Designing a fixed-off-time PFC

AN3112

12/36

Doc ID 16820 Rev 3

Equation 14

Inductor peak current:

Equation 15

It is also possible to calculate the RMS current flowing into the switch and into the diode,

needed to calculate the losses of these two elements.

RMS switch current:

Equation 16

RMS diode current:

Equation 17

It is worth remembering that the accuracy of the approximate energetic relationships

described here is quite good at maximum load for low values of parameter k, that is, at low

line voltage, but worsens at high line and as the power throughput is reduced. Since, in the

design phase, current stress is calculated at maximum load and minimum line voltage, their

accuracy is acceptable for design purposes.

4.3

Power section design

4.3.1

Bridge rectifier

The input rectifier bridge can use standard slow recovery, low-cost devices.

Typically a 600 V device is selected in order to obtain a good margin against mains surges.

An NTC resistor, limiting the current at turn-on, is required to avoid overstress to the diode

bridge.

The rectifier bridge power dissipation can be calculated using equations

Equation 18,Equation 19,Equation 20. The threshold voltage (Vth) and dynamic resistance

(Rdiode) of a single diode bridge can be found in the component datasheet.

max

PK

r

r

pk

I

k

3

8

k

6

IL

⋅

⋅

−

⋅

=

∆

A

04

.

2

A

98

.

6

34

.

0

3

8

34

.

0

6

IL

pk

=

⋅

⋅

−

⋅

=

∆

max

PK

r

max

pk

I

k

3

8

8

IL

⋅

⋅

−

=

A

01

.

8

A

98

.

6

34

.

0

3

8

8

IL

max

pk

=

⋅

⋅

−

=

π

⋅

−

⋅

⋅

=

3

k

16

2

V

k

P

ISW

min

out

min

in

rms

A

22

.

4

3

32

.

0

16

2

V

400

32

.

0

W

400

ISW

rms

=

π

⋅

−

⋅

⋅

=

π

⋅

⋅

=

3

k

16

V

k

P

ID

min

out

min

in

rms

A

57

.

2

3

32

.

0

16

V

400

32

.

0

W

400

ID

rms

=

π

⋅

⋅

⋅

=