Panasonic NN-S550WF Schematic

Panasonic Microwave Oven Inverter HV Power Supply

By David Smith VK3HZ (vk3hz (*at*) wia.org.au)

This particular power supply comes from a circa-2000 Panasonic Microwave model NN-S550WF.

Nearly all Panasonic microwave ovens now use an Inverter, and are always labelled with “Inverter” on
the front.

The High Voltage Power Supply Unit (HV PSU)

The HV PSU measures 165mm x 105mm x 60mm and weighs 650g.

At left is the control daughter board. In front of that on the main board are the opto-isolators for the
control and status signals brought out to the green connector. Back left is the rectified mains filter choke.
The mains rectifier and switching transistors can just be seen on the heatsink behind the transformer.
The mains filter capacitor is at right rear. The HV rectifiers and filters (doubler) are right front – white
wires are the HV output from the transformer. The green wire is for grounding the HV +ve. The two lugs
at right are for connecting HV -ve and heater to the magnetron. The winding that can be seen on the
transformer is the primary and is made from 3mm finely stranded wire.

Here’s a view of the control end:

This is the high voltage end:

Circuit Notes

The circuit for the HV PSU is below (taken from the Panasonic Service CD):

Notes about the circuit:

1. Apart from the block diagram, there is no information on the Inverter cont(o)rol circuit. The circuit
itself is centred on one large, unmarked IC, so no help there.

2. The control and status signals seem to be a digital stream (2-3v suggests a 5V data stream).
They are opto-isolated because the majority of the circuit is at mains potential (**BEWARE**).
The part that isn’t is at 4kV (*** REALLY BEWARE ***)

3. The mains input side is monitored for both current and (under) voltage. No indication of what the
control circuit does with this information.

4. The mains filter capacitor (C702) is very small – only 4uF. In a “normal” switching supply, there
is usually 220 or 470 uF in this position.

5. Q701 that does all the hard work is a very heavy duty IGBT – a GT60N90 - 900V @ 60 A. Q702
forms some sort of flywheel circuit. This circuit from a Toshiba IGBT application note looks
similar: