Designing a voltage regulator power supply - Page 1
Forum section: electronics / beginners

author: PythonGuython
author_rank: Contributor
timestamp: March 15, 2021, 12:25:51 am
content: I'm designing a 12v power supply for a class project, and I finally have a working design, but my mega capacitor and quadruple cascaded transistors seem a bit silly.Requirements:-Step 120v AC 60 Hz to 12v DC-supply 6A peak load-less than 10% ripple at peak load-must only use fundamental components (transformer, LRC, transistors, diodes, etc.)My current design:Explanation of my design:I step down 120v with an 6:1* transformer. Sent to a full wave rectifier, then filtered. I could only get this to work with an absurd amount of capacitance (at least I think this is an absurd amount of capacitance) 2x 8000uF caps in parallel. That filtered output is then sent to voltage regulator made with a summing amplifier. I've been having the most trouble with the series pass transistor. At 6A, I could only get this to work with 4 power NPN transistors cascaded.What I've tried already:-capacitance multiplier at the bridge filter. Maybe I wired it wrong, or I didn't use a beefy enough transistor, but I couldn't any voltage out of it.-power MOSFET as series pass element - When I tried this, the voltage would start to drop around 2A load-power MOSFET as a darlington pair - got ringing. When I found a dirty solution to the ringing, the voltage would still drop around 3A/-darlington pair with pnp at the 1st stage, then 2 npns - this was recommended by a TI paper on regulators. This gave no improvement over just 2 npns.(I've gotten the most improvement in output by modifying the pass element, so I think this is where I could improve the most)Is there anything obvious I'm doing wrong?*fixed typo in edit

author: bob91343
author_rank: Super Contributor
timestamp: March 15, 2021, 01:37:49 am
content: The 2N3055 transistors, except the main one and perhaps its driver, are operating at too low a collector current. Instead, use smaller transistors. Also include B-E resistors to tame the circuit at low currents. When the load is removed and the pass transistor is hot, you may not be able to keep the output voltage down.With 6A load current, you probably will require about 600 mA drive. Divide that by the gain of the two low-current power transistors and that will tell you how much drive you need from the IC.The rectifiers are being abused with so much surge current.Your text says 8:1 ratio but the diagram says 6:1.You may need some bypassing of the power transistor bases to keep it stable.

author: wizard69
author_rank: Super Contributor
timestamp: March 15, 2021, 02:30:33 am
content: There is a very similar thread here:https://www.eevblog.com/forum/beginners/lm317hv-with-pass-transistor/and some interesting circuits here:https://www.qsl.net/yo5ofh/hobby%20circuits/power_sourse_circuits.htmQuoteIn this case I would have to say you are selecting the wrong transistors to drive your base. Consider figuring out how much current you need to drive the base at the required current level. That should help you zero in on a transistor with acceptable gain levels.Your circuit isn't fancy but I doubt that is the reason for the exercise. In fact it is probably the instructors goal to get you to compute the value of the different components in the circuit. In other words to figure out how to bias a transistor in the analog region.As for the primary filter caps 8000F is nothing really in a traditional analog power supply. I really don't see sizing the caps as the main point of the exercise either.

author: PythonGuython
author_rank: Contributor
timestamp: March 15, 2021, 02:34:42 am
content: I replaced the first 3 transistors with 2N4401s instead and it now works (well, simulates) like a charm, thanks for the insight. When you say surge current, are you referring to when the device is first turned on and the capacitors haven't fully charged yet?

author: srb1954
author_rank: Super Contributor
timestamp: March 15, 2021, 05:44:03 am
content: Quote from: PythonGuython on March 15, 2021, 12:25:51 amSome things to check:(1) the power dissipation in the series pass transistors and compliance with safe operating area for those transistors. You have all the output current flowing though one 2N3055, which likely won't last verylong unless you have some very substantial heatsinking attached.(2) the overall efficiency of the power supply.(3) the transient response of the supply in response to sudden changes of load.(4) the performance with a model of a commercially available op amp rather than an idealised op amp(5) the accuracy of the output voltage taking into account component tolerances etc.It is easy to simulate a circuit and get satisfactory results because simulated components don't ever overheat and catch fire unlike real world components. Simulated components also have perfectly precise values and no parasitic elements unlike real world components.You should design and simulate your circuit as though you intend to actually build it and take into account real world issues like power dissipation, efficiency, accuracy, component ratings and lifetime. You will possibly get extra marks for your project if you can demonstrate, and document, that you have thought a bit further about the design than just getting a satisfactory simulation result.

author: bob91343
author_rank: Super Contributor
timestamp: March 15, 2021, 05:52:12 am
content: Surge current takes a couple of forms. Yes there is the initial surge until the capacitors have reached full charge. But there is a repetitive surge every half cycle due to the load draining the capacitors during diode off time. This creates heating due to ESR of the capacitors and thus raises their temperature, shortening life.In addition, that surge current goes through the diodes and creates heating there as well, not to speak of the transformer winding. Short pulses of current are more destructive than steady currents because the power wasted is greater per pulse than the average.

author: mvs
author_rank: Frequent Contributor
timestamp: March 15, 2021, 09:27:07 am
content: Quote from: PythonGuython on March 15, 2021, 02:34:42 amThe Darlington configuration is used to increase current gain. It lowers speed and you should use as few stages as possible.General purpose opamps usually can drive 20mA w/o any problems, so you need a gain of around 300 to achieve your 6A target. You should be able to do it with just a basic darlington pair.2N4401 is too small for the task, look for BD139-16 or something similar.Current gain (hFE) of BD139-16 is larger then 100, current gain of 2N3055 is larger then 15, current gain of basic darlington pair (BD139-16 + 2N3055) is then at least 100*15=1500.If you wish to split load across multiple transistors, you can use multiple 2N3055 in parallel with a 0.2Ohm resistor in series with each emitter.You can also use faster transistors like D44H11 to replace 2N3055. They have also a higher current gain.

author: Zero999
author_rank: Super Contributor
timestamp: March 15, 2021, 09:38:21 am
content: A quadruple Darlington is way overkill. You'll probably find the BD131 + 2N3055 is enough. If it isn't, add the BC338. Connecting many 2N3055 in parallel, with emitter resistors, will also help to increase the current gain, since the hFEof the 2N3055 decreases, at higher collector currents.

author: perieanuo
author_rank: Frequent Contributor
timestamp: March 15, 2021, 10:16:18 am
content: +1 to Zero999 's solution, couldn't say it better

author: Zero999
author_rank: Super Contributor
timestamp: March 15, 2021, 11:18:30 am
content: A few optimisations.Power the zener from the output of the op-amp, to ensure it has a roughly constant current through it, irrespective of the intput voltage. This will only work if you desire a fixed output voltage. If you want variable, you could use another op-amp, for the zener.Add some base-emitter resistors, R6 & R7 in this case, to speed up the turn-off time of the transistors.Include a current limit, so if the output is short-circuited, it won't blow-up. Q4 cuts off the drive to the pass transistors, when the voltage across R3 starts to reach about 0.6V to 0.7V. The transistors will need to be on a good heat sink.A decoupling capacitor is required on the output, to improve transisent response.You'll might also require a frequency compensation capacitor between the op-amp's inverting input and the output. It depends on how fast the op-amp is, compared to the rest of the circuit. Look at the output on an oscilloscope, set to AC coupled and briefly apply a 6A load. It shouldn't ring, or oscillate, when the load is connected/disconnected. A MOSFET and 2R resistor, with the gate connected to a pulse generator, can be used to generate 6A load transients.Don't rely on SPICE simulation too much. Build it on a breadboard. The high current paths mustn't go through the breadboard, otherwise it'll melt.