P6KE + fast Silicon clamp is slow?

[741]

The series clamp network does have some effect - but the diodes do not seem to be very fast, or to clamp much. Scope leads directly across the clamp diodes (see image).

I used a BYV27-200-TR (fast silicon) in series with a P6KE18A-R0G (Unidirectional 18V TVS). Aside: I was unable to find a so-called "soft recovery" type for the silicon except at high cost.

One thing does confuse me - the P6KE data sheet from LittelFuse headlines the fast response
• Fast response time: typically less than 1.0ps from 0 Volts to BV min

then shows a response plot in milli-seconds.

[T3sl4co1l]

Show layout.

What they are plotting is the waveform used to measure surge capability.  I don't know that anyone actually specifies response time, in terms of having a fixture to actually measure it; I think it's just assumed.  Rest assured, avalanche breakdown is very fast -- the limiting factor in circuit, is the path to the diode, not the diode itself.  In particular, at these impedances, stray inductance.

So, it's very likely that layout is to blame for the lumpy waveforms.

Tim

[Yansi]

So, it's very likely that layout is to blame for the lumpy waveforms.

Or simply the probing technique 

[T3sl4co1l]

So, it's very likely that layout is to blame for the lumpy waveforms.

Or simply the probing technique 

Currently putting that as second most likely.  All the more reason for layout (including probing technique) pics.

Tim

[AndersJ]

Could this happen if the FET turn off is slow?

[T3sl4co1l]

Also, curious what's on the other side of the transformer, and what the small-signal parameters of the transformer are, or its frequency response, or just winding schedule, something.  Those waveforms look pretty lumpy at best..

Tim

[741]

Thanks for the feedback 

I wrongly thought that because the scope was right across the TVS, the layout was not a concern 

Now I've added a 56uF 0.2R ESR capacitor close to the TVS so as to provide a low-Z path back to Vdd.

Success!!