Calculating controller power input capacitance

[HighHopes]

some musings:
ESR... equivalent series RESISTANCE. and what is resistance? a thing that limits the peak current. note you did not say ESL which is inductance that limits the rate of change of current. no, resistance is only the peak.

so your question does high or low ESR help mosfets? its like you are saying does having some sort of limit to the amount of peak amps help mosfet? in theory you could answer yes becuse too much amps = mosfet explode. but really.. ESR, what value we are normally talking milliohms. so it takes more than just a cap's ESR to limit the peak amps to some value that would be safe. in reality what you'll find is that the ESR of the cap plus all the other resistance like in the copper busing and lug or solder connections etc. will not actually limit the amps to a value that is survivable.. the ESR is still too low!

so was ESR good or bad for the mosfet? from peak amps point of view, it is irrelevant.

so what else does ESR do other than limit peak amps? well, it helps to dissipate energy that would otherwise oscillate. ahh.. now that is useful
this also means that if you build with electrolytics and then you have high(er) ESR then you are not so prone to oscilliations. that's good for DIY'er actually. if you build with film capacitors as i do, your ESR is low(er) and so you better not have a crappy power pass geometry or you will oscillate and there aignt nothing much to limit the oscillations..

 

[madin88]

so what else does ESR do other than limit peak amps? well, it helps to dissipate energy that would otherwise oscillate. ahh.. now that is useful

i would like to know what would happen if we have lowest devisable ESR together with highest capacitance on a controller power stage (so "stiffest" possible power supply till the FET's source)?

now would that make things better or worse for the FET's?
(higher peak current when the FET turns on VS lower voltage spikes when the FET turns off).

as far as i can see the peak current mainly depends on the motor resistance and inductance (including battery voltage and PWM duty cylce), so the advantages of having "more caps" should be higher as the disadvantages.

 

[Arlo1]

so what else does ESR do other than limit peak amps? well, it helps to dissipate energy that would otherwise oscillate. ahh.. now that is useful

i would like to know what would happen if we have lowest devisable ESR together with highest capacitance on a controller power stage (so "stiffest" possible power supply till the FET's source)?

now would that make things better or worse for the FET's?
(higher peak current when the FET turns on VS lower voltage spikes when the FET turns off).

as far as i can see the peak current mainly depends on the motor resistance and inductance (including battery voltage and PWM duty cylce), so the advantages of having "more caps" should be higher as the disadvantages.

You are not really understanding what HH is trying to tell you.

The ESL is what you need to look at. Not the ESR. ESR might limit peak amps but we are talking about limiting it at a few orders of magnitude more the the MOSFTETs can handle so ESR is not something to look at from a FET standpoint.
ESL limits how fast the current can transition and ESL will limit the current flowing through the fet if and ONLY IF the ESL is to high. But on the other end of the spectrum the MORE IMPORTANT END OF THE SPECTRUM the ESL will limit how fast the cap(s) can absorb the inductive spike at turn off and this spike gets higher with higher amperage. If the cap is to high ESL the inductive spike will cause a over voltage from Drain to Source and kill the FETs This is also the reason you need a MOSFET rated for more then the max charged DC voltage on the battery and the CAP ELS or is ability to absorb energy fast partially determines how much over head you need.

 

[madin88]

The ESL is what you need to look at. Not the ESR.

Thanks Arlo for bringing this up.
now, is ESL something that can be calculated from ESR together with other values of a cap, or is a special measuring process necessary to get it?
i ask because most caps are specified with the ESR, and so i took a look at this value for choosing them..

How important is it to have such high frequency filtering on controllers?

 

[Arlo1]

ESR (resitance) and ESL (inductance) are 2 different things. You can find the ESL in some data sheets. As a rule any cap with short leads are likely something they have done to keep L down.
Poly caps have far lower ESL so that is one reason to look at only using poly caps.

You ask a loaded question without any basses for data. The caps job is to protect the Power transistors and to feed the energy into the motor when the power transistors turn on at first.
It needs to be seen as all the power comes from the cap(s) to the motor but from the battery to the cap(s). The caps job is very important.

 

[HighHopes]

alro is correct except for small statement about caps being there to protect the fets. its not the caps primary job to protecdt fets though you could in round-aabout way make such a claim, its not really why the caps are there.

madin88 , i think you should read the PDF in this thread as it will tell you what you need to know for cap sizing. don't worry what you read elsewhere, it doesn't make any sense and will drive you crazy. stick to the basics, and build from there. the PDF contains the basics.