Originally posted by Bruno Putzeys in another thread
Well if you are very patient and have enough power devices to spend on failed amplifier you could just possibly get a UcD based ampliverter working.
Let's say I would not recommend it but just for the heck of it, here's how it could be done (mono amplifiers only):
1. Make a normal UcD loop.
2. Feed the comparator output to a state machine that alternatively switches the primary and secondary sides.
But here's the annoying bit:
3. Add a protection that will override the comparator output if it hasn't switched for the last few microseconds. Otherwise the primary side will blow instantly when the amp is clipped.
I'm not a fan of ampliverters though. The idea is very elegant, but the bilateral switches on the secondary side make things a bit more complicated in reality than in concept.
Hi,
Ampliverter is basically a transformer isolated class D amplifier, so it can be connected directly to the rectified mains voltage. Effectively it is just a SMPS with bidirectional synchronous rectifiers.
Compared to a classical class D amplifier it has some advantages (at least on paper):
- power supply is integrated, so it can be made small
- both pairs of switches operate at one half of the switching frequency, 200kHz in this case.
- there is no stored energy on the secondary, so DC speaker protection is not necessary.
- auxiliary supply voltages require only few turns windings on isolation transformer
I have read of only one commercial amplifier using this principle (Peavey). There are several patents on this topology, but the main one (US 4479175) has already expired. The most recent patents came from Nuforce (class N amps). All designs that I have seen use constant switching frequency and that seemed like a good idea at first. Only when I tried to close the feedback loop in simulation (using triangle derived from the secondary winding) I realized that delays in the modulator and driver chain produce DC offset at the output. I was then toying with PLL used as a modulator until I started to think about Bruno's idea. Variable frequency is not such a problem as it seems. At 200kHz transformer would operate at 0.05T because of the losses, so lowering primary frequency to 50kHz is no problem.
So here is a simulation of proof of concept UcD type ampliverter. It shows that the whole thing is not much more complicated than a commercial UcD amplifier which also incorporates current limiting, soft start and overvoltage protection. Simulation uses S elements as switches to speed up simulation, but i believe that all necessary parts are included. E elements with unity gain represent optocouplers or other digital isolators. Simulated circuit is stable into open and short circuit and is also protected against clipping. The whole concept of variable frequency ampliverter is not limited to the UcD type of modulator, it can also be used with leapfrog modulator or even self oscillating hysteresis modulator. Before I proceed with the final design I would like to know how much some second order effects can be detrimental to the performance of this circuit.
By second order effects I mean unmatched switching times in the primary and the secondary and unmatched delay from the modulator comparator to the primary and secondary switches. As can be seen from UcD vs ZapPulse thread switch transition times have far greater influence on distortion that I have originally thought. Any comments from Bruno, Lars, analogspiceman and all others are welcome. Also many thanks to Bruno for the original idea.
Best regards,
Jaka Racman