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dB-r 23rd November 2006 07:07 AM

Questions about mosfets IRF3205 IRFZ44N
This is my first post on this forum, I have been reading it from time to time for 2 years or so now. I wish I had time to read it all. There is some real know-how in this forum, and I see it is very popular judging by the number of posts and numbers of users online at any given time.

I guess I should introduce myself, get it out of the way anyhow. I am a computer tech, I have been doing computers mostly Unix servers since 1994. I was into car audio back in the late 80's - mid 90's. I honestly believe I have seen the golden years of car audio already pass by. I got out of car audio when I got into computers and the car audio scene turned into a bunch of cheap mass produced junk.

I went to college for 2 years for EET, but dropped out to take a good job with an ISP back in the ISP boom days. That fell out in 1999 and ever since then I have been self-employed, working contract with a local Unix reseller. Unix has now come and gone, and so has a lot of my work, so I decided to open my own business and store, selling and working on computer and networks.

So anyway, about 2 years ago someone brought me an amp to repair, and I repaired it, it was simply a blown capacitor, and easy fix for me since I had been trained in soldering and had been tinkering around with electronics since I was a kid, plus a little DC circuits class in college helped too. I stumbled across Perry Babin's CD teaching about amplifier repair. I paid $85 for it. After reading it I decided it was something I could definately do. So I added that to my business and have been repairing amplifiers locally for almost 2 years now, with good success. Haven't made a ton of money of course, but it's good for extra money for a little side work and diversity in my business. I can also repair other items like power supplys, some RF amplifier equipment etc. I was always into radio too.

Anyway, glad that is out of the way I won't have to post it again. I am no expert on amplifiers, just because I repair amps, I know that. I think of myself as a begginner compared to some ofthe things I have read here. I am not that good or interested in theory and design, but I like to do hands on things like repairs.

So I got a dilemma (sp?). Most amps I have worked on use the IRFZ44N mosfets in the power supply. I have read I can replace with IRF3205 in high current demand situations. I have an old MTX thunder 280 that kept blowing the power supply fets (2) when cranked up with lots of bass in 4 Ohms bridged. It is rated to handle this. I could not understand why they kept blowing. Then common sense kicked in and since the output section consisted of 2 pairs (4 total) of IRFZ44N (same mosfets as in the power supply, but twice as many) I came to the conclusion that when I max the amp out, the 2:1 ratio of output fets vs. power supply fets must have been causing an overload on the 2 fets in the power supply. So I replaced the 2 IRFZ44N in the power supply with 2 IRF3205. Problem solved. Amp gets hot, but doesn't blow the fets anymore.

The IRF3205 are rated at twice the current capability of the IRFZ44N. I am wondering a few things about this swap, that I am certain that many of you here can help me figure out:

1. Input capacitance - The amp in question uses a TL594CN which drives directly the 2 fets in the power supply. The IRF3205 have a higher input capacitance. Could this overload the 594 without the addition of external gate drive transistors? It seems to be working fine however.

2. Transformer Overheating - I know that the transformer is only capable of handling a certain amount of current passing through it, therefore, if I am enabling the amp to pass more current by upgrading the power supply fets do I run a risk of frying the transformer?

3. Output fets need to be upgraded as well? - Is it possible to also replace the IRFZ44N on the output section with IRF3205 or would that likely result in the same situation as when it was running 4 output fets with only 2 power supply fets (of equal capability, back to 2:1 ratio).

4. Gate drive resistors - On the power supply, since I have changed the fets, should I change the gate drive resistors? How would I go about mathimatically calculating what gate drive resistor values would be needed?

Also, I am currently working on a Planet Audio TT2250D Class D amp that is a fairly powerful amplifier in my opinion, but the guy was running it at .5 ohms and I know that won't work. When it get's reinstalled it will be either 2 ohms (my preference) or 1 ohms at the very minimum with regular cooling off periods to help it cool. It really needs more heatsink in my opinion. I am wondering can I swap in some IRF3205's in place of the IRFZ44N's on the power supply. It uses 16 IRFZ44N on the power supply, dual transformers, 4 rectifiers and 10 IRFB31N20D on the output section. The entire amp was smoked. I got it back up and running with 4 IRFZ44N on the PS and 2 IRFB31N20D so I know it is salvageable. Also, it has a bad pop when turned on and off, I noticed that this amp has been worked on several times by other shops and they did a really crappy job too, and I think the pop is coming from the fact that IC302 is missing. I am pretty sure it has something to do with the muting circuit, which I don't know much about. Is there a general replacement I can use on this amp for the missing IC? Next to where it should be on the PCB are 2 other 8 pin IC's that are white in color. I am wondering if the missing IC should be the same part as the other 2 next to it. Planet Audio would NOT give me a schematic....

Thanks in advance for any help. I look forward to reading and participating in this forum. I feel like I am light years behind some of the people in here who design their own amps. I hope to one day have enough grasp of the technology to be able to do the same.

P.S. I also I noticed this site takes donations, and believe me, I have run some message boards and I understand why the donations are necessary. I will be sure to donate to the board as I can.

dB-r 6th December 2006 07:09 AM

Did I say something wrong? Post too long? Just wanting opinions on this because I put the project on hold to wait for some advice.

ilimzn 6th December 2006 11:18 AM

I believe your post is in the wrong section, there is a whole car audio subsection here, I'm sure you will have better luck there.

dB-r 6th December 2006 12:36 PM

Should I get an admin to move it or just repost it in that section?


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