Earthquake PHD2 seems to lower output???

[ppia600]

Well since I have the amp out of the car, I'm going to test it with a power supply. I'll use one of my smaller power supplies (10 amps) since I won't be driving any load. I'm not exactly sure where to test for rail voltage but it probably won't take long to find. Currently I'm using my "pro mos" 50 to run the subs (and I'm also wondering if that amp has a built in subsonic filter....) I'm off to open the PHD2 now, be back in a little while...

[ppia600]

Ok tested a few points:

-diodes soldered on ic - 77.4 vdc regulated
-rail supply output filter capacitors - 81.2 vdc
-output of both power supply rectifiers -78.something vdc??(why is this lower than at the caps??)
-gate leads of power supply fets .5 to .7 vdc
-gate leads of audio output fets about 80 vdc...

I actually had it connected to a small sub playing low so I could watch for any fluctuations even though this was nothing compared to what it would do in the car. Just watching it play those super low notes makes me want to swap it back in and take the ppi out, haha. It is a lot noisier than a class a/b amp on the output. You can actually hear switching sounds through the sub even with no music signal. Its nothing you'd hear inside the car though.
Apparently the diodes aren't connected between the gate and source of the power supply fets, they are connected at the audio output fets which have a really high voltage. Funny thing is they read about 3.75v across the anode and cathode, but if you read them from ground to either side, one side is about 77 and the other about 81. They must be really high voltage zeners or just for reverse polarity protection of some sort. Its pretty kewl that the gate voltage is so high, almost the same as the source voltages. This is apparently the extreme side of switching design which explains the switching output noise.

[Perry Babin]

The rectifier is before the inductor. The ripple is probably causing the error. When the duty cycle increases, they will likely be closer.

The zeners are rated at 20v or less. The ones near the outputs are 18v zevers. I'm not sure why they have them on the IC also. They seem to be doing the same thing. Maybe they're rated lower for better protection.

The voltage on the gate of the high side FET is actually greater than rail. It has to be at least ~5v higher if you want to drive the FET to the rail.

[ppia600]

I'm still wondering about the diodes, going to have to find out more about them..

[Perry Babin]

What are the numbers on them. They are likely 1n52xx zeners. The ones near the outputs are 1n4746 if I'm not mistaken.

[ppia600]

THe ones on the chip look like 1N5234 (6.2v??)... man I need my eyes done! I can't tell about the ones across the output leads because they are soldered down, but there are two on each fet, one tiny and one larger, both are the clear glass type...

Ok, there are tiny diodes on each output fet, but only one large diode for half of the output fets.. the large ones appear to be 1N4740 (10v??) but its hard to tell, they are all soldered where I can barely see the part number.

I don't understand why the large diodes are there unless they are in series.. The gate voltage is so high, there's no way those are paralled???

[Perry Babin]

The smaller diodes are switching diodes. They allow the drive circuit to switch the FET off quickly. When turning the FET on, the drive signal passes through the resistor. When turning the FET off, the drive signal goes through the diode (a tiny current will flow through the resistor on turn off also).

I think the Zeners are there to protect the FETs in case of a short circuit. If the output was shorted and the source was dragged down quickly, the gate may see significantly more than 20v and the FET would fail. The low side doesn't need them.

[ppia600]

I was looking around on the ampguts site and noticed the phd2 there doesn't have the diodes soldered to the ic.. I'm still searching and haven't found out for sure. I am running the amp currently (giving the ppi a rest) but I'm taking it easy on the alternator for now.

[Formas]

i have a amp with a problem unlike yours its a "shredder" phd2, my six of my power fets have blown, it still power up and play too but very weak my outputs is good no resistor or diodes have burn on the circuit, my question is what do i do..im a electronics technician but i dont have access to the original parts but im planning on ordering from the net, thats my personal amp and i want it to be placed back in my car...do i change the tc44cpa? the PWM ic which you identified as 4080? and add my new powerfet to the old ones they are p80ne-06?
i need the help in a hurry please thanks guys.

[Perry Babin]

If the audio output transistors have not failed, the 4080 isn't likely to be defective.

When the power supply transistors fail, the 4424 almost always fails and will need to be replaced. Check the 10 ohm gate resistors also.

Were the 6 failed FETs on the same side of the amp?