Switcher PCB design

08.04.2007 21:09

Here is an interesting circuit I came across today. It's from an older "Standard Charger" for Ericsson mobile phones (in other words, it can't be used on recent models). The circuit itself is a pretty standard flyback switching power supply (240V AC input, 5V DC output at 450mA), however the PCB layout has two interesting features:

PCB photo

  • Designer used a "ground mecca". Connections from all elements to the ground on both high voltage (left) and low voltage (right) sides radiate from one point that is near a large electrolytic capacitor. This significantly reduces noise problems in the control circuitry (I had to use the same technique when I designed a switcher last year).
  • There's a feature resembling a spark gap between the high and low voltage sides. I'm guessing it is some kind of a safety feature. On both sides it is connected to ground, so perhaps it makes sure that if a spark makes a galvanic connection between the high and low voltage sides it first connects both grounds. However I'm not sure what good that would be, since the "ground" on the high voltage side may be at 380V in reference to the earth (or the unsuspecting user) depending on how the charger was plugged in.

Update: after some research it looks like this spark gap may in fact be used to protect the circuit from user not vice versa. For example if someone touches one of the low voltage terminals of the charger (which I guess may be connected to a metal phone case), it may bring the low voltage side a couple of kilovolts higher than the high voltage side (which is connected to the earth through the mains). The resulting ESD might damage the transformer or the optocoupler. The spark gap ensures that the discharge instead happens harmlessly in the air.

Posted by Tomaž | Categories: Analog

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