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Old 2nd January 2014, 07:27 PM   #11
shepo1 is offline shepo1  United States
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Sorry I only have a old scope to work with .. so I can take captures.. Can you explain the boot strap section.. I am wondering about that.. for now I have the FETS out.. so I don't keep blowing stuff up..
Thanks
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Old 2nd January 2014, 08:17 PM   #12
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Quote:
Originally Posted by shepo1 View Post
Sorry I only have a old scope to work with .. so I can take captures.. Can you explain the boot strap section.. I am wondering about that.. for now I have the FETS out.. so I don't keep blowing stuff up..
Thanks
I'm no professional at this, ive learned alot here from others and reading constantly the same issues others experienced.

Brush up and read these papers they enough to educate you and set you on the right path, don't just replace with new parts, the key points are what's happening at the mosfets the waveforms can really help you understand what the root of the problem is, there are only a few core reasons why they blow up.

http://www.irf.com/product-info/audi...sdtutorial.pdf see Boot Strap High Side Power Supply and study current flow during this transition.
http://www.ti.com/lit/ml/slup169/slup169.pdf Design And Application Guide For High Speed MOSFET Gate Drive Circuits.
http://www.irf.com/technical-info/appnotes/an-1144.pdf IRS20957S App note from the above two papers you can have a clear idea what the root issues are.

You don't really need a diagram, just reverse engineer the circuit, most of them are replicated from the app notes. (except maybe the modulator in this case)
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Last edited by Reactance; 2nd January 2014 at 08:24 PM.
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Old 2nd January 2014, 09:31 PM   #13
shepo1 is offline shepo1  United States
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Thanks.. I have been reading and researching. I missing probably one stupid thing.. and yes.. I have been reverse engineering.. hard to trace crap on these boards though. I think I have a good understanding of the Class D theory. but a little hazy on application of the IRS20957 ( yes I have the data sheets and have seen the test board schematics0 .. and a little confused on self modulation ( which I'm quite sure this amp uses) and the boot strap thing.. but I'm getting there.. Thanks for the links.. I will dive in.. I appreciate the advise..
I thought I had it nailed when I found the H04 ic was just outputting DC to the Driver chips.. but that appears to be a secondary issue cause when the IRS20975 smokes
anyway.. thanks again
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Old 2nd January 2014, 09:44 PM   #14
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Quote:
Originally Posted by shepo1 View Post
Thanks.. I have been reading and researching. I missing probably one stupid thing.. and yes.. I have been reverse engineering.. hard to trace crap on these boards though. I think I have a good understanding of the Class D theory. but a little hazy on application of the IRS20957 ( yes I have the data sheets and have seen the test board schematics0 .. and a little confused on self modulation ( which I'm quite sure this amp uses) and the boot strap thing.. but I'm getting there.. Thanks for the links.. I will dive in.. I appreciate the advise..
I thought I had it nailed when I found the H04 ic was just outputting DC to the Driver chips.. but that appears to be a secondary issue cause when the IRS20975 smokes
anyway.. thanks again
no problem dude, good luck.!
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Old 2nd January 2014, 09:51 PM   #15
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After semiconductors there is usually little to go wrong in class d amps.
Unless a resistor has burned out or a cap gone short circuit.

The old 100watt mains lamp in series with the amp trick is a good idea.
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Old 2nd January 2014, 10:50 PM   #16
shepo1 is offline shepo1  United States
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I agree.. and since this is a production design I would think it would be not that big a deal.. If I recall. .when I was doing some testing.. it seemed like the Hi side Fets were coming on.. and I read some place that LO side should first.. Also one question.. with no audio input.. should there still be a Square wave at pin three ( In) this is one area I am confused about.. I understand that the input to pin 3 needs to be some type of square wave modulated from the input sine wave.. But if there is nothing at pin 3. will the outputs still start to oscillate ? My understanding is that the Fets are always switching on and off so if that is the case.. I should think I would see that at pin 3 from the feed back I assume must be there.. I am wondering if i'm missing the feed back.. thanks again..
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Old 2nd January 2014, 10:52 PM   #17
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The amp should oscillate with no input signal.

I know the irs2092 wont oscillate unless a load is connected. It just sits at +17v !
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Old 2nd January 2014, 10:59 PM   #18
shepo1 is offline shepo1  United States
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Ok.. so if there is feed back loop then you should see that at pin 3 right ?? or no..
This last time I replaced all the bad parts.. I made sure to hook up to a load.. just in case.. still did same thing.. I see VS go to about 11 V.. then Fets try to turn on.. and then it starts cycling for protection . .. Although CLIP led was not coming on..this time around.. ( it was when the input to pin 3 was stuck at about 4 VDC due to the bad H04 IC) so I wonder if there is some external protection that is wired in and cause the Protection to engage.. idk.. Im having fun though.. lol.. thanks again.
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Old 2nd January 2014, 11:03 PM   #19
shepo1 is offline shepo1  United States
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Its possible this is open loop too.. idk... I know it must be a full bridge design cause there are 2 sets of Half bridge amps ..
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Old 6th January 2014, 11:32 PM   #20
shepo1 is offline shepo1  United States
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Hi.. well still fighting this thing.
I put new 20957/s in.. check everything with out FETS out ... Still getting OC cycle.. I assume because with no FETS VS never changes and IC thinks OC condidtion. ?? I put FETS in still get OC cycle.. I tried to check Vgs and see firing signal.. I see Trigger LED flicker and scope when checkin LO output.. but cant really see anything happening voltage wise.. Upper IC doesn't seem to output any LO signal .. HO signals also don't seem to trigger scope.. I pulled out the Upper IC and lower still cycling the OC thing.. Then lower fets shorted again.. Grrr..

I have a schematic of a Ampeg amp that seem to have the same schematic.. almost. exactly.. LOL ( Like how they provide VDD from the +15 V supply) anyway.. here are some readings I took while it was in the OC cycle condition.
Pin 1 VDD 10.2
pin 2 CSD cycle from 2.5 up to about 7..
pin 3 IN 0 or 5 v.. ( no audio input)
pin 4 VSS 0V
pin 6 Vref 5.17 (ref to -B
Pin 7 OC SET 4.85 (ref to -B
pin 8 DT 8.68 ( ref to -B)
Pin 9 -75 to GND 0 to -B
Pin 10 LO same as 9
pin 11 VCC 12 V to -B -63 to GND
pin 13 VS +9.57 to GND 84.57 to -B
pin 14 same as 13
pin 15 15 V to VS
pin 16 2.1 V

So I think that all seems normal..
How do I current limit that mains.. ?? I think I can insert something in series between the PSU and the power output pcb
Oh mains were at +75 and -75 and stable ..

Thanks

NOTE page one of schematic is PSU page 2 is Power ouput and signal input section.
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