| Electronics > Beginners |
| Need tips on building a variable power supply |
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| not1xor1:
--- 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. --- End quote --- The sziklai pair is not relevant. The circuit works and has been simulated for loop gain stability and transient response, both with sziklai and darlington pass elements and with an assortment of power transistor models. Real circuits of course are affected by parasitic capacitance/inductance and are another matter of fact. Nothing so difficult to handle, but in any case that is the reason why I'll show a complete circuit with various possible options only after I'll have built and tested a prototype. |
| mvs:
--- Quote from: not1xor1 on November 25, 2018, 11:19:28 am ---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. --- End quote --- Perhaps we have just different expectations and needs. I have simulated similar circuit with LT1013 and CC-CV transient response was really bad. --- Quote ---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. --- End quote --- To get good DIP parts is difficult nowadays... Look at MC34072, perhaps it will fit your needs... --- Quote ---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 --- End quote --- Pinout of LT1013 is normal in DIP-8 package, but in SO-8 it is very unusual. --- Quote ---BTW I just noticed you suggested the usual bad ebay kit. That is one of those ugly circuit design I was referring to. --- End quote --- Design is quite old and it was not made to be beautiful... Voltage error amplifier connected after current error amplifier do not saturate, like in your simplified design. This improves transient response in CC->CV transition. TL081 are also a bit faster, then LT1013. --- Quote ---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 --- End quote --- Circuit design limits input range of voltage err. amplifier to Vee+5.1V | Vcc-4.2V, other amps are even more limited, so no problems hier. --- Quote ---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. --- End quote --- Nobody is perfect, cheap TL0xx opamps are not always better then expensive LT parts. :) |
| David Hess:
--- Quote from: mvs on November 25, 2018, 07:54:37 pm ---Pinout of LT1013 is normal in DIP-8 package, but in SO-8 it is very unusual. --- End quote --- It took me 20+ years to learn where that weird SO-8 dual operational amplifier pinout came from and I had to figure it out myself. The large LT1013 die is rectangular and has to be rotated 90 degrees to fit inside the SO-8 package. |
| spec:
--- Quote from: David Hess on November 26, 2018, 03:27:07 am --- --- Quote from: mvs on November 25, 2018, 07:54:37 pm ---Pinout of LT1013 is normal in DIP-8 package, but in SO-8 it is very unusual. --- End quote --- It took me 20+ years to learn where that weird SO-8 dual operational amplifier pinout came from and I had to figure it out myself. The large LT1013 die is rectangular and has to be rotated 90 degrees to fit inside the SO-8 package. --- End quote --- You learn something new everyday :) |
| not1xor1:
--- Quote from: mvs on November 25, 2018, 07:54:37 pm ---Perhaps we have just different expectations and needs. I have simulated similar circuit with LT1013 and CC-CV transient response was really bad. --- End quote --- Thanks for having highlighted the problem. :scared: At 7.5V output, on short-circuit recovery, I got about 22% overshot with LT1013 and 19% with faster opamp (tl072, lt1056). I doesn't look like a so great improvement IMHO. Of course the overshot as % is much worse for low output voltages On the other side it is less when instead of short-circuit (output voltage to 0V) is just CV/CC switch. A great improvement comes instead from reducing the opamp saturation. Just a schottky diode (BAT46) connecting the inverting input to the positive ground reduces LT1013 overshot to about 20%. More complex solutions are beyond the purpose of keeping the circuit as simple as possible. --- Quote --- --- Quote ---In any case I'll be glad to know about any other faster opamp with the following features: [...] --- End quote --- To get good DIP parts is difficult nowadays... Look at MC34072, perhaps it will fit your needs... --- End quote --- I have it. But it is only just a bit faster. --- Quote --- --- Quote ---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 --- End quote --- Pinout of LT1013 is normal in DIP-8 package, but in SO-8 it is very unusual. --- End quote --- I apologize. I just forgot to check the smd case. |O --- Quote --- --- Quote ---BTW I just noticed you suggested the usual bad ebay kit. That is one of those ugly circuit design I was referring to. --- End quote --- Design is quite old and it was not made to be beautiful... Voltage error amplifier connected after current error amplifier do not saturate, like in your simplified design. This improves transient response in CC->CV transition. TL081 are also a bit faster, then LT1013. --- End quote --- I'll try to simulate it to check how much better (in any) it is. :) Thanks for your suggestions. |
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