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| Preregulation of a linear bench PSU |
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| Atom:
--- Quote from: not1xor1 on December 27, 2018, 08:25:01 am --- 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. --- End quote --- 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. |
| Kleinstein:
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. |
| Atom:
--- Quote from: Kleinstein on December 27, 2018, 12:39:08 pm ---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. --- End quote --- i know that you are a really good EE and your knowledge is far greater than mine..i wish my professors could teach the things that i learn here. yes the preregulator proposed at the start has the problem that makes the transformer buzz and has a continuos adjustment creating high current spikes, but the circuit of not1xor1 switches only once in a time when i need to switch between taps so there would be a single current spikes at the mosfet turn on, what i'm trying to say it's that if the mosfet takes a current spike once there shouldn't be a problem since this circuit doesn't switches continuosly. |
| Kleinstein:
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. If going from lop tap to high tap there could be quite some current spike, as the filter cap is charged fast. This could be more than the 60-100 A seen at start. Here switching near zero would still help, as the charging is no faster than the sine wave. Still some 10 V/ms and 10mF of filter capacitor would be 100 A in theory, if there is no other limit. The other point would be rare cases where the switches are active both at the same time, if 2 Fets are used (using a diode instead of the 2nd FET (at COM) could avoid this). These peaks could be damaging. Another point that could help a little against high current spikes could be a fuse before the rectifier - it's resistance also limits the current a little. As rectifiers sometimes fail short, it is usually a good idea to have a fuse to protect the transformer. |
| not1xor1:
--- Quote from: Atom on December 27, 2018, 10:44:01 am --- --- Quote from: not1xor1 on December 27, 2018, 08:25:01 am --- 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. --- End quote --- 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. --- End quote --- The problem of the previous circuit I showed is that in some instances both mosfet are on and that should be avoided. The current spikes through the capacitor are less relevant as there are plenty of mosfets able to withstand that. |
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