I'm drive a couple of solenoids with MOSFETs, and so of course - it's an inductive load.
Besides the plethora of clamping techniques out there; Zeners to ground, Zeners to supply, RC snubbers, RCD snubbers, lossless inductive snubbers etc - it would appear more and more devices are beginning to be sold as 'self clamped', in that they have a Zener from the source to the gate, to switch the device on when source voltage exceeds the Zener voltage. (and of course a Zener from the gate to ground, to prevent overvoltage)
However, I'd like to replicate this myself as these devices tend to have higher on resistance values than really available with 'normal' MOSFETs - I guess a side effect of squeezing everything onto the same die. So, these devices have the following internal structure - so i've replicated it as a schematic.
The question is, knowing the energy stored in the inductive load - does anybody have any idea how to calculate the extra power dissipated in the MOSFET when it switches on to dissipate the voltage spike?
EDIT - I just found an app note regarding this...
The active clamp feature is shown in Figure 5. During the off-state, the power MOSFET is turned back on when Vds > Vzener +Vf,diode + V threshold,MOSFET. Also, there are two large resistances, the resistor and the MOSFET, within this current path to help dissipate energy. (Note that during the active clamp the FET is in a linear, or high-resistance, mode.) The load demagnetizes quickly during active clamp because the stored energy in the load is dissipated across a large po- tential [Vcc - Vclamp]. The larger the difference, the faster the demagnetization. The energy dissipated by the IPS during the Active Clamp sequence is Vclamp x ∫Ids(t)dt.
The energy stored by the inductance is [Vclamp - Vcc] x ∫Ids(t) dt. Thus, it is important to notice that during the active clamp sequence, the device dissipates more power than the load. (The current through the load and the IPS is the same but the voltage across the IPS is higher than the one across the load).
Here's the app note for anybody interested http://www.irf.com/technical-info/designtp/dt99-4.pdf