Class D output filter high impedence

Thread Starter

artmaster547

Joined Jan 6, 2016
409
Hi all
I have an issue I want to use a class D amplifier to drive a piezoelectric load, I am modelling this as just a resistive load of 400 ohms, (37.5W, voltage swing between +175V and -175V). Could anyone suggest what kind of filter I should use on the output, the class D FETs will be driven at a minimum frequency of 100kHz and I need a AC sinusoidal output of 1kHz? I have seen it being traditionally with an LC filter when calculating these values I got an extremely large inductance of 59mH, and wondered if anyone had any suggestion if I could use anything else that uses components of smaller magnitudes and are more accessible?

Kind Regards
Art
 

Hymie

Joined Mar 30, 2018
983
With the parameters you have given (+/-175V switching at 100kHz into a 400Ω resistive load), using LTspice - with a filter made up of a 10mH inductor followed by a 0.1uF capacitor (to 0V) reduces the switching voltage to around 5V (peak to peak) at the load.

If you play around with these values (or add a second filter stage), you could reduce this voltage further.
 

Thread Starter

artmaster547

Joined Jan 6, 2016
409
With the parameters you have given (+/-175V switching at 100kHz into a 400Ω resistive load), using LTspice - with a filter made up of a 10mH inductor followed by a 0.1uF capacitor (to 0V) reduces the switching voltage to around 5V (peak to peak) at the load.

If you play around with these values (or add a second filter stage), you could reduce this voltage further.
By reducing the switching voltage do you mean the filter attenuates? So rather than the load seeing +/- 175V it sees +/- 5V?
 

Hymie

Joined Mar 30, 2018
983
The D class amplifier is switching the output at high frequency; the audio frequency modulates the high frequency such that at 1kHz, the above filter is applying virtually no attenuation, so the 400Ω load will see 1kHz (350V peak to peak) – but the high frequency component is removed.
 
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