Measuring diode voltage drop in SEPIC - Page 1
Forum section: electronics / projects

author: PeterF
author_rank: Contributor
timestamp: October 09, 2017, 02:45:28 pm
content: Hi guys!Thanks for the help on my previous topic regarding the supercapacitor charging circuitry.I ended up using the LT3956 in a SEPIC configuration, which I had already prototyped a bit.I am now in the process of verifying the boards, and I am getting some headache over the forward voltage drop of the diode I'm trying to measure.In my simulations and in specs, I should be getting somewhere around 0.4-1 V drop when passing a couple amps through the diode.But I tried putting the scope on the board, and measure over the diode - both by using one probe and grounding one end (with separate supply ground), and by using two probes and doing the diff thing.I am measuring about 3-4 Volts drop on the diode..This is the part I'm using: SD2114S040S8R0https://www.digikey.se/product-detail/en/avx-corporation/SD2114S040S8R0/478-7817-1-ND/3749523Could you guide me towards what I could be missing? I can't really be seeing 4 volt drop on a schottky?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 09, 2017, 02:48:22 pm
content: Post the individual waveforms (that are diff'd to get that measurement), and a picture of the probing setup you are using.Also, what V, F, I is the circuit operating at? Maybe 4V is normal, we have no clue what you're doing.Tim

author: Tomorokoshi
author_rank: Super Contributor
timestamp: October 09, 2017, 05:21:38 pm
content: I have a similar application using the LT3958. Here is the Schottky I settled on after testing out several parts:https://www.digikey.com/products/en?keywords=FSV10120VCT-NDFor instance, this part, which is SMC, had a similar failure mode to yours:https://www.digikey.com/products/en?keywords=MBRS4201T3GOSCT-NDI have a lower current draw than yours, but note how the package of the FSV10120V is larger than the SMB, and I had to be careful about the heatsinking. Schottky diodes have a way of going resistive when they get hot. Also, my capacitors and the coupled inductor are all about half of yours. You may get getting some peak currents that are beyond the rating of the part. It may be getting a bit punch-drunk.How hot does yours get? How much heatsinking is there on the board? Can you also post a picture of the layout?

author: PeterF
author_rank: Contributor
timestamp: October 11, 2017, 09:27:51 am
content: Quote from: T3sl4co1l on October 09, 2017, 02:48:22 pmI will update you with more information when I've had the opportunity to get back to the lab and measure again.I did not document at what point of state this measurement was performed.But the use case is charging super capacitors from a 24V input.The output current was probably at this state around 2A, and the output voltage somewhere between 5-15 V.I will perform a more thorough measurement with details and post more info when I get to it.

author: PeterF
author_rank: Contributor
timestamp: October 11, 2017, 09:35:12 am
content: Quote from: Tomorokoshi on October 09, 2017, 05:21:38 pmI've attached an image of the layout surrounding the diode.Can you tell me more about your failure?I have only measured the temperature with a cheap flir camera, which resulted in a maximum of about 50 C.I will try to measure the peak currents more thoroughly.I've only so far measured average current with a multimeter, but haven't tried investigating what peak currents are flowing.I could also perhaps try to hack in a beefier diode ( I have some beefier D2pak diodes in the lab with similar packaging as your part) and see if there is a big difference, even if I won't be able to heatsink it too well.

author: sean0118
author_rank: Frequent Contributor
timestamp: October 11, 2017, 12:11:10 pm
content: Does this have an internal GND plane? From the images posted I can't see the GND path between anything.edit: Also is C21 the coupling capacitor? Looks small, can it handle the ripple current?Sorry I know I'm probably not helping much with the diode issue.

author: PeterF
author_rank: Contributor
timestamp: October 11, 2017, 12:43:42 pm
content: Quote from: sean0118 on October 11, 2017, 12:11:10 pmI'm very thankful for all input of courseYes I hid some layers to simplify screenshot clearness. I have a GND plane and an internal power plane as well (4 layer stackup).C21 is the coupling capacitor. It's a 1210 50V X7R 10uF. I haven't put too much attention to the coupling capacitor rating I'm afraid. I believe I based it on the app note I've copied the design from.

author: sean0118
author_rank: Frequent Contributor
timestamp: October 11, 2017, 12:52:50 pm
content: Oh, 1210 is larger than I thought looking at it, it'sprobablyokay then.Good that you have a GND plane as well.

author: PeterF
author_rank: Contributor
timestamp: October 11, 2017, 04:02:46 pm
content: I've now performed some more measurements.I connected a resistive load to the output instead of my capacitors.Vin 24 V, Vout ~12.5V, Iout~2.1A, Iin 1.1A.Ive attached two images of zoomed in waveforms and a very rough phone image of the connection.I'm using two probes with GND leads tied to GND and the probes to each side of the diode.The output voltage is at about 12V, while the other side of the diode goes between ~ -3V to -35 V. Never can I see any point where there seems to be a positive forward voltage at all.I tried switching the diode out to one with higher reverse voltage and current specs, but its behaving much the same.EDIT: Added an image from LTspice with the result I was kindof expecting to see. Voltage is positive at about 0.5 V at the "high side" of the waveform.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 11, 2017, 04:14:26 pm
content: Check probe compensation?Sort of equivalently: does it look the same if you swap probes?Tim

author: PeterF
author_rank: Contributor
timestamp: October 11, 2017, 04:21:10 pm
content: Quote from: T3sl4co1l on October 11, 2017, 04:14:26 pmChecked compensation and both looked fine.

author: TimNJ
author_rank: Super Contributor
timestamp: October 11, 2017, 05:22:01 pm
content: I don't know a whole lot about SEPIC converters, but are you using a buck/boost design and "accidentally" operating it as a boost converter? I'm not exactly surehowyou would do that, but I think that would explain why your cathode voltage > anode voltage?Might be way off on my understanding though.Edit: Ah wait no Vin = 24V

author: fourtytwo42
author_rank: Super Contributor
timestamp: October 11, 2017, 05:35:31 pm
content: Quote from: T3sl4co1l on October 11, 2017, 04:14:26 pmOr put both probes on same signal and check there identical

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 11, 2017, 06:27:55 pm
content: You're zoomed in quite a lot, maybe it's a scope offset issue? Try reading the diode voltage, with the probe grounded to the output node? (Needs isolation in suitable places, of course.)Tim

author: PeterF
author_rank: Contributor
timestamp: October 24, 2017, 02:30:35 pm
content: Update:I tried using another scope and got much better results

author: RES
author_rank: Regular Contributor
timestamp: October 24, 2017, 03:17:53 pm
content: Switching converter means a PWM (alternating) signal across the diode. Can your scope show rms values? Test the (schottky)diode on dc(0 Hz) see what the voltage drop does at different current values.