AN-42047

APPLICATION NOTE

2

REV. 0.9.0 8/19/04

input voltage frequency, the current will look much closer to

the input waveform (load dependent); however, any little

interruption on the mainline will cause the entire system to

react negatively. In saying that, in designing a SMPS, the

continue to power its load.

Figure 4. SMPS Input Without PFC

the circuit in Figure 4) with a very light load, and hence, very

peaks, but only a small percentage with respect to the overall

greater than its stored voltage, meaning that a non-PFC cir-

cycle time.

After 90 degrees (Figure 6), the half cycle from the bridge

the bridge, inhibiting current flow into the capacitor (via

is. All the circuitry in the supply chain (the wall wiring, the

diodes in the bridge, circuit breakers, etc) must be capable of

carrying this huge peak current. During these short periods

to average this spike out so it can use the rest of the cycle to

accumulate energy, in essence smoothing out the huge peak

current, by using power factor correction.

Figure 6. Voltage and Current Waveforms in a

Simple Rectifier Circuit

cycle rather than just a small portion of it. Today’s non-linear

loads make it impossible to know when a large surge of cur-

rent will be required, so keeping the inrush to the capacitor

constant over the entire cycle is beneficial and allows a much

correction.

Boost Converters the Heart of Power

Factor Correction

Boost converter topology is used to accomplish this active

power-factor correction in many discontinuous/continuous

modes. The boost converter is used because it is easy to

implement and works well. The simple circuit in Figure 7 is

a short refresher of how inductors can produce very high

voltages. Initially, the inductor is assumed to be uncharged,

inductor, as it is defined by the current direction (inflow side

is positive). When the switch opens causing the current to

slope). Looking at it mathematically:

or L times the change in current per unit time, the voltage

approaches negative infinity (the inductor reverses polarity).

Because the inductor is not ideal, it contains some amount of

series resistance, which loads this “infinite” voltage to a

finite number. With the switch open, and the inductor dis-

charging, the voltage across it reverses and becomes additive

connected to the output of this circuit, the capacitor would

charge to this high voltage (perhaps after many switch

cycles). This is how boost converters boost voltage, as shown

in Figure 8.

V1

D1

Cin

R1

+

–

RTN

Vo (to PWM)

50

Time (s)

100

0

0

100

130

-100

-130

Vin(t)

Vc(t)

V

Input Voltage

(Full Rectified)

Input Current

Charging Bulk Input

0

180

90

270

360

Deg

∫

=

dt

V

L

I

L

L

1

.

t

i

L

dt

di

L

V

L

∆

∆

≈

=

,