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Preregulation of a linear bench PSU
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Atom:

--- Quote from: duak on December 26, 2018, 07:56:29 pm ---The first ranging power supply I encountered was an hp 6002A.  It used a state machine to select the best combination of transformer secondary taps for the output voltage and current.  The design was written up in the June 1977 hp Journal: www.hpl.hp.com%2Fhpjournal%2Fpdfs%2FIssuePDFs%2F1977-06.pdf&usg=AOvVaw18_7cLX_j2ET9_i8jT4dl8

Figure 3 on page 4 shows the basic transformer and rectifier circuit.  The takeaway from this is that for the low voltage range, a standard bridge rectifier provides all the current and when a higher range is needed, a higher voltage tap is switched in and the LV rectifier just doesn't conduct.  In this supply, thyristors were used for tap switching but I don't see any reason that a MOSFET can't be used.  The hp supply is complicated because it has four ranges and a 200 W maximum power limit.  I have one that I repaired but I don't use it much so I can't say if its complexity is worth the capability.  I can't even say if its output is clean compared to a non-ranging or SCR supply.

Cheers & Best Wishes,


--- End quote ---

since the voltage drop of an scr is 1V more or less even a relay soultion is better from that standpoint. without considering the power losses of it. nonetheless an intreasting reading.

the link seems broken here's one working:http://www.hpl.hp.com/hpjournal/pdfs/IssuePDFs/1977-06.pdf

sadly even if i wanted to replicate the circuit i coudn't because i don't have power scr on hands.
Kleinstein:
An extra delay could avoid having both MOSFETs on at the same time. Howver there is a second possible problem, and that is a high peak current when switching from the lower tap to the high in a moment when die voltage is high. It gets less dramatic if the switching is done at a low voltage.

It is still the question if the switching should be done as hard switching between the taps or also with fast turn off if the required voltage (+ some overhead) is reached. This would give an added continuous pre-regualtion  similar (but other part of the wave)  to the SRC regulators.

The SCR could also replace a diode, to the 1 V drop is not that bad, though a little more than a diode.
soldar:
I have a commercially purchased 0 ~30 Volt, 5 Amp, adjustable bench power supply.  It switches the taps of the secondary with two relays and it has three possible positions: 0-10, 10-20 and 20-30 volts sensed on the output.

The sensing and switching is done with a couple OpAmps which have no hysteresis designed and it has never been a problem. It would be easy though to add a couple resistors to add some hysteresis if I ever find it necessary.

The relays switch fast enough that it is not a problem.  Only one switches at a time.

The unit has given me many years of good service now. It cost me about $30 ten years ago.
not1xor1:

--- Quote from: duak on December 26, 2018, 07:56:29 pm ---The first ranging power supply I encountered was an hp 6002A.  It used a state machine to select the best combination of transformer secondary taps for the output voltage and current.  The design was written up in the June 1977 hp Journal: http://www.hpl.hp.com/hpjournal/pdfs/IssuePDFs/1977-06.pdf

Figure 3 on page 4 shows the basic transformer and rectifier circuit.  The takeaway from this is that for the low voltage range, a standard bridge rectifier provides all the current and when a higher range is needed, a higher voltage tap is switched in and the LV rectifier doesn't conduct.  In this supply, thyristors were used for tap switching but I don't see any reason that a MOSFET can't be used.  The hp supply is complicated because it has four ranges and a 200 W maximum power limit.  I have one that I repaired but I don't use it much so I can't say if its complexity is worth the capability.  I can't even say how clean its output is compared to non-ranging or SCR supplies.

Cheers & Best Wishes,

--- End quote ---

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.
not1xor1:

--- Quote from: Atom on December 26, 2018, 08:36:05 pm ---
--- Quote from: not1xor1 on December 26, 2018, 04:18:05 pm ---I made a more complete simulation with a 1Hz 0-30V triangle wave to simulate the variation of a PSU output voltage.
I found that even with a 1ms deadtime there may be cross conduction if the switch doesn't occur on zero-crossing.

So to work properly the circuit would need a zero-cross detector, a histeresis comparator and a flip-flop to be set/reset when both AC is 0V and voltage is over/below the given threshold.

--- End quote ---

So you are telling me that switching should be done at the zero crossing point...and i would need some external circuitry to get everythig to work ... that adds substantial complexity to the circuit..i'll se what i can do.

stupid idea but if i can induce a proper delay and drive the mosfet gates from a stabilized dc supply(without any ripple) those problems would go away right? or i'm still missing something

since the trasformer is a toroidal one i can add an aux winding pretty easily for driving mosfet gates. Could you share the LTspice file i'm not familiar with it but since it's what everyone uses it's time to learn something new.

--- End quote ---

I think zero-crossing switching is safer.
Later I'll see if I can find a simple solution.
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