55:141 Advanced Circuit Techniques Switching Regulators

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1 55:141 Advanced Circuit Techniques Switching Regulators Material: ecture Notes, Handouts, and Sections of Chapter 11 of Franco A. Kruger 55:141: Advanced Circuit Techniques The University of Iowa Switching Regulators, Slide 1

2 Sidebar: Parts Per Million (PPM) Part Per Million an abbreviation for parts-per-million and is commonly used when very small changes from a nominal value of some sort is involved, for example temperature coefficients (TCRs) of precision resistors. To convert from ppm to %, divide the number by 100,000 To convert from % to ppm, multiply by 100,000 Thus, a TCR of 100 ppm/ o C is the same as a TCR of 0.01%/ C A. Kruger 55:141: Advanced Circuit Techniques The University of Iowa Switching Regulators, Slide 2

3 Resistors Specifications Temperature Coefficient. The resistance of a conductor changes with changes in temperature. This change is in general nonlinear, but for small temperature changes may be approximated as linear, and is commonly expressed as a temperature coefficient (TCR). The units are ppm/ C TRC is relevant for reversible changes in resistance as a function of temperature. Assume a TRC of +150 ppm/ C for a 1K resistor. If the ambient temperature rises from 25 C to 30 C, then the resistor s value will change with 150 R = 1,000 (30 25) = ,000,000 If the temperature drops to 25 C the resistor s value returns to 1K A. Kruger 55:141: Advanced Circuit Techniques The University of Iowa Switching Regulators, Slide 3 Ω

4 Why Study Power Supplies? Electronic Design 01/4/10 page 33. A. Kruger 55:141: Advanced Circuit Techniques The University of Iowa Switching Regulators, Slide 4

5 Why Study Power Supplies? We will look at this A. Kruger 55:141: Advanced Circuit Techniques The University of Iowa Switching Regulators, Slide 5

6 Switching Power Supplies Primary switching power supplies Convert line (120/240 VAC) to DC Switchers, SMPS, medical switchers, etc. Efficient Small Uses smaller transformer, inexpensive compared to a non-switching power supply with the same power-handling capability Cell-phone chargers, laptop power supplies, medical equipment, etc. Secondary switching power supplies Convert DC to DC Also called DC/DC converters A. Kruger 55:141: Advanced Circuit Techniques The University of Iowa Switching Regulators, Slide 6

inear Power Supply Bulky, expensive transformer I Ripple voltage is V pp I 2 fc Where f is the line frequency (50/60 Hz) Thus for good pre regulation, need large C (bulky, $$$)

7 inear Power Supply Bulky, expensive transformer I Ripple voltage is V pp I 2 fc Where f is the line frequency (50/60 Hz) Thus for good pre regulation, need large C (bulky, $$$) inear regulator, often dissipates significant power and need expensive heat sinks. A. Kruger 55:141: Advanced Circuit Techniques The University of Iowa Switching Regulators, Slide 7

Switch Mode Power Supply Chop DC at 50 khz 1 MHz Ripple voltage is V I 2 fc High frequencies require small C pp The rest of the circuit can tolerate significant ripple, so C can be small Recall the

8 Switch Mode Power Supply Chop DC at 50 khz 1 MHz Ripple voltage is V I 2 fc High frequencies require small C pp The rest of the circuit can tolerate significant ripple, so C can be small Recall the universal transformer equation 2 fnab = π 2 fnab A. Kruger 55:141: Advanced Circuit Techniques The University of Iowa Switching Regulators, Slide 8 E peak rms High frequencies require smaller transformers

Differential equation for a capacitor Some Basic Circuit Theory ii CC (tt) = CC ddvv CC(tt) dddd The voltage across a capacitor cannot change instantaneously.

9 Differential equation for a capacitor Some Basic Circuit Theory ii CC (tt) = CC ddvv CC(tt) dddd The voltage across a capacitor cannot change instantaneously. Instantaneously means ddvv CC dddd, so would mean ii CC, which is physically impossible. Consider a capacitor that is initially charges to VV 0 V, that is then connected to a battery with voltage VV mm through a resistor RR. When the switch is closed, the voltage right after the switch is closed is still VV 0. The capacitor charges exponentially to VV mm with a time constant ττ = RRRR. vv CC (tt) VV mm VV mm R C vv cc tt = KK 1 + KK 2 ee tt ττ t Switch closed here A. Kruger 55:141: Advanced Circuit Techniques The University of Iowa Switching Regulators, Slide 9

Some Basic Circuit Theory Differential equation for an inductor vv (tt) = ddii (tt) dddd The faster the current through an inductor changes, the bigger the voltage across the inductor: if ddii dddd

10 Some Basic Circuit Theory Differential equation for an inductor vv (tt) = ddii (tt) dddd The faster the current through an inductor changes, the bigger the voltage across the inductor: if ddii dddd 0, then VV. This idea is use to generate high voltages, for example ignition systems in our motor vehicle. In transient circuits, we say that the current through an inductor cannot change instantaneously. 10 V 10Ω 10 Ω 10 V 10Ω 10 Ω In steady state, with switch open current through inductor is 1 A Right after switch is closed, the inductor behaves as a 1 A current source A. Kruger 55:141: Advanced Circuit Techniques The University of Iowa Switching Regulators, Slide 10

11 Some Basic Circuit Theory Capacitors Inductors i( t) = C dv( t) v( t) = dt di( t) dt dc dv(t)/dt = 0 i = 0 Capacitor is an open circuit to dc dc circuits, replace capacitors with open circuits dc di(t)/dt = 0, v = 0 Inductor is a short to dc Dc circuits, replace inductors with short circuits A. Kruger 55:141: Advanced Circuit Techniques The University of Iowa Switching Regulators, Slide 11

12 Back to Basics v - + v = di dt I = t A. Kruger 55:141: Advanced Circuit Techniques The University of Iowa Switching Regulators, Slide 12

13 Back to Basics Relay inductance: 0.3 H, 150 Ω resistance About 60 ma flows when the transistor is on What happens when the transistor turns off? Answer: the collapsing magnetic field induces a voltage with the polarity shown. The magnitude is v = di dt - + Assume the transistor turns off in 0.1 ms. Then v = 0 3 = 180 V Is 0.1 ms for turn-off a good assumption? A. Kruger 55:141: Advanced Circuit Techniques The University of Iowa Switching Regulators, Slide 13

14 Back to Basics - + This diode provides a low impedance path and protects the transistor from the voltage spike. This diode provides a low impedance path and protects the rest of the circuit from the voltage spike Diode will not work, since it will conduct on alternate cycles. Use RC snubber A. Kruger 55:141: Advanced Circuit Techniques The University of Iowa Switching Regulators, Slide 14

15 DC/DC Converter inear Regulator Switching Regulator A. Kruger 55:141: Advanced Circuit Techniques The University of Iowa Switching Regulators, Slide 15

16 Switching Regulator Topologies 1 V O = DV VO = VI I 1 D Buck Boost V D = 1 D O V I Note that the average output voltage is essentially independent from the value of and C Buck-Boost A. Kruger 55:141: Advanced Circuit Techniques The University of Iowa Switching Regulators, Slide 16

Boost Converter For inductor VV = ddii dddd For fixed voltage across Δii = VV Δtt

The ON time is then tt ONN = DDDD and the OFF time is tt OFF = 1 DD TT.

VV II, and Δii (OOOO) = tt OOOO ΔVV = DDDD VV II (increase) + During tt OOOOOO, the

II VV oo (decrease) In steady state Δii (OOOO) = Δii (OOOOOO).

17 Boost Converter For inductor VV = ddii dddd For fixed voltage across Δii = VV Δtt Assume the switch is driven by a square wave with period TT and cycle DD. The ON time is then tt ONN = DDDD and the OFF time is tt OFF = 1 DD TT. We will assume VV OO is constant and VV OO > VV II During tt OOOO, voltage across is VV II, and Δii (OOOO) = tt OOOO ΔVV = DDDD VV II (increase) + During tt OOOOOO, the voltage across is VV OO VV II, and Δii (OOOOOO) = tt OOOOOO VV II VV oo = 1 DD TT VV II VV oo (decrease) In steady state Δii (OOOO) = Δii (OOOOOO). Combining equations leads to VV OO = VV II 1 DD A. Kruger 55:141: Advanced Circuit Techniques The University of Iowa Switching Regulators, Slide 17

18 Buck Converter - ON ON D = = = t ON t + t OFF t T S f S t ON What factors affects efficiency? A. Kruger 55:141: Advanced Circuit Techniques The University of Iowa Switching Regulators, Slide 18

Switch Closed - Switch Open Assume output and input voltages are constant. V SAT is the voltage across the (BJT/FET) switch when closed, and V D is the voltage cross the diode when conducting.

19 Switch Closed - Switch Open Assume output and input voltages are constant. V SAT is the voltage across the (BJT/FET) switch when closed, and V D is the voltage cross the diode when conducting. With switched closed, voltage cross the inductor is V I - V o V SAT and the current though the inductor increases linearly. After t ON, the inductor current has increased by i = V o VI V satt t ON When the switch opens, current cannot change instantaneously, voltage across inductor reverses, and cathode swings to below ground. The voltage across the inductor is V o +V D and the current decreases linearly as the magnetic field collapses. After t OFF, the current has decreased by i = V o + V D t OFF In the steady state, the increase during t ON should balance the decrease during t OFF, so that V V V V o I SAT o + ton = V D t OFF Solving for V o and recognizing that t ON + t OFF = 1/f and that D = t ON + t OFF is the duty cycle D, then V o ( I VSAT ) ( D) VD DVI = D V 1 Note that this is independent of the load current,, and C A. Kruger 55:141: Advanced Circuit Techniques The University of Iowa Switching Regulators, Slide 19

20 Buck DC/DC Converter - Called continuous conduction mode (CCM), where the inductor current never goes to zero In the CCM, output voltage is independent of the load current A. Kruger 55:141: Advanced Circuit Techniques The University of Iowa Switching Regulators, Slide 20

21 i = V o Buck Regulator: Discontinuous Mode VI V satt t ON i OFF This is called discontinuous conduction mode (DCCM) = V o + V When load current drops, the output voltage stays constant, and Δi stays the same. When the load current falls below a critical value I omin the inductor goes to zero during a cycle. T V I omin Vo 1 2 V = i In DCCM, more energy is stored than extracted in each cycle and the output voltage rises (and some of our assumptions thus far are not valid ) The controller will reduce D to keep V o fixed o D t A. Kruger 55:141: Advanced Circuit Techniques The University of Iowa Switching Regulators, Slide 21

22 Buck Switching Regulator V O = DV I Only valid for CCM When the converter goes into DCCM, output voltage rises and controller reduced duty cycle to keep voltages fixed A. Kruger 55:141: Advanced Circuit Techniques The University of Iowa Switching Regulators, Slide 22

23 Buck Ripple Voltage - Capacitor smoothes the output voltage. The capacitor charge current is I I o. The charge applied and removed during one cycle corresponds to the hatched area (remember Q = I t). The change in capacitor voltage is thus V o = Q C = 1 ON OFF C 1 t 2 2 t + 2 I 2 = T 8C I Note: both ΔI and ΔV o are peak-to-peak values. A. Kruger 55:141: Advanced Circuit Techniques The University of Iowa Switching Regulators, Slide 23

24 Buck Ripple Voltage - ESR V o = Q C = C 1 t 2 2 t ON OFF I 2 T = I 8C However, the capacitor has an ESR, that also contributes to the ripple voltage. The voltage across the ESR is v = ESR ESR i C Note that the voltage is not in phase with the voltage across the capacitor: V r = v ESR + j V o A. Kruger 55:141: Advanced Circuit Techniques The University of Iowa Switching Regulators, Slide 24

25 Buck Capacitor Selection - ESR V o = Q C = C 1 t 2 2 t ON OFF I 2 = T 8C I v = ESR ESR i C V r = v ESR + j V o Both capacitance and ESR contribute to ripple voltage. Capacitor dissipates (ESR) i 2 C(rms) and must be able to handle this Capacitor must be able to handle ripple current ΔI Good capacitor can cost $$$ A. Kruger 55:141: Advanced Circuit Techniques The University of Iowa Switching Regulators, Slide 25

26 Buck Coil Selection Assume V SAT = V D = 0 Continuous Mode i t ON Vo V = i = V V o I I t ON Vo i = t t OFF i = V OFF o t + t = 1 ON OFF f s = V O (1 VO f i S / V I ) Suggested Design Equation i = 0. 2 I i = V o VI V satt t ON i = V o + V D t OFF Coil must handle I P without saturating core A. Kruger 55:141: Advanced Circuit Techniques The University of Iowa Switching Regulators, Slide 26

Efficiency If all the components are lossless: ESR = 0 VD = 0 ES = 0 Switch resistance = 0 Then the buck DC/DC converter can provide 100% conversion η = P o Po + P

27 Efficiency If all the components are lossless: ESR = 0 VD = 0 ES = 0 Switch resistance = 0 Then the buck DC/DC converter can provide 100% conversion η = P o Po + P diss P = V o o I o P = P + P + P + P + diss SW D coil cap P controller A. Kruger 55:141: Advanced Circuit Techniques The University of Iowa Switching Regulators, Slide 27

28 Efficiency η = P o Po + P diss P = V o o I o P + D = VDI F ( avg ) f s ( V I t ) R F RR P = P + P + P + P + diss SW D coil cap P controller P D = V I + 2 v i t 2 P = ESR SAT SW SW SW Conduction loss Switching loss SW cap I C(rms) P 2 coil = Rcoil I + ( rms) Pcore( f ) A. Kruger 55:141: Advanced Circuit Techniques The University of Iowa Switching Regulators, Slide 28

Buck Coil Selection w = 1 2 2 i = V O (1 VO f i S / V I ) Suggested Design Equation i = 0.

29 Buck Coil Selection w = i = V O (1 VO f i S / V I ) Suggested Design Equation i = 0. 2 I Continuous Mode Discontinuous Mode A. Kruger 55:141: Advanced Circuit Techniques The University of Iowa Switching Regulators, Slide 29

30 Coil Selection Coil must carry some rms current I to feed the load Coil must handle I P without saturating the core. I = I O Buck I = V V O I I O Boost I V = 1 V O I I O Buck-boost Continuous Mode Discontinuous Mode A. Kruger 55:141: Advanced Circuit Techniques The University of Iowa Switching Regulators, Slide 30

31 Switching Regulator: Controller Both Pulse Frequency Modulation and Pulse Width Modulation (PWM) can be used for control. Most commercial ICs use PWM control with f s between 10 khz and 1 MHz Two-types of control: voltage and current-mode control A. Kruger 55:141: Advanced Circuit Techniques The University of Iowa Switching Regulators, Slide 31

32 Voltage Mode PWM Control Comparator Sawtooth generator running at f S Voltage Mode Control Waveforms Question: what type of feedback? How does it affect the output resistance? A. Kruger 55:141: Advanced Circuit Techniques The University of Iowa Switching Regulators, Slide 32

33 Voltage Mode PWM Control Comparator Sawtooth generator running at f S Voltage Mode Control Waveforms A. Kruger 55:141: Advanced Circuit Techniques The University of Iowa Switching Regulators, Slide 33

34 Error Amplifier in CCM Voltage Control Assume changes in output voltage is much slower than f s Equivalent for buck converter operating in CCM with voltage control Model as a linear system Will analyze later A. Kruger 55:141: Advanced Circuit Techniques The University of Iowa Switching Regulators, Slide 34

35 Gated Oscillator Converter Uncommitted op-amp Switch Reference Comparator Oscillator, fixed duty cycle A. Kruger 55:141: Advanced Circuit Techniques The University of Iowa Switching Regulators, Slide 35

36 Gated Oscillator Converter Comparator with hysteresis ~ 10 mv Non-linear element non-linear feedback More stable feedback, more efficient converter A. Kruger 55:141: Advanced Circuit Techniques The University of Iowa Switching Regulators, Slide 36

37 Gated Oscillator Converter Comparator with hysteresis ~ 10 mv Can be uses to terminate switch cycle prematurely Very low supply current oscillator switched only when needed, when FB drops below reference Adaptive base drive to make sure switch is not overdriven improve efficiency Hysteresis ensures loop stability without complex compensation networks D Duty cycle optimized for circuits where V in and V out differ by factor D A. Kruger 55:141: Advanced Circuit Techniques The University of Iowa Switching Regulators, Slide 37

T1173 DC/DC Converter 100 μa in standby mode Uncommitted op-amp Step-up or step down Supply 2.

38 T1173 DC/DC Converter 100 μa in standby mode Uncommitted op-amp Step-up or step down Supply V 1 A internal switch Switch frequency ~ 24 khz, duty cycle ~ 50% Comparator with ~ 5 mv hysteresis 50% Duty cycle A. Kruger 55:141: Advanced Circuit Techniques The University of Iowa Switching Regulators, Slide 38

39 Wiring as a buck regulator A. Kruger 55:141: Advanced Circuit Techniques The University of Iowa Switching Regulators, Slide 39

40 V I V o D C A. Kruger 55:141: Advanced Circuit Techniques The University of Iowa Switching Regulators, Slide 40

41 V I D R 2 V o C R 1 A. Kruger 55:141: Advanced Circuit Techniques The University of Iowa Switching Regulators, Slide 41

42 V I D R 2 V o C R 1 Choose either R 2 or R 1, solve for other one. Choose a value > 50K A. Kruger 55:141: Advanced Circuit Techniques The University of Iowa Switching Regulators, Slide 42

43 V I D R 2 V o C R 1 Choose either R 2 or R 1, solve for other one. Choose a value > 50K A. Kruger 55:141: Advanced Circuit Techniques The University of Iowa Switching Regulators, Slide 43

44 V I C 1 D R 2 V o C R 1 Choose either R 2 or R 1, solve for other one. Choose a value > 50K A. Kruger 55:141: Advanced Circuit Techniques The University of Iowa Switching Regulators, Slide 44

45 V I 1 C 2 C 1 D R 2 V o C R 1 Choose either R 2 or R 1, solve for other one. Choose a value > 50K A. Kruger 55:141: Advanced Circuit Techniques The University of Iowa Switching Regulators, Slide 45

46 V I 1 C 2 C 1 V o C = ow ESR D R 2 C D = Schottky R 1 = No saturation Choose either R 2 or R 1, solve for other one. Choose a value > 50K A. Kruger 55:141: Advanced Circuit Techniques The University of Iowa Switching Regulators, Slide 46

47 V I Wiring as a boost regulator D V o R 2 C R 1 Choose either R 2 or R 1, solve for other one. Choose a value > 50K A. Kruger 55:141: Advanced Circuit Techniques The University of Iowa Switching Regulators, Slide 47

48 V I C 1 D V o R 2 C R 1 Choose either R 2 or R 1, solve for other one. Choose a value > 50K A. Kruger 55:141: Advanced Circuit Techniques The University of Iowa Switching Regulators, Slide 48

49 V I C 2 1 C 1 Wiring as a boost regulator D V o C = ow ESR D = Schottky = No saturation R 2 R 1 Choose either R 2 or R 1, solve for other one. Choose a value > 50K C A. Kruger 55:141: Advanced Circuit Techniques The University of Iowa Switching Regulators, Slide 49

50 Increasing Output Drive V I What if this switch can not handle the peak and rms currents? D R 2 R 1 V o C Buck Regulator A. Kruger 55:141: Advanced Circuit Techniques The University of Iowa Switching Regulators, Slide 50

51 Increasing Output Drive, Take 1 V I Add external power transistor to make this a Darlington Transistor D R 2 R 1 V o C A. Kruger 55:141: Advanced Circuit Techniques The University of Iowa Switching Regulators, Slide 51

52 Increasing Output Drive, Take 1 V I D R 2 R 1 V o C A. Kruger 55:141: Advanced Circuit Techniques The University of Iowa Switching Regulators, Slide 52

53 Increasing Output Drive, Take 2 V I Add external high-side switch D R 2 R 1 V o C A. Kruger 55:141: Advanced Circuit Techniques The University of Iowa Switching Regulators, Slide 53

54 Increasing Output Drive, Take 2 V I D R 2 R 1 V o C A. Kruger 55:141: Advanced Circuit Techniques The University of Iowa Switching Regulators, Slide 54

55 Increasing Output Drive, Take 2 V I D R 2 R 1 V o C A. Kruger 55:141: Advanced Circuit Techniques The University of Iowa Switching Regulators, Slide 55

56 Increasing Output Drive, Take 2 Application Notes for a particular chip often contain a wealth of information A. Kruger 55:141: Advanced Circuit Techniques The University of Iowa Switching Regulators, Slide 56

57 Increasing Output Drive, Take 3 A. Kruger 55:141: Advanced Circuit Techniques The University of Iowa Switching Regulators, Slide 57

58 T1173 DC/DC Converter A. Kruger 55:141: Advanced Circuit Techniques The University of Iowa Switching Regulators, Slide 58

with voltage control Model as a linear system A.

59 Error Amplifier in CCM Voltage Control Assume changes in output voltage is much slower than f s Equivalent for buck converter operating in CCM with voltage control Model as a linear system A. Kruger 55:141: Advanced Circuit Techniques The University of Iowa Switching Regulators, Slide 59

60 Error Amplifier in CCM Voltage Control What is the of the gain of Mod? v o = DV I v D = V v v o = V v c c sm sm V o H Mod = = vc I V V I sm A. Kruger 55:141: Advanced Circuit Techniques The University of Iowa Switching Regulators, Slide 60

Error Amplifier in CCM Voltage Control Equivalent for buck converter operating in CCM with voltage control Assume that the switching frequency is high enough so that PWM can be regarded as a

61 Error Amplifier in CCM Voltage Control Equivalent for buck converter operating in CCM with voltage control Assume that the switching frequency is high enough so that PWM can be regarded as a continuous process over range of (load) frequencies. One can show that the control-to-output transfer function is (this assumes R >> ESR, normally a very good assumption) H CO = v v O C = V V I sm 1 1+ j 2 ( ) + ( j ) Q 0 z = z = C 1 ( ESR) C Q = ( R coil 1 + ESR) C Note that and C in loop create a complex pole pair, and ESR creates as zero. Purpose of EA is to provide good regulation and good phase margin to ensure stability. A. Kruger 55:141: Advanced Circuit Techniques The University of Iowa Switching Regulators, Slide 61

62 Error Amplifier in CCM Voltage Control Purpose of EA is to provide good regulation and good phase margin to ensure stability. H EA = v v C O = ( j ( 1+ j 1)( 1+ j 2 ) )( 1+ j )( 1+ j ) With C << 2 >> C1 R3 R2 then = 2 = 3 = 4 = 5 R C R C R C R C = 1 R C 2 2 A. Kruger 55:141: Advanced Circuit Techniques The University of Iowa Switching Regulators, Slide 62

63 A. Kruger Switching Regulators, Slide 63 55:141: Advanced Circuit Techniques The University of Iowa Switching Regulator ( )( ) ( )( ) ) ( 1 1 j j j j j v v H O C EA = = ( ) ( ) Q j j V V v v H z sm I C O CO = = = = CO H EA H T Overall loop gain: ( )( ) ( )( ) Q j j j j j j j z ) / / ( ) / ( 1 / ) (

64 A. Kruger Switching Regulators, Slide 64 55:141: Advanced Circuit Techniques The University of Iowa Error Amplifier in CCM Voltage Control For fast response the crossover frequency f x should be as high as possible. A common choice is f x ~ f s /5 = = CO H EA H T Overall loop gain: ( )( ) ( )( ) Q j j j j j j j z ) / / ( ) / ( 1 / ) (

65 Current-Mode Control Pulse-by pulse current-limiting Fast transient response A. Kruger 55:141: Advanced Circuit Techniques The University of Iowa Switching Regulators, Slide 65

66 T1070 Switching Regulator A. Kruger 55:141: Advanced Circuit Techniques The University of Iowa Switching Regulators, Slide 66

67 T1070 Switching Regulator V Boost configuration 1 + R O = 2 V REF R1 Soft start: build up duty cycle A. Kruger 55:141: Advanced Circuit Techniques The University of Iowa Switching Regulators, Slide 67

68 Flyback Regulator Triple-output flyback regulator A. Kruger 55:141: Advanced Circuit Techniques The University of Iowa Switching Regulators, Slide 68

69 Switched Capacitor Converters Also called charge-pump converters Flying capacitor converters Generally used for low-power (~ 100 ma or less) A. Kruger 55:141: Advanced Circuit Techniques The University of Iowa Switching Regulators, Slide 69

70 Switched Capacitor Converters Basic Circuit Flying Capacitor Non-overlapping switches Switch frequency khz A. Kruger 55:141: Advanced Circuit Techniques The University of Iowa Switching Regulators, Slide 70

Switched Capacitor Converters Voltage Inverter Step 1: Charge C1 to V in Step 2: All switches open Step 3: Transfer charge With proper capacitors and

71 Switched Capacitor Converters Voltage Inverter Step 1: Charge C1 to V in Step 2: All switches open Step 3: Transfer charge With proper capacitors and switches, the inversion efficiency can be close to 100% A. Kruger 55:141: Advanced Circuit Techniques The University of Iowa Switching Regulators, Slide 71

It seems wrong with the + side connected to ground, but remember, pin 5 is

72 Classic Switched Capacitor IC (1) Assume 6 V Here (2) Then -6 V here Carefully note the polarity of this capacitor. It seems wrong with the + side connected to ground, but remember, pin 5 is at -6V A. Kruger 55:141: Advanced Circuit Techniques The University of Iowa Switching Regulators, Slide 72

73 Voltage Doubler Reverse biased => can remove from circuit A. Kruger 55:141: Advanced Circuit Techniques The University of Iowa Switching Regulators, Slide 73

74 Voltage Doubler Note, only one set of switches are used. One can put the other set of switches to generate other voltages A. Kruger 55:141: Advanced Circuit Techniques The University of Iowa Switching Regulators, Slide 74

75 TC962 Charge Pump Oscillator Uncommitted Zener diode Switches A. Kruger 55:141: Advanced Circuit Techniques The University of Iowa Switching Regulators, Slide 75

TC962 Charge Pump Oscillator Charges the

V REF V H /2 and the The square wave here has a

76 TC962 Charge Pump Oscillator Charges the capacitors until V + exceed V REF +V H /2 and the comparator trips. Discharges the capacitors until V + is less than V REF V H /2 and the comparator trips. The square wave here has a frequency that depends on the charge/discharge currents and capacitor A. Kruger 55:141: Advanced Circuit Techniques The University of Iowa Switching Regulators, Slide 76

77 TC962 Charge Pump Oscillator Increases the frequency owers the frequency A. Kruger 55:141: Advanced Circuit Techniques The University of Iowa Switching Regulators, Slide 77

78 TC962 Charge Pump Oscillator Can supply one s own clock here A. Kruger 55:141: Advanced Circuit Techniques The University of Iowa Switching Regulators, Slide 78

79 TC962 Charge Pump A. Kruger 55:141: Advanced Circuit Techniques The University of Iowa Switching Regulators, Slide 79

80 More Circuits A. Kruger 55:141: Advanced Circuit Techniques The University of Iowa Switching Regulators, Slide 80

81 More Circuits (1) Assume 6 V Here (4) The GND is the V+ supply for this one (2) Then -6 V here (5) So here we get -12 V (3) Which is the V dd for this one A. Kruger 55:141: Advanced Circuit Techniques The University of Iowa Switching Regulators, Slide 81

82 More Circuits A. Kruger 55:141: Advanced Circuit Techniques The University of Iowa Switching Regulators, Slide 82

83 More Circuits Increase output current capacity Noisy output NOR gate helps synchronize switching and reduce output ripple noise A. Kruger 55:141: Advanced Circuit Techniques The University of Iowa Switching Regulators, Slide 83

84 Switched Capacitor Building Block Charge transferred each cycle Charge transferred per unit time is Rewriting I ( V = 1 I = 1 V 2 ( f C ) s ( V R eq 1 ) 1 V 2) R 1 eq q = C V = q = T 1 f C s 1 f s C ( V ) V = I A. Kruger 55:141: Advanced Circuit Techniques The University of Iowa Switching Regulators, Slide 84

85 Implementation The switches are commonly implemented with FETs. While SC ideas have been around for a long time, modern IC manufacturing allows for commercially-viable implementations. The switches are not ideal and has an on resistance of a few Ohms A. Kruger 55:141: Advanced Circuit Techniques The University of Iowa Switching Regulators, Slide 85

86 Performance Thevenin equivalent Switched-capacitor inverter Note that the output is unregulated Output voltage is a function of the load Size & characteristics of capacitors? A. Kruger 55:141: Advanced Circuit Techniques The University of Iowa Switching Regulators, Slide 86

87 Performance RSW ~ 20 Ω (sum of all switch resistances) R O =? R R ESRC O ( RSW1 RSW 3 ESRC1) 2( RSW 2 RSW 4 ESRC1) + f pump C1 1 2 ESRC O RSW + + 4ESRC1 + f pump C1 2 2 Example C 1 = C2 = 10 µ F, fosc = 10 khz R O ~ ESR Ω Pump frequency, and switch resistances depend on temperature and the supply voltage. A. Kruger 55:141: Advanced Circuit Techniques The University of Iowa Switching Regulators, Slide 87

88 Performance Segment A is the voltage drop across the ESR of C 2 at the instant it goes from being charged by C 1 (current flowing into C 2 ) to being discharged through the load (current flowing out of C 2 ). The magnitude of this current change is 2I O, hence the total drop is 2I O ESRC 2. Segment B is the voltage change across C 2 during time t 2, the half of the cycle when C 2 supplies current to the load. The drop at B is I OUT t 2 /C 2. The peak-to-peak ripple voltage is the sum of these voltages V RIPPE 1 IO + 2ESRC 2 2 f pumpc2 To reduce ripple, increase pump frequency, increase C 2 and reduce ESR of C 2. ESR of C 1 is less important. The assumption is that C 1 and C 2 can charge/discharge during a pump cycle. If they are too big, this can contribute to the ripple as well. A. Kruger 55:141: Advanced Circuit Techniques The University of Iowa Switching Regulators, Slide 88

89 Switched-Capacitor Building Block Assume C A and C B are initially uncharged. When the switch is thrown to (a), C A charges and attains a charge C A V i. When the switch is thrown to (b) the capacitors are in parallel with value C T = C A +C B, and charge C A V i on them, so the output voltage after the first cycle is The charge on C B after the first cycle is When the switch is returned to (a), C A again attains a charge C A V i. When the switch is thrown to (b), the capacitors are in parallel and has a charge and output voltage after the second cycle: One can see that the output voltage is a geometric series or A. Kruger 55:141: Advanced Circuit Techniques The University of Iowa Switching Regulators, Slide 89

90 Switched-Capacitor Building Block A. Kruger 55:141: Advanced Circuit Techniques The University of Iowa Switching Regulators, Slide 90

91 Switched Capacitor Subtractor A. Kruger 55:141: Advanced Circuit Techniques The University of Iowa Switching Regulators, Slide 91

92 Switched Capacitor Instrumentation Amplifier TC1043 IC Good CMRR A. Kruger 55:141: Advanced Circuit Techniques The University of Iowa Switching Regulators, Slide 92

93 Switched Capacitor Difference Integrator A. Kruger 55:141: Advanced Circuit Techniques The University of Iowa Switching Regulators, Slide 93

94 Switched Capacitor Difference Integrator A. Kruger 55:141: Advanced Circuit Techniques The University of Iowa Switching Regulators, Slide 94

95 A. Kruger 55:141: Advanced Circuit Techniques The University of Iowa Switching Regulators, Slide 95

55:141 Advanced Circuit Techniques Switching Regulators

55:141 Advanced Circuit Techniques Switching Regulators Material: ecture Notes, Handouts, and Sections of Chapter 11 of Franco A. Kruger 55:141: Advanced Circuit Techniques The University of Iowa Switching