Class D low frequency RF amp question

[Chris Wilson]

Push pull output, Class D low frequency (136kHz) RF amp, circa 500 Watts. If I want to test it for a few hours into a dummy load does it matter if the low pass filter network usually used to get a sine wave, harmonic free output into an antenna, is not between the output transformer and the dummy load? I want to leave it running overnight and I am modding the LPF and it's in bits until next week. Thanks.

[MagicSmoker]

If the dummy load is sufficiently resistor-like up to, say, 5-10MHz, then it should be fine. The RMS value of the unfiltered PWM is going to be a little bit higher since there aren't any losses from the filter network, but the only component I'd really watch carefully for the first hour or so is the output transformer as it will definitely run hotter (particularly the windings) as a result of higher harmonic content.

[awallin]

Push pull output, Class D low frequency (136kHz) RF amp, circa 500 Watts.

Can you share the circuit/chip you are using for this?
I'm interested especially if it has analog input...(i.e. it takes a low-power 136kHz AM/PM-modulated signal as input)

[Chris Wilson]

Sure, here's the schematic attached

[Chris Wilson]

If the dummy load is sufficiently resistor-like up to, say, 5-10MHz, then it should be fine. The RMS value of the unfiltered PWM is going to be a little bit higher since there aren't any losses from the filter network, but the only component I'd really watch carefully for the first hour or so is the output transformer as it will definitely run hotter (particularly the windings) as a result of higher harmonic content.

OK, that's great, thanks very much for the reply.

[T3sl4co1l]

Yes, the filter network affects the reactive power seen by the switching transistors and therefore the switching (commutation) transient.

Tim

[Chris Wilson]

Yes, the filter network affects the reactive power seen by the switching transistors and therefore the switching (commutation) transient.

Tim

OK, so in simplistic terms running the above schematic into the dummy load without the filer network is a bad idea then? Cheers Tim, LPF shown in attachment.

[KJDS]

The LPF has been designed to present an open circuit to harmonics. Without that the amp won't work as efficiently as intended.