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https://designers-guide.org/forum/YaBB.pl Design >> High-Power Design >> Randomization for PWM DcDc-Converter https://designers-guide.org/forum/YaBB.pl?num=1219666020 Message started by Berti on Aug 25th, 2008, 5:06am |
Title: Randomization for PWM DcDc-Converter Post by Berti on Aug 25th, 2008, 5:06am Hi all, I would like to reduce EMI of an existing PWM DcDc-Converter. Therefore I added randomization, which improved EMI. However, the randomization signal appears at the converter output (see figure). Does anybody has suggestions how to improve that? Thank & Regards |
Title: Re: Randomization for PWM DcDc-Converter Post by NiceFeng on Apr 12th, 2009, 5:16am Hi, Berti, How did you modulate the duty cycle? I think your output waveform is related to the period of your modulation signal. It seems it is too fast. Regards, Feng |
Title: Re: Randomization for PWM DcDc-Converter Post by Berti on Apr 14th, 2009, 11:27pm Hi Feng, The period of the modulation signal is given by the length of the LFSR which is 8bit in my case. However, in the meantime I found several publications which confirm the presence of low frequency noise if randomization is applied. And since the DcDc-Converter output is finally regulated by an additional LDO this is not really a problem. Especially since the PSRR of the LDO is very good at low frequencies. Thank you. Cheers. |
Title: Re: Randomization for PWM DcDc-Converter Post by krab on Jul 8th, 2009, 1:30am Hi Berti, I have just started a DC-DC IC design where +-3% variation in the switching frequency is needed for spread spectrum control. I considered using a 8 bit LFSR just like you but I am wondering if the period of the register (8 cycles) is too short. I plan to use the current clock edge to drive the LFSR and the new state of the LFSR will determine the next cycle frequency. I am curious, after your research what is the reason why you have low frequency noise on the output? I am also considering maybe using a low frequency sawtooth to modulate the duty cycle, but I am not sure how I should choose the freq of my sawtooth. Any suggestions? Also any references on how one might implement this sawtooth modulation on chip will be much appreciated. Cheers |
Title: Re: Randomization for PWM DcDc-Converter Post by Berti on Jul 9th, 2009, 12:14am Hi, Quote:
I came to the conclusion that the low frequency noise is inherent to randomization. I found several publications which report on that. Basically it makes sense: If you add dither to a system, the dither will typically also appear at the output. However, since the DcDc-Output is further regulated by an additional LDO in my case, low frequency noise will be suppressed by the LDO. Quote:
The choice of the switching frequency is related to your system (spectral interference planing). Often a low frequency is preferred, which however translated into a large inductor. Cheers |
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