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Spread spectrum of power supply

Having lead design efforts for very sensitive instrumentation with high frequency A/D converters with greater than 20-bits of resolution my viewpoint is mainly concerned about the noise in the regulated supply output. In these designs fairly typical 50-mV peak-to-peak noise is totally unacceptable and some customers cannot stand 1-uVrms noise at certain frequencies. While spread spectrum may help the power supply designer it may also raise havoc with the user of the regulated output. The amplitude of the switching spikes (input or output) as some have said, are not reduced by dithering the switching frequency. Sometimes locking the switching time, where in time, it does not interfere with the circuits using the output can help. Some may also think this is cheating but as was said it is very difficult getting rid of most 10megHz noise. This extremely difficulty applies for many of the harmonics above 100kHz. (For beginners who think that being 20 to 100 times higher than the LC filter will reduce the switching noise by 40 to 200 are sadly wrong as once you pass 100kHz many capacitors and inductors have parasitics making it very hard to get high attenuation in one LC stage and often there is not room for more. More inductors often introduce more losses as well.) We should be reducing all the noise we can and then use other techniques as necessary. With spread spectrum becoming more popular we may soon see regulation on its total noise output as well.

One form of troublesome noise is common mode noise coming out of the power inputs to the power supply. If this is present on the power input to the power supply it is very likely it is also present in the “regulated” output power if floating. Here careful design of the switching power magnetics and care in the layout can help minimize this noise enough, that filters may be able to keep the residual within acceptable limits. Ray discusses some of this in his class but many non-linear managers frequently do not think it is reasonable or necessary for the power supply design engineer to be involved in layout or location of copper traces. Why not, the companies that sell the multi-$100K+ software told their bosses the software automatically optimizes and routs the traces.

Spread spectrum is a tool that may be useful to some but not to all. I hope the sales pitch for those control chips do not lull unsuspecting new designers into complacency about their filter requirements.

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