[andy55]

Hi, I'm trying to design a servodrive using the IRF IRAMX powerstage.
The datasheet mentions the need for filtering caps between the high voltage dc bus and ground. It also mentions that 'more is better', but does not go into detail of what size capacitors etc.

The drive is a sinusoidally commutating PWM drive with max DC bus voltage of 200 VDC and maximum current of 15-20 A momentarily.

On another forum I heard that it is customary to use a number of different sized caps with say 50x size difference, so for example
500 uF
10 uF
200 nF
4 nF

Now if my dc voltage is 200 V and I want to play it safe I guess I should choose 300 or 400 V caps. A 500uF 400V cap is really large and might not fit on the board.
What will I loose if I start with the 10uF and use only three caps ?

Should the smallest or largest cap be closest to the drive IC pins ?

naturally there will be larger filter caps on the powersupply, these are just the caps on the servodrive board, closest to the powerstage.

[Al_The_Man]

Generally Large (electrolytic) capacitors are used for DC filtering i.e. reduction of DC ripple.
Because Electrolytics posses a certain amount of inductance they are not considered ideal for high frequency filtering, this can often be taken care of by placing a high grade low value e.g. .05µfd capacitor across the DC supply.
Also, the problem with 'more is better' is that with the higher the capacitance used, although the ripple is lower the energy in the charging pulse is higher requiring the use of a larger VA rating of the transformer, another thing to consider is that an allowance should be made that you are unlikely to draw maximum current for extended periods of time on all axis with typical CNC operations.
Al.