Electromagnetic Compatibility. This one has kept me busy recently.
At some point I’m going to have to address the issue of CE marking. That’s the little logo that electronic devices must have if they’re to be sold in the EEC. Two of the directives are of interest; the Low Voltage Directive and the EMC Directive.
Fortunately the Low Voltage Directive doesn’t apply to devices that won’t see more than 75 volts DC. The EMC Directive, however, definitely does apply. It states that the device musn’t interfere with other electrical devices and shouldn’t be susceptible to interference from other devices (or words to that effect).
There’s a lot to this subject, a lot to read and much to understand. But it got me thinking. What sort of interference is the charge controller chucking out? Best way to find out; an AM radio.
Ouch, it was pretty noisy. What to do? Compare it with the charge controller I bought. That was noisy too and it has a CE logo. I think we can do better.
First I tried a snubber. A capacitor and resistor in series connected across the switch contacts (the FET drain and source pins). Nothing. Absolutely nothing dispite trying a wide range of capacitances and variable resistors.
Then a crazy thought. What if I modulate the PWM frequency with a random number generator. Actually quite easy to do, but not very effective.
Finally I noticed a commercial SMPS chip featured slow rise and fall times to minimise RFI when driving MOSFETS. Let’s give that a try. Just one resistor in series with the FET gate did the trick (the gate itself being a capacitor of course).
There must have been ringing on each edge of the FET signal which my gate resistor effectively snubs out. Very effective too, just 1 metre up cable from the controller no interference at all (radio right against solar panel cable). I am confident this will fulfil the EMC requirements.