Low Freq Impedance Match to capacitive element

[riscy00]

I have an pizo load element of which is capacitive nature (ZL = 1.2 -J-0.85) normalized at 15KHz.

I have Class D power amp driving into an impedance match element. It is basically H bridge with series inductor and capacitor as smoothing filter.

Has anyone have lumped model representation of the power amp and how this is represented in smith chart (from say 1KHz to 50Khz)?

Does the class-D output have purly resistive output or resistive/inductive output, I'm not sure what the source impedance actually is?

By understanding the impedance of the Class-D power amp, would provide way to optimize the impedance matching with load element, using smith chart.

Any example using smith chart on that respect would be very useful, but also interested for other method.

 

[WimRFP]

The output impedance of a full bridge when fed from a stiff supply voltage nearly equals twice the Rdson of the mosfets, so it is very low.

You should provide the amplifier with the correct impedance so that current is not too high and that it is close to zero current switching. Don't try to match the load impedance to the amplifier's output impedance.

Say you have 80V supply and you want 150W output from the full bridge:

80V supply results in about 80*1.27V = 100Vpk first harmonic voltage component. From P=Vpk^2/(2R), R = 100^2/(2*150) = 33 Ohms.

So you need to design your load to show 33 Ohms for the first harmonic. If you have sufficient series-L (to block higher harmonic current components), this will result in a near sinusoidal current through it.

 

[riscy00]

Thank for the reply

What about reluctance element in a form of complex number.

I'm more interested in form of complex number over frequency. so impedance matching can be designed between source impedance and load impedance.

Approximation or general information is sufficient, ie what is look like?

 

[WimRFP]

If you have a large Im(Zload) at the operating frequency, the switching moments will be under relatively high current (as first harmonic voltage component is no longer in phase with the first harmonic current component). So your circuit will run under hard switched instead of soft switch condition. Hard switching will increase the switchting loss.

Forget the load/source impedance match issue. Your H-bridge amplifier just wants to see a certain load to deliver the power and to be happy That load is NOT equal to the actual output impedance of the amplfifier. I don't know any numbers for your application, but very likely you can consider it as a voltage source.

See it as your audio amplfiier at home, It wants to see 4 or 8 Ohms, but the actual output impedance of the amplifier is very likely < 1 Ohm.