. Thi circuit i ued for application that require up to W. deally, TN33 P9 D R4 TX MBR45 uh + - m = = 4 TD = n TR = m TF = n PW = m PER = m C R uf m R 5m R3 m D MBR45 C R5 uf 5m 3Adc M _TURNS = _TURNS = 34.8uH : 8.7uH = = TD = n TR = n TF = n PW = 99n PER = u RF4 Forward converter 7

72 5.m.4m.6m.8m.m.m.4m.6m.8m.m.m.4m.6m.8m 3.m (TX:) (C:) nput and put waveform at tart-up 5 SE>> 4.A (TX:) (C:) (M:g) (R:) (D:) (D:) (D:,:).A A.994m.9945m.995m.9955m.996m.9965m.997m.9975m.998m.9985m.999m.9995m 3.m -(C) D(M) (D) (D) () -(C) Thi converter ha an input of 4 and an put of 5. The put current i 3A o the put power i 5W. Tranient waveform for CCM The MOSFET i modeled with the following parameter: W=.97 =u. 7

73 Forward converter tate TN33 P9 D R4 TX MBR45 uh m = = 4 TD = n TR = m TF = n PW = m PER = m C R uf m R 5m R3 m C R5 uf 5m 3Adc M _TURNS = _TURNS = 34.8uH : 8.7uH = = TD = n TR = n TF = n PW = 99n PER = u RF4 On-time tate During the on-time the MOSFET i on. The forward diode i on and the freewheeling diode i off. TN33 P9 TX uh R4 m = = 4 TD = n TR = m TF = n PW = m PER = m C R uf m R 5m R3 m D MBR45 C R5 uf 5m 3Adc _TURNS = _TURNS = 34.8uH : 8.7uH = = TD = n TR = n TF = n PW = 99n PER = u Off-time tate During the off-time the MOSFET i off. The forward diode i off and the freewheeling diode i on. The circuit look like a Buck converter during the offtime. 73

74 The pecification for the Forward converter in CCM are the following: in 6 5 3A D. 44 % 5m % 3mA f 5kHz D max.5 87 / N A nh pf R m DS ( ON ) 7 C o 55 C ref 3 pf where C o i the put capacitance of the MOSFET and C ref i the etimated capacitance acro the econdary ide reflected to the primary ide. The turn ratio i defined a follow: N N p in(min) D max D N N p N.5 N p The average current through the MOSFET i: N ( ave) 3A.5. N p 5 p A The RF4 MOSFET ha an R DS(ON) of 7mΩ and an average of.5a will flow through it during the on-time o the voltage drop acro the device i: The voltage acro the primary ide of the tranformer at in =4 i: p in FET 4 5m The duty cycle i: D N N D p.455 D p The period i: T f 5 khz 74

75 On-time and off-time are: ton DT.455 9n t DT The inductance i calculated at high line which i when the voltage acro the MOSFET i larget, the duty cycle i mallet and the off-time i larget: p(max) in (max) FET 6 5m off D min p(max) N N D p D min.4 t off (max) D T min t D toff (max) H 3mA H The wort cae ripple current through the inductor happen with the maximum off-time: D toff (max) 5.44 t.6 (max) 87mA H N p i choen to be and N i choen to be. The inductance of the econdary ide of the tranformer i: p A N p nh 87 N 34.8H The inductance of the primary ide of the tranformer i: A N nh 87 N 8.7H The peak current through inductor and diode i given by: (max) (max) 3A 87mA 3. 43A 75

76 The maximum ripple current through the inductor reflected to the primary ide of the tranformer i given by: N (max)( p) (max) 87mA.5 43mA N p The maximum ripple current on the primary ide of the tranformer i calculated at high line which i when the voltage acro the MOSFET i larget, the duty cycle i mallet and the on-time i mallet: ton(min) Dmin T.4 84n p ton(min) n T ( p) 65mA 34.8H p Combining the ripple current of inductor and tranformer on the primary ide of the tranformer produce thi: TOT (max)( p) T ( p) 43mA 65mA 769mA The peak current through the MOSFET i given by: N TOT 769mA DS(max) (max)( p) 3.43A A N p The RMS ripple current through the capacitor ha an inductive hape: 87mA C ( rm) 83mA 3 Therefore, a capacitor with an RMS current rating of at leat 83mA mut be elected. Selecting a μf capacitor, the corner frequency of the C put filter i: f o kHz C H F 76

77 During the off-time the voltage at the drain of the MOSFET increae dramatically. The voltage at the anode of the forward diode doe the ame thing but in the oppoite direction. Thi behavior i caued by the tranfer of the energy tored in the magnetizing inductance of the tranformer to the paraitic capacitance mentioned above. The tranformer reet a it drain voltage goe back to the input voltage well before the end of the off-time a hown below: m.9964m.9968m.997m.9976m.998m.9984m.9988m.999m.9996m 3.m (TX:) (C:) (M:g) (R:) (D:) (D:) (D:,:) The drain voltage of the MOSFET goe back to the input voltage (tranformer reet) During the off-time the MOSFET ee a voltage of The anode of the forward diode dip to -3.. The put capacitance of the RF4 MOSFET i 55pF and the junction capacitance of the MBR45 diode at 3 (revere) i 75pF The maximum voltage at the drain i given by the um of the maximum input voltage on the primary ide of the tranformer and the maximum reonant reet voltage: The revere voltage at the anode of the forward diode i given by the voltage acro the primary ide to the econdary ide of the tranformer: f the voltage at the drain of the MOSFET i near or above the drain-to-ource rating of the MOSFET, an RC or RDC nubber can be placed acro the MOSFET or the forward diode. 77

78 The put ripple voltage i produced by the reitive component of the put capacitor (ESR): (max) (max) ESR 87mA 5m 4. 34m f the ESR i relatively mall the put will look inuoidal. f the ESR i relatively large the put will look triangular m.9945m.995m.9955m.996m.9965m.997m.9975m.998m.9985m.999m.9995m 3.m (C:) Output ripple voltage 78

79 An example of a Forward circuit i the inear Technology T83, a 6- input Forward controller. Typical application Note that the forward diode i referenced to ground and placed at the bottom of the chematic in the oppoite direction. When the diode are replaced by MOSFET they make the circuit ynchronou and therefore more efficient. By placing the MOSFET on the low ide of the power tranformer both MOSFET are referenced to ground and they are eaier to drive (turn on and off). Block diagram 79

80 Puh-pull converter Thi circuit ue two witche which are typically implemented by MOSFET. t alo ha two tranformer. The econdary ide of the tranformer i centertapped. Thi circuit i ued for application that require up to W. D R SD5 3uH m RCXN 3 Q TX XFRM_NONN/CT-PR/SEC C R uf Adc C m = = TD = n TR = n TF = n PW =.44u PER = 5u = = 48 TD = n TR = m TF = n PW = m PER = m R3 uf m P_TURNS = 3 P_TURNS = 3 S_TURNS = S_TURNS = RP_AUE = 3m RS_AUE = 5m D SD5 RCXN Q 3 = 3 = TD =.5u TR = n TF = n PW =.44u PER = 5u Puh-pull converter deally, D in N N p 5 4.m.4m.6m.8m.m.m.4m.6m.8m.m (:+) (:+) nput and put waveform at tart-up 8

81 5 (:+) (:+) (Q:GATE) (Q:GATE) (Q:DRAN) (Q:DRAN) A A A SE>>.99m.99m.99m.993m.994m.995m.996m.997m.998m.999m.m -(C) (Q:DRAN) (Q:DRAN) (D) (D) () -(C) Thi converter ha an input of 48 and an put of 4. The put current i A o the put power i 48W. The MOSFET i modeled with the following parameter: W= =µ. Tranient waveform 8

82 Puh-pull converter tate 3uH R m RCXN 3 Q TX XFRM_NONN/CT-PR/SEC C R uf Adc C m = = TD = n TR = n TF = n PW =.44u PER = 5u = = 48 TD = n TR = m TF = n PW = m PER = m R3 uf m P_TURNS = 3 P_TURNS = 3 S_TURNS = S_TURNS = RP_AUE = 3m RS_AUE = 5m D SD5 = 3 = TD =.5u TR = n TF = n PW =.44u PER = 5u Puh tate During the puh tate the Q MOSFET and the D diode are on. Meanwhile, the Q MOSFET and the D diode are off. D SD5 3uH R m TX XFRM_NONN/CT-PR/SEC C R uf Adc C m = = TD = n TR = n TF = n PW =.44u PER = 5u = = 48 TD = n TR = m TF = n PW = m PER = m R3 uf m P_TURNS = 3 P_TURNS = 3 S_TURNS = S_TURNS = RP_AUE = 3m RS_AUE = 5m D SD5 = 3 = TD =.5u TR = n TF = n PW =.44u PER = 5u Dead-time tate 8

83 During the dead time both MOSFET are off. Both diode are on and they hare current. D SD5 3uH R m TX XFRM_NONN/CT-PR/SEC C R uf Adc C m = = TD = n TR = n TF = n PW =.44u PER = 5u = = 48 TD = n TR = m TF = n PW = m PER = m R3 uf m P_TURNS = 3 P_TURNS = 3 S_TURNS = S_TURNS = RP_AUE = 3m RS_AUE = 5m RCXN Q 3 = 3 = TD =.5u TR = n TF = n PW =.44u PER = 5u Pull tate During the pull tate the Q MOSFET i and the D diode are on. Meanwhile, the Q MOSFET and the D diode are off. The duty cycle i given by: The period i: f khz T 5 Therefore, the on-time i:

84 Q and Q turn on alternately and there i a dead time between and 3. When Q i on and Q i off, current flow through D. When Q i on and Q i off, current flow through D. When either of the diode i on, it carrie the entire load current which in thi cae i A (average). During the dead time both diode carry current. Q /D on Q /D on 5 dead time (Q:GATE) (:+) A A A SE>>.99m.99m.99m.993m.994m.995m.996m.997m.998m.999m.m (D) (D) () -(C) MOSFET gate ignal (top) and inductor, diode and capacitor current (bottom) m.99m.99m.993m.994m.995m.996m.997m.998m.999m.m (C:) Output ripple voltage 84

85 An example of a Puh-Pull circuit i the inear Technology T3999, a.7-36 input Puh-Pull converter. Typical application Block diagram 85

HIGH VOLTAGE DC-DC CONVERTER USING A SERIES STACKED TOPOLOGY

HIGH VOLTAGE DC-DC CONVERTER USING A SERIES STACKED TOPOLOGY Author: P.D. van Rhyn, Co Author: Prof. H. du T. Mouton Power Electronic Group (PEG) Univerity of the Stellenboch Tel / Fax: 21 88-322 e-mail: