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PPI amp (power precision) blew out
PPI amp (power precision) blew out
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Old 14th October 2007, 03:02 AM   #111
ppia600 is offline ppia600  United States
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Default Now I'm almost (but not quite) totally confused, haha

Wow, I really opened a can of worms with that question. Yeah Perry, I might just use the z48v's like the ones I had to replace in the audiobahn.
First off, the original gate resistors on the ppi are 475 ohms. They are yellow(4), purple(7), brown(0), and the tolerance band is gold (5%). I'm pretty sure I'm not confusing bipolars with fets. I understand how sensitive most fets are at the gate, what I don't understand is how I can drop from a 470-475 ohm resistor to a 47 or even 22. That is quite a large jump. What in the architecture of a z44 for example makes the gate lead different than the original 25no5e's?
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Old 14th October 2007, 04:08 AM   #112
Perry Babin is offline Perry Babin  United States
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Brown is 1.

They were 470 ohms. 475 ohms would have had another band (yellow, violet, green, black, brown).

I tried to find the datasheet for the 25n05 but wasn't successful. I'd guess that a very low input capacitance and a lot of dead time allowed them to work with the 470 ohm resistors. I've only seen one other amp use values over 100 ohms (as far as I can remember). 10-100 ohms are the most common.

If you use Z48s, you need to use 47 ohm resistors (that's my suggestion anyway).
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Old 14th October 2007, 04:15 AM   #113
EnvisionAudio is offline EnvisionAudio  United States
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Default Re: Now I'm almost (but not quite) totally confused, haha

Quote:
Originally posted by ppia600
Wow, I really opened a can of worms with that question. Yeah Perry, I might just use the z48v's like the ones I had to replace in the audiobahn.
First off, the original gate resistors on the ppi are 475 ohms. They are yellow(4), purple(7), brown(0), and the tolerance band is gold (5%). I'm pretty sure I'm not confusing bipolars with fets. I understand how sensitive most fets are at the gate, what I don't understand is how I can drop from a 470-475 ohm resistor to a 47 or even 22. That is quite a large jump. What in the architecture of a z44 for example makes the gate lead different than the original 25no5e's?
The 25N05E is a 50V 25Ampere device. A direct replacement for this obsolete device is an IRFZ34N. It's possible that PPI was intentionally trying to slow the turn-on of these FETs to reduce dv/dt - but I can't state that with any sort of proof. Who knows - maybe that is what the manufacturer of the FET recommended? Remember, this was at a time when using MosFets in the power supply was new. Maybe their research led them to believe that was a good thing.
Looking at the schematic...the dead time resistor is indeed 100 ohms.
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Old 14th October 2007, 03:38 PM   #114
ppia600 is offline ppia600  United States
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Default Re: Re: Now I'm almost (but not quite) totally confused, haha

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Originally posted by EnvisionAudio


The 25N05E is a 50V 25Ampere device. A direct replacement for this obsolete device is an IRFZ34N. It's possible that PPI was intentionally trying to slow the turn-on of these FETs to reduce dv/dt - but I can't state that with any sort of proof. Who knows - maybe that is what the manufacturer of the FET recommended? Remember, this was at a time when using MosFets in the power supply was new. Maybe their research led them to believe that was a good thing.
Looking at the schematic...the dead time resistor is indeed 100 ohms.

Yeah Perry, I was using the resistor color code wheel and it read 470, that is where I got the value from. The meter also read .470-.475k. (I wish it would just read 470 ohms but it doesn't have that range for some reason) The reason I said 470-475 ohms may be because my meter leads are crapping out, so sometimes it will fluctuate a little, but it could also be because of the 5% tolerance allowing the slight variance.

I still don't understand the reason for the metal can transistor guy saying the original gate resistors were 47 ohms..
Envision... where is the dead time resistor you're talking about located? And what is the exact reason they used the transformer/inductor between the two sets of gate resistors? Kind of interesting

I've also already ordered the z44 resistors, so which would be the optimum resistor to use at the gate now? Also, would it be a good idea to find some 1% tolerance resistors? Thanks for the help
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Old 14th October 2007, 03:57 PM   #115
EnvisionAudio is offline EnvisionAudio  United States
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Default Re: Re: Re: Now I'm almost (but not quite) totally confused, haha

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Originally posted by ppia600



Yeah Perry, I was using the resistor color code wheel and it read 470, that is where I got the value from. The meter also read .470 k. (I wish it would just read 470 ohms but it doesn't have that range for some reason) The reason I said 470-475 ohms may be because my meter leads are crapping out, so sometimes it will fluctuate a little, but it could also be because of the 5% tolerance allowing the slight variance.

I still don't understand the reason for the metal can transistor guy saying the original gate resistors were 47 ohms..
Envision... where is the dead time resistor you're talking about located? And what is the exact reason they used the transformer/inductor between the two sets of gate resistors? Kind of interesting

I've also already ordered the z44 resistors, so which would be the optimum resistor to use at the gate now? Thanks for the help
The exact reason? I'd have to build their circuit to find out, but I think it's designed to reject common mode noise that might trigger both phases of the FET bank at the same time causing shoot-through. The schematic also shows a third winding connected to ground, bypassed with a 220uF cap to +12V to ostensibly reduce flux density in the core during dead time.

I have to admit - I've never seen this used before since the 3525 IC includes exclusive-OR logic lockouts to prevent double triggering of the outputs. Somebody must have thought it was a good idea, but it is obviously less of an issue that, say, transformer saturation or flux walking of a push-pull design; two mechanisms that definitely contribute to the cause of blown FETs.

The dead-time resistor is connected between pins 7 and 5 of the SG3525AN IC.

Regards,
Aaron Hammett
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Old 14th October 2007, 05:13 PM   #116
lumanauw is offline lumanauw  Indonesia
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Hi, Perry,

You're right, I forgot to include the dead time factor of the PWM IC. If the dead time is adjusted, 100ohm may be OK. My bad experience with 100ohm Rg on IRFZ44 is not with SG3525, but with TL494 with 0V at pin4.
Long time ago, I calculated the optimum Rg for IRFZ44 (with the formula found in one SMPS handbook, it turns to be about 12ohm. But practically, the totempole curent of the driver will be too big.
It is the discharge when OFF that makes the problem with too big Rg. IRFZ48 has bigger gate charge than IRFZ44, it will have different optimum Rg than IRFZ44.
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Old 15th October 2007, 01:34 PM   #117
ppia600 is offline ppia600  United States
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Well I have the irfz44s on the way and some 100 ohm 1% tolerance resistors. Hopefully that is the right combo
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Old 22nd October 2007, 12:12 AM   #118
ppia600 is offline ppia600  United States
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Default awesome

Okay, got all the z44's and gate resistors in. Double checked the outputs and found one bad 6490. Luckily I had a bag of them I ordered a couple of years ago and never used. Unfortunately I have no way to match them to the originals as far as hfe is concerned. Luckily, regular transistors are more forgiving than fets. I installed one of the new 6490's (central semi) and got it all regreased. So far, it works perfectly. I don't hear any unusual turn on or turn off noises either. The output section is getting warm at low power levels and the power supply fets aren't even making the sink warm to the touch. I'm currently running it at a 2 ohm mono load (yeah I know, not optimum) at a pretty low volume level to warm it up. If it can handle this stress test, it will surely do well at 2 ohms stereo. I'm glad the amps I love all have normal transistors for the output sections!
Thanks a lot for the help, guys! My friend is going to love this amp, he's never had a ppi and its going to make his four eclipse 6.5's shine like he's never heard. I hope he can truly appreciate the improved sound.
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Old 22nd October 2007, 12:44 AM   #119
ppia600 is offline ppia600  United States
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Update: Now that the amp has been running a little while I've swapped it to a 4 ohm load and the heatsink is staying about 115 farenheit. Looking good so far.
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Old 22nd October 2007, 03:52 AM   #120
EnvisionAudio is offline EnvisionAudio  United States
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Cool, mang - good to hear it!
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