AN3111

Circuit design

Doc ID 16815 Rev 2

9/23

From Equation 6, we have:

Equation 12

A 1

current, two SMT 2

Operating principles:

The operation of the converter can be described as follows:

●

The period of t0 to t3 is near the line voltage zero crossing. The period of t4 to t7 is

around the peak line voltage.

●

[t0,t1], Q1 is turned on at time t0. The inductor current reaches its peak at t1. Near the

line voltage zero crossing, the peak amplitude is lower than the constant value Ipk.

●

[t1,t2] Q1 is turned off at t1. The inductor current decreases to zero at t2.

●

[t2,t3] the drain voltage of Q1 starts to fall at t2, and reaches zero at t3. The ZCD pin of

the controller detects the ZCD signal low and turns on Q1 again at t3. Q1 is turned on

at zero voltage.

●

[t4,t5], Q1 is turned on at time t4. The inductor current reaches its peak at t4. The peak

amplitude is the constant value Ipk.

●

[t5,t6] Q1 is turned off at t5. The inductor current decreases to zero at t6.

●

[t6,t7] the drain voltage of Q1 starts to fall at t6, and reaches its minimum value at t7,

but it does not reach zero. The ZCD pin of the controller detects the ZCD signal low and

turns on Q1 again at t7. Q1 is turned on at reduced voltage.

Figure 4.

Illustration of key waveforms of the converter

The MOSFET Q1 operates zero voltage turn-on when the instant input voltage is lower than

the output voltage. It is turned on at reduced voltage when the input voltage is higher than

the output voltage. Therefore, it is a partial soft-switched converter.

)

Vout

Vpk

Vpk

*

Vout

(

Ipk

max*

fsw

1

L

+

=

L200

μH

=