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Replacing Mosfet with no markings. General guidelines

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fastline

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I am replacing a mosfet in a PWM circuit. I know the gate measured 9V to it, I know the amperage should not exceed 30A, and the Voltage will not exceed 22-25VDC. Problem is the mosfet that burned up has a proprietary number that cannot be found. I will have to take a stab.

The mosfet is a TO-220 package. I realize many would not really be rated for this much current alone and it does have a single copper heat sink. I realize that it really just comes down to temp and heat dissipation but wondering if there are any general guidelines for surface area of heat sink relative to increased amperage of mosfet and how much you can push them? ie, 3A mosfet with no sink, 30A with 10sq in copper sink...? I assume that might be pushing it a bit..
 
The heat sink required is determined by the amps it is switching and the "on" resistance of the MOSFET. So you would likely want a MOSFET (N-channel ?) with a voltage rating of a least 50V and low "on" resistance at 30A in a TO-220 package (if you can find it).
 
Be sure you understand the maximum voltage. A flyback circuit can be several times its DC supply voltage. There is going to be a high voltage spike on drain for a short period where the output winding rectifier is getting into full conduction to dissipate coil charge. It the snubber's job to keep the voltage spike below MOSFET breakdown voltage.

Second, be a little careful about going for a higher then needed current (with associated lower Rds-ON). Lower Rds-ON and higher breakdown voltage MOSFET's come with higher gate drive capacitance. You can get into a situation where the original circuit's gate driver can't make the risetime with the beefier MOSFET.
 
Is this a flyback circuit?
 
Thanks for the replies guys and sorry about the delay in getting back. I cannot confirm the flyback circuit here because the board is soldered in place and I cannot get to the back of it. I can replace the obviously failed mosfet in an effort to salvage it. After some investigation, this mosfet drives a DC motor with .1ohm windings at 20V so I think it would be safe to assume that I need something that is at least rated for the locked rotor amps of 200A? Keep in mind that locked rotor is worse case but should not instantly fail the mosfet in this condition. POssible in several seconds but not instantly.

Regarding the voltage, I am still looking at specs but looks like 50-100V is pretty common. I realize some of this is more or less a guess so just trying to get any ideas here on how to decide what to try in this circuit. I was astounded to see a price of 7 bucks for a 55V/120A mosfet and cannot find anything rated over 200A yet. Am I chasing my own tail by considering the locked rotor amps for a ballpark current rating for the mosfet? Ie, if they are rated for 100A continuous, they "should" be fine at 200A for a few seconds?
 
Don't forget that the mosfet is probably being driven with PWM at high frequency and so the impedance of the motor will define the current flowing.

Mike.
 
Something doesn't sound quite right. I'm assuming the PWM & motor worked together once? It is hard to find a FET over about 60 Amps in a TO 220 package. So If yours has a single TO 220 it is unlikely to be over 60 Amps. Maybe you could post a picture of the circuit & motor?
 
Here is a pic of the board. A little fuzzy so let me know if I need to do better. There is 1-2 power diodes on the bottom and the buss bars under it are soldered in place so I would have to desolder several things to get it out.

You can see where the mosfet mounted in the center. the 3 large solder globs are for the mosfet and the screw tab is the retailer. You can see the mess it made when it gave up.

I would think like a fuse, a mosfet should have a current/time graph to look at peak performance. ie, continuous load and then peak current/time. I think typicaly load on this would be around 50-80A max but still have to consider how many seconds it can handle at locked rotor before cooking the mosfet.


The motor is nothing special. A DC motor with a pair of brushes. Yes, this is a PWM circuit but I did not understand how that would have any variance on the impedance of the system and how that might relate other than to reduce current draw?
 

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Mosfet

Here is about the biggest one you will find in the TO220 package. You can get it from Mouser.
https://www.st.com/stonline/books/pdf/docs/9969.pdf
If you look at figure 2 & 16 you can get an idea of the pulsed limits. The thing that usually limits them in PWM mode is the bond wire. This is what hooks the legs of the IC to the die or in this case "the fuse".
Heat sink: From the data sheet you can get Rds on. This is the resistance of the fet when it is fully turned on. In this case .0046Ω. At 30 amps that is 4 watts. For a TO220 package the temperature rise is 62 C per watt, so the junction temperature would be 255 C or so. From the data sheet the maximum is 175C, so you need a heat sink. A rule of thumb is to keep the case temperature at 50C or below so it would be nice to have a heat sink with a temperature rise of 6C per watt (or there abouts) Another seat of the pants number for heat sinks is 50C/√sq. cm so about 69 sq.cm or 10 sq. in. If your willing to let it run hotter you can use less. These are rough numbers so leave margin or read up on the details.
 
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