Lecture 2 - Overview of power switching devices. The Power Switch: what is a good power switch?

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1 Lecture 2 - Overview of power switching devices The Power Switch: what is a good power switch? A K G Attributes of a good power switch are: 1. No power loss when ON 2. No power loss when OFF 3. No power loss during turning ON or OFF 4. Little power required to turn it ON or OFF 5. Bi-directional? 6. Adequate voltage and current ratings 7. Low Turn-on and Turn-off times Lecture 2 - Overview 2-1 F. Rahman

2 Lecture 2 - Overview 2-2 F. Rahman

3 Lecture 2 - Overview 2-3 F. Rahman

4 More on classification of power semiconductors Source: S. Bernet, Recent developments of high power semiconductors for industry and traction applications, IEEE Transactions on, Vol. 15, No. 6, Nov. 2000, pp Lecture 2 - Overview 2-4 F. Rahman

5 Diodes of various sizes (d) Thyristor (e) Thyristor (f) 3-φ IPM with IGBTs 50A, 400V 1200V, 1000A 1200V, 100A Typical power device encapsulations Key Attributes of power semiconductor switches 1. High breakdown voltage (BV) 2. Low On-state resistance 3. Fast switching times There is not much freedom to enhance one without affecting the others two. 4. Low gate drive requirement Lecture 2 - Overview 2-5 F. Rahman

6 Static I-V characteristics of power semiconductor switches BV I R ON V sat For power electronic converters, only the saturation region and blocking regions of the characteristic (shaded part) are used. Majority-carrier devices such as MOSFETS, and Schottky diodes have low breakdown voltage in order to have acceptable On-resistance. These have very fast switching times. Minority-carrier devices such as BJTs, IGBTs, GTOs, SCRs, MCTs have low On-resistance, due to injection of large quantities of minority carries into the depletion layer when switched on. Such devices can have high breakdown voltage. These are however much slower than majority carrier devices. Lecture 2 - Overview 2-6 F. Rahman

7 DEVICE RATINGS SUMMARY (Mohan, 1995) Lecture 2 - Overview 2-7 F. Rahman

8 DEVICE RATINGS SUMMARY (SEMIKRON, 2004) Lecture 2 - Overview 2-8 F. Rahman

9 Power Devices and Applications Power rating VS frequency range of power semiconductor devices (Courtesy of Powerex. Inc) Lecture 2 - Overview 2-9 F. Rahman

10 The Power Diode Chapter 19 of Mohan gives a good introduction to physics of pn junctions. Junction p-n diode: static characteristic 1 R ON Signal PN diode structure Lecture 2 - Overview 2-10 F. Rahman

11 The avalanche breakdown voltage BV BD is inversely proportional to the impurity densities. BV BD d 17 2 BD ε E = V N 2qN d where N d = donor atom densities/m 3 ε = dielectric constant of the depletion layer q = electronic charge in Coulomb E BD = field strength for avalanche breakdown 20 MV/m / cm / cm 10 / cm Power diode structure The n layer gives the high breakdown voltage capability. The lightly doped n layer supports most of the reverse blocking voltage. The breakdown electric field in the n VAK,max layer W giving avalanche breakdown in the n d layer is determined by width W d of the n layer and the impurity density of this layer. These are selected for the required maximum V AK, or BV BD or V RRM. Lecture 2 - Overview 2-11 F. Rahman

12 The wide n layer does not necessarily mean increased conduction voltage drop across the diode. The relative levels of impurity densities ensure that adequate conductivity is achieved when minority carriers are injected across the junctions. Refer to diode data sheets 1-3 in the Lecture Notes webpage for diode parameters. Switching Characteristics di F dt di R dt trr nsec for fast recovery diodes. several 100 μsecs for line-frequency power diodes Q rr = Reverse recovery charge, C Lecture 2 - Overview 2-12 F. Rahman

13 S = snappiness factor = t t 5 4 ( di / dt) di t di 2Q di I t 2 I dt S+ 1 dt S+ 1 dt t R rr R rr R R rr = 4 = = < τ d0 rr ( + ) 2Qrr 1 S ) 2τ Id0 = < di / dt di / dt R 12 2 τ 4 10 BV BD Schottky diodes Power Schottky diodes are formed from metalsemiconductor junctions, with an n -1 layer as shown in figure below. Note: only majority carriers. Anode, A Metal contact SiO 2 SiO 2 R p p Depletion layer w/o p guard rings n 1 Depletion layer with p guard rings n Metal contact Cathode, K Lecture 2 - Overview 2-13 F. Rahman

14 Compared to junction (bipolar) pn diodes Schottky diodes, being a majority carrier device, have 1. much lower forward on-state voltage drop, V 2. much smaller turn-off times; no reverse recovery current 3. low reverse blocking voltage ratings, up to 200V 4. higher junction capacitance; the charging current of this capacitance at turn-off is comparable to the reverse recovery current of junction (bipolar) pn diodes. 5. high efficiency and switching frequency. Lecture 2 - Overview 2-14 F. Rahman

15 The Power MOSFET Very fast, t off 50 nsec nsec. R dson increases with 500V, 15A device. 2.6 V BD ; typically, R ds 40 mω for a Turned-on and -off by V GS. V GS,TH 5-20V. These devices are easily connected in parallel. N-Channel MOSFET (a) symbol, (b) v-i characteristic, (c) idealized characteristic. MOSFET with parasitic reverse diode MOSFET with blocked parasitic blocking and fast recovery diode Lecture 2 - Overview 2-15 F. Rahman

16 Switching of Power Devices (Resistive load) Gate input power: t on p = v i dt f G G G s 0, W Switching power loss = p SW = shaded area f s, W Lecture 2 - Overview 2-16 F. Rahman

17 Switching ON/OFF a resistive-inductive circuit T i T v T L I o V d FW Resistor R FW R LOAD The turn-off problem Suppose the switch T is ON for a while and the inductive Vd load current is Io =. R LOAD Assume that the free-wheeling resistor R FW, is not present. When T is turned OFF, I o falls to zero in time t fi or t rv (whichever is longer). If V d = 5V and R LOAD = 0.1Ω, L = 30 mh, t fi = t rv = 150 nsec, (i) What is voltage V ds across the switch when it is turned off? (ii) What role could the free-wheeling resistor play in limiting this voltage? (iii) What role could R FW play in turning off the load current quickly? Lecture 2 - Overview 2-17 F. Rahman

18 Switching a diode-clamped circuit The load current I o is assumed to remain constant while the switch turns ON and OFF with a duty cycle at the switching frequency f s. T i T v T V d Ideal Diode, D i D I o We assume: ideal devices and no reverse recovery current in the diode. I o = i T + i D at all time. Voltage v T (t) and current i T (t) transients at turn-on and turn-off occur along straight tines. When the switch T is turned on, the diode continues to carry part of I o as i T builds up. Until i T becomes equal to I o, the diode remains in the conducting state (i.e., forward biased). The diode starts to become reverse biased after I o fully commutates to T in time t ri. Thus, while i T rises from zero to I o, v T remains clamped to V d. Once I o fully commutates to T, the voltage v T (or v AK ) across switch falls to zero in time t fv. Lecture 2 - Overview 2-18 F. Rahman

19 When the switch T is turned off, the diode does not conduct until the potential at the cathode of diode D rises to V d. Thus, diode D remains reverse biased until v T rises to V d in time t rv. Thereafter, the diode becomes forward biased and starts to conduct. Thus, the current through T remains clamped to I o until v T rises to V d in time t rv. Until then i T = I o. Once v T rises to V d, current i T falls to zero in time t fi. Note that diode clamping increases the areas of overlap during both transitions. The power loss in the switch is given by the product p = v T * i T, given by the shaded areas. Note also that the reverse recovery current of the diode at turn-on of T increases the peak current loading of switch T. The following analysis assumes ideal diode, i.e., the reverse recovery current of the diode is neglected. The energy dissipated (power lost) in the switch during turn-on and turn-off transients can be found by multiplying the voltage v T (t) and current i T (t) of the switch and integrating the product over the duration of each transient. The calculation is simplified if the origin t = 0 for each case is shifted to time when the respective transitions begin, and if the on-state voltage of the switch is neglected in comparison with the DC supply voltage V d. Lecture 2 - Overview 2-19 F. Rahman

20 T s = 1 f s = T ON + T OFF T OFF T ON v T V d I O V d i T t don t doff t ri t fv t rv t fi I O i D V d I O W 1 = V I t 2 coff d o coff p T W con = 1 V I t 2 d o con W on = V I t on o on t con =t ri + t fv t coff = t rv + t fi Lecture 2 - Overview 2-20 F. Rahman

21 Thus, for turn-on transient, W tri I tfv = o Vd tdt + I ov d 1 0 t 0 ri t t s on fv 1 1 = VIt + VIt 2 2 d o ri d o fv dt 1 1 = VI d o( tri + tfv) = VIt d o con J 2 2 Joules Similarly, it can be shown that for turn-off transient, W trv V tfi = d I o tdt + Vd I o 1 0 t 0 rv t t s off fi dt 1 1 = VI d o( trv + tfi) = VIt d o coff J 2 2 J The total switching power loss is thus given by sw d o ( tcon + tcoff ) fs 1 P = 2 V I W If the turn-on and turn-off transients are not short compared to T s, the average power loss in the switching process, P s, may become large compared to the loss during the ON time. 1 P = V I f t + t 2 ( ) sw d o s con coff The on-state power loss is given by Lecture 2 - Overview 2-21 F. Rahman

22 P V I t f = W on on o on s where t on is the on-time of the switch in a switching period. Note that P sw increases proportionately with f s, while P ON does not, since T on and f s are inversely proportional to each other. Lecture 2 - Overview 2-22 F. Rahman