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RE: Distance from rectifier diodes to smoothing capacitors

Posted by tlenthe (A) on November 16, 2009 at 05:26:05

In Reply to: Distance from rectifier diodes to smoothing capacitors posted by megalomaniac on October 8, 2009 at 13:01:30:

The benefit of "soft" rectifiers is that they reduce the amount/rate of current change (dI/dt in engineering terms). Every wire has some inductance... that is that flowing current ("I") creates a magnetic field (and vice-versa). Recall the electro-magnet you made as a kid by wrapping wire around a nail and connecting it to a battery.

Every component has some amount inductance, even the capacitors. The longer and thinner the wire/PCB trace the greater the magnetic field and therefore the greater the inductance.

This magnetic field has two adverse effects in audio electronics. Firstly, it can interact with other conductors and induce a current in it. This is called "cross talk", where a signal (even from power switching) is generated in a neighboring circuit from current in another (try listening to an battery transistor AM radio near to a running vacuum cleaner to hear this effect).

Secondly, even in a lone, isolated circuit, the magnetic field will "resist" a change in the current flow. This "resistance" to a sudden increase or decrease is manifested by creating an opposing voltage.

Back to the power supply diode, the "soft" rectifiers reduce the peak reverse current spike when turning off. This reduced maximum current spike directly reduces the voltage resulting spiking on the power line itself as well as any radiated magnetic field.

Although the these current reversals occur only once per power cycle (50 or 60 times/second), the duration of the current spike is on the order of 10's or 100's of nano-seconds, which corresponds to frequencies in the 10's to 100's of Mhz. Think of is as a burst of RF noise once every power cycle (using a rectifier bridge means that one of the diodes is turning off TWICE in each power cycle).

So, where do these peak currents flow in an audio power supply? Yes, you are correct that there will be some effect due to the length of the conductors between the capacitor and the diode, but think about where that current goes... though power transformer, it's leads, the internal AC connection, the AC power line, plug, house wiring, etc. Unless some extraordinary measures are taken, it's likely that the inductance of those elements outweighs the inductance from cap to diode by at least 1000 to 1.

Another way to address this issue is to use "snubber" circuits which "wrap" directly across the diodes to locally supply the shut off current so that the amount of current flowing through the wiring to the diode is minimized. There are many articles on these if you want more info.

Good luck!

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