Electronics > Beginners

Need tips on building a variable power supply

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spec:

--- Quote from: mvs on November 24, 2018, 03:23:02 pm ---
--- Quote from: not1xor1 on November 24, 2018, 12:44:25 pm ---BTW would a new thread in the project section would be more appropriate?

--- End quote ---
If you plan to implement and verify your design in hardware... maybe yes. Otherweise it makes not much sense.

There is almost nothing special in your design... emitter folower and 2 error amplifiers. LT1013 is also strange choise, since it is quite slow and has odd pinout for dual opamp.
The only thing i liked, is usage of Sziklai pair instead of Darlington.

just my 2 cents

--- End quote ---

The Sziklai pair is not advisable. While its DC performance is superior on paper, it suffers from a poor frequency response and very poor turn off characteristics, which can lead to odd behavior and frequency instability. The Darlington pair although less good from a DC point of view is much better from a ton/toff and frequency point of view.

And that is my point of view ;D

 

David Hess:

--- Quote from: spec on November 24, 2018, 05:17:54 pm ---The Sziklai pair is not advisable. While its DC performance is superior on paper, it suffers from a poor frequency response and very poor turn off characteristics, which can lead to odd behavior and frequency instability. The Darlington pair although less good from a DC point of view is much better from a ton/toff and frequency point of view.
--- End quote ---

In my experience, the Sziklai pair is more prone to parasitic oscillation with difficult loads unless local frequency compensation is used so the design and testing requirements are more stringent.  (1) Other than that, the difference in performance between the Sziklai and Darlington configurations is insignificant in most cases.

Where this would matter the most is a power supply design which includes the minimum of output capacitance for good current limiting performance.  The low output capacitance makes frequency compensation of the feedback loop and Sziklai pair more difficult.

In the example below from the Tektronix PS503A bipolar output power supply, the top Sziklai pair has an extra 0.01 microfarad capacitor to ground presumably to prevent local oscillation and the 50 microfarad output capacitors are integral to maintain stability.

(1) Essentially a transient response or network measurement should be made on the Sziklai pair to verify stability independently of the tests made on the error amplifier's frequency compensation.  With some experience, one can get an idea of what is required to stabilize the Sziklai pair; usually just a small capacitor is required for local feedback.  This is a good place to use an RC substitution box during testing.

spec:

--- Quote from: David Hess on November 24, 2018, 06:38:37 pm ---
--- Quote from: spec on November 24, 2018, 05:17:54 pm ---The Sziklai pair is not advisable. While its DC performance is superior on paper, it suffers from a poor frequency response and very poor turn off characteristics, which can lead to odd behavior and frequency instability. The Darlington pair although less good from a DC point of view is much better from a ton/toff and frequency point of view.
--- End quote ---

In my experience, the Sziklai pair is more prone to parasitic oscillation with difficult loads unless local frequency compensation is used so the design and testing requirements are more stringent.  (1) Other than that, the difference in performance between the Sziklai and Darlington configurations is insignificant in most cases.

Where this would matter the most is a power supply design which includes the minimum of output capacitance for good current limiting performance.  The low output capacitance makes frequency compensation of the feedback loop and Sziklai pair more difficult.

In the example below from the Tektronix PS503A bipolar output power supply, the top Sziklai pair has an extra 0.01 microfarad capacitor to ground presumably to prevent local oscillation and the 50 microfarad output capacitors are integral to maintain stability.

(1) Essentially a transient response or network measurement should be made on the Sziklai pair to verify stability independently of the tests made on the error amplifier's frequency compensation.  With some experience, one can get an idea of what is required to stabilize the Sziklai pair; usually just a small capacitor is required for local feedback.  This is a good place to use an RC substitution box during testing.

--- End quote ---
Very informative :-+

not1xor1:

--- Quote from: mvs on November 24, 2018, 03:23:02 pm ---
--- Quote from: not1xor1 on November 24, 2018, 12:44:25 pm ---BTW would a new thread in the project section would be more appropriate?

--- End quote ---
If you plan to implement and verify your design in hardware... maybe yes. Otherweise it makes not much sense.

There is almost nothing special in your design... emitter folower and 2 error amplifiers. LT1013 is also strange choise, since it is quite slow and has odd pinout for dual opamp.
The only thing i liked, is usage of Sziklai pair instead of Darlington.

just my 2 cents

--- End quote ---

I do plan to build the real circuit.
I showed just the simplified one, but I've been running thousands of simulations on this kind of circuit (with models of real parts) in order to check all the possible conditions (voltage/current control loop gain stability, start-up, switch-on/off, remote sense, fine/coarse regulation, short-circuits, master/slave tracking, transformer-tap switching, etc.)

LT1013 speed is more than adequate for a power supply, and in particular mode for a beginners' one.

In any case I'll be glad to know about any other faster opamp with the following features:
- cheap and easy to find as LT1013
- possibly in DIP package (some old guy like me cannot struggle with those small SMD arthropods :()
- low voltage offset/drift
- maximum supply voltage >= 44V
- ability to work with input/output at least close at one of the supply rails without phase reversal
- and yet I might have forgotten something  ;D
So please suggest one.

The most funny thing is the hint to the "odd pinout" of LT1013... I just checked... most dual opamp I got here are the same:
AD8066, LM358, TL0*2, MC3**02, microchip ones, NE5532... and so on... You must be kidding  :-DD

BTW I just noticed you suggested the usual bad ebay kit.
That is one of those ugly circuit design I was referring to.

While in most cases TL071/4-TL081/4 do work with input voltages close to the positive rail they are not granted to do that.
E.G. for ±15V supply the datasheet grants only ±11V of input voltage range and AFAIK there is no mention about phase reversal protection.
Besides that TL0* output range is much more limited than that of LT1013 (3V less than supply rails) and so is the maximum supply voltage.
I would not comment further on that other weak design choices as that was already discussed here in past.

Advantages of the circuit I propose (some are evident only in the completee circuit I show later):
- it is cheap and yet offers much better performance than many other circuits you find on the net
- it is simple to understand and to build for a beginner
- needs only one transformer winding
- no need for an additional differential amplifier (only one opamp in the loop of the current control)
- the current control reference is supplied by a basic constant current circuit that also works as down-programmer without loading the current sense resistor
- the ability of LT1013 to work with voltages close to the negative rail allows a really low dropout
- probably the cheap dual V/I panel meters sold on ebay would fit easily and might even be used directly for the current sense (unfortunately I do not have one here to ckeck)

Of course I'm not pretending that it is the best circuit.
There are much better circuit using more parts and separate rails for the control circuit (Harrison's design) widely discussed, for instance, in the HP's DC power supply handbook.

not1xor1:

--- Quote from: spec on November 24, 2018, 05:07:00 pm ---
--- Quote from: not1xor1 on November 24, 2018, 12:44:25 pm ---It looks like every day there is a new thread about a beginner willing to build an unrealistically featured PSU based on various dubious schematic diagrams, at best outdated, at worst with no chance of working.
--- End quote ---
I don't see how this sweeping statement applies. Neither can I see any reason for it. The OPs circuit is quite satisfactory and simpler than the circuit you have outlined. It is these kinds of statements that confuse and discourage newbies and forces then to seek help via PM to avoid the flac. On one forum I was doing more PM advice than forum advice because of this kind of statement.

--- End quote ---

What I wrote was meant as a general statement.
In any case the OP circuit is outdated, not much simpler than mine and with the usual coarse single BJT current limiter rather than a real current control.

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