# Bulk capacitance after 12 V battery input

I am working on a new project that will be mounted on my racing car. I have to make a power distribution module using e-fuse and high switch modules to control the various loads.

Exaggerating, they will flow (if all the outputs are active) maximum 90-100 A (oh yes a good current!).

On the PCB there will be two converters from 12 V to 5 V and from 12 V to 3.3 V (for the MCU). I thought of using switching since they are now at a good price. As input I thought I would use one of those screw connectors suitable for over 200 A (if I remember correctly it produces the redcube and they should be press-fit). Immediately after the input a nice power MOSFET that acts as a reverse battery protection.

Is it recommended to use bulk capacitors after the reverse battery protection? How do I calculate the capacitance I need?

With the kind of capacitances needed to have any effect at a current of 100A, you will need to have some kind of charging circuit for the capacitors, which take them up to battery voltage using a controlled current, before the main battery supply is switched to the capacitors.
This charging circuit also needs to have a protection function to avoid switching the battery to the capacitors unless they are charged to the correct voltage, to avoid blowing things upā¦

Most of the math is easily deduced with a bit of logical thinking.
The problem however is finding the parameters you want to compensate for.

For example, when your batteries are only rated for a 50A peak current, but your application needs a peak current of 100A (for some time) then you can rate your capacitors for the 100A over the time you need.

Also consider that every electronic component (battery, capacitor, wiring) has some inherent resistance, and with a 100A current the losses add up quickly. If you double the current though a resistor, then the loss quadruples. (For this reason itās a very bad idea to use peltier elements with a pulsed current. the extra loss introduces extra heating of the element).

Are your āHigh switch modulesā creating a PWM current for your motor?
Maybe have a look at the hobby market for Remote controlled stuff, skateboards etc. controllers for those (most often BLDC) motors are often capable of delivering (peak?) currents in excess of 100A.

But this is a KiCad forum, dedicated to the use and issues of working with KiCad itself. For more generic electrical questions there are plenty of more appropriate forums.

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@paulvdh
Hello no no pwm they just have to turn on or off some loads: for example: Fans, heaters etc. Then each output of the pcb will be max 10A but when summed they come out 90-100 A.

Ah ok but the current will not immediately be 100A. With the high side switches I turn on or off loads such as: Fan, heater, light. All of these things almost never come on together. Since there are 10 outputs, if the current is added, the total will be about 90A if everything is turned on.

I assume you say that the load current will not be 100A immediately (or continously).
That might or might not affect your choice or need of capacitance in your device.

What I was talking about was the inrush current from the battery to the capacitor when you connect the battery to the device with the capacitor.
This inrush current is only limited by the ESR/ESL of the capacitor, resistance/inductance of the circuit between battery and capacitor (which must be very low for 100A) and the inner resistance of the battery.

Given a load current of 100A, the potential for 1000A or more of inrush current is certainly possible.
If the ESR of the capacitor is so low that you donāt get much inrush current, it potentially wonāt do anything useful in the circuit, so this need of charging circuit in power DC equipment is very commonā¦

The total current with all the switches turned on will correspond to 90-100A as already mentioned. I was thinking of putting some capacitors after the battery reverse protection (mosfet p with a zener). Seeing on the web they are called bulk capacitance but the problem I have right now is how do I calculate what I need? Most loads are inductive.

Let me take a step back: you have some low-current stuff (controllers), which need smoothed supply. But do the REST (fans?? heaters???) really need smoothed supply? I suspect not.

Iām thinking you need to somehow āisolateā the low current stuff (dropper resistor? Diode?) and then use enough capacitance for just the ālocalā needsā¦ For INPUT caps, the normal āfinger in the airā figure I use is around 2000uF per amp.

(The ITV1205SA has a 12 volt input, will work from 10.8v to 13.2v. If your input is a car battery, it can be close to 14v, so having a dropper / diode is possibly essential)

Read the specs for the relevant DC:DC converters to see the limits for smoothing capacitors AFTER the converterā¦ for example the ITV1205SA has a 200mA output, max cap is 220uF.

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The longer these thread go the more chance they get further off the rails which is why we donāt encourage them. I understand you may get less ānoiseā on this channel but thatās why we limit it.

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Hello ah ok thanks yes in fact the āloadsā do not need some stabilized voltage because they are fans, lights, etc. Then seeing that on the pcb there will be a mcu, some Ina sensors (for current measurement) and some e-fuse the power supply part of this stuff, I will isolate it from the rest of the circuit

I am sorry that you think so even if you are partly right. Since I am going to create my board with kicad, it seemed right to share some doubts with you.

Yes, but we expect direct link to KiCad. Screenshots, design files, how to do something with KiCadā¦

Ah ok. Thanks for letting me know

In shortā¦ You do not need a capacitor after the pass FET, itās acting like a piece of wire and you have tonnes of that as you distribute the current to the loads - the loads should have an input bulk capacitor to decouple the lead inductance

Your biggest problem will be that FET rippling itself to bits when it turns off and there is high current flowing (eg a MCU reset due to noise ā¦)
Force commutating an inductive load is asking for a lot of volts and a lot of smoke unless you provide means to dissipate the 0.5LI^2

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