17/42

AN1262 APPLICATION NOTE

and its power rating has to be:

Usually the resistor value will be selected much higher than the minimum to reduce losses. The clamp capacitor

will then be quite larger than the minimum as well.

The blocking diode must be not only very fast-recovery but also very fast-turn-on type to avoid additional drain

suggested ST parts.

Table 12. Recommended ST parts for blocking diode.

(16)

To take this into consideration, as a rule of thumb the stand-off voltage can be selected as high as 70% of the

desired clamp level. Please refer to [2] and [3] to see how these problems are handled.

The Transil or zener must have an adequate power handling capability in steady state operation:

.

Table 13 lists some recommended devices available from ST.

The same recommendations as in the RCD clamp case apply to the blocking diode in series to the Transil. Only

Table 13. Recommended ST parts for clamping.

Diode

Package

Diode

Package

BYT01-400

400

F126

STTA106

600

F126

SMBYT01-400

400

SMB

STTA106U

600

SMB

Pz

≤ 0.75 W

Pz = 1W

Pz = 1.5W

≤100 V

BZW04-154

BZW06-154

SMAJ154A-TR

BZW04-154

BZW06-154

SMBJ154A-TR

P6KE180A

1.5KE180A

SMCJ154A-TR

130 V

BZW04-188

BZW06-188

SMAJ188A-TR

BZW04-188

BZW06-188

SMBJ188A-TR

P6KE200A

1.5KE200A

SMCJ188A-TR

P

R

V

R

2

R

min

------------

1

2

--- L

LK

I

lim

2

f

sw

⋅⋅

⋅

+

=

P

Z

1

2

---

V

CL

V

CL

V

R

–

------------------------ L

LK

I

li m

2

f

sw

⋅⋅

⋅

⋅

=