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| Designing a Differential and Common Filter Circuit for DC DC Converter Output |
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| MarcusS:
Hi everyone, Does anyone have experience designing an output differential and common mode filter using a common mode choke for the output of a dc dc converter power supply. The dc dc converter I am using is a shielded unit capable of 12V output at 2.5 amps. It has a switching frequency of 360Khz and will be driving a 12V 300mA load. I want to put a common mode and differential mode filter to filter out as much noise as possible and have some 1.7mH common mode chokes with 10uH leakage inductance I would like to utilise as a starting point. The load has an onboard 1000uF electrolytic capacitor that could be used as the dampening capacitor of the filter. Is anyone familiar with the maths for these kind of filter designs. I have watched the Biricha videos on you tube but the calculations seem to assume things that I don't have available in the specification of the dc dc converter. |
| T3sl4co1l:
Right, you can't design a filter without all the numbers. You need: Converter isolation capacitance Converter output noise and spectrum, DM and CM Source impedance (what AC return path is there) Maximum noise spectrum (i.e., subtract the spectra to get the required attenuation) You are perfectly welcome to just guess -- indeed, most of us do. But you need some way to test whether it's done what you think it should do. You can measure noise in a simple fixture, for example, preferably with a spectrum analyzer but a scope will do in a pinch, with some hand-waving over the waveforms. Tim |
| MarcusS:
Not sure I understand what the converter isolation capacitance is for this dc dc converter. At the very least I don't see anything mentioned in the spec sheet. The converter is a TDK lambda CCG30-48-12S. My calculations tell me with a 40 ohm load (which is assumed from 12V 300mA output power draw from the converter with my intended load) that a 1.7mH common mode choke with 0.56uF capacitors between each leg of the common mode choke and the ground reference to my pcb will give me a cutoff off frequency of 5Khz with a filter Q of 0.7. If I wanted to make a differential mode filter making use of the leakage inductance of the common mode choke which is 10uH then I would get a cutoff frequency of 900KHz with a filter Q of 0.7 providing the differential mode capacitor is 3300pF. |
| MarcusS:
Also worth mentioning is the 12V dc dc converter is running off the output of a 36V switchmode power supply running off mains. The 12V dc dc converter is for a secondary rail in the device with the main part of the device running off 36V. Its an off the shelf item this switchmode power supply but I don't think I necessarily have an ac return path, as the output is not mains referenced from what I can tell. |
| T3sl4co1l:
Could you draw a diagram of the system, including chassis grounding (if applicable), the nature of this 36V supply, the 12V load, and so on? Is this actually common ground, the 12V is just stepped down 36 and a buck converter would be fine but you're using a module just because..?? --- Quote from: MarcusS on November 14, 2019, 04:09:26 pm ---I don't think I necessarily have an ac return path, as the output is not mains referenced from what I can tell. --- End quote --- There is ALWAYS an AC return path. Even if it's battery powered, there is some capacitance from the body of the device to its surroundings. This is... a bit un-obvious when you haven't seen it before; understanding the common mode equivalent circuit will turn your brain sideways at first. :) Tim |
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