A lightweight switching power supply of 2000W/kg
The Panasonic microwave oven HV inverter is a remarkable design, because it is a half-bridge converter of 1000W with a low weight of just 470g. That is much lower than other supplies with the same power. Because, I wanted to develop a lightweight battery charger for electric bicycles, I wanted to know if it would be possible to build a battery charger in the same way as the Panasonic HV inverter, so with the same low weight.
Note, that battery chargers are allowed to have a lot of output ripple, in contrast to switching power supplies, where the output voltage must be clean. The same is the case with a microwave oven, it doesn’t matter if the high voltage has lot of ripple. Therefore, the Panasonic HV inverter can be made simple and lightweight because no additional chokes are needed for filtering.
For the reason mentioned above, I bought a Panasonic microwave oven HV inverter and re-engineered it.
Panasonic microwave oven HV inverter board circuit
How to safely test the Panasonic microwave oven HV inverter
Here I will tell you how you can check the operation of the Panasonic HV inverter on a safe and easy way, by contactless testing by measuring the magnetic field.
If the microwave oven doesn't work, either the microwave tube itself or the HV inverter is defect. The first test is whether the microwave tube works, here you can see that it produces light:
Than, the HV inverter has to be checked. The high voltage on the circuit board is very dangerous and therefore it is safer not to carry out measurements on the circuit board itself, if it is not strictly necessary. See here how to do it:
Make a wire loop around the transformer core and connect a diode 1N4007 and resistor of 10Ω / 1W in series. If the HV inverter works well, the voltage should be:
- 1.6V with the microwave tube connected.
- 1.9V with the microwave tube disconnected.
You can also use an oscilloscope; the signals are identical with a disconnected and connected microwave:
Extreme danger if the HV PSU is not grounded
The high voltage power supply must always be grounded to the chassis ground with the ground cable. If not, it may expose the user to 4000V and cause extreme DANGER!
See the Microwave oven repair safety precautions and the Panasonic safety repair rules.
Automatic high voltage capacitor safety discharge resistor
After power off, a high voltage resistance of 100MOhm discharges the 3000V capacitors, which are (in my case) 5.6nF and 4.3nF. The RC time is 0.3s, so the high voltage is disappeared after 1 s.
