PCB Layout, Strategies

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bruinbear714
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PCB Layout, Strategies

Post by bruinbear714 » Thu Aug 14, 2003 9:44 pm

Are there any good books or sites on the net that give advice on PCB layout for best performance and minimal EMI noise for a switching power supply?<p>Also, is there info on analog circuit pcb layout in general?<p>I am getting more switching noise/ground problems on my switching supply than I expected.

rshayes
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Re: PCB Layout, Strategies

Post by rshayes » Fri Aug 15, 2003 1:07 am

Unfortunately, I don't know of any how-to books for analog or switching supply layout. A fair fraction of the professional printed circuit board designers have problems with this type of layout.<p>Most of it is basic physics, but the devil is in the details.<p>The critical areas in a switching supply are the paths carrying high currents and being switched at high rates. On the primary side, this will be the path from the bypass on the power source to the transformer winding, from the other end of the transformer winding to one terminal of the switching device, and from the second terminal of the switching device back to the other lead of the bypass. There is a similar path on the output side between the transformer, rectifiers, and filters. These paths should have broad traces and be as short as possible. Move the parts around to make them that way.<p>Bypasses and filters need to be effective at both high and low frequencies. Electrolytic capacitors are usually necessary at low frequencies, but start to become resistive or inductive at higher frequencies (between 10 and 100 KHz). Ceramic and some plastic capacitors can be used to bypass high frequencies, but they are ineffective at low frequencies, since they do not have enough capacitance. Usually you want to use two parts (one low frequency, one high frequency) in parallel for bypassing. The high frequency capacitor should be located closest to the transformer or switching device. The other capacitor should be as close as possible after that.<p>The traces in these areas should not be connected into a ground plane. Only connect to a ground plane after the switching currents have been diverted around a closed loop.<p>It helps to place traces carrying currents in opposite directions parallel to each other on opposite sides of the board. This allows the magnetic fields to cancel and lowers the effective inductance of the trace.<p>Once the inputs and outputs have been bypassed, some series filter inductance may be desirable to reduce noise conducted out of the power supply. At best, the noise across the bypass capacitors will still be hundreds of millivolts, and this is still enough to cause problems in other circuits. Don't expect the output of a switching supply to have less than tens of millivolts of noise. If you need a lower noise level, consider using a linear post regulator.<p>Although they don't look much like normal switching supplies, charge pump power converters can also be a serious noise source, since very high peak currents flow as charge is transferred between the capacitors.

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Dave Dixon
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Re: PCB Layout, Strategies

Post by Dave Dixon » Fri Aug 15, 2003 1:32 pm

Lien,
National Semiconductor's website has a
document called "Layout Guidelines for Switching
Power Supplies" with pictures and everything :)
Search for their application note AN-1149.
www.national.com/an/AN/AN-1149.pdf
Good luck,
Dave

bruinbear714
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Re: PCB Layout, Strategies

Post by bruinbear714 » Fri Aug 15, 2003 2:35 pm

Thanks.<p>[ August 15, 2003: Message edited by: Lien ]</p>

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