dontz125 wrote:I've been doing some quick reading on the LFPAK mosfets you mentioned; while they do seem very well designed for thermal loading and reduced resistance, I noticed a few things that I hope wiser heads can comment on. The first is that none of the pieces I looked at had any sort of voltage or current clamping features; the VND is a fairly complex circuit unto its own, while the LFPAK devices seem to be "dumb" designs - just big lumps of silicon etc. Especially considering that the cause of the failures remains unknown, this may or may not be a selling feature - YMMV.
The second point is the "avalanche" rating. The VND has a claimed Eas of 200mJ at 20v, while the NXP LFPAKs I've been looking at range from 70-100mJ (at various voltages). I have only the most basic understanding of avalanche ratings; "more" would seem to be better than "less", but are the devices operating in the same manner, and are the flyback loads experienced in 12v / 12-16 ohm injectors sufficient to bring these characteristics into play?
I noticed that the body diodes of the LFPAKs were all indicated as "diodes", while that of the VND is a Zener diode. At what point does this become significant, if ever?
You are correct, it is a dumb device.
The avalanche is a condition when the drain-source voltage exceeds the bulk break down of the MOSFET, adding an external clamp handle the fly-back takes this condition away from the device. There is no strain on the device with an external diode/TVS/Zener to clamp that energy. Now the external diode must be able to handle that of course.
The MTD3055VL that Saab uses as mentioned by Keithg is rated at 72mJ (Vdd=25V Id=12A) and Fairchild states that the device has a rugged diode internal diode that can eliminate the need for an additional Zener.
Must be good enough, he says they rarely fail.
The IR part (IPS1041R) that is similar to the VND has an Id de-rating for inductive loads. At 7mH inductance the max Id current approaches 1A. Now this part is not an exact copy of the VND device but I am sure that the VND should be similarly be de-rated for high inductive loads also.
I am not trashing anyone for their choice with the VND5N07 part. I probably would have picked the same part, internal clamp, thermal protection, etc... It looks good.
I will get some of those FETs next week and try to drive a really big inductive load with the external clamp.
Reliability and fail safe is important to me.