RFI &&|| EMF

Eugeneo · Joined: 30 Aug 2005 Posts: 155 Location: Calgary, AB

I've used a pic 12f629 (still a 14 bit part) as a variable AC chopper circuit.

I do NOT have MCLR enabled, but for some reason during times of heavy rfi or emf, the processor resets itself. So I tied MCLR to gnd(not VCC) and the the thing works perfectly.

Is it possible that the rfi is causing enough voltage to send the chip into programming mode (+12v)?

kender · Joined: 09 Aug 2004 Posts: 768 Location: Silicon Valley

I had this problem too in a circuit, where PIC was controlling a variable load for an RF generator. The PIC would reproducibly reset in 38 seconds after RF was turned on. The voltage on MCLR can go up to +12V or down to logic-zer0 level, given enough EMI. In my case I didn't have control over EMI source, so I isolated the PIC from EMI with optocouples. PIC stopped resetting.

You, however, have design control over the EMI source. You could reduce EMI, if you put small resistors in series with your FETs' gates (10-100Ohm, depending on the gate capacitance of your FETs). These resistors together with the gate capacitances will form RC filters. These RC filters will lower the raise time on the gates. It will remove the sharp edges from the output of your chopper, which cause harmonics.

There are a few drawbacks to this approach:
(1) The raise time and the fall time of the gate voltage is higher. You might need to increase the deadband.
(2) The efficiency of your chopper will be lower, and you might need to heat-sink the FETs better.

Cheers,
Nick

Eugeneo · Joined: 30 Aug 2005 Posts: 155 Location: Calgary, AB

My whole circuit uses optical isolation with the exception of zero crossing sense. But I did enough test to confirm that it is not from there. But RFI and EMF seem to be painfully unpredictable.

Thanks for the info.

Ttelmah · Guest

I think you will find the chip is just doing a MCLR reset.
This has been mentioned on another group. The MCLR pin, does not have the protection diode present on other inputs. If MCLR is disabled, the circuitry is still 'there' inside the chip, but isolated by a CMOS gate. If -.6v is present on the input, this gate becomes reverse biased, still conducts, and resets the chip. If the MCLR line is not being used as an I/O pin, simply tie it to a power rail. If it is being used as an input, add your own protection.

Best Wishes