1

EE462L, Spring 2014

DC−DC Boost Converter

2

V

in

+

V

out

–

C

i

C

I

out

i

in

Buck
converter

i

L

L

+ v

L

–

Boost
converter

V

in

+

V

out

–

C

i

C

I

out

i

in

i

L

L

+ v

L

–

3

Boost converter

This is a much more unforgiving circuit than the buck

converter

V

in

+

V

out

–

C

i

C

I

out

i

in

i

L

L

+ v

L

–

i

D

•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 I

out

= 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

V

in

+

V

out

–

C

i

C

I

out

i

in

i

L

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

5

Boost converter

Using KVL and KCL in the average sense, the

average values are

+ 0 V –

I

out

V

in

+

V

out

–

C

I

out

L

0 A

I

in

V

in

+

V

out

–

C

i

C

I

out

i

in

i

L

L

+ v

L

–

i

D

Find the input/output equation by examining the

voltage across the inductor

6

Switch closed for DT seconds

Reverse biased, thus the
diode is open

L

V

dt

di

in

L



for DT

seconds

V

in

+

V

out

–

C

I

out

i

in

i

L

L

I

out

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

+ V

in

−

7

Switch open for (1 − D)T seconds

Diode closed. Assume
continuous conduction.

L

V

V

dt

di

out

in

L





V

in

+

V

out

–

C

I

out

i

in

i

L

L

for (1−D)T seconds

(i

L

– I

out

)

+ (V

in

− V

out

)

−

8

Since the average voltage across L is

zero



 



0

1















out

in

in

Lavg

V

V

D

V

D

V

in

in

in

out

V

D

V

D

V

D

V















)

1

(

D

V

V

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,

L

V

dt

di

V

v

in

L

in

L





,

L

V

V

dt

di

V

V

v

out

in

L

out

in

L









,

sec

/

A

L

V

in

DT

(1 − D)T

T

I

max

I

min

I

avg

= I

in

I

avg

= I

in

is half way between

I

max

and I

min

sec

/

A

L

V

V

out

in



ΔI

i

L

10

Inductor current rating

 

2

2

2

2

2

12

1

12

1

I

I

I

I

I

in

pp

avg

Lrms















2

2

2

2

3

4

2

12

1

in

in

in

Lrms

I

I

I

I









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

in

in

Lrms

I

I

3

2



2I

in

0

I

avg

= I

in

ΔI

i

L

Use max

11

MOSFET and diode currents and current

ratings

in

rms

I

I

3

2



Use max

2I

in

0

2I

in

0

Take worst case D for
each

V

in

+

V

out

–

C

i

C

I

out

i

in

i

L

L

+ v

L

–

i

D

12

Capacitor current and current rating

2I

in

−I

out

−I

out

0

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

out

out

Crms

I

I



Use
max

i

C

= (i

D

– I

out

)

V

in

+

V

out

–

C

i

C

I

out

i

in

i

L

L

i

D

See the lab document for the
derivation

13

Worst-case load ripple voltage

Cf

I

C

T

I

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

V

in

+

V

out

–

C

I

out

i

in

i

L

L

V

in

+

V

out

–

C

I

out

i

in

i

L

L

15

Continuous current in L

sec

/

A

L

V

V

out

in















f

L

D

D

V

T

D

L

V

D

V

T

D

L

V

V

I

boundary

in

boundary

in

in

boundary

in

out

in







































1

1

1

1

1

1

1

2

f

I

D

V

L

in

in

boundary

2



2I

in

0

I

avg

= I

in

i

L

(1 − D)T

f

I

V

L

in

in

2



guarantees continuous
conduction

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

use max

use min

,

2

f

L

D

V

I

boundary

in

in



16

Impedance matching

out

out

load

I

V

R



equiv

R













load

out

out

out

out

in

in

equiv

R

D

I

V

D

D

I

V

D

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





in

out

I

D

I



 1

17

Example of drawing maximum power

from solar panel

I

sc

V

oc

P

max

is approx.

130W (occurs at
29V, 4.5A)







44

.

6

5

.

4

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

18

Connect a 100Ω resistor directly, extract only

14W

130W

6.4

4Ω

res

ist

or

100Ω

resistor

14W





75

.

0

100

44

.

6

1

1

,

1

2















load

equiv

load

equiv

R

R

D

R

D

R

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

5A

10A

10A

120V

120V

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

20

5A

1500µF50kHz

0.067V

BOOST DESIGN

MOSFET. 250V, 20A

L. 100µH,
9A

C. 1500µF, 250V, 5.66A
p-p

Diode. 200V, 16A

21

40V

2A

50kHz

200µH

BOOST DESIGN

MOSFET. 250V, 20A

L. 100µH,
9A

C. 1500µF, 250V, 5.66A
p-p

Diode. 200V, 16A