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Linear Power Supply based on HP/Agilent E3610A
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timsu:
Hello,
I'm trying to build a power supply based on the HP E3610A (15V/2A).
You can find the Schematics as attachement to this post or http://sites.fas.harvard.edu/~phys191r/Bench_Notes/A1/agilent_e3610a.pdf on page 16.
On the pages before there are also the component values.

I think I understood most of the circuit, except Q2/CR8( (HP schematic) Q3/D7 (my schematic).
The simulation seems to work fine even without it.
Can somebody explain to me in what situation the transistor is passing current and when not? What is the point of it?

In a service note HP is recommending to remove CR8/D7 to prevent oscillations.
But here https://www.eevblog.com/forum/testgear/hp-agilent-e3610a/ it is suspected, that this modification can destroy the pass transistors because of a too high base-emitter voltage.
Also, should I include the modification, the added cap in the voltage error amplifier, from the linked thread?

Did HP make more "bad" choices which I can prevent?

Kleinstein:
The transistor Q3 is working as an emitter follower - so it's always in a linear range, essentially never off. The diode limits the lower output voltage to something close to -100 mV. It also provides higher current sinking capability. This should not be important with the shown BJT power stage, but is can be important with HPs MOSFET stage. The high sinking current allows to turn off the FETs fast and the 1.2 K resistor limits the speed when turning them on. 

The values for C1, R2 look odd - looks way to slow. I would more expect something in the 500 pF and 5 K range. It depends on the output caps and power stage too.

One poor choice HP made was adjusting the voltage by changing the feedback divider. It's better to keep the divider fixed and adjust the set voltage like indicated. However with a variable ref voltage for the divider one might need an extra minimum load (e.g. constant current) to bring the output back to zero.   

Also the supply voltage for the OPs could have be chosen lower, not to allow the OPs to go way to high in output voltage. Limiting the voltage excursion for the regulation part that is not active can speed up the switch over from CC to CV mode and back. Especially with a BJT output stage the voltage range is well predictable, so it's easy to chose more reasonable limits. So going for a Rail-Rail OP might be one option - the LM1001 is rather slow anyway.

The output caps should be a combination of a low ESR (e.g. ceramic / foil in the 100 nF to 1 µF range) and low ESR electrolytic. HP might not have modern low ESR electrolytic as an option back then. So one might get away with lower capacitance - the large caps chosen by HP might be needed to get the ESR down in the 1 Ohm or lower range, not the capacitance itself. The old style high ESR caps are usually not helping - so I would prefer lower capacitance low ESR (e.g. 0.1-0.5 Ohms range).
timsu:
So why would HP decide to remove the diode because of oscillations?
That could cause harm to the transistors, right?

--- Quote from: Kleinstein on May 04, 2016, 06:57:19 pm ---The values for C1, R2 look odd - looks way to slow. I would more expect something in the 500 pF and 5 K range. It depends on the output caps and power stage too.

--- End quote ---
I thought this was also confusing, because in the high voltage model E3612 they used 22 pF and no resistor in the voltage feedback loop.

--- Quote from: Kleinstein on May 04, 2016, 06:57:19 pm ---However with a variable ref voltage for the divider one might need an extra minimum load (e.g. constant current) to bring the output back to zero.   

--- End quote ---
The 330 Ohm R9 should work, right? This was also added by HP in a later revision.

--- Quote from: Kleinstein on May 04, 2016, 06:57:19 pm ---Also the supply voltage for the OPs could have be chosen lower, not to allow the OPs to go way to high in output voltage.

--- End quote ---
I would prefer going for -5V/+5V because for other parts of the circuit (DAC/ADC) I will need 5V.

Also why use a voltage divider with 1R and 500K before the current shunt?
Are they compensating some error with this?
Kleinstein:
Removing the diode (CR8) to limit negative control voltage should not change much with oscillations - it could be an issue if the output stage (gate capacitance) is setting much of the compensation. The diode might limit the ability to go all the way to 0 V output voltage, especially if the transistors are hot. However the more logical step would be using two diodes in series instead of removing it all together. Allowing the voltage to go to negative (much below -7 V) without the diode might cause trouble for the output transistors (to negative a base-emitter voltage), so it is not a problem with only a +-5 V supply there.

R9 could to some degree act as a minimum load, though it is not very efficient at low voltages and gets quite hot at high voltage. A constant current sink might be more appropriate.

The divider at the shunt looks odd - though it could be just to use the same meter with different supply models.
timsu:
I think HP was limited in their ability to change something aftwerwards because they had no space on the board for two diodes, or a constant current sink, so they added just a big resistor as load.

I started to draw up the schematic (attachement). The OP-Amps are just placeholders, maybe someone has a recommendation.

I will have to do some more simulations to determine the right component values for the voltage error loop and the output cap.
Also the digital stage is still missing, it will be 12 Bit ADC/DAC and an ATmega.

I would appreciate feedback.
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