AN-6961

APPLICATION NOTE

© 2009 Fairchild Semiconductor Corporation

www.fairchildsemi.com

Rev. 1.0.2 • 4/8/09

2

Basic Operation of the Boost Converter

The typical boost converter and its operational waveforms

are shown in Figure 2, 3, and 4, respectively.

()

g

vt

+

−

()

L

vt

+−

D

Co

Ro

o

V

+

−

()

L

it

Q

b

L

Figure 2. Boost Converter

()

g

vt

+

−

()

L

vt

+−

()

L

it

Q

b

L

()

g

vt

+

−

()

L

vt

+−

Co

Ro

o

v

+

−

()

L

it

b

L

(a) Switch Q is ON

(b) Switch Q is OFF

Figure 3. Switching Sequences of the Boost Converter

()

og

b

vv t

L

−

()

L

vt

on

t

off

t

Q

()

L

it

()

g

b

vt

L

()

g

vt

o

vv t

−

T

,

(t)

Lavg

i

Figure 4. One-cycle Waveform of the Boost Converter

Operation Principle

Switch Q is ON:

When Q turns on, the rectifier diode D is

and can be expressed as:

b

g

on

L

L

(t)

V

t

I

=

)

(

(1)

Switch Q is OFF:

reversed. The diode D is forward-biased in this stage. The

be expressed as:

b

g

o

off

L

L

(t)

V

-

V

t

I

=

)

(

(2)

Controlled On-Time:

The on-time of the power MOSFET Q

is determined by the output of the error amplifier that

monitors the preregulator output voltage. With a low-

bandwidth error amplifier, the feedback signal is almost

constant during a half AC cycle, resulting a fixed on-time of

the power MOSFET at a specific AC voltage and some

certain output power level. Therefore, the peak inductor

achieving a natural power factor correction mechanism.

Figure 5 shows the typical inductor current waveform during

a half AC cycle.

on

Off

,

()

Lavg

it

()

g

vt

max

T

Gate

,

Lpk

i

min

T

-

fixed On Time

Figure 5. Controlled On-Time Inductor Current Waveform

Referring to Figure 4, considering one switching period the

average area of triangle waveform of inductor current:

s

2

b

s

on

g

T

L

2

T

t

)

t

(

V

)

(

•

•

⎥

⎦

⎤

⎢

⎣

⎡

•

⎥

⎥

⎦

⎤

⎢

⎢

⎣

⎡

+

=

(t)

V

-

V

(t)

V

t

I

g

o

2

g

avg

L,

(3)