VAXmate PSU

Den s?n 5 apr. 2020 kl 23:53 skrev Rob Jarratt via cctalk < cctalk at classiccmp.org>: When you use a variac the developed voltage on the secondary side will be much lower due to the turn ratio of the transformer. You measured 0.4V on the 5V line if I understand you correctly. That means approx 1/12 of nominal output. That would then lead to 1/12 current on the secondary side and 1/144th of the power developed. With this little power developed on the secondary side there is no chance you can exceed the power transistor ratings. I do this all the time. I measure transistor current when doing this and there are not a ten fold or so increase in current as Brent claim. By using a variac and you can measure and study waveforms and detect problems in driving circuits without killing the main switch transistor. You can study actions of crowbar functions. There are two different basic variants for SMPS mode PSUs. Forward or Flyback. Forward are normally used in higher power designs while flyback is lower power. A forward mode PSU is like creating a square wave AC current and feeding it to a transformer. In forward mode the duty cycle cannot be more than 50% since the core need to reset before next cycle. The output will be a in relation to the input by the turn ratio of the transformer. A forward converter need a way to reset the transformer, either by various diodes on the primary diode or using various bridge topologies for the switching system. There are also filtering inductors on the secondary side. However your PSU look very much like a flyback design. It means that the transformer has an airgap. It is actually both and inductor and transformer. A flyback converter stores energy in the inductor when the transistor is turned on and then when turned off the energy is transfered to the load. When the transistor is switched on the current will increase linearly in the primary winding. This will continue until the transistor is switched off or the core is saturated. When saturated the current will rise sharply. This also means that the duty cycle can be almost 100% only limited by the design of the transformer so that it does not saturate. Saturation is a condition you don't want to have. What could be interesting is to study the waveform of the drain current in more detail. You need to set timebase of your oscilloscope to a much shorter time span to in detail study the increase of current in the drain. The UC3842 does allow up to 100% duty cycle while the other cousins in the same family, UC3844 and UC3845, only go up to 50%, thus made for forward converters. It can be interesting to investigate if the feed back circuits works as expected. Usually one can inject lab supply voltages on the outputs and find out if the control circuitry reacts properly. The problem with your supply is that it is fairly complicated on the secondary side. While reading the schematic it is not immediately clear that the op-amps will be operating properly by feeding in +5V and +12 V backwards, unfortunately. Then another option is to cut open the feed back path and inject a feedback voltage from a labsupply at the input of the UC3842. You can then vary it while there are supply voltage to the UC3842 and you can then monitor the duty cycle of the UC3842. If you cut away the secondary side output stage you have to bear in mind that this is most likely a flyback design. Without any load at the output of the transformer the energy stored in the magnetic field has no where to go. You could get very high voltages on the drain. I suggest that you have a load resistance over each output of the transformer. By doing this together with breaking up the feed back loop as described above you could monitor how the primary side is doing its switching. You should not try to do this without a variac. While having the output stages cut away from the transformer you could also test the output stages by powering them from the transformer side by a bench supply. What happens if you turn up the voltage slowly? Will the crowbar trigger or not. Is a proper feed back voltage generated. A lot of things to test, but one common note is that you need to study your waveforms on a cycle basis not 100 ms/division. More like 10 us/division. Good luck! /Mattis