victor mosfets
 

Does anyone know what transistors are in the Victors? I was thinking of possible reasons that one of ours failed, but I need to look up some specs on the transistors.

I'd open one up, but I don't have access to any at the moment.

Re: victor mosfets
 

The FETs used are IRL3103 (International Rectifier) or an equivalent like FDB6035AL (Fairchild Semiconductor) for our competition victors.

http://www.irf.com/product-info/data...ta/irl3103.pdf

Digi-Key or Mouser have them.

This old thread discusses the victor's pretty in depth.

Re: victor mosfets
 

Go to the IRC website. Lookup the data sheet on the IRL3103 and then go to the automotive applications and look up thru the hole automotive rated FETS. As you'll see there maybe some better choices for fets. The fets rated for automotive applications are designed to take more abuse that is found in cars. Would a fet swap in the victors be legal? Adding better heat sinking?

Re: victor mosfets
 

As to swapping fets, I can't see how the rules could be interpreted as allowing this.

As to adding heat sinks, I am not so sure. It may be legal, but why would you? I don't think the current ratings of the circuit breaker give much of a workout to the fets.

I say this, but I have never actually touched a fet after a match. Does anyone have any data how hot the fets run?

Joe J.

Re: victor mosfets
 

<R27> and <R59>, read together, appear to preclude using any controller other than a Victor 884 and any custom circuit component that directly alters the electric power pathway. A FET different than the ones originally provided in the Victor 884 would not comply.

I haven't found a rule against a custom cooling device; however, keep in mind that the tabs on a TO-220 package FET are common to the Drain terminal (middle pin), so some tabs are common to a motor output and others are common to the +12V power input. This means that heatsinks attached directly to FET tabs could become electrical fault hazards. I would not want to have to inspect Victors that had been modified by adding heatsinks directly to the FET tabs.

IMO the fan provided with each Victor is the most effective and safest way to cool it.

Re: victor mosfets
 

Let's say that we backdrive a motor clockwise. Would the back EMF be the same polarity as that which we would need to apply to the motor leads to make the motor turn clockwise? Or does backdriving it reverse the polarity? I want to cite Lens' Law, but I'm not sure if it completely applies here.

On a side note, I noticed that the MOSFETs are rated for 64A. Where does the Victor's 40A limit come from? It's not the power limit because at 40A, it's only 19.2W and it's rated for 94W.

Re: victor mosfets
 

I pulled this note off a document somewhere, but don't ask me where.

"Typically the FET case's ability to dissipate heat limits this number(64A) to around 40 A per FET."

Re: victor mosfets
 

Lenz's Law does apply here. A motor that develop's positive back-EMF when backdriven clockwise will also spin clockwise when positive voltage is applied at its terminals, assuming the mechanical load allows it to spin.

Check out Fig. 9 on page 5 of the IRF3103 datasheet; it shows maximum drain current vs. case temperature. If your cooling system can hold the case temperature at 25 degrees Celsius then you can operate the FET at 64 Amperes drain current. At higher case temperatures, the maximum drain current decreases; e.g., at about 115 degrees Celsius case temperature the maximum drain current is 40 Amperes. Thermal analysis of FET installations can get complicated; however, in general the datasheet current rating is going to be higher than the current at which you can really operate the FET in a cost-effective system.

Re: victor mosfets
 

Don't forget that each leg of the h-bridge is really 3 fets. So, 40A per fet means 120A for the h-bridge total.

Lots of good things happen with parallel paths for the current.

Joe J

Re: victor mosfets
 

As I said above, thermal analysis of FET installations can get complicated. To see how, let's use the Victor as an example:

As an extreme case, let's consider operating the FETs in free air with no convection. For this case, the data sheet provides a figure of 62 Kelvins (Celsius degrees) junction temperature rise (over ambient) for each Watt dissipated in the FET junction. A reasonable maximum for usable junction temperature might be 150 C, and ambient temperature on the FRC field might be 30 C, so the rise would be 120 Kelvins and the maximum dissipation would be 120/62 = 1.94 Watt. At 150 C junction temperature the FET's on-state resistance is Rds(on) = 0.02 Ohm [this was obtained by multiplying the nominal value 0.012 Ohm by 1.7, taken from Fig. 4 on the data sheet] so the maximum current in the FET is sqrt(1.94/0.02) = 9.7 Amperes. There are three FETs per leg, so the maximum leg current is about 29 Amperes. We can improve on this by running the cooling fan, which will reduce the thermal resistance from junction to ambient by a considerable factor, probably at least two. Using 31 Kelvins per Watt of dissipation and repeating the calculations above gives 3.87 Watts maximum dissipation for each FET and therefore 13.9 Amperes per FET and 41.7 Amperes per leg.

Of course this analysis predicts far less than the maximum 120 Amperes arrived at in the earlier posts; to get there, we would need to improve the thermal resistance from junction to ambient still further. A practical lower limit might be about three times the thermal resistance from junction to case (given as 1.6 degrees K/W by the data sheet) or 4.8 Kelvins per Watt. Using this figure, the maximum dissipation per FET is 25 Watts, the maximum FET current is 35 Amperes, and the maximum leg current is 105 Amperes. Based on the Victor's 40 Ampere rating, I'd guess that this level of FET cooling is probably beyond the capability of the Victor design. For reason's I gave earlier, it is probably not a good idea to try to retrofit the Victor with home-made heatsinks in an attempt to improve its thermal performance.

Re: victor mosfets
 

I am not arguing that the heat analysis was simple, I am just reminding folks that there were 3 fets per leg.

As to the heat analysis, I would suppose that 31 Deg K per Watt is too high for forced air convection. This relatively small, clip on heatsink has rise of less than that based on natural (unforced) convection. A little bit of wind goes a long long way with respect to cooling.

Unless something happened that I am not aware of, Victors were not dropping left and right due to fet failure. Based on the years of service we have had with the Victor, I would guess that the 40A breaker is not allowing the fets to get anywhere near their temp limit. Based on that and assuming the math above is correct, I would estimate the effectiveness of the cooling to be higher than the 31 Deg K per Watt.

For Watt it's worth... ...;-)

Joe J.

Re: victor mosfets
 

I agree, 31 K/W is a very conservative figure; IFI's 40A rating probably includes significant safety margin -- which I think is appropriate given their customer base! ;)

Re: victor mosfets
 

Alright, now just for a sanity check, MOSFETs short when they fail from too much current, right? I've seen several failed Victors and I'm reasonably sure that's what they do, but I can't remember.

Also, I have an internship this summer and I'm running tests on various devices and it seems that MOS shorts when it fails(unless of course you attack the gate). I'm working at the wafer level, but I don't see why things would change for a power transistor.

Re: victor mosfets
 

Short-circuit failure is typical on the wafer level. When large die are mounted and bonded to a single-device leadframe (e.g., TO-220) the weak link after such a failure is usually the source bonding filaments (for an n-channel device). Assuming sufficient fault current is available, these will burn open fairly quickly after the die has failed short. The result is that most discrete package power MOSFET failures are open-circuit by the time things cool off and you get to inspect the damage.