Pick my bench power supply design apart - Page 1
Forum section: electronics / projects

author: mpicker21
author_rank: Contributor
timestamp: August 16, 2016, 03:23:13 am
content: Attached is the schematic that I've spent a weekend or two developing. It's yet another bench power supply design. I was inspired by Dave's design but discouraged by his, Ian Johnston's, and other's results with the LT3080. I considered going the "by scratch" method but it appears that getting a stable supply would take a lot of tuning so I'm going with the classic LM317. Here's what I'm trying for:constant current/constant voltage regulation1.5A output0-24(ish) volts outputcontrollable using a micro-controllerI'm still a ways from finalizing this schematic but before I go through and try to put values on the passives and such I was looking for some honest advice about the general design. Here's the overall design thoughts:Power comes in from a standard 24 volt transformer. The voltage set pin on the LM317 is set using a DAC and buffer. The LM2576 SMPS tracks the output of the LM317 to help keep the power dissipated by the LM317 to a minimum and help me get that 1.5 amps out rather than the half amp or so if it was dropping all that voltage itself. I'm getting a negative voltage using a charge pump from the transformer and -5.6 volts using a zener. I'll use this supply for the negative rail on the op amp and then to get my negative reference voltage so I can offset the LM317 down to 0 volts. I have a current sense amplifier tied to another op amp to provide some constant current control that draws the adjust pin down. Another DAC sets the current limit.I may get ambitious and put two of these in the supply with a transformer that has two secondary coils. I'd put the micro-controller on another small transformer and isolate the buses to the DAC/ADCs with opto-couplers or something. I haven't gotten there yet. My thought is this would allow me to double the voltage, double the current, or just give me two independent supplies. Would this work like this?Thanks in advance for the suggestions.

author: MosherIV
author_rank: Super Contributor
timestamp: August 16, 2016, 06:57:00 am
content: QuoteIf the LM2576 cannot provide 1.5A then the LM317 cannot magically create and provide the extra current !It is actually harder to add current control to a voltage regulator then to design and build your own voltage regulation circuit. Your design already has op-amps to control voltage, replace the LM317 with a power transistor, like a 2N3055, in emitter follower configuration and you have a voltage reglation circuit. There will be problems with high freq oscillations in the op-amp, add filter to the op-amp between output and feedback as close to op-amp as possible.Hybrid designs will be noisier than a linear, are you ok with the increased noise?

author: CJay
author_rank: Super Contributor
timestamp: August 16, 2016, 06:58:57 am
content: Zener D10 is reverse polarity.I'd drop the shunt resistor R1 and the INA193 in favour of one of the Hall effect current sensors like ACS712What's the D2/R8/R11 combo meant to do if you're feeding the output from the DAC U6 to the non inverting input of U4?

author: CJay
author_rank: Super Contributor
timestamp: August 16, 2016, 07:02:10 am
content: Quote from: MosherIV on August 16, 2016, 06:57:00 amLM2576 is 3 amp capable, it's the LM317 he's trying to get the current from, while the LM317 is capable of 1.5A it's only able to deliver that within its power dissipation limits so having a tracking pre regulator, the LM2576, should allow him to keep the LM317 within that power limit and supply the 'rated' 1.5 amps.

author: mpicker21
author_rank: Contributor
timestamp: August 16, 2016, 02:02:09 pm
content: Yeah the LM317 datasheet says it can only provide the minimum 1.5 amps down to a 15 volt drop from it's supply voltage. I'd rather just drop a few volts to help keep it cool and maintain the 1.5 amp output capability.I'll look at the 2N3055 option though. I'm not married to using an LM317. I just thought using an adjustable regulator/SMPS combo would be a good way of getting the linear performance without the heat and wasted power issues. I figure most of the usage of this supply will operate at 12 volts or below. That could be a lot of heat at even an amp.Quote from: CJay on August 16, 2016, 06:58:57 amIsn't that the way it should be so that I get a negative 5.6 (or whatever the zener value is) volts out the junction of D10 and R17?Quote from: CJay on August 16, 2016, 06:58:57 amI will definitely look into that.Quote from: CJay on August 16, 2016, 06:58:57 amWhat I'm trying to do here is allow the output of U4 go down to -1.25. D2 would be a 1.25V zener. I'm trying to subtract that value from the output of the DAC. If the output of the supply is 0-25 volts then I need the output of U4 to be -1.25 to 23.75 volts. I get the basics of op amps but I struggle with some applications of it. I got this configuration from somewhere on the internet.Thanks for the feedback guys. I could always copy someone's design online, and trust me I've considered it at times, but there is something cool about coming to the same conclusions yourself. It's how I learn.

author: StillTrying
author_rank: Super Contributor
timestamp: August 16, 2016, 02:16:27 pm
content: Quote from: mpicker21 on August 16, 2016, 03:23:13 amNope, components are in the wrong places. I don't think you'll do it with less than this.Edit: Added LT version, use at own risk etc.

author: CJay
author_rank: Super Contributor
timestamp: August 16, 2016, 02:20:34 pm
content: Quote from: mpicker21 on August 16, 2016, 02:02:09 pmKeep the tracking pre regulator, they work and are used in a few commercial designs too, all you're doing with the 2N3055 is building your own linear regulator so it'll still benefit.Quote from: mpicker21 on August 16, 2016, 02:02:09 pmNo, the zener still needs the anode still needs to be more positive than the cathode for it to work. The way you have it connected it will bring the negative rail to whatever the forward voltage drop of the Zener is, not the Zener voltage.Quote from: mpicker21 on August 16, 2016, 02:02:09 pmWatch this:Quote from: mpicker21 on August 16, 2016, 02:02:09 pmIt's a great way to learn, especially if you actually build the thing. I'm nowhere near as knowledgeable as some of the people here, I'm always learning too.

author: Mechatrommer
author_rank: Super Contributor
timestamp: August 16, 2016, 04:24:25 pm
content: Quote from: StillTrying on August 16, 2016, 02:16:27 pmi agree with wrong place of resistor and zener, but i believe from your diagram, leftmost cap and diode are not needed in OP's application. rightmost cap only for better Vzener regulation, less better without, but workable. from the OP's diagram, his charge pump V will be = Vt + Vz = -18.4V, if Vt = -24V, Vz = 5.6V. C9 will develop negative voltage Vt for zener to be in reverse mode.

author: Kleinstein
author_rank: Super Contributor
timestamp: August 16, 2016, 04:50:06 pm
content: The SMPS / linear combinaton is nice if you need a lot of power from a small package. For just 1.5 A and maybe 25-30 V, one could still go with a pure linear regulation for lower noise / ripple. It's also easier to build - so I would try that first, even if it's only 0.5 or 1 A. The feedback circuit at the LM2976 will not work this way. It usually needs an extra transistor to get the SMPS to follow the linear output plus somthing like 3 V.In most aspects the LM317 is just replacing a power transistor. The only benefit is having SOA, thermal and extra overcurrent protection. The regulation loop of the LM317 (like other 3 pin regulators) is not really suited for a bench / lab supply as it is not made to cope with any load. So it is fast, but may oscillate under certain circumstances. So normal power transistors may be easier.

author: CJay
author_rank: Super Contributor
timestamp: August 16, 2016, 05:13:04 pm
content: I like the pre-regulator, makes it more efficient and it may not need any forced cooling so it can be silent.Persnally I'd build it linear only first and then integrate the pre-regulator followed by the ADC/DACs but that may not be everyone's choice, it just seems like a logical path to me and I could be wrong.

author: StillTrying
author_rank: Super Contributor
timestamp: August 16, 2016, 05:38:25 pm
content: Quote from: Mechatrommer on August 16, 2016, 04:24:25 pmWithout them there are no -ve voltages, the bridge rectifier clamps anything -ve going to GND, and anything +ve going to +24V.