6-19-2007 An additional part may be added to the PNP feedback transistor on the isolated side to improve it. Place a switching diode in series with the upper resistor of the voltage divider for temperature compensation. That transistor can have the double duty of also providing current limiting if a very ;low value resistor is placed in the positive output current path and sensed with that PNP transistor. 3-12-2007 I have discovered two additions to the ZVS USMPS circuit which may help performance in many cases. The first change resulted from my trying to improve loop stability. The second was to address the weakness of the charging/discharging of the resonant capacitor on the primary side during light output load. I have traced much of the loop stability trouble to the slow response of the optocoupler. A couple of years ago I placed an extra transistor after the optocoupler. That made the optocoupler drive a lower impedance at its collector. It also increased the gain of the circuit, allowing tighter regulation. But it was the harder voltage tug on its collector which helped the frequency compensation. Recently, I decided that there still was not enough frequency compensation. I then had to concentrate on the diode side of the optocoupler. I simulated the addition of a TL431 but was not happy with the result. Instead, I have used a PNP transistor combined with a zener diode. An NPN transistor should also have worked with a rearranging of the ancillary components, but I have a lot of extra PNPs on hand. The old circuit configuration on the diode side provided a certain asymmetrical application of the frequency compensation (often referred to as phase compensation and phase lead). The asymmetrical action tends to damp oscillation and provide quick over-voltage protection. The phase compensation capacitor can charge through the transistor base but it can discharge only through the upper voltage divider resistor. That situation is somewhat like the one before I added the new components. Then, the capacitor charged directly through the optocoupler diode, but could only discharge through the feedback resistor in series with the zener diode. I did simulate the feedback improvement on both the ZVS and non-ZVS versions. I hope to also get the opportunity to try it on the real bench-test circuit. In fact, I plan on adding both of the improvements to the bench-test circuit to see how they really work. The second main recent improvement to the USMPS pertains only to the ZVS version. The problem was that under light load there was not enough energy stored in the leakage inductance of the transformer to charge/discharge the resonant capacitor in parallel with the primary all the way to each DC voltage rail. Then the MOSFET had to finish the task when it turned on, wasting energy as heat. The best solution is not only to place an inductor in parallel with the secondary of the power transformer, but to place a capacitor in series with that extra inductor. Without the series capacitor, the inductor draws more energy linearly when we do not want it to. The extra energy then only works to increase the slew rate on the primary side of the transformer under heavy load when we do not want it. The extra capacitor in series with the inductor limits the energy the inductor can store. What's more, it saves it until the end of the duty cycle when the primary side resonant capacitor needs its energy boost! I have also added two anti-resonant diodes to go along with that extra secondary-side capacitor as I have likewise done for the coupling capacitor in series with the primary transformer winding. Now, I must admit that I have studied these two major new ideas only on the LTspice simulator. I hope to bench test both of them during the summer of 2007 by adding them. to the circuit I have been trying different configurations on. But already as of now, I am very pleased with the ZVS USMPS circuit, at least in its theory. The ZVS addition to the original USMPS has the main purpose of reducing electromagnetic interference(EMI) production. A remaining source of such interference from it now is the diodes. They produce interference mainly when their junctions are recovering during current reversal. To reduce that problem, they still may need snubbing in some cases. They don't need extra snubbing if they are sandwiched between two capacitors as in the case of the primary coupling capacitor clamping diodes. Additional high frequency harmonic generation in the circuit, as with all square wave transformer drive circuits, occurs when the drive waveform hits each power rail. The slower rise times of the ZVS USMPS reduces this interference source. The clamping diodes on the primary side prevent resonance of the primary winding with the coupling capacitor. Besides, without these, the capacitors cannot be used in current limiting since they will be able to charge/discharge beyond the power rails. The current would be able to gradually increase as the voltage swings progressively extend further and further beyond the power rails with each half cycle. Eventually the voltage rating of the capacitor would be exceeded, too. 10-27-06 My bench test circuit became more stable in the feedback loop when I connected the phase compensation capacitor to bypass only the feedback resistor instead of both the feedback resistor and the zener diode series combination. I was surprised at the improvement and am unsure if it holds for all circuit implementations. I still see no real need for active current limiting in the ZVS USMPS. The output inductor alone can provide enough current limiting. Additional current limiting action can occur if the transformer primary winding coupling capacitor is small enough in value. In that case, shunt diodes need to clamp capacitor/transformer resonance peaks to the power supply rails. If implemented well, as current through the primary winding increases, the slew rate of the primary voltage can actually begin to decrease, as the circuit moves toward zero current switching, theoretically lowering EMI. However, the switching noise of the extra diodes may negate this reduction to a certain degree.