# Downside to using mosfets for freewheel diodes?



## ngrimm (Oct 19, 2007)

I tried to find where this has been discussed but no luck. If the mosfets I'm using cost say $1 ea and the freewheel diodes cost $2 ea for the same or less amperage than the mosfets, can I also use the same model mosfets for freewheel diodes? Am I overlooking something? Thank you


----------



## Tesseract (Sep 27, 2008)

Depends... the anti-parallel diode in MOSFETs is a byproduct of the fabrication process and is, in fact, part of a parasitic bipolar transistor. Manufacturers have gotten pretty good at optimizing the characteristics of this diode over the years through selective doping and other gobbledygook but in the end this diode will never be as "good" as a purpose-built one.

You have to short the gate to the source to keep the MOSFET from turning on from Miller charge - MOSFETs conduct equally well in both directions.

Or you can make your life really miserable by trying to turn them on at the proper time to short out their anti-parallel diode (aka, synchronous rectification).

Part number you are considering here?


----------



## MPaulHolmes (Feb 23, 2008)

I finished the control board for a SR controller, which wasn't too compicated because I'm using a dsPic30F4011 which has 6 pwm outputs, with programmable dead time. I'm using these for "freewheel mosfets" and mosfets:

http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=IXFX230N20T-ND


----------



## ngrimm (Oct 19, 2007)

Thank you for the replies. Jeff, the mosfets I am using for this little controller are ftp61n20. Paul, those are some mean mosfets. I may consider them on the controller I'm building for the 71 Toyota Hilux. One thing I noticed is that they are rated at 230 amps continuous but are limited to 160 amps by the external leads. Even at 160 amps though, I would only need six for 960 amps. I think the cost and power would be about the same as the fifteen 62a 200v mosfets on my Ninja. I need to figure out the gate drive required for them since my tc4422 mosfet driver may be at it's limit. Back to the original subject, I think I will try hooking the ftb61n20s up as diodes and see how it goes. This little controller isn't driving a load with much inductance so I don't think it needs too much for a freewheel. Thank you again.


----------



## Tesseract (Sep 27, 2008)

ngrimm said:


> Thank you for the replies. Jeff, the mosfets I am using for this little controller are ftp61n20. Paul, those are some mean mosfets. I may consider them on the controller I'm building for the 71 Toyota Hilux. One thing I noticed is that they are rated at 230 amps continuous but are limited to 160 amps by the external leads. Even at 160 amps though, I would only need six for 960 amps.


You probably meant F_D_P61N20, a Fairchild part number.

And Paul's IXFH230N20 can't even manage 160A... square that and multiply by the Rds[on] of 0.0075 ohms and you still get a temp rise of 192C... Since Tj shouldn't exceed 100C, 160A is clearly unattainable regardless of what the leads might otherwise limit the current to.

Note, also, that Rds[on] usually has a strong positive temperature coefficient, typically rising 1.5X the 25C value at 75C. If you assume the junction temperature in steady state operation to be 75C and work backwards you find that a TO-220 package MOSFET can usually take 20-30A without requiring an unrealistic heatsink while the TO-247 package will allow 50-60A.

That's a rough rule of thumb, but it turns out to be surprisingly accurate.



ngrimm said:


> This little controller isn't driving a load with much inductance so I don't think it needs too much for a freewheel. Thank you again.


The amount of inductance in the load is, practically speaking, irrelevant. If there is any you will need a freewheeling diode, and this diode carries just as much current as the switch (at the opposite duty cycle, is all).

The diode in this part number seems reasonably capable.


----------



## ngrimm (Oct 19, 2007)

The mosfets seem to be working fine as freewheel diodes so I thank you for that. But I have a somewhat related ongoing problem. I am using a tc4422 mosfet driver to drive lowside mosfets (half bridge) and they work well with the following exception. Lets say I am driving a 100 amp load (battery tester) at 50% duty cycle at 12 volts. One end of the load is continually connected to the positive supply and of course the pwm is pulsing the negative side. Any time I remove the B+ to the pwm controller without first either turning the pulse width to zero or removing B+ from the load, the mosfet driver shorts out. You can actually here it fry. I have checked and it only does this when the freewheel diodes are in place but of course I need to have them. Is this what you would expect to happen? So far my solution has been to either switch of the B+ power to the load or reduce the pulse width to zero before cutting power to the controller. Any ideas?


----------



## Tesseract (Sep 27, 2008)

ngrimm said:


> ... Any time I remove the B+ to the pwm controller without first either turning the pulse width to zero or removing B+ from the load, the mosfet driver shorts out. You can actually here it fry. I have checked and it only does this when the freewheel diodes are in place but of course I need to have them. Is this what you would expect to happen?...


Yeah, I would expect the gate driver IC to blow if you remove power to it with the output stage still connected. A pulse of Miller current (proportional to Crss) will back bias the "weakly off" driver IC, likely destroying it.

Simple solution here is to make sure your PWM/driver circuit turns on first and turns off last!


----------

