Series Transistor Power Supply Design - feedback appreciated - Page 1
Forum section: electronics / projects

author: radian
author_rank: Contributor
timestamp: September 08, 2013, 04:24:45 am
content: I'm working on building a lab power supply for home use. I don't have any particular performance requirements, thinking 0-12V, 0-1A with "decent" load regulation and "minimal" ripple. Initially I was thinking about using a programmable voltage regulator, but I've read that these aren't best suited for varying outputs. So I've now considering using a transistor (BJT) along with op amps to control voltage and current output. Note, I'm not that great with BJT's and op amps, so please go easy on me. Will using a BJT allow me to control down to 0V?I'm attaching the circuit and simulation output. In the simulation I've specified three operating regions (in each voltage set to 5V, current limited to 400mA). In the first and third region the load is drawing 300mA and hence is in CV mode. In the second region the load attempt to draw 700mA, which exceeds the current limit, so the PSU is in CC mode.In the first and third region (CV), based on simulation results, the output is fairly stable at 5V. Now in CC mode, things are a little strange. First, current overshoots a bit, then settles to around 400mA. I'm also concerned is the voltage, which is very low at around one volt during CC. Sure, I expect voltage to drop somewhat, but this much? Also not that impressed with the response time. If I reduce the capacitor values, things start oscillate violently. I'm not sure if I've implemented CC correctly. I don't know if pulling the base to ground is the right way to go about this?I haven't breadboarded this yet, afraid it's going to burst into flames, so hoping to get some expert feedback first before going any further. Note that I haven't added any protection circuitry, just the basics for now to keep things simple. Thanks!NOTE: Green trace is output voltage, red trace is output current

author: mmagin
author_rank: Frequent Contributor
timestamp: September 08, 2013, 05:52:13 am
content: For a start:Certainly a 2N2222 can't dissipate >12 watts at close to 0 volts, 1 amp outAs far as old NPN jellybean transistors, TIP31 would probably be sufficient. Or just look at your favorite parametric parts search.I'm not sure I see the purpose of C2.

author: miceuz
author_rank: Frequent Contributor
timestamp: September 08, 2013, 07:24:27 am
content: You've missed a current limiting resistor on the base of Q2

author: radian
author_rank: Contributor
timestamp: September 08, 2013, 02:46:22 pm
content: Quote from: mmagin on September 08, 2013, 05:52:13 amYup, I chose the 2N2222 arbitrarily. It only can support up to 800mA I believe, which is adequate for this demonstration. I've got some alternatives in mind, but didn't bother looking for SPICE models quite yet.If I remove C2 the current during CC mode oscillates very quickly. See attached screenshot (stopped simulation early, was taking a while).Quote from: miceuz on September 08, 2013, 07:24:27 amTrue, good catch.

author: dannyf
author_rank: Super Contributor
timestamp: September 08, 2013, 05:14:22 pm
content: QuoteYou are far more likely to be successful with your project if you had used one of those simple 3-terminal regulators. It will get you started so you can learn more about electronics in the future to design your own power supply.

author: mmagin
author_rank: Frequent Contributor
timestamp: September 08, 2013, 05:46:29 pm
content: Quote from: radian on September 08, 2013, 02:46:22 pmIt may work fine in simulation, I was merely pointing out the power dissipation problem:You often cannot reach the rated collector current when a transistor is used in a linear regulator like this -- if you output 1 volt and say have 13 volts in, at 800 mA you dissipate 9.6 watts in the transistor, and even with heatsinking the little metal-can 2N2222 cannot survive thatI won't discourage you from taking the approach of building a power supply from transistors and op-amps, it's certainly an interesting project, though you won't get some of the convenience or protections built in to a three-terminal device, such as over-temperature protection.