kiev wrote:It looks like the initial current kick is about 14 Amps in ~2 msec,
Yes; this is straight Ohm's law, with the capacitors initially a short circuit. 340 V / 24 Ω = 14 A.
then current tapers back down to zero as the 800uF cap fills in about 120 msec, which is 6*RC time constants with R = 24 Ohms.
Everybody seems to be ignoring the extra 220 μF in the back end of the charger, making the total capacitance 1020 μF. So RC = 1.02×10⁻³ x 24 = just under 25 ms. After 5 time constants (τ=RC), the current is less than 1%. It's less than 5% after 3 τ. Using 5 τ and 1020 µF, we get 122 ms, as observed.
The resistor has to dissipate the energy stored in the capacitor over the precharge timeframe, which is considered to be 6 time constants, where one time constant is R*C = 24R*800uF =~20msec, so Tpc = 6 time constants is 0.120 sec. This can be seen in the scope trace where the current drops back down to nearly zero.
That's all more or less true.
The energy stored in the cap charging up to 360 Volts is E = 0.5 *C * V^2 =~52 joules.
For 360 V and 1020 μF, it comes to 66 J.
So the resistor power during precharge is E/Tpc = 52j/0.12s = 435 Watts , which is a short-time overload factor of about 11x for only 0.120 seconds.
That's the average over the pre-charge time, with the majority of it nearly zero. But the instantaneous peak really is 5400 W. Using my figures, I get 541 W average. This is 13.5 × nominal. And the initial peak power, which is only for milliseconds, is 5400 W or 135 × nominal. It has to suffer that eye-watering overload for non-zero time. It's not obvious to me that any random resistor not designed for pre-charging or at least for high peak loads will stand up to this treatment.
The JRM datasheets indicate they are designed to handle 10x overload for 5 seconds, so that's why Mits is using it.
It's presumably up to the job. The Tyco HSC series I've used before are rated for only 5× overload for 5 seconds, and 25× for 1 second. It's a pity the graph only starts at 1 second.
As for wanting a gentler pre-charge, 14 or 15 A is a doddle for these capacitors. Also, a higher value pre-charge resistor will have lower peak power, but for a longer time, and it ends up dissipating the same energy (the energy in the capacitors, ½CV²). So it's all about safely handling the initial peak power.