Input stage of the AC/DC converter

AN4205

12/23

Doc ID 023929 Rev 1

The choice of the correct size of the post filter has been an iterative process that has

required some trials before finding the optimal level of the desired ripple.

2.3

Clamp circuit

Figure 5.

Clamp circuit

The drain pin of the Power MOSFET device (STD3NK80Z) has to be properly clamped to

prevent a spike due to the transformer leakage inductance from exceeding the breakdown

voltage. An RCD clamp is a cheap solution but it dissipates power. Therefore we have

preferred to use a Zener or Transil™ clamp which is recommended to minimize the power

losses during the Power MOSFET commutation.

Imposing a safety margin of 15%, we have:

Equation 29

Hence:

In accordance with this specification the Transil™ is STP6KE300A.

DZ1

D1

T1

M1

R3

AM16371v1

V

ZENER

V

DCmax

V

DSS

15

–

=

+

V

ZENER

300V

≅