Ringing on gate signal

brandes.cm · 21st January 2012, 02:17 AM

What causes ringing on the upper and lower portions of a output gate signal? The output config is the same as Sinuslive 1500, opto-isolated half bridge.

A picture from each channel. Do I worry about this?

Perry Babin · 21st January 2012, 02:23 AM

Is this with or without the outputs in the circuit?

brandes.cm · 21st January 2012, 02:26 AM

I have two fets per side.

Perry Babin · 21st January 2012, 02:29 AM

I doubt that it's a problem. It's likely something caused by the output filter inductor. If you have two probes, you can use the scope in differential mode to see if that gives you a cleaner signal.

brandes.cm · 21st January 2012, 02:39 AM

It's actually me using the wrong gnd for the lower fets. It is referenced to -rail not the gnd point I was using. Isolation transformer on your scope is a must, for me at least.

Perry Babin · 21st January 2012, 02:48 AM

An isolation transformer is dangerous. If you had it grounded to a rail, the case of the scope was at rail. For some amps, that's more than 150v DC. You need to learn to use the differential setup. It's on the DVD. Tech tips 8, the last item on the page.

brandes.cm · 21st January 2012, 12:38 PM

I know how to setup differential using 2 probes. I guess never really thought about what happens when using a isolation transformer.

One other question on this amp. I set the supply voltage to 13.1Vdc coming in, idle current is ~2A. When I increase the voltage to 14.4V, idle current goes up to ~7A. Is this right? That seems high for something that should be in the operational voltage range.

The pwm frequency doesn't change both in the supply and output carrier. Just the amplitude of the signal.

Perry Babin · 21st January 2012, 03:16 PM

That does seem too high. It could be that the gate drive isn't quite right for the power supply FETs and with higher drive voltage, the driver transistors aren't able to get the FETs switched off properly.

brandes.cm · 23rd January 2012, 12:50 AM

I bought this amp used and it was apparently worked on by a previous person. It uses an IRFB31n20 output with 47ohm gate resistor. I found the diodes attached to the gates replaced with more 47 ohm resistors. I would assume this diode could be critical to get a faster turn off time of the fet. Is this correct?

Perry Babin · 23rd January 2012, 08:55 AM

Yes. The diodes connected in parallel with the gate resistors allow the FETs to be driven off more quickly.

21st January 2012, 02:23 AM	#2
Perry Babin diyAudio Member Join Date: Nov 2003 Location: Louisiana	Is this with or without the outputs in the circuit? __________________ Links >> Basic Car Audio Amp Repair * Basic Car Audio Electronics * New Site * Basic Switching Power Supply Design * Basic Computer Skills << Links

21st January 2012, 02:29 AM	#4
Perry Babin diyAudio Member Join Date: Nov 2003 Location: Louisiana	I doubt that it's a problem. It's likely something caused by the output filter inductor. If you have two probes, you can use the scope in differential mode to see if that gives you a cleaner signal. __________________ Links >> Basic Car Audio Amp Repair * Basic Car Audio Electronics * New Site * Basic Switching Power Supply Design * Basic Computer Skills << Links

21st January 2012, 02:48 AM	#6
Perry Babin diyAudio Member Join Date: Nov 2003 Location: Louisiana	An isolation transformer is dangerous. If you had it grounded to a rail, the case of the scope was at rail. For some amps, that's more than 150v DC. You need to learn to use the differential setup. It's on the DVD. Tech tips 8, the last item on the page. __________________ Links >> Basic Car Audio Amp Repair * Basic Car Audio Electronics * New Site * Basic Switching Power Supply Design * Basic Computer Skills << Links

21st January 2012, 03:16 PM	#8
Perry Babin diyAudio Member Join Date: Nov 2003 Location: Louisiana	That does seem too high. It could be that the gate drive isn't quite right for the power supply FETs and with higher drive voltage, the driver transistors aren't able to get the FETs switched off properly. __________________ Links >> Basic Car Audio Amp Repair * Basic Car Audio Electronics * New Site * Basic Switching Power Supply Design * Basic Computer Skills << Links

23rd January 2012, 08:55 AM	#10
Perry Babin diyAudio Member Join Date: Nov 2003 Location: Louisiana	Yes. The diodes connected in parallel with the gate resistors allow the FETs to be driven off more quickly. __________________ Links >> Basic Car Audio Amp Repair * Basic Car Audio Electronics * New Site * Basic Switching Power Supply Design * Basic Computer Skills << Links


	Please consider donating to help us continue to serve you. Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving

21st January 2012, 02:26 AM	#3
brandes.cm diyAudio Member Join Date: Sep 2009 Location: Wisconsin	I have two fets per side.

21st January 2012, 02:39 AM	#5
brandes.cm diyAudio Member Join Date: Sep 2009 Location: Wisconsin	It's actually me using the wrong gnd for the lower fets. It is referenced to -rail not the gnd point I was using. Isolation transformer on your scope is a must, for me at least.

21st January 2012, 12:38 PM	#7
brandes.cm diyAudio Member Join Date: Sep 2009 Location: Wisconsin	I know how to setup differential using 2 probes. I guess never really thought about what happens when using a isolation transformer. One other question on this amp. I set the supply voltage to 13.1Vdc coming in, idle current is ~2A. When I increase the voltage to 14.4V, idle current goes up to ~7A. Is this right? That seems high for something that should be in the operational voltage range. The pwm frequency doesn't change both in the supply and output carrier. Just the amplitude of the signal.

23rd January 2012, 12:50 AM	#9
brandes.cm diyAudio Member Join Date: Sep 2009 Location: Wisconsin	I bought this amp used and it was apparently worked on by a previous person. It uses an IRFB31n20 output with 47ohm gate resistor. I found the diodes attached to the gates replaced with more 47 ohm resistors. I would assume this diode could be critical to get a faster turn off time of the fet. Is this correct?

Thread Tools	Search this Thread
Show Printable Version Email this Page	Search this Thread: Advanced Search

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
mmats d200hc and earthquake d2 gate drive signal	spooney	Car Audio	1	24th June 2011 03:35 AM
Gate capacitance and gate charge	rtarbell	Parts	1	22nd January 2007 05:11 PM
Ringing midrange	JP Haggar	Multi-Way	7	17th April 2006 02:33 PM
best way to generate Gate-Drain or Gate-Source short in electret capsule JFET?	capslock	Parts	2	27th September 2002 08:07 AM
Acoustic ringing?	grataku	Solid State	1	15th March 2001 04:55 AM

New To Site?	Need Help?
Register to Participate Search Privacy Statement Contact Us	Frequently Asked Questions Did you forget your password? Mark Forums Read