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| Designing a Differential and Common Filter Circuit for DC DC Converter Output |
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| T3sl4co1l:
Oh, so it's audio, and there is a common ground elsewhere? So there would probably be some interest in minimizing ground loops to preserve audio quality? The 300mA load is pretty small, I would route the 36V to the power amp and connect the converter there (with filtering as needed). This keeps the loop smaller than having a separate power supply section, and keeps the loop current low. --- Quote from: MarcusS on November 16, 2019, 02:17:53 am ---According to the calculations I don't understand how the differential filter has a transmission peak at 880kHz. That is indeed the frequency Fc but I expected for the differential mode filter of 10uH and 3300pF that would give me a Q of 0.7 so that shouldn't be a peak. --- End quote --- Taking the circuit at face value, there's no load (or, well, the 4.7k I guess), so the Q can be quite high. I don't know what kind of load a miniDSP board is like so I might assume the worst. If it's a typical load, it probably has a few uF onboard, in which case having a C11 doesn't really matter. You would still have to figure out the Q factor though. You generally want a filter impedance much less than the load resistance, because the filter impedance (with a factor of Q higher or lower, depending on which equivalent resonance the filter happens to manifest) is how much AC voltage (supply ripple) appears at the filter for a given change in load current. For a better than 10% regulation under 300mA load step change, you need less than 4 ohms, and so on. Electrolytics of modest values (a few uF) have ESR comparable to this (an ohm, or fractional ohms), making them appealing to ensure low Q. A smaller ceramic can be placed in parallel to maintain good HF performance. (The high-Q (ceramic or film) capacitance should be at most a third of the lossy bulk capacitance. So, a 10uF electrolytic with <= 3.3uF ceramic in parallel, should be pretty well behaved.) Tim |
| MarcusS:
The 300mA load I have quoted is what the minidsp draws and is also what it says on the datasheet. I have measured it and it is pretty constant and doesn't change much. The first thing on the minidsp board is a 1000uF electrolytic so I haven't added any additional electrolytics on the load side of the dc dc converter. The primary reason being is that the 12V dc dc converter has a 1200uF maximum capacitive load rating. So in other words the common mode and differential mode filters are being designed for a 300mA load at 12V. The 4700 ohm resistor is for some additional damping but I may get rid of it completely. It was basically there to stop the filter ever having no load at all and is not necessary with the minidsp connected as it will be on the final pcb. With my calculations, the Q of the filters with a 40 ohm load (e.g. 12V @ 300mA) is about 0.7 for both the differential and common mode filters. Cutoff frequency is about 900Khz in differential mode and 5Khz in common mode. Do you agree with my maths on that one. Probably also worth mentioning is the filter design doesn't use electrolytics. The 0.56uF capacitors are film caps and the capacitors that have pF values (C11 and C10) are all ceramics. I have just used the wrong symbols on my schematic and I probably should have used a non polarised symbol. C10 is meant to be the "pi" capacitor. C11 + leakage inductance of the common mode choke is the differential mode filter and C6, C7 and the common mode choke that is 1.7mH in common mode is the common mode filter. The other thing that is important is the amplifier is in a completely different chassis running off a completely different power supply and will remain that way. In real world operation due to the fact the audio signal from this dsp chassis connects to the audio input of the amplifier chassis, there will be a connection between the DSP V- and the amplifiers power supply ground. Hopefully that clears things up a bit. |
| MarcusS:
Also C8 across the input of the 12v dc dc converter on the 36V side is also a ceramic. The 2x 330uF capacitors on the input side of the dc dc converter are nippon chemicon low esr electrolytics. |
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