Linear regulator
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eltonseemann
diyAudio Member

Join Date: Oct 2006
Linear regulator

Hi folks,

Im trying to project a linear regulator for +-47V and current limit of 20A pulse (protect the transistors).
Mains devices will be mj15003/4
Transformer is 42+42AC >1KVA

This is the circuit that I think and simulate. Is it OK?
Attached Images
 regulator.png (31.8 KB, 516 views)

 14th October 2007, 04:27 PM #2 Eva   diyAudio Member     Join Date: Oct 2003 Location: Near the sea Learn about "foldback" current limiting and "SOA" (safe operating area) protection. Constant current limiting is not usually suited for linear circuits because it can still lead pass transistors to huge power dissipation and instantaneous failure. The trick is to make the current limiting threshold dependent on the voltage across the pass transistor, time, and on its temperature if overcurrent conditions are frequent or sustained. Bipolar power transistor datasheets contain SOA curves. __________________ I use to feel like the small child in The Emperor's New Clothes tale
 14th October 2007, 08:25 PM #3 eltonseemann   diyAudio Member   Join Date: Oct 2006 thanks Eva, I was see previously the SOA of MJ15003 DC current is 20A below 10V Vce Pulse is 30A Second breakdown is above 60V Vce The only think that I need to avoid is 60Hz peak capacitors charge current that exceed 30A (I adopt 20A threashold) at full load or at start. I will operate whit fuses of 15A at mains (will open in case of short or overcurrent). The principal objective is to drop peak voltage from +-58V to +-48V to feed Ucd modules. I want the minimum possible drop at BJTs, minimum dissipation. YES, my last post circuit can instantaniously damage MJ15003/4 wich a peak power at start capacitor charge but current not ultrapasses 20A. Is this true? I will always be a series NTC at 220V Mains I need a example of foldback, without opamps
 30th October 2007, 11:38 AM #4 18thell   diyAudio Member   Join Date: Jan 2007 From the cct. U posted, both output transistor cannot deliver 20A, why? this due to not enough Ib, by R1, and R6 to supply the base current !!!. Suppose mj1500X Beta = 20, Ib must have 1A, another problem is if the current limited tr. turns on, can it stand 1A, otherwise it will breakdown and burnt. the Zener diode connected to the error amp. tr. E and B term. ??? mistake. SOA, the BVce > 60V, it seems not enough, usually we only use 1/3 of the BVce, U have also consider heat dissipation, continue operation--- many factors.
 30th October 2007, 12:27 PM #5 AndrewT   R.I.P.   Join Date: Jul 2004 Location: Scottish Borders Have you considered getting the amp working with a conventional transformer + rectifier + smoothing caps? Then learn how to build a regulator for low current/low voltage duty. Expand your knowledge to include high voltage, then try high current + high voltage and finally go back to investigating current limiting when you have some knowledge of temperature de-rated SOAR. Once you have all that knowledge look to adding in a current limited HiV HiI PSU for your amp. Oh, and keep in mind that the final 4.7mF cap will provide a very Hi current to the amp for a very short time using up all or most of the transient capability of your output devices. __________________ regards Andrew T.
 30th October 2007, 06:38 PM #6 jan.didden   diyAudio Member     Join Date: May 2002 Location: The great city of Turnhout, BE I think the original poster was talking about the SOA of the supply series transistor, while you guys are talking about the SOA of the amp that is supplied by this circuit. If so, this can develop into an interesting discussion Jan Didden __________________ Cable: a potential source of trouble interconnecting two other potential sources of trouble. Linear Audio pubs and articles . The SilentSwitcher now at diyaudio store SilentSwitcher
AndrewT
R.I.P.

Join Date: Jul 2004
Location: Scottish Borders
Quote:
 Originally posted by janneman I think the original poster was talking about the SOA of the supply series transistor, while you guys are talking about the SOA of the amp that is supplied by this circuit. If so, this can develop into an interesting discussion
Hi,
I was specifically referring to the regulator SOAR, but my comment
Quote:
 back to investigating current limiting when you have some knowledge of temperature de-rated SOAR.
could equally well apply to the amplifier SOAR.
Understanding both allows the design capability to be optimised economically rather than throwing resources at it and hoping to achieve reliability.
__________________
regards Andrew T.

jan.didden
diyAudio Member

Join Date: May 2002
Location: The great city of Turnhout, BE
Quote:
 Originally posted by AndrewT Hi, I was specifically referring to the regulator SOAR, but my comment could equally well apply to the amplifier SOAR. Understanding both allows the design capability to be optimised economically rather than throwing resources at it and hoping to achieve reliability.
Andrew,

Correct, but in the reg the pass transistor normally has only a few volts Vce so SOA would not be an issue there. In the amp it is, of course.

Jan Didden
__________________
Cable: a potential source of trouble interconnecting two other potential sources of trouble.
Linear Audio pubs and articles . The SilentSwitcher now at diyaudio store SilentSwitcher

 31st October 2007, 09:34 AM #9 AndrewT   R.I.P.   Join Date: Jul 2004 Location: Scottish Borders Temperature de-rated SOAR is always an issue for an economic and fast and current limited PSU regulator. If it weren't them fold back current limiting would not have needed to be invented. High capacitance on the output of the PSU puts the amplifier at risk and can make the design more troublesome. Reduce the output capacitance to keep those amplifer SOAR risks low and you find that the regulator must meet the peak current demand. Now the foldback becomes even more important. Consider what happens if the mains is running high through periods of low demand. PSU Vce (and dissipation) can easily exceed double what might occur during periods of high power demand. __________________ regards Andrew T.
jan.didden
diyAudio Member

Join Date: May 2002
Location: The great city of Turnhout, BE
Quote:
 Originally posted by AndrewT Temperature de-rated SOAR is always an issue for an economic and fast and current limited PSU regulator. If it weren't them fold back current limiting would not have needed to be invented. High capacitance on the output of the PSU puts the amplifier at risk and can make the design more troublesome. Reduce the output capacitance to keep those amplifer SOAR risks low and you find that the regulator must meet the peak current demand. Now the foldback becomes even more important. Now consider what happens if the mains is running high through the night. PSU Vce (and dissipation) can easily exceed double what might occur during periods of high power demand.
Andrew,

I don't disagree on these issues, but even with a steady state 10V across the series pass xsistor, SOA is no issue, especially since those 20A loads will be few and long between. And if Vce is more, you have the wrong transformer

With foldback limiting you are basically looking at a single transition from low Vce and high Ic to low Ic and high Vce, unlikely that SOA would be a problem.
I agree it has to be looked at carefully to be sure, but I would first worry about the amp SOA.

Jan Didden
__________________
Cable: a potential source of trouble interconnecting two other potential sources of trouble.
Linear Audio pubs and articles . The SilentSwitcher now at diyaudio store SilentSwitcher

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