DC to DC boost converter, Study notes of Power Electronics

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EE462L, Spring 2014

DC−DC Boost Converter

Vin

V

out

C

iC

Iout

iin

Buck converter

iL

L

• vL –

Boost converter

Vin

V

out

C

iC

Iout

i in

iL

L

• vL –

Boost converter

Vin

V

out

C

iC

Iout

i in

iL

L

• v L

i D

• (^) Modify your MOSFET firing circuit for Boost Converter

operation (see the MOSFET Firing Circuit document)

• (^) Limit your output voltage to 120V

Boost converter

Using KVL and KCL in the average sense, the average

values are

• 0 V – Iout

V

in

Vout

C

I

out

L

0 A

Iin

Vin

Vout

C

i C

I

out i in

i L

L

• vL –

i D

Find the input/output equation by examining the voltage

across the inductor

Switch open for (1 − D)T seconds

Diode closed. Assume

continuous conduction. L

V V

dt

di L in out





Vin

V

out

C

Iout

i in

iL

L

for (1−D)T seconds

(i L

– I

out

)

+ (V

in

− V

out

Since the average voltage across L is zero

    1       0 Lavg in in out

V D V D V V

out in in in

V  ( 1  D )  V  D  V  D  V

D

V

V

in

out





1

The input/output equation becomes

A realistic upper limit on boost is 5 times

!

Inductor current rating

2 2 2 2 2

I I I I I

Lrms avg pp in

    

2 2 2 2

3

4

2

12

1

Lrms in in in

I  I   I  I

Max impact of ΔI on the rms current occurs at the boundary of

continuous/discontinuous conduction, where ΔI =2I in

Lrms in

I I

2I in

0

I avg

= I in (^) ΔI

i L

Use max

MOSFET and diode currents and current ratings

rms in

I I

Use max

2I in

0

2I in

0

Take worst case D for each

V

in

Vout

C

i C

I

out i in

i L

L

• vL –

i D

Worst-case load ripple voltage

Cf

I

C

I T

C

Q

V

out out

The worst case is where C provides I out

for most of the period. Then,

−I out

0

i C

= (i D

• I out

)

14

Voltage ratings

Diode sees V out

MOSFET sees V out

C sees V out

• (^) Diode and MOSFET, use 2V out
• (^) Capacitor, use 1.5V out

Vin

Vout

C

Iout

i in

iL

L

V

in

Vout

C

Iout

i in

iL

L

16

Impedance matching

out

out

load I

V

R 

equiv

R

 

    load

out

out

out

out

in

in

equiv

D R

I

V

D

D

I

D V

I

V

R

2 2 1 1

1

1

   





 

DC−DC Boost

Converter

V in

−

−

I in

V in

−

I in

Equivalent from

source perspective

Source D

V

V

in

out 



1

out in

I  1  D I

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 - amps

I sc

V oc

P max

is approx. 130W

(occurs at 29V, 4.5A)

  6. 44 

4. 5

29

A

V

R load

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

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

in

I

3

(^2) 1. Vout I (^) out 2 Vout

in

I

3

2

10A 120V 5A 120V 10A

Likely worst-case boost situation

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

Our components

9A 250V

MOSFET. 250V, 20A

L. 100μH, 9A

C. 1500μF, 250V, 5.66A p-p

Diode. 200V, 16A

BOOST DESIGN

Comparisons of Output Capacitor Ripple Voltage

Converter Type Volts (peak-to-peak)

Boost

Cf

I

out

5A

1500μF (^) 50kHz

0.067V

BOOST DESIGN

MOSFET. 250V, 20A

L. 100μH, 9A

C. 1500μF, 250V, 5.66A p-p

Diode. 200V, 16A