You could try using a ferrite bead in the supply to the PIC. Use as high an
impedance as your circuit allows and follow it with 4.7uF, 100N and 100P all
to ground.

Are you using a shielded inductor?

National has a switcher that appears to try to reduce the spike when the FET
turns on. You might give that a try. Not sure they support the voltages you
need.

http://dkc1.digikey.com/us/en/tod/NationalSemiconductor/SimpleSwitcher/index
.html

David
 =


> -----Original Message-----
> From: rlistas [mailto:rlistas@gmail.com] =

> Sent: Saturday, March 10, 2007 9:31 AM
> To: Microcontroller discussion list - Public.
> Subject: [PIC]Buck converters and Noise
> =

> Hi;
> =

> I need some advices about noise in DC-DC Buck converters
> controler by a PWM generated with a PIC 16F88. Any help
> or ideas will be great. =

> =

> The buck circuit is capable of 2KW power output, using
> 4 power mosfets in parallel. It accepts a voltage in
> from 50 to 300V and output 50V.
> =

> The problem is that with about 500W output =

> the generate electrical noise resets the PIC
> or corrupt the data in the LCD displays that
> show the voltage in and power output.
> =

> Observing the signal at the gate of the mosfets,
> the is a small spike in the turn on fase of the PWM
> that increases with power until the circuit resets.
> =

> The gate of each mosfet is protected by a 15V zener, =

> a series 33ohms resistor and a pull down 10kohms resistor.
> =

> The driver circuit is a TC4420 and an 6N135 opto.
> =

> The PIC circuit has decoupling capacitors of 100Uf and 100nf
> very near de power pins.
> =

> All the cables from pic to power circuit are coaxial with =

> only one side of
> the coaxial connectec to gnd.
> =

> The buck inductor is a closed "E" type with gap, 100uh.
> =

> I try LC filters in the PIC power lines, but didn=B4t work.
> =

> The PIC circuit is about 10cm far for the power circuit.
> =

> =

> Thanks very much for the attention,
> =

> =

> Rubens


-- =

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