Have I got this straight - zobel\snubber questions

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mr coffee

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#1
As I understand it, the "zobel network" on the output of a Class D or Class T switching amp is for stability and to prevent the amp from self-destructing if someone decides to run it into little or no load, i.e., without a speaker connected to the output, which can then cause the output filter to ring like crazy and blow MOSFETS in the output stage. Correct?

And the "zobel network" on the output of a Class D or Class T switching amp does so by damping the high impedance "load" (or more correctly, the lack of one), at high audio (and up) frequencies (10khz and up). Correct?

Hopefully I've got this straight. One more question...

Is the small amount of inductance in a wirewound resistor in the R-C "zobel" or "snubber" circuit an issue - I measured the inductance of 3 and 5 watt wirewound resistors (the sand box type) and some round 5 watt resistors in 8 ohm to 20 ohm values, and my cheap multimeter inductance function shows a small inductance, ranging from 20 to 40 uh. Is this a problem?

mr coffee
 
#2
Yes, inductance matters. Snubbers are there to dampen fast transients. Inductance slows them down and makes them less useful in their intended role. Do not use wirewounds if you can help it...... And the inductance will resonant with the snubber cap, potentially causing more headaches.

Please consider buying Ohmite or Stackpole 1 watt carbon composition resistors from Digikey and others. Pricey but I know but they will get the job done for a snubber.
See: PN OA100KE-ND at Digikey, for example.
Do you really need 5 watts? What value cap are you using in the snubber?
 
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#3
stoc005 said:
Yes, inductance matters. Snubbers are there to dampen fast transients. Inductance slows them down and makes them less useful in their intended role. Do not use wirewounds if you can help it......Please consider buying Ohmite or Stackpole 1 watt carbon composition resistors from Digikey and others. Pricey but I know but they will get the job done for a snubber.
Click to expand...

Yes, low inductance resistors are much better for this application. The power rating can be a watt or so, if you don't do full power bench testing
or have an excessively large snubber cap.
 
#4
Thanks for replies.

It's for a guitar amp, so there is always the chance somebody might try to play through it without a speaker connected. The app note I am working from specs a 3 watt resistor, and non-inductive winding wirewound resistors are hard to find, though maybe I can look harder. I found only 1 watt in carbon composition, although I guess I could use 3 of them in series. Kinda bulky. The other non-inductive ones I found were ceramic composition, which appear to be a bulk resistance construction similar to Carbon Comp., although I've never used them before. I tried two wirewounds in bucking position in series, but it didn't seem to do anything to lower the parasitic inductance as read on my cheapo meter.

So a few tens of uh. would put my output transistors at risk if somebody forgets to plug in the speakers? Darn.

The app note recommends .33uf and 15 ohms in series, so for ruggedness, I would think 3 watts would be the minimum. I've seen guitar players turn the volume control up all the way and stand there riffing waiting for a tech to "fix the problem", oblivious of what a stress they are putting on the power amp. Murphy's law says it will happen to this amp sooner or later.
 
#5
mr coffee said:
The app note recommends .33uf and 15 ohms in series, so for ruggedness, I would think 3 watts would be the minimum.
Click to expand...

The you'd better use those values, but for stability you need a noninductive resistor, if necessary three 1W 47 Ohm (or four 1W 62 Ohm)
carbon comps in parallel, to keep inductance low. Use a low inductance cap as well, extended foil types are good.
 
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