optocoupling power stage - hiss with 6N137

ionutgaga · 7th February 2008, 07:04 AM

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

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

ionutgaga · 7th February 2008, 08:29 AM

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

This is top layer:

ionutgaga · 7th February 2008, 02:47 PM

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.

7th February 2008, 07:04 AM	#1
ionutgaga diyAudio Member Join Date: Jul 2006	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

7th February 2008, 07:56 AM	#2
Eva diyAudio Member Join Date: Oct 2003 Location: Near the sea	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). __________________ I use to feel like the small child in The Emperor's New Clothes tale

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7th February 2008, 01:52 PM	#5
fredos diyAudio Member Join Date: Mar 2005 Location: Quebec	Synchronize your floating power supply to amplifier clock....This should fix your probleme!

7th February 2008, 02:30 PM	#6
ionutgaga diyAudio Member Join Date: Jul 2006	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)

7th February 2008, 03:01 PM	#9
lumanauw diyAudio Moderator Emeritus Join Date: Oct 2002 Location: Bandung	Could you try change C20 to 4n7 and R11 to 1k8?

7th February 2008, 03:08 PM	#10
ionutgaga diyAudio Member Join Date: Jul 2006	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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