Hello to all electronics fans, I hope someone has come across a similar situation and can give me some advice. I had a truckload of IRFB3207 handy (N-channel MOSFET, 75V, 130 to 180A, 3.6 mohms, Ciss=7600pF) and a large ferrite (EE75) transformer with 2x12V very thick, multi-strand primaries and a 400V secondary tapped at 200V. I Decided to make a DC-DC converter with an SG3525 voltage mode PWM chip at about 40kHz (only without any PWM function, using it as a simple oscillator with complementary, totem-pole driven outputs). The diagram is similar to this Elliot-89 project often mentioned on the internet: http://sound.westhost.com/project89.htm I am currently at 5xIRFB3207s on each side, directly mounted on two large heatsinks (no isolation, the heatsink aluminum body is used as the conductor between the MOSFET drains and the x-former primaries), and have enough space left on the heatsinks to mount another 2x5 MOSFETS. I am using Japanese 30V/3A/>100MHz transistor pairs (2SB772, 2SD882) as external totem poles to boost the SG3525 output current, instead of the BD139/140 in the Elliot schematic. Bipolar totem-poles are not supposed to suffer shoot-through at pulse crossover, so the emitters are shorted together, no limiting resistors, with the cathode of a 1A 30V schottky(1N5818) connected to the emitters and the anode to ground - allegedly to shunt negative drive pulse ringing. The totem pole emitters drive the paralleled MOSFETS (drains and sources are shorted together) through one gate resistor of 22 ohms and a 15nF polyester capacitor in parallel per MOSFET. The secondary is rectified with a bridge made of 12A 1200V fast (FRED) diodes, without filtering capacitors, and various loads have been connected. Up to 600W 220V filament lamps have been tried, tied alternatively to both the 400V and the 200V windings through the rectifier bridge - yeah, I know, 400V is a bit too much for 220V lamps, but they are supposed to take 311V from the 220 AC anyways, and I keep my PWM chip at a low Duty Cycle of 5uS and they seem to sweat it out shining a bit brighter than normal. A universal motor 220V 500W angle grinder has also been tried at the rectified DC of 200 and 400V, works fine at both windings. The whole setup is put together without a PC board (save the SG3525 and totem poles), but heavy copper bars are used for ground and transformer connections to avoid stray inductances. For the most part, I would say the load-dependent spikes I get at the switching recoveries are within reason. I am using some simple RC snubbing, but does not seem to have much of an effect. I am attaching two scope shots (the scope ground is connected to the negative side of the car battery used to supply the 12V to the inverter, with two 1000uF electrolytics between the +12V primary center tap and the ground): In the first picture (DSCN0516-small.jpg), the lower trace shows the MOSFET drain waveform of one side, while the upper trace is the opposite side gate pulse, with a resistive load of 300W lamps at the 200V secondary. We can discern: 1)a 5uS at 0V when the MOSFETs are on, followed by 2) 7.5uS of recovery with ringing at some mid voltage close to 12V, and 3) another 5uS at 2x12V reflected when the other winding is conducting, 4) dropping back to 12V for another 7.5uS, before the next MOSFET conduction period of 5uS. Question 1: the recovery voltage, at pulse interval (2), even after settling down from parasitic ringing, is higher than 12V, (the more so the larger the load), while the dropping back voltage of interval (4) seems correct at 12V. Why is that so ? The second picture, (DSCN0517-small.jpg) shows the waveforms at the same points, only with a 500W universal motor angle grinder connected to the 200V secondary winding. One can see that the pulse is totally screwed up. Question 2: instead of recovering to 12V after ringing at interval (2), the pulse recovers close to the reflected voltage of 2x12V of interval (3). Interval (4), on the contrary and unexpectedly, goes below 12V. Why is that so ? Thanks in advance for your responses.