Problem Description

I am trying to design a BLDC inverter and I am in doubt of what would be the right approach for estimating the input capacitor size/type. Doing it empirically or throwing everything in SPICE is one approach, but I would like to understand what is going under the hood and at least start from somewhere.

Since there are infinite possible solutions, I know I need to fix some variables before going any further. The initial thought was to allow for "V" volts of ripple at the input. After I fixed this and knowing what the value of ripple current is, "Iripple", I can get the max allowed resistance "R" of the input capacitance network because ripple current is what is causing the input voltage ripple. Therefore I have:

Rcap_max = Vripple/Iripple

Ok, still no clue about the capacitance value so I need to move forward.

We know about "i = C * dv/dt" - if I rearrange this in order to find the minimum capacitor value I have:

Cmin=I* D /(fsw*Vin_max); "D" being the duty cycle

Based on "Cmin" and "Rcap_max" I can choose the input capacitance.

But...

Previous calculations assumed zero impedance coming out of the external cables connected to the DC-link power terminals. With the increase of the cable length, cable impedance would increase and will form a pole at:

f=1/(2pi*sqrt(Lcable * Cinput))

This would cause ringing which could potentially damage the inverter.

Questions

(1) How can I determine the maximum cable length(or inductance, to make the problem simpler) which can be used given the input capacitance value? Do I set the pole at i.e. "fsw/10"(Hz) and then work backward?

(2) Is there a better way of approaching it using the "napkin math" approach?

posted about 3 years ago

CC BY-SA 4.0

9mo ago by Lorenzo Donati‭

2kind‭

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