Dr.EM said:
Ok, thanks for the responses. I have the gate driving base covered as i'm using smaller MOSFETs as drivers. I have the system on breadboard and using a 4 amp heater (and only one fet) it seems to work well, showing fast switching times, full rail to ground switching and the MOSFET staying nice and cool with a very small heatsink. But then the heater has no inductance issues. I have used a 1A DC motor with it, and although it works properly, there are lots of spikes showing up on the scope. Seeing as how I intend to use 4 IRF540, with a load of 20A hopefully possible, max voltage only 35v and plan on heatsinking to the case, is there any other way to protect the MOSFETs from inductive loads? Can diodes be placed around the FET somehow to protect it, 36v zeners?
You could use a Zener, but you will just cook it (dont under estimate the power of the backEMF
)
Three options
1) propper power-cct layout taking into concideration.
This should be your main driving force! keep all hte power-components close (but not so close that arcing is a possibilty - you are working at low-voltage so not really an issue). DO NOT just use some leads to connect up, if you can make a busbar, you say you want 20A you are going to start to have some REAL problems if your power cct is not layed out to minimise stray inductance.
2) slow down yr switching!!! you will be seeing spikes when switching an inductive load due to turn-off voltage overshoot as stary inductance try's to keep the current flowing through them flowing. By slowing down the speed that the FET turns-off (normally by having a different turn-OFF gate resistor) it allows the strays to dissipate their stored energy by the time the millar-region has been reached.
This however, results in more losses (increased switching losses) in the FET
#1 is by far the prefered option BUT even the best layout will still have stray inductance in very bad places, this is where #2 comes in to "tune" the switching characteristics of the switch's to match the present implementation
now we get onto #3
3) Snubbers!! There was a time when these were needed for forced-commutated devices (1st generation IGBT's) but that time has now gone with 4th being fantastic, Snubbers are still used main due to extreamly poor power-layout resulting in either too much switching-loss or too high of a voltage overshoot.
a snubber's job is basically to shift some of the switching losses from the switch to the snubber dissipating element.
in its simplest form a snubber is a RC in parallel with the FET/IGBT and get more elaborate
#1 with #2 is by far the best way