Mohit M. wrote: > I am designing a peltier heater/cooler using PWM (10kHz, 12VDC @ 8A) > and an H-bridge. To filter the ripples across the peltier, I plan to > use an LC filter: > > ____ Peltier > ----|____|---- > | | > | | > L | C1 | L > ___ | || | ___ > V+ o---UUU--o-----||-----o--UUU---o V- > | || | > | | > | | > C2 --- --- C2 > --- --- > | | > ^^^ ^^^ > GND GND > > (created by AACircuit v1.28 beta 10/06/04 www.tech-chat.de) > > C1, C2, C3 values are 10uF. > > I am able to get multi-layer ceramic capacitors 16V,10uF in SMD1210 > package from Kemet/Murata. I have a few questions regarding these... > > 1. How much should I derate these capacitors for my use? Is 25% okay > (12V PWM over 16V)? The dielectric is X7R. > 2. Are these suitable for use in filters in an 8A application? Or > should I be paralleling them to increase the current? These answers strongly depend on the value of L, which you did not give. > The manufacturer (and past discussions on this list) say less > than 10% ripple across the cooler. With a 1.5-ohm Peltier device, the ripple will be within spec. only if the total inductor impedance is greater than 15 ohms at the drive frequency. At a switching frequency of 10 kHz, this requires a total of 240 uH. I assume you're not driving the Peltier with a 24 Vp-p signal, right? In which case, the worst case for the capacitors is at 50% duty cycle, with 6V average across the Peltier. If you allow 10% voltage ripple, or 0.6Vp-p, then at a drive frequency of 10 kHz, a 10 uF capacitor would be passing an AC current of about 120 mA, which doesn't sound too bad. -- Dave Tweed -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist