On 15/05/2021 13:37, Paul wrote:
Jeff Layman wrote:
Inverter microwave power supplies are effectively continuous at the
power selected. They don't switch on/off in the way that conventional
power supplies do, although they do use pulse-width modulation to
control the power.

This is from http://educypedia.karadimov.info/library/Inverter.pdf
"Inverter Technology In inverter-equipped microwave ovens, the power
transformer is replaced by a circuit board, which converts the 60Hz
incoming line frequency to a variable rate of 20 KHz to 45 KHz. A
relatively small transformer is then required to increase the voltage to
the level required by a magnetron. By varying the pulse width, the
output power can be linearly controlled for more precise cooking and
defrosting levels. The bulky power transformer is replaced by a small,
lightweight circuit board; and, because less heat is dissipated, power
efficiency is increased. Conventional technology uses just a single
power level, which is regulated by switching pulses. In contrast,
inverter technology directly controls the power output. This constant
soft penetration of microwave energy prevents the common problems of
shrinkage, overcooking, and loss of nutrients. The result is even food
temperature and textures throughout."


My inverter microwave, operates PWM from 70% power to 100% power.
It operates in "relay operated ON and OFF" duty cycle, to achieve
levels such as 40% power.

The LG microwave at the store earlier this year, is able to use
PWM below 70%. It represents an improvement on the range the
Panasonic shows.

But at least the earlier inverter based ones, have two operating
modes, PWM for continuous adjustment, then interrupted-mode for
when PWM can't go low enough.

And being an inverter, there's still a diode to rectify the output.
According to this, two diodes, D701 and D702. And the magnetron
filament runs off the inverter transformer.

https://fccid.io/ACLAP4T01/Schematic...atic-77643.pdf

The circuitry is also shown in that pdf I linked to. What I find
interesting is that magnetron filament voltage must also vary depending
on the output from the inverter. It obviously isn't critical at all,
and, assuming that at full power its "correct" voltage is being applied,
then at lower powers it must run cooler if the inverter voltage, and so
the output from the transformer filament winding, must be at lower voltage.

--

Jeff