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ionutgaga 7th February 2008 07:04 AM

optocoupling power stage - hiss with 6N137
I'm wondering why 6n137 is rated at 100v/us in some datasheets, or 10000v/us in other places. Certainly, after i remove other variables (like pcb, power fet, inductor, etc), a small hiss is presented at output, even if i drive only power stage with clean square wave 250khz (offcourse, with deadtime).
It is a posible cause for hiss this 6n137 optocoupler?

Description on final stage: 6n137+tc429+power fet+floating 15v supply

Eva 7th February 2008 07:56 AM

The hiss may be just due to the sum of time errors of all components (jitter). Power supply noise is a direct contributor too. When doing open loop tests, note that output filter resonance may boost some frequencies unles it happens well above 20Khz.

6N137 performance depends on manufacturer. I think that Avago (formerly Agilent (formerly Hewlett packard)) supplies the one rated at 10KV/us. You may consider trying other fast shielded optocouplers.

Anyway, if erratic switching due to false optocoupler triggering is actually taking place, the oscilloscope should show it. False triggering only happens during transients and may create a self oscillating system (this has happened to me).

ionutgaga 7th February 2008 08:25 AM

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Thanks Eva for your quick answer. This is my sch:

ionutgaga 7th February 2008 08:29 AM

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And this is my layout. The yellow traces is from top ground plane (unfigured for clarity reason). May i ask to you to analyse this and give your valuable advice?

fredos 7th February 2008 01:52 PM

Synchronize your floating power supply to amplifier clock....This should fix your probleme!

ionutgaga 7th February 2008 02:30 PM

Thanks Fredos!
But i'm already checked that... even if i synch, even if i adjust the Fosc of smps, same white noise... (ok, small, but perceptible)

ionutgaga 7th February 2008 02:46 PM

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This is top layer:

ionutgaga 7th February 2008 02:47 PM

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And bottom:

lumanauw 7th February 2008 03:01 PM

Could you try change C20 to 4n7 and R11 to 1k8?

ionutgaga 7th February 2008 03:08 PM

In fact, i tryiet with semireglable in place of R11, modifying Fosc, but give more noise (other type of noise, more like radio interference or something like this). Indeed, i tryiet before to sync SG with 62.5kHZ, but result if the same with RC with values showed in the sch.

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