EE 462 L Fall 2011 DCDC Buck Converter

EE 462 L, Fall 2011 DC−DC Buck Converter 1

Objective – to efficiently reduce DC voltage The DC equivalent of an AC transformer Iin + Vin − Iout DC−DC Buck Converter + Vout − Lossless objective: Pin = Pout, which means that Vin. Iin = Vout. Iout and 2

Here is an example of an inefficient DC−DC converter The load R 1 + Vin − + R 2 Vout − If Vin = 39 V, and Vout = 13 V, efficiency η is only 0. 33 Unacceptable except in very low power applications 3

Another method – lossless conversion of 39 Vdc to average 13 Vdc Stereo voltage Switch closed Switch open 39 + 39 Vdc – Rstereo 0 Switch state, Stereo voltage Closed, 39 Vdc DT T Open, 0 Vdc If the duty cycle D of the switch is 0. 33, then the average voltage to the expensive car stereo is 39 ● 0. 33 = 13 Vdc. This is lossless conversion, but is it acceptable? 4

Convert 39 Vdc to 13 Vdc, cont. + 39 Vdc – Try adding a large C in parallel with the load to control ripple. But if the C has 13 Vdc, then when the switch closes, the source current spikes to a huge value and burns out the switch. Rstereo C L + 39 Vdc – C Rstereo Try adding an L to prevent the huge current spike. But now, if the L has current when the switch attempts to open, the inductor’s current momentum and resulting Ldi/dt burns out the switch. lossless L + 39 Vdc – C Rstereo By adding a “free wheeling” diode, the switch can open and the inductor current can continue to flow. With highfrequency switching, the load voltage ripple can be reduced to a small value. A DC-DC Buck Converter 5

Taken from “Waveforms and Definitions” PPT C’s and L’s operating in periodic steady-state Examine the current passing through a capacitor that is operating in periodic steady state. The governing equation is which leads to Since the capacitor is in periodic steady state, then the voltage at time to is the same as the voltage one period T later, so or The conclusion is that which means that the average current through a capacitor operating in periodic steady state is zero 6

Taken from “Waveforms and Definitions” PPT Now, an inductor Examine the voltage across an inductor that is operating in periodic steady state. The governing equation is which leads to Since the inductor is in periodic steady state, then the voltage at time to is the same as the voltage one period T later, so or The conclusion is that which means that the average voltage across an inductor operating in periodic steady state is zero 7

Taken from “Waveforms and Definitions” PPT KVL and KCL in periodic steady-state Since KVL and KCL apply at any instance, then they must also be valid in averages. Consider KVL, KVL applies in the average sense The same reasoning applies to KCL applies in the average sense 8

Buck converter + v. L – i. L iin Iout L Vin C • Assume large C so that Vout has very low ripple i. C + Vout – • Since Vout has very low ripple, then assume Iout has very low ripple What do we learn from inductor voltage and capacitor current in the average sense? +0 V– iin Vin Iout L C + Vout 0 A – 9

The input/output equation for DC-DC converters usually comes by examining inductor voltages + (Vin – Vout) – iin Switch closed for DT seconds Vin i. L L C Iout + Vout (i. L – Iout) – Reverse biased, thus the diode is open for DT seconds Note – if the switch stays closed, then Vout = Vin 10

Switch open for (1 − D)T seconds – Vout + i. L Vin L C Iout + Vout (i. L – Iout) – i. L continues to flow, thus the diode is closed. This is the assumption of “continuous conduction” in the inductor which is the normal operating condition. for (1−D)T seconds 11

Since the average voltage across L is zero The input/output equation becomes From power balance, , so Note – even though iin is not constant (i. e. , iin has harmonics), the input power is still simply Vin • Iin because Vin has no harmonics 12

Examine the inductor current Switch closed, Switch open, From geometry, Iavg = Iout is halfway i. L between Imax and Imin Imax Iavg = Iout ΔI Imin DT Periodic – finishes a period where it started (1 − D)T T 13

Effect of raising and lowering Iout while holding Vin, Vout, f, and L constant i. L Raise Iout ΔI ΔI Lower Iout ΔI • ΔI is unchanged • Lowering Iout (and, therefore, Pout ) moves the circuit toward discontinuous operation 14

Effect of raising and lowering f while holding Vin, Vout, Iout, and L constant i. L Lower f Raise f • Slopes of i. L are unchanged • Lowering f increases ΔI and moves the circuit toward discontinuous operation 15

Effect of raising and lowering L while holding Vin, Vout, Iout and f constant i. L Lower L Raise L • Lowering L increases ΔI and moves the circuit toward discontinuous operation 16

Taken from “Waveforms and Definitions” PPT RMS of common periodic waveforms, cont. Sawtooth V 0 T 17

Taken from “Waveforms and Definitions” PPT RMS of common periodic waveforms, cont. Using the power concept, it is easy to reason that the following waveforms would all produce the same average power to a resistor, and thus their rms values are identical and equal to the previous example V V 0 0 0 -V V 0 0 0 V 0 18

Taken from “Waveforms and Definitions” PPT RMS of common periodic waveforms, cont. Now, consider a useful example, based upon a waveform that is often seen in DC-DC converter currents. Decompose the waveform into its ripple, plus its minimum value. the ripple = + the minimum value 0 19

Taken from “Waveforms and Definitions” PPT RMS of common periodic waveforms, cont. Define 20

Taken from “Waveforms and Definitions” PPT RMS of common periodic waveforms, cont. Recognize that 21

Inductor current rating Max impact of ΔI on the rms current occurs at the boundary of continuous/discontinuous conduction, where ΔI =2 Iout Iavg = Iout 0 i. L ΔI Use max 22

Capacitor current and current rating i. L Iout L C Iout 0 −Iout i. C = (i. L – Iout) Note – raising f or L, which lowers ΔI, reduces the capacitor current ΔI Max rms current occurs at the boundary of continuous/discontinuous conduction, where ΔI =2 Iout Use max 23

MOSFET and diode currents and current ratings i. L iin Iout L C (i. L – Iout) 2 Iout 0 Use max Take worst case D for each 24

Worst-case load ripple voltage Iout 0 −Iout i. C = (i. L – Iout) C charging T/2 During the charging period, the C voltage moves from the min to the max. The area of the triangle shown above gives the peak-to-peak ripple voltage. Raising f or L reduces the load voltage ripple 25

Voltage ratings i. L iin Switch Closed Iout L Vin C i. C C sees Vout + Vout – Diode sees Vin MOSFET sees Vin i. L Switch Open Vin Iout L C i. C + Vout – • Diode and MOSFET, use 2 Vin • Capacitor, use 1. 5 Vout 26

There is a 3 rd state – discontinuous Iout Vin L C Iout + Vout – • Occurs for light loads, or low operating frequencies, where the inductor current eventually hits zero during the switchopen state • The diode opens to prevent backward current flow • The small capacitances of the MOSFET and diode, acting in parallel with each other as a net parasitic capacitance, interact with L to produce an oscillation • The output C is in series with the net parasitic capacitance, but C is so large that it can be ignored in the oscillation phenomenon 27

Inductor voltage showing oscillation during discontinuous current operation v. L = (Vin – Vout) Switch closed v. L = –Vout Switch open 650 k. Hz. With L = 100µH, this corresponds to net parasitic C = 0. 6 n. F 28

Onset of the discontinuous state 2 Iout Iavg = Iout i. L 0 (1 − D)T Then, considering the worst case (i. e. , D → 0), use max guarantees continuous conduction use min 29

Impedance matching Iout = Iin / D Iin + + Source DC−DC Buck Converter Vin Vout = DVin − − Iin + Vin Equivalent from source perspective − So, the buck converter makes the load resistance look larger to the source 30

Example of drawing maximum power from solar panel Pmax is approx. 130 W (occurs at 29 V, 4. 5 A) Isc For max power from panels at this solar intensity level, attach Voc I-V characteristic of 6. 44Ω resistor But as the sun conditions change, the “max power resistance” must also change 31

Connect a 2Ω resistor directly, extract only 55 W 130 W 2Ω res isto r 55 W or 4Ω 4. 6 t sis re To draw maximum power (130 W), connect a buck converter between the panel and the load resistor, and use D to modify the equivalent load resistance seen by the source so that maximum power is transferred 32

Buck converter for solar applications The panel needs a ripple-free current to stay on the max power point. Wiring inductance reacts to the current switching with large voltage spikes. ipanel Vpanel + v. L – i. L Iout L C i. C + Vout – Put a capacitor here to provide the ripple current required by the opening and closing of the MOSFET In that way, the panel current can be ripple free and the voltage spikes can be controlled We use a 10µF, 50 V, 10 A high-frequency bipolar (unpolarized) capacitor 33

BUCK DESIGN 9 A 10 A 250 V 5. 66 A Our components 200 V, 250 V 16 A, 20 A 40 V 10 A 40 V Likely worst-case buck situation 10 A Our L. 100µH, 9 A Our C. 1500µF, 250 V, 5. 66 A p-p Our D (Diode). 200 V, 16 A Our M (MOSFET). 250 V, 20 A 34

BUCK DESIGN 10 A 0. 033 V 1500µF 50 k. Hz Our L. 100µH, 9 A Our C. 1500µF, 250 V, 5. 66 A p-p Our D (Diode). 200 V, 16 A Our M (MOSFET). 250 V, 20 A 35

BUCK DESIGN 40 V 200µH 2 A 50 k. Hz Our L. 100µH, 9 A Our C. 1500µF, 250 V, 5. 66 A p-p Our D (Diode). 200 V, 16 A Our M (MOSFET). 250 V, 20 A 36