Questions about MosFET's IRF3205 and IRFZ44N swapping - diyAudio
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Old 6th December 2006, 04:53 PM   #1
dB-r is offline dB-r  United States
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Default Questions about MosFET's IRF3205 and IRFZ44N swapping

Originally posted in Solid State: I was informed I should post this here.



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. Hey Perry! Good to see you are here! 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 of the 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.
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Old 7th December 2006, 10:15 AM   #2
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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.

**** The IC can drive the FETs 'on' but the external circuit is generally responsible for turning the FETs off. You may need to make sure that the FETs are being swiched off quickly enough. With the FETs not clamped to the sink, do they get hot within 30 seconds (amp on, no load, no signal)? If they get hot, you may need to lower the value of the pull down resistor and/or the gate drive resistor. 47 ohms is typically good enough for IRF3205s.


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?

**** The difference in the current flowing with the Z44s or the 3205s is minimal. If, however, you were using the 3205 so you could run it into a lower ohm load, then you could run into a problem with the transformer overheating.


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).

**** The number of components isn't really important. There are many variables that determine the number of components used in the audio and power supply sections. The current carrying capacity of the component, the power dissipation (class AB very different from class D), the ohm load and the rail voltage are likely the most significant.



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?

**** 47 ohm resistors are almost universally safe as gate resistors. You should not change the value of the gate resistors if you're using the original part number components (for others reading this). Sometimes, using a value significantly lower than needed can result in a noisy supply. Again, to be sure that the resistors are getting the FETs turned off properly, have the amp on, at idle, no signal and see if the FETs quickly heat up. If they remain cool, the resistors have a sufficiently low value. For amps with regulated power supplies, you must get the amp to maximum duty cycle. If the regulation has the duty cycle cut back, the additional dead time will mask the problem. If you can turn the power supply down to ~10.5 volts, the duty cycle should be at the maximum. IF the FETs get hot at the lower voltage, you may need to lower the value of the gate resistors.


**** On the 2250, the gate resistors are likely 100 ohms and will not work well with the 3205s. The drivers are likely the next weak link. The 3205s may be too much of a load for them.

**** The missing IC is probably an LM393. If I'm not mistaken, the first half of the IC does provide a time delay. In this circuit, you need to use an op-amp capable of accepting input near the power supply voltages (including ground). If you have an LM358, that should also be a good replacement.
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Old 7th December 2006, 10:17 AM   #3
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dB-r, email me. babin_perry@yahoo.com.
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