thanks for the link...
I'll have a look later, but as far as I remember HP uses triacs not scr, and they are switched on zero-crossing so a simple mosfet cannot replace them.
You need back-to-back mosfets, zero-crossing detector and insulated drivers.
If the MOSFET is turn on hard with capacitors on both sides, there can be quite some peak current spike. The thread started with the problem of high current spikes that looked like up to 100 A.
The current spike may be OK for the MOSFET if it does not get much lager. However with so large a current there is noise and also possible voltage spikes. I would be more worried about loss at the capacitors and also some of those peaks coming through to the output. The audible noise is a second problem. On fast load changes (short to open, CC to CV) there could be also quite some current spike - though usually only at a low frequency.
For the switching there are several options: Triac before the transformer, relays before the rectifier, SCRs as part of the rectifier and a MOSFET behind the rectifier. The switching can be hard between taps or in some cases also with a continuous output. The circuit from the beginning is turning on in zero crossing and doing some continuous adjustment. The circuit does not look so bad - I would expect even more current spikes from the continuous adjusting SCR version.
thanks for the link...
I'll have a look later, but as far as I remember HP uses triacs not scr, and they are switched on zero-crossing so a simple mosfet cannot replace them.
You need back-to-back mosfets, zero-crossing detector and insulated drivers.
Yes my bad, hp uses triacs and not scr since they are switching the ac portion and not a dv voltage. By the way beginner question here why spikes of current are harmful for mosfets? Mosfet rated for 60A continous current are for 200A spikes so even a large current spike of 60 amps or more shouldn't damage them.
For example the irf has a continuos current of 110amps and a peak current of 390A , i know that those are values that you can't even approach but we aren't going anywere near them.
If the MOSFET is turn on hard with capacitors on both sides, there can be quite some peak current spike. The thread started with the problem of high current spikes that looked like up to 100 A.
The current spike may be OK for the MOSFET if it does not get much lager. However with so large a current there is noise and also possible voltage spikes. I would be more worried about loss at the capacitors and also some of those peaks coming through to the output. The audible noise is a second problem. On fast load changes (short to open, CC to CV) there could be also quite some current spike - though usually only at a low frequency.
For the switching there are several options: Triac before the transformer, relays before the rectifier, SCRs as part of the rectifier and a MOSFET behind the rectifier. The switching can be hard between taps or in some cases also with a continuous output. The circuit from the beginning is turning on in zero crossing and doing some continuous adjustment. The circuit does not look so bad - I would expect even more current spikes from the continuous adjusting SCR version.
For the circuit from not1xor1 it still depends on how the switching is controlled. It could be switching with hysteresis and thus not so often or it could still be fast control within a line period.
hysteresis plays no useful purpose here because even if the comparator would "misfire" at 50 Hz (i.e. every other cycle) the capacitor voltage would be higher than the center tap voltage and that alone would prevent the center tap mosfet conduction.
I'll try to show that tomorrow...
hysteresis plays no useful purpose here because even if the comparator would "misfire" at 50 Hz (i.e. every other cycle) the capacitor voltage would be higher than the center tap voltage and that alone would prevent the center tap mosfet conduction.
I'll try to show that tomorrow...
i'll look forward to it.
These last two screenshots show the peak current and voltage spike when switching from low to high voltage.
The simulation has been run with a huge 47.000µF capacitor and a load of 10Amps.
If you only need one switchpoint than take a look at this schematic, it uses a Schottky diode and a PowerMOSfet.
www.bramcam.nl/Diversen/CO-2016-PSU.pdf
Hi,
If you only need one switchpoint than take a look at this schematic, it uses a Schottky diode and a PowerMOSfet.
www.bramcam.nl/Diversen/CO-2016-PSU.pdf
...
That version of tap switching looks nice and simple.
The nice point with the diode is that, there can be no cross conduction. The voltage loss only happens at in the lower tap case, where is does not hurt much. The relatively slow turn on of the MOSFET limits peak currents, when going from the center tap to the full voltage. There would be some heat loss at the MOSFET, but not that much (e.g. comparable to 1/4 the energy in the filter caps) and more of a transient mode.
Moving the switching from low to high tap close to the zero crossing could further reduce the current peak a little.
However the MC34063 is not a good choice for higher power - if at all it is acceptable when the internal switch is sufficient and efficiency is not important.
I agree that Blackdog's circuit is much simpler (I made something even simpler as it was for regulation on the negative rail).
When converting from AC to DC you have a fixed voltage loss (max peak-to-peak ripple voltage + max dropout of regulator) plus diode bridge and transformer loss.
In the case of center tap transformer the diode bridge and transformer losses are halved, but the other aren't so saving that fraction of volt wouldn't be so bad.
In any case such a complicated circuit is obviously an overkill for a single tap. I designed that just for the puzzle solving fun.
But in future I would start from there to replace the triacs in the HP 3 taps switching design.
A full wave voltage doubler could be added on to that to give two extra extended ranges. There is one disadvantage I can see
though as it's halving the output capacitance (since 2 caps in series) on the lower voltage (higher current) ranges where its needed most.
Notice on the third range how the top caps gets charged upto double the voltage of the bottom cap to get us the third range (sw1 closed sw2 open).
The available outputs currents and voltages I give on the schematic are what should be theoretically available from the
transformer and don't include rectification losses.
Happy New Year .
why dont use one chip solution? like 50¢ 34063? i once try to build my own discrete pre regulator from some comparator and/or schmitt trigger and/or mcu pwm. but with all the hassle, on another circuit, i just use the one chip solution, all i have to take care of is the noises spike and ground loop. in fact i'm building another one right now. ymmv.
Hi Atom, seems that you are on the similar journey as myself, searching for power pre-regulator solution. I've started with "blackdog" type of stuff, and gave up (possibly too early, since at least @tombi from this forum reported good results independently from @blackdog). I spent some time after that with LM5118, LM5088, and finally end up with LTC3864 for EEZ H24005 power supply. I didn't stop on DC-DC converter as the final solution since it still require a) huge mains transformer or b) an AC-DC coverter to get "DCbus" voltage for pre-regulator (48 Vdc in my case). Therefore I finally decide to make an AC-DC converter that can works with fixed or widely adjustable output voltage as pre-regulation. Progress is reported here.
I'd like to mention once again what was said here, if you are in the phase to try DC-DC converter as pre-regulator first.
Finally not completely on-topic (sorry for that): It's interesting that people when someone mention bench power supply start to talk about low noise and ripple. That could be indeed beneficial, but question is for what kind of applications? Does everyone is working with sensitive RF circuits, have ultra-precise voltage reference, or whatever? I'm still didn't find a good article or post about noise levels/categories with explanation when low PARD become of paramount importance and present a possible show stopper or not.
Keep up with good work, and all the best in New (Western counting) Year to you, and all members of the forum.
i know that's not the right mosfet for the job but i wanted to try it, it works, but there is a problem the voltage measured from drain to source is 11V pkpk so 5.5v. @ 6.6A load
power disspiantion is Rdson = 5.5/6.6 = 0.83ohm not good power disspiation is 0.83*36= 30W (the mos was on a heatsink).
yes that's not the right mosfet but it works, i'm waiting for low voltage high current mos.
the project of the power supply that you linked (the one on the diy audio forum is really intresting)
that said i think a full smps prereg is far out of my capabilities (for now )
i know that's not the right mosfet for the job but i wanted to try it, it works, but there is a problem the voltage measured from drain to source is 11V pkpk so 5.5v. @ 6.6A load
power disspiantion is Rdson = 5.5/6.6 = 0.83ohm not good power disspiation is 0.83*36= 30W (the mos was on a heatsink).
yes that's not the right mosfet but it works, i'm waiting for low voltage high current mos.
mesuring the output of the psu there is a 100mvpkpk 1mhz wave (ac copuling).. i think is the mosfet oscillating ? probably because it isn't the right type of mos for this application, that would explain the large power dissipation and the fact that i wasn't able to go to the full 30vdc on the output, anyway i have to wait or the better mos to arrive.
Hi Atom, seems that you are on the similar journey as myself, searching for power pre-regulator solution. I've started with "blackdog" type of stuff, and gave up (possibly too early, since at least @tombi from this forum reported good results independently from @blackdog).