Current Mode Class D Questions
How do I calculate the optimum inductance of each RFC?
What determines each MOSFET's drain current when each is switched ON?
I've studied the equations in this research paper:
http://sigma.ucsd.edu/research/articles/2001/2001_2.pdf
but I can't see what really sets the current. It seems like the paper talks in circles around what really sets the current.
Maybe my dyslexia is blocking me from seeing the answer.
Is it the supply voltage divided by the accumulated series resistances in the MOSFET, RFC resistance, tank inductor and interconnections that determine the current?
Thanks if someone can enlighten me.
73, AF5IE, George
OK, I think I found the DC current in equation 3.8 here:
https://inforum.oru.se/PageFiles/148...-mag102_06.pdf
But I still don't know how to calculate the optimum RFC inductance.
I think there isn't any optimization procedure for this kind of amplifiers because RFC is used to block RF currents and it does not contribute to matching or anything else.
RFC should be as much as possible to suppress RF currents but limited to its own self resonance frequency.
That's why, if this amplifier will work in a narrow bandwidth, I suggest you to use lambda/4 circuit.Otherwise a wide-band RFC can be found from-for instance-Coilcraft.
There's not much optimization to be done on the RFCs. To little inductance and you'll get excessive RF current in the supply, and the chokes will effectively become part of the tank circuit. Too much and you get excessive losses from its ESR.
What sets the current is, to a first order, the drain bias voltage and the load impedance. At least that's the case when the load impedance is much greater than the parasitic losses in the inductors and FETs. For an approximate formula, look at equation 3. If your efficiency is perfect then Pin=Pout, and so you can figure out ID by ID=Pin/VD.
edit: yeah that equation in the thesis is it.