I'm trying not to get lost in the details, but I want to have some math behind, preferable simple and conservative within reason. For example, for calculation of MOSFET switching power dissipation for inductive load I use Pd = (tr + tf) * 0.5 * Vbat * Id * fpwm , which is an equation that overestimates losses but it's simple and easy to understand (KISS KISS KISS). I want to build a reliable controller; I plan to ride a motorcycle with a DC motor, and I don't want a MOSFET failing short (as they like to) and have full power suddenly applied to the wheel, or sudden rear wheel brake. I know I must make it fail safe through other means, but my philosophy to always start with a sound design.
The guidance I gathered so far for the snubber cap design is:
1) Minimum RMS ripple current
Assume the capacitor provides all the current during the MOSFET switching time, so
[pre]Iripple > Imax * (tr + tf) * fpwm[/pre]
(1st written by rithee5 above in this topic)
Example: Imax = 60A (peak, not continuous), Vbat = 43.2V (almost discharged 16s LiFePO4), switching time tr = tf = 500ns, switching frequency = 16KHz -> Iripple -> 1Arms
2) Minimum capacitance
2.a) The capacitor must take/provide half (triangle area) the max current during the switching time while keeping the rail at +- a percentage w of Vbat, so C = i * dt / dv -> [pre]C > Imax/2 * tr / (Vbat * w)[/pre]
This gives me values much higher than it seems needed. Example: Imax = 60A, Vbat = 43.2V, 5% max Vbat ripple and 500ns switching time -> 7uF.
2.b) Should store enough charge as to provide Irr while keeping the rail at +- a percentage j of Vbat, so Q = CV -> [pre]C >> Qrr_diode / (Vbat * j)[/pre]
Example: IRFP4110: max 210nC Qrr @ 75A 85V 100A/us 125ºC, Vbat 43.2V, j = 5% => C >> 0.1uF; x 10 -> 1uF + 7uF from 2.a) = 8uF
3) Maximum ESL
As small as possible
HighHopes said:
for snubber cap, i think the theory is clear that it is there to balance the stray inductance.
It seems that Irr through the parasitic inductance (layout + caps) is the main problem.
HighHopes said:
but for me i always chose the value "by feel" and didn't really use math for that. i had no snubbers for <1kW drive, and 2.2uF for 50kW continuous drive.
With what kind of MOSFET switching time?