# How to calculate the correct values ​of the coil and capacitor for an LC circuit and what type of capacitor to use?

I designed a complete H-bridge using four MOSFETs, to be able to control the supply voltage of a Peltier cell. I put a 1 μH coil in series with the Peltier to work as a small filter and I use an INA current sensor op-amp from Texas Instrument to measure the current going into the H-bridge. The PWM frequency is 2.5 kHz.

The circuit works fine but the coil gets very hot, and when I look at the output signal of the current sensor I see that 10 A spikes appear when the signal of the PWM goes from 0 to 24 V. I don't understand why that peak current of 10 A appears when the Peltier cell has a maximum consumption of 7 A according to its data sheet (the Peltier cell continues to work correctly after having tested the circuit for 1 full week without being turned off).

simulate this circuit – Schematic created using CircuitLab

I decided to completely filter the PWM signal to get a constant voltage proportional to the PWM duty cycle, so I put in an LC filter, did some simulations, and got acceptable values ​​for the inductance and the capacitor. The problem is that the ripple current of the capacitors is a bit high. Can I use X8L ceramic capacitors instead of the electrolytic ones?

As I mentioned, I had to simulate a bit to find the values ​​but I remember that there are formulas to theorize the ripple current of the capacitors, the values ​​of the capacitor and the coil, and others. Where can I find information for this type of filter? Is the theory for designing LC filters for AB amplifiers valid here?

In the simulation, increase the frequency of the PWM signal to 80 kHz; that will be the new frequency that I will use in the new design.

• Commented Mar 21, 2023 at 21:33
• Would you please post links to the data sheets of the relevant components? What is the current rating of the inductors? Commented Mar 21, 2023 at 21:34
• Sorry for the extra hoop to jump through, but this is StackExchange - please edit your question to incorporate those links, and any other new information it needs as it evolves. StackExchange wants both questions and answers to be complete, without any important information buried in the comments. Commented Mar 21, 2023 at 22:51
• Your schematic does not show a capacitor, so we can't know if you've got it in the right place. Note that there's a schematic editor that you can use if you want -- find the little circuit icon (just to the right of the "picture" icon) and press it. Commented Mar 21, 2023 at 22:55
• Please share a complete schematic. With every component you've used. And with a picture of the assembled circuit as well (breadboard, protoboard, whatever you got). I gave you a starting point for a readable schematic. You can edit it further to include the actual components you've used, like mosfets etc. Once you've done that, I can copy it into an answer and set up some simulations to demonstrate what is wrong (because there invariably is). Commented Mar 22, 2023 at 2:43