AN1262 APPLICATION NOTE

16/42

10

CLAMP CIRCUIT DESIGN

The drain pin of the IC needs to be properly clamped to prevent the spike due to the transformer leakage induc-

tance from exceeding the breakdown voltage (700V minimum). An RCD clamp (see fig. 5a) is a popular cheap

across the clamp resistor at all times. If minimizing the light load losses is a must, the use of a zener or transil

clamp (see fig. 5b) is recommended whenever possible. Such circuit gives also a better defined clamping level

but dissipates more power at full load.

Figure 5. Suggested clamp circuit topologies

to check carefully the spike under overload and start-up conditions to make sure that the voltage rating of the

MOSFET is never exceeded.

The capacitor must be low-loss type (with polypropylene or polystyrene film dielectric) to reduce power dissipa-

tion and prevent overheating due to the high peak currents it experiences.

The minimum value of the clamp resistance is:

,

a)

R

C

D

RCD CLAMP

L6590

L6590D

L6590A

Drain

GND

b)

D

ZENER

CLAMP

Drain

GND

L6590

L6590D

L6590A

C

min

L

LK

I

lim

2

⋅

V

R

V

spik e

+

()

2

V

R

2

–

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

=

R

min

1

f

sw

C

min

1

V

sp ik e

V

R

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

+





ln

⋅⋅

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

=