Please Teach Me How Amspire's PSU Works - Page 1
Forum section: electronics / beginners

author: Mechatrommer
author_rank: Super Contributor
timestamp: January 19, 2013, 03:21:21 am
content: i'm trying to figure out how it works. its from amspire's psu..fromhttps://www.eevblog.com/forum/projects/general-purpose-power-supply-design-7488/i want to highlight the following region...1) what the R5 (330ohm) is doing there?2) what the yellow region PNP is doing? i can think during startup the base is pulled low (Q3 on) just for a moment and then pulled high by the diodes (Q3 off). and then what?3) i believe the green region is the voltage regulation. correct? how do the mechanism works? and how do we come up with such circuit? ie 10nF cap, R6.8K pulling the base down and why R13 is 22ohm?4) if i want to increase amperage of this psu. can i just add another ZTX1048 paralleled to the Q1? without affecting stability?5) can i replace the parts (ZTX1048A, BC557B, 2N3904) with another parts that i have in stock? whats the critea for all those parts?i wish i could ask "how to compensate for unconditionally stable" psu, but i think thats another long story. thank you.

author: Kremmen
author_rank: Super Contributor
timestamp: January 19, 2013, 10:08:01 pm
content: -Q3 seems to be a constant current source and provides the base current for Q1. Q2 shunts excess current to ground.-U1 is the voltage control. U3 is high side current sense amp and U4 is current limiter. It drives Q2 to shunt Q1 base current away when overcurrent is detected. U1 works similarly.-As to the 330 ohm resistor, it is not immediately obvious why it is there. My guess is that it provides a pole in the loop transfer function to stabilize the control loop(s). PNP/NPN combo LDO regulators suffer from a zero in the transfer function, necessitating compensation but this circuit does not really look like that. So without a more careful analysis it is hard to be sure what the resistor is actually doing.

author: Mechatrommer
author_rank: Super Contributor
timestamp: January 19, 2013, 10:15:53 pm
content: hmm. with the lack of interest i guess this is not a normal joe method of doing PSU. Ampire alone may answer these questions precisely, but i dont think i'm going to disturb him. looks like i have to test it myselfthanks Kremmen for some hints. i kind of have a picture on other areas, its just the transistor based regulator that i have problem with (colored area), and i believe its the main engine (regulating) in the whole circuit.

author: IanB
author_rank: Super Contributor
timestamp: January 19, 2013, 11:01:03 pm
content: It's not perhaps that there is a lack of interest, but rather perhaps a lack of comfort in trying to answer your questions. That's not a simple circuit and it wouldn't help for people to speculate on the design details without having built the circuit and studied its behaviour in detail, or having repeated all the calculations that presumably Richard did.One detail that you seem to have missed in your earlier post is that U1 is the core of the voltage regulation circuit. You missed it out entirely from your green shaded region. U1 is part of the feedback loop that compares the output voltage with the desired voltage and adjusts the current through Q1 to make them equal. The other components near U1 like C2, C9, R9, R17, etc. are there to adjust the loop response time and stability. Q2 is an amplifier that provides enough current to drive the base of Q1.I don't know what the yellow part of the circuit around Q3 is for, but it may perhaps be there to avoid an output voltage spike on switch on? In an ideal power supply the output voltage will ramp up in a controlled way to the set point and not swing wildly.

author: Mechatrommer
author_rank: Super Contributor
timestamp: January 20, 2013, 05:25:19 am
content: Quote from: IanB on January 19, 2013, 11:01:03 pmhow about a simple explanation in "layman's words". just the functionality of each region... but i think you already did that pretty well, thank you. yes i think you are right that i am wrong. probably the regulation is indeed done by U1, the rest of transistors are just to provide the "base current". i'm going to need all the possible "speculations" since maybe sooner or later i'm going to build it but i dont have strong mathematical and conceptual skill to analyze this transistor circuit, so i'm doing just a "poor man" method... try and error and luck. sigh.i did my 1st psu sometime ago using opamp feedback something like above but maybe lack of proper compensation. i was using darlington pair, ie all npn's collectors are tied together, i dont know how efficient, how good or how poor it is. amspire's config where Q2's collector is tied to Q1's base is pretty strange to me. thats something i need to learn to advance my skill.

author: IanB
author_rank: Super Contributor
timestamp: January 20, 2013, 08:14:01 am
content: Quote from: Mechatrommer on January 20, 2013, 05:25:19 amIt's really not so strange if you rearrange it like this:Q3 is turned on via R2 and provides base current to Q1, turning on Q1 and passing current to the load. (The two diodes D1 and D2 ensure the base voltage of Q3 is lower than the emitter voltage, causing it to turn on.)Q2 now provides control over the output load current. As Q2 is turned on by the control circuitry (not shown) it sinks current from the base of Q1 and progressively turns it off, reducing the load current. As Q2 is turned off it allows Q1 base current to rise and increases the load current. By measuring the load voltage and applying appropriate feedback to the base of Q2 the load voltage may therefore be regulated.

author: SeanB
author_rank: Super Contributor
timestamp: January 20, 2013, 09:11:31 am
content: R5 serves as a resistor to ensure Q1 turns off when the supply is loaded with no or zero load and is delivering lowest output voltage at high heatsink temperature. It sinks leakage current to keep BE of Q1 at same voltage, otherwise with a hot transistor and high voltage across it the leakage current could be enough to drive the loop out of it's regulation range. It also does some damping of the transistor.As to replacing components, Q3 can be any PNP capable of 100mA and with a minimum Vceo of 60V or more, as it is basically just supplying a 8mA constant current that is then either sunk by Q2 ( the controlling transistor) or drives the power darlington Q1 to provide the required output voltage and current. Q1 can be almost any darlington capable of supplying the required output current with the Vceo being more than 100V, and where the gain is over 1000 at the maximum output current. It must also have a DC SOA that is larger than the power limits of the power supply, plus it must be in a big enough case and attached to a large enough heatsink so that the power dissipation at full load into a short circuit does not raise junction temp to the max recommended temp, as then leakage can cause thermal runaway and a shorted device.The regulation acts by sinking current away from the output device to lower the output voltage, the components around the voltage loop like R4 C4, R9 R17 C2 C9 are there to prevent the loop oscillating, R12 keeping the LM324 in the class A operating region by sinking a little current so that the output stage distortion does not have too much effect on the output. R13 is a protection resistor for Q3, it keeps the peak current through the transistor to a low enough level on step changes that it will not drive the loop into oscillation.Component values in most places here are not terribly critical, 55 either way on most will not have much effect on stability, though capacitor values are not the optimum values as they have to fall in the E6 series, so use the recommended values and do not use ceramic unless they are NP0, use film units, as they do not have varying capacitance with voltage. for all those capacitors in the voltage regulation loop use a 100V unit minimum. C5, C7 will have to be electrolytics, use 100V there as well, and add the same value or anything up to 100uF 100V direct on the input side, especially if the main supply capacitor is more than 5cm of wire away. R5 direct across Q1 terminals, and thick wire from Q1 to D5 and then to the current sense and output. keep this wiring short if possible.

author: Mechatrommer
author_rank: Super Contributor
timestamp: January 20, 2013, 01:24:57 pm
content: now those are "very good enough" explanations for me. well done IanB your diagram certainly simplify things. and also SeanB for explaining components replacement and construction aspects of it etc, now i have more confidencenow let me refer to the diagrams below. i want to compare between my PSU i built earlier to the simplified IanB's diagram... here they are again...1) mine...2) IanB's & Amspire'swhats the pro and con between them? i can only tell the pro for (1) is, easier to understand, its just darlington, the earlier transistor will provides enough current to the next transistor. now (2), not just we need darlington Q1, but also pnp-npn pull-push like type at the base. the main difference i can see is, instead of just providing enough current to the next stage transistor's base (method 1), in (2) the base current provided by Q3 is drained through Q2 in order to regulate, so whats the catch in earlier (1) method? if someone can explain more detail of it i will appreciate it.and also the pro and con in feedback system, mine (1) is just simple votage follower i thought not much can go wrong with it, but i dont know i didnt examine that closely while building that. Amspire's is using somesort of division in -ve input of U1, ie (R7 and R9+R17) to get the balance. now this can expand the discussion, so i dont expect to dig deeper into feedback mechanism, if anyone can spot the obvious (but simple to explain) issue, then be pleased to highlight it. thanks.

author: SeanB
author_rank: Super Contributor
timestamp: January 20, 2013, 01:52:45 pm
content: Amspire design makes it easy to have multiple control loops, so that it is easy to add eg constant current and constant voltage, with little interaction with each other. Both have the same control range so they will go from one to the other without oscillation. Also makes possible a constant power option where the third loop is a multiplier that feeds it. Also enables a soft power switch by a simple saturated switch, and this can be used for thermal limiting as well, which will not interfere with the other loops, as any current sink will drop voltage, and the highest current sink is the control element. The control is by current only, the voltage is, within limits, ignored so the control performs the same at the highest as well as the lowest voltage and has very little non linear response.Also it has a low dropout voltage, essentially supply less 4 diode drops. Yours has none of that, and dropout voltage is the opamp high voltage ( normally Vcc less 3V) less 4 diode drops.The Amspire one can also drive a power linear mosfet with little change other than needing a 15V supply over the existing main power rail to provide the enhancement voltage to the fet, and this extra supply only has to deliver a constant current as well so is easy to design.

author: onlooker
author_rank: Frequent Contributor
timestamp: January 20, 2013, 03:55:32 pm
content: QuoteThis is what I can see from IanB's drawing. As SeanB noted, Q3 is a ~8mA constant current source. Therefore, Q2 is always working in the active region independent of the load. While yours may have the 2 pre stages of the darlington working in or near the cut-off region especially at small load.The other thing I can guess is that if the Q3 constant current source can be made variable by user control, it is already a way to have user settable current limit.

author: Mechatrommer
author_rank: Super Contributor
timestamp: January 20, 2013, 04:22:57 pm
content: yup thanks guys! now, time to rigging up, until then

author: IanB
author_rank: Super Contributor
timestamp: January 20, 2013, 06:46:26 pm
content: Quote from: Mechatrommer on January 20, 2013, 01:24:57 pmIn addition to the detailed comments provided by Sean, consider what happens when C3 is included in the picture:Now when power is first applied to the circuit the base of Q3 will be pulled high by C3 ensuring it is turned off, thereby ensuring Q1 is turned off. The circuit will therefore power up in a safe state with no output to the load, and will gradually increase the output as C3 charges up. (The RC time constant between C3 and R2 is about 1 ms.)As to why this is desirable, look at a test I did with my TTi EL302P power supply. I set the voltage to 10 V, I set the current limit to 30 mA, I connected a 5 mm LED to the output, and I turned on the output. Now normally if you connect 10 V to an LED you will blow it up in short order, but in this case nothing bad happened at all. Here is an oscilloscope plot of the voltage across the LED terminals:See how the voltage ramped up slowly to 2.3 V with no danger of overshoot.Inthis threadthere is an example of someone doing a similar experiment and their video shows how the LED momentarily flashes as the current goes higher than the limiting current when they connect the LED. That shows how ideally you would connect the load first and then turn on the power supply, rather than turning on the output first and then connecting the load. The power supply should be designed so that the output is safe when you turn it on.