yet another bench power supply project - Page 1
Forum section: electronics / projects

author: sigmatechnica
author_rank: Contributor
timestamp: July 17, 2014, 08:42:57 pm
content: So the internet is littered with unfinished bench power supply projects and it seems the more usable and better specified they are the higher the chances of them being unfinished and abandoned.So I thought I'd go ahead and start another one.Requirements* dual (+-) channel* tracking. That is for what one one channel is + the other is -. Only one voltage and current control needed for both channels* not to fussed about the max voltage, will probably end up being about +-30v looking at the available transformers* 2 amps or so.* actual voltage and current readout* CV and CC on both channels, but if one channel hits the current limit and has the voltage reduced, I want the other channel to do the same so it stays symmetrical about 0v. At least I think that's what I want. Thoughts?* digitally controlled. MCU (arduino, because its easy and i'm lame) controlling SPI DACs for the voltages and ADCs for measurement of the actual voltage.* MCU NOT in the feedback loop (because that would suck) its just there for an interface* tracking pre-regulators can go away. I'll just throw capacitors, power transistors and heatsinks at it until it performs adequately* optionally an additional +5V railI anticipate the hardest part will be the analogy control loop.I'm thinking of using the MAX4081 for current feedback, but it has a couple of issues. Firstly it needs the common mode voltage about 4.7V, so a dual supply will need extra +- 5(ish)v lines, to offset the ground on these chips in order to reach an output of 0v Secondly the bandwidth isn't so great. 150kHZ (i assume that's -3db) seems a bit slow, no idea if that will be a issue though.Thoughts and suggestions welcome

author: cellularmitosis
author_rank: Unknown
timestamp: July 17, 2014, 10:38:35 pm
content: I am about to try out a design using the INA193, which has 500kHz bandwidth. Did you take a look at that one?http://www.ti.com/lit/ds/symlink/ina193.pdf

author: David Hess
author_rank: Super Contributor
timestamp: July 18, 2014, 03:12:44 pm
content: Quote from: sigmatechnica on July 17, 2014, 08:42:57 pmThis will make the design more complex unless high voltage operational amplifiers are used. +/- 20 volt is more typical.QuoteOffhand I do not know of any tracking supplies which do this. One way to implement it is to use a floating 0 to 60 volt single supply with a power amplifier generating a virtual ground but that is not how I would try it first.QuoteOn my Tektronix PS503, the high current +5 volt output is referenced to chassis ground while the variable +/- 20 volt tracking supply is floating.QuoteThe input to the opposite supply can be used to generate the bias for the MAX4081. You may want to do this anyway for the control circuits.I am leery of specialized ICs like this so I would try to use a single ended current limit like in the PS502 or PS503 although this would present other difficulties for DAC control.I would consider is using integrated regulators to drive the power pass transistors in order to take advantage of their thermal shutdown, current limit, and safe operating area protection.

author: sigmatechnica
author_rank: Contributor
timestamp: July 19, 2014, 02:28:15 pm
content: cellularmitosis:hmm actually yes, the INA193 does look better, thanksDavid:Thanks for the advice. I was indeed considering using integrated regulators, some experimentation needed there i think though.Thanks all,next step: schematic

author: David Hess
author_rank: Super Contributor
timestamp: July 19, 2014, 08:47:06 pm
content: Quote from: sigmatechnica on July 19, 2014, 02:28:15 pmYour requirement for a 2 amp output from 0 to 30 volts will cause problems regarding safe operating area of the pass element whether it is integrated or discrete at low output voltages and high currents. This especially includes output shorts when current limiting is active.A discrete pass element will have to be derated significantly beyond just its rated power to handle it. I doubt modern 15 amp 115 watt 2N3055 transistors for instance would meet your 30 volt and 2 amp requirement (The old but slow, expensive, and low gain MESA process ones are tougher.) once temperature is taken into account. Integrated regulators have protection built in for this but it just creates a different problem since high current will not be available at low voltages. While not widely advertised, most power MOSFETs, at least the vertically constructed ones, suffer from a similar problem at high but surprisingly low Vds and there is often little data available about their limitations when operating in linear mode.The above explains why power supplies and audio amplifiers often have what appears to be at first glance an excess of paralleled output transistors when only the current and power ratings are taken into account.This is just something to be aware of. Peruse the safe operating area curves carefully and derate them for expected worst case junction temperature.One trick I like to use is thermally coupling the integrated regulator used as a pass element driver to the power pass transistors by mounting them to the same heat sink and then using its built in safe operating area protection and thermal shutdown to protect them but that should not necessary with a carefully designed 30 volt 2 amp power supply.If failure of the pass transistors causing damage to an expensive load is unacceptable as they will tend to fail with a short from input to output applying the full input voltage to the load, then adding a variable voltage output crowbar circuit would be a good idea. A suitably selected current sense resistor in this case may serve as a fuse.

author: sigmatechnica
author_rank: Contributor
timestamp: July 20, 2014, 06:55:44 pm
content: QuoteIndeed, I confess SOA isn't something I had thought about particularly. Since it is going to be MCU controlled though, overtemp shut of or warning could be incorporated into the firmware with a big chunky relay disconnecting the output before things get too toasty.Its probably overkill, but I have some 2SC5200/2SA1943 in the parts box not doing anything which I was intending to use for the pass devices. If I am reading the datasheet right, maximum current at 30v is 5A, derating by 40% for a 75deg temperature (its going to be nowhere near that hopefully!) gives me 3A which should be plenty...heatsinks will have a fan also.

author: SeanB
author_rank: Super Contributor
timestamp: July 20, 2014, 07:11:27 pm
content: That SOA curve is only valid at die temp of 25C, you will have to derate according to the next table and this gives a 40% derating for you, so you probably will only be able to have 1.5A per device at 30V. You will need to parallel around 3 to get the desired SOA curve, but this will be a bonus as then you will run with lower dissipation per device.Horrid how the headline specs get slashed when you want to use the device at anything like real world operation. But that is always going to happen.

author: David Hess
author_rank: Super Contributor
timestamp: July 20, 2014, 09:07:42 pm
content: Quote from: SeanB on July 20, 2014, 07:11:27 pmIt is actually worse than this because the input will be higher than 30 volts to support the dropout voltage of the regulator and the ripple of the input filter.

author: dannyf
author_rank: Super Contributor
timestamp: July 20, 2014, 09:21:18 pm
content: The 5200 1943 are fabulous devices for audio. For what you do, a cheap mosfet would work.you may also think about CPU heatsinks for cooling. Some xeon heatsinks can easily handle 100w or more. They are also small in size.

author: David Hess
author_rank: Super Contributor
timestamp: July 20, 2014, 09:30:44 pm
content: Quote from: sigmatechnica on July 20, 2014, 06:55:44 pmThe usual solution is to implement foldback current limiting which is pretty simple if it is designed for worst case temperature. A lot of power supplies in older Tektronix instruments include this so a simple short does not cause a catastrophic failure. Integrated regulators include this function anyway so when used as a driver in the right design, they can be used to protect the pass transistors.QuoteI like putting variable speed thermostats on fans when I use them like this. It just takes an operational amplifier, small power transistor or integrated regulator, diode as a temperature sensor, and one trimmer although there are no-trim ways to do it also. I am slightly tempted to make a little board just for this function to stick into things.

author: dannyf
author_rank: Super Contributor
timestamp: July 20, 2014, 11:51:54 pm
content: A simple temperature switch shorting a power resistor in serial with the fan will create a two speed fan controller.

author: sigmatechnica
author_rank: Contributor
timestamp: July 21, 2014, 09:19:22 pm
content: Quote from: David Hess on July 20, 2014, 09:07:42 pmOk i'm confused now. If the maximum safe current at 30v is 5A and you derate by 40% that is 3A, not 1.5. Are you applying an extra 50% as a safety factor?I had neglected the dropout voltage, looking at the transformer, it states 19v, so that's going to be about 28v present on the caps. I should really measure it though before getting stuck into the design since transformers in the UK, especially wobbled ones can be designed for mains voltages of anything between 220 and 250v.Quote from: dannyf on July 20, 2014, 09:21:18 pmmosfets would be an option too. I just have these transistors doing nothing and it would be a shame to waste them. datasheet notes they are obsolete so i am unlikely to use them in any audio design.CPU heatsinks are exactly what i am intending on using!Quote from: David Hess on July 20, 2014, 09:30:44 pmI did something similar back when i made more PCBs. In this case i was planning on using a 4 wire fan and controlling the speed directly from the MCU. I have some lovely SanAce fans that fit perfectly in the box i'm intending on using

author: David Hess
author_rank: Super Contributor
timestamp: July 21, 2014, 11:49:46 pm
content: Quote from: sigmatechnica on July 21, 2014, 09:19:22 pmI was just including the dropout and ripple voltage. If the input to the pass element is an average of 35 volts then with an output short and 2 amp current limit, the pass element has to support 35 volts at 2 amps even though the maximum output voltage is 30 volts.The 2SC5200 datasheet says it must be derated linearly above Tc=25C. If its maximum current is about 4 amps at 40 volts, then it will be 3 amps at 56.25C and 2 amps at 87.5C.I am a little fuzzy about how Toshiba considers the interaction between Tc and Tj but I think that is right.