Neglecting switching characteristics for a moment, the only 2 parameters for a free-wheeling diode are (1) it must safely handle the peak inductor current, and (2) the reverse-breakdown voltage must be greater than the supply voltage. In operation, it acts as a regular diode to provide a path for the inductor current until it decays to zero, or a switching event happens.
Now, if we consider switching effects, the diode has a finite turn-on time, on the order of several nanoseconds. As the driver MOSFET turns-off, the current thru it decays (di/dt < 0) and the inductance of the solenoid produces a counter-voltage (= Ldi/dt). If the MOSFET turns of slowly enough, the negative 'spike' produced by the solenoid might be negligibly small and you wont even need the diode. But that's a big 'IF' and requires detailed analysis of all operating conditions. If the MOSFET turns off very fast, it's possible the free-wheeling diode will not turn-on sufficiently fast to clamp the spike to a safe level; given the currents involved in this circuit, I doubt this would happen. But if this was an electric vehicle, engineers will be spending a lot of time optimizing the design tradeoffs and probing around with a scope. Lastly, there's reverse-recovery. In this design, the inductor current decays to zero long before the MOSFET turns on so you can ignore reverse-recovery. But in many circuits, such as motor-controllers and inverters, the freewheeling diode is still conducting current when the driver turns on again. Not only does the MOSFET need to supply the inductor's current, it also has to handle the reverse-recovery current (technically, charge or Qrr) of the free-wheeling diode; neglecting Qrr would result in an under-sized MOSFET which could get zapped. -- You received this message because you are subscribed to the Google Groups "neonixie-l" group. To unsubscribe from this group and stop receiving emails from it, send an email to neonixie-l+unsubscr...@googlegroups.com. To view this discussion on the web, visit https://groups.google.com/d/msgid/neonixie-l/ca0d5dc1-0da1-4c08-96df-fa40d84b272f%40googlegroups.com.
