Isolated power supply voltage spike issue - Page 1
Forum section: electronics / projects

author: spencer2004
author_rank: Contributor
timestamp: July 13, 2014, 05:04:00 pm
content: Hello.I have made an isolated power supply to boost 12V to +20V and -20V (but it can be adjusted to almost anything up to +-30V). The design is based on one that I found on the internet which can be seen below.I have managed to get it working with little trouble and it seems to work quite well, however I hooked it up to my oscilloscope and I discovered that it has quite a bad voltage spike problem on the output as seen below.200mV, 5uS per division:Using the reference design the voltage spikes were about twice as bad but I have made come changes which has helped but there still is something wrong here.I have also tried placing a pull down resistor on the base of the opto-isolator transistor, a small cap between the primary and secondary ground and an RC filter on the Vin pin of the LM2587 but these either do nothing or make the problem worse.I am all out if ideas now so could someone please throw some suggestions my way?

author: void_error
author_rank: Frequent Contributor
timestamp: July 13, 2014, 05:13:18 pm
content: Try using an LC filter on the output, it should smooth things out. For best results use a powdered iron core inductor.

author: SeanB
author_rank: Super Contributor
timestamp: July 13, 2014, 05:25:26 pm
content: Those 4700uF caps are very much on the large side. Try replacing them with a 470uF low ESR one, and use a LC filter on the output side and your noise will be a lot lower along with having improved transient response. Look up the TL431 applications to see how to use it properly in a power supply feedback loop as well, as the photodiode typically is connected to the high side, as it is now you will have terrible transient response. I see no reason to have the 20V zener and the 51V one on the primary side either, you should remove one, probably the 51V one.

author: nickm
author_rank: Regular Contributor
timestamp: July 13, 2014, 05:27:55 pm
content: It is most likely layout or measurement error. If you are using the scope probe ground clip that is the first thing to suspect.

author: spencer2004
author_rank: Contributor
timestamp: July 13, 2014, 06:32:04 pm
content: Thanks very much for your replies.void_error:Adding an LC filter on the output has helped a fair bit, I used a 66uH inductor (2 33uH in series) and a 4700uF capacitor just for testing purposes, it's just held together with crocodile clips at the moment so I imagine that it will be even better when everything's soldered properly. These are just random values that I picked so could someone suggest any better values please?SeanB:I did suspect that the capacitance value might be too high, I guess I was just thinking that more was better when I built it lol. I will have to order me some low ESR caps tomorrow. I shall remove the 51V zenner as it is indeed pointless. I will also re-arrange the feedback circuitry as you suggested.nickm:Yes the scope is probably making things seem worse than they actually are, it is over 20 years old after all. Also little to no thought went in to the layout, I just made it up as I went along so that's probably another issue that needs to be addressed in version 2.0.

author: eneuro
author_rank: Super Contributor
timestamp: July 14, 2014, 10:50:21 am
content: Quote from: SeanB on July 13, 2014, 05:25:26 pmIt will depend what LM2577 expects on its Comp pin and which is frequency it operates while it looks like in this second (changed) circuit those 2x 0.1uF capacitors gives very low cutoff frequency on this optoisolator diodeQuoteIt makes no sense use quite fast 4N27 optoisolator with 2us raise/fall time when there is such low frequency.In oryginal design it might work, while there is no capacitors on this optoisolator input diode.I think key is to understand LM2577 logic and than adjust TL431I.If we know what LM2577 expects as feedback, than using this TL431 cathode current vs cathode voltage datasheet graphone can design something such looks like overvoltage detection on output stage.However, to get very sharp overvoltage output from optoisolator using TL431 proper selection of resistors driving optoisolator diode are critical.For example in this prototype overvoltage detection circuit which switches very sharp output loadto get it working @ 6V and @ 15V those resistors close to optoisolator input have to be diffrent and this TL431 datasheet cathode current vs voltage is critical to get desired effect-sharp On/OFF needed for this simple mosfet driver for loads about 100W only.Even slow PC817 works well in this case up to some input voltage change frequency, but it depends what we want.Do not analysed LM2577 in this case but if we a looking for 5us spikes and have such slow feedback on this optoisolator diode, those changes in second circuit does not make any senseUpdate: According tohttp://www.ti.com/product/lm2577its Switching Frequency (Max) 52 (kHz) which gives 20us period.However, they recomend alternative parts like:LM2585 - Higher Switching Frequency (100Khz - 200Khz)LM2587 - Higher Switching Frequency (100Khz - 200Khz), Higher Switch Current (5A) etc.OK i see LM2587 is used in changed circuit.Just trying to figure out why Feedback pin is shorted to ground there and Comp pin used for output voltage sensing?In not optoisolated version it looks like this in LM2587 datasheet:It makes sense while instant output voltage is compared with internal LM2587 1.23Vref.This is what I did using eg. microcontrolers and what expect to have (voltage difference) be able to controll this output voltage by adjusting PWM duty cycle, but now I'd like to do the same using Lm2587But how this resistor 3k and 0.33uF capacitor is determined for Comp pin 1 in LM2587?It looks like it affects LM2587 duty cycle comparator and it is input to cycle-skipping comparator set at 0.3V so affects PWM itself, but how to calculate needed voltage on this Comp pin when feedback is shorted?

author: David Hess
author_rank: Super Contributor
timestamp: July 17, 2014, 09:11:42 pm
content: Quote from: spencer2004 on July 13, 2014, 06:32:04 pmThe problem will be with the probes and not the oscilloscope. Leaking magnetic flux, parasitic coupling, and ground loops will make accurate measurements difficult.When working with switching power supplies, I often end up soldering a short coaxial BNC pigtail with minimum lead lengths to the ground and low impedance node to be measured and then using a coaxial probe tip to BNC adapter with a x10 probe. Even the shortest probe ground lead will likely be too long.