boost converter class notes

1

EE462L, Spring 2014DC−DC Boost Converter

2

Vin +

Vout–

C iC

Iout iinBuck converter

iL

L

+ vL –

Boost converter

Vin +

Vout–

C iC

Iout iin

iL

L

+ vL –

3

Boost converter

This is a much more unforgiving circuit than the buck converter

Vin +

Vout–

C iC

Iout iin

iL

L

+ vL – iD

• If the MOSFET gate driver sticks in the “on” position, then there is a short circuit through the MOSFET – blow MOSFET!

• If the load is disconnected during operation, so that Iout = 0, then L continues to push power to the right and very quickly charges C up to a high value (250V) – blow diode and MOSFET!

• Before applying power, make sure that your D is at the minimum, and that a load is solidly connected

!

4

Boost converter

Vin +

Vout–

C iC

Iout iin

iL

L

+ vL – iD

• Modify your MOSFET firing circuit for Boost Converter operation (see the MOSFET Firing Circuit document)

• Limit your output voltage to 120V

5

Boost converter

Using KVL and KCL in the average sense, the average values are

+ 0 V – Iout

Vin +

Vout–

C

Iout

L

0 A

Iin

Vin +

Vout–

C iC

Iout iin

iL

L

+ vL – iD

Find the input/output equation by examining the voltage across the inductor

6

Switch closed for DT seconds

Reverse biased, thus the diode is openL

Vdtdi inL

for DT seconds

Vin +

Vout–

C

Iout iin

iL

L

Iout

Note – if the switch stays closed, the input is short circuited!

+ Vin −

7

Switch open for (1 − D)T seconds

Diode closed. Assume continuous conduction.L

VVdtdi outinL

Vin +

Vout–

C

Iout iin

iL

L

for (1−D)T seconds

(iL – Iout)

+ (Vin − Vout ) −

8

Since the average voltage across L is zero

01 outininLavg VVDVDV

inininout VDVDVDV )1(

DVV in

out

1

The input/output equation becomes

A realistic upper limit on boost is 5 times

!

9

Examine the inductor current

Switch closed,

Switch open,

LV

dtdiVv inL

inL ,

LVV

dtdiVVv outinL

outinL

,

sec/ ALVin

DT (1 − D)T

T

Imax

Imin

Iavg = Iin

Iavg = Iin is half way between

Imax and Imin

sec/ ALVV outin

ΔI

iL

10

Inductor current rating

22222121

121 IIIII inppavgLrms

2222342

121

inininLrms IIII

Max impact of ΔI on the rms current occurs at the boundary of continuous/discontinuous conduction, where ΔI =2Iin

inLrms II3

2

2Iin

0Iavg = Iin ΔI

iL

Use max

11

MOSFET and diode currents and current ratings

inrms II3

2

Use max

2Iin

0

2Iin

0

Take worst case D for each

Vin +

Vout–

C iC

Iout iin

iL

L

+ vL – iD

12

Capacitor current and current rating

2Iin −Iout

−Iout

0

Max rms current occurs at the boundary of continuous/discontinuous conduction, where ΔI =2Iout

outCrms II

Use max

iC = (iD – Iout)

Vin +

Vout–

C iC

Iout iin

iL

L

iD

See the lab document for the derivation

13

Worst-case load ripple voltage

CfI

CTI

CQV outout

The worst case is where C provides Iout for most of the period. Then,

−Iout

0

iC = (iD – Iout)

14

Voltage ratings

Diode sees Vout

MOSFET sees Vout

C sees Vout

• Diode and MOSFET, use 2Vout

• Capacitor, use 1.5Vout

Vin +

Vout–

C

Iout iin

iL

L

Vin +

Vout–

C

Iout iin

iL

L

15

Continuous current in Lsec/ A

LVV outin

fL

DD

VTD

L

VD

V

TDL

VVIboundary

in

boundary

inin

boundary

inoutin

11

11

1112

fIDVLin

inboundary 2

2Iin

0Iavg = Iin

iL

(1 − D)T

fIVLin

in2

guarantees continuous conduction

Then, considering the worst case (i.e., D → 1),

use max

use min

, 2fL

DVIboundary

inin

16

Impedance matching

outout

load IVR

equivR

loadout

outout

out

in

inequiv RD

IVD

DIVD

IVR 22 11

1

1

DC−DC Boost Converter

+

Vin

−

+

−

Iin

+

Vin

−

Iin

Equivalent from source perspective

Source DVV in

out

1

inout IDI 1

17

Example of drawing maximum power from solar panel

PV Station 13, Bright Sun, Dec. 6, 2002

0

1

2

3

4

5

6

0 5 10 15 20 25 30 35 40 45

V(panel) - volts

I - a

mps

Isc

Voc

Pmax is approx. 130W (occurs at 29V, 4.5A)

44.65.429AVRload

For max power from panels, attach

I-V characteristic of 6.44Ω resistor

But as the sun conditions change, the “max power resistance” must also change

18

Connect a 100Ω resistor directly, extract only 14W

PV Station 13, Bright Sun, Dec. 6, 2002

0

1

2

3

4

5

6

0 5 10 15 20 25 30 35 40 45

V(panel) - volts

I - a

mps

130W

6.44Ω

resistor

100Ω resistor

14W

75.0100

44.611 ,1 2 load

equivloadequiv R

RDRDR

To extract maximum power (130W), connect a boost 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

So, the boost converter reflects a high load resistance to a low resistance on the source side

19

Worst-Case Component Ratings Comparisons for DC-DC Converters

Converter Type

Input Inductor

Current (Arms)

Output Capacitor Voltage

Output Capacitor Current (Arms)

Diode and MOSFET Voltage

Diode and MOSFET Current (Arms)

Boost inI3

2 1.5 outV outI 2 outV inI3

2

5A 10A10A 120V 120V

Likely worst-case boost situation

5.66A 200V, 250V 16A, 20AOur components

9A 250V

MOSFET. 250V, 20A

L. 100µH, 9AC. 1500µF, 250V, 5.66A p-p

Diode. 200V, 16A

BOOST DESIGN

20

Comparisons of Output Capacitor Ripple Voltage

Converter Type Volts (peak-to-peak) Boost

CfIout

5A

1500µF 50kHz

0.067V

BOOST DESIGN

MOSFET. 250V, 20A

L. 100µH, 9AC. 1500µF, 250V, 5.66A p-p

Diode. 200V, 16A

21

Minimum Inductance Values Needed to Guarantee Continuous Current

Converter Type For Continuous

Current in the Input Inductor

For Continuous Current in L2

Boost fI

VLin

in2

–

40V

2A 50kHz

200µH

BOOST DESIGN

MOSFET. 250V, 20A

L. 100µH, 9AC. 1500µF, 250V, 5.66A p-p

Diode. 200V, 16A