Circuit protection causing drop in voltage under load? - Page 1
Forum section: electronics / beginners

author: bitman
author_rank: Unknown
timestamp: September 29, 2020, 04:27:04 pm
content: I've begun to add basic protection to my PCBs - at this point it's very basic stuff like reverse current protection and over-voltage protection. I've attached a diagram of what I'm using. I need this circuit to drive up to 1.5 amps in short bursts, and the mosfet I'm using is rated 3.5->5amps - should be plenty for my need.When I just power on with no load, I see a perfect 12V on the output as I wanted. But using a DC load, the moment I add a load the voltage drops. To less than 10V when I pull 1.5Amps. I've put the sense line right next to the DMG2305UX on the PCB and it still measures just below 10V at 1.5Amps. The power-supply shows a nice 1.5Amp drawn at 12V but it seems like the output isn't close to that under load.I could be measuring wrong of course - I have the load on the ground and plus by the C2 cap - although connected through a 20cm long solid core wire. I have the sense measurement right next to the DMG2305UX - and it's ground using the same line as the DC load. Am I measuring the load in the right way? Right now I'm just using a bench power-supply and it does not show a drop in voltage at all. What I'm interested in testing is the MOSFET and if it's able to handle the load.I'm wondering if this is just a bad choice of MOSFET? It's what I had in the drawer that could handle more than 2Amps at 12v - but that doesn't mean it's right. If it's wrong, what am I missing in the datasheet to show it's wrong?

author: bitman
author_rank: Unknown
timestamp: September 29, 2020, 05:30:34 pm
content: Ok - I now realize I suck at using DC Loads. Or rather, didn't look more careful at what I was doing. Just plugging the DC load directly into the bench power-supply I see the discrepancies and it has nothing to do with my circuit. My guess is that I need to put my sense-line where the power is provided and not where it's measured - as the amps and resistance indicated by the DC Load provides the voltage it claims - but since it measures over that resistance it's no wonder there's a drop that's relative to the current.So back to school for me - learn how to connect the sense-lines better next time. Leaving this here just in case others make the same mistake as I did.

author: wizard69
author_rank: Super Contributor
timestamp: September 29, 2020, 05:36:11 pm
content: Quote from: bitman on September 29, 2020, 04:27:04 pmI'm not an engineer like many around these haunts but the first thing that come to mind is that FET operating like a switch with that circuit? I don't see it if this is your overvoltage protection circuit. I'd check the voltage across the transistor when it is under load.

author: ledtester
author_rank: Super Contributor
timestamp: September 29, 2020, 05:48:12 pm
content: The maximum gate-source voltage is +/- 8V:

author: bitman
author_rank: Unknown
timestamp: September 29, 2020, 08:35:00 pm
content: Quote from: ledtester on September 29, 2020, 05:48:12 pmI have to ask - so the Vds is 20v but the Vgs is only 8? What good is the other 12V then? Also, the +/- is a bit confusing here. I understand why p-channel often are shown as negative numbers, but in this case why would it be written as +/-?Obviously I failed to see this value (well, I saw it but thought it was something else - I was wrong).

author: ledtester
author_rank: Super Contributor
timestamp: September 29, 2020, 09:04:29 pm
content: I'm sure the +/- 8V means that the gate voltage should never rise more than 8 volts above the source or more than 8 volts below the source.I'm not sure that limits on Vds have have anything to do with Vgs limits.Try a lower voltage zener. I would also use a smaller gate resistor to make sure enough current flows through the zener for it to regulate. At least initially I would use at least 1 mA of zener current. It's possible you can get by with less.I postulate that you were seeing the 1.5V drop because the current was only flowing through the body diode -- just a guess though.

author: Zero999
author_rank: Super Contributor
timestamp: September 29, 2020, 09:34:04 pm
content: It's possible you've damaged the MOSFET, by applying more than 8V to the gate, but hopefully the low zener current would have reduced the voltage drop to a safe level. If you don't have a lower voltage zener handy, just use a potential divider to drive the gate with around 4V.MOSFETs often have very low gate-source voltages, compared to the drain-source voltage ratings. A part might be rated to VDS600V, but only VGS+/-20V. As a general rule, the lower the threshold voltage, the lower the gate-source voltage, but always check the data sheet.

author: ArthurDent
author_rank: Super Contributor
timestamp: September 29, 2020, 10:04:38 pm
content: Here's a good video on reverse voltage protection. Also to avoid having to worry about drops through your protection you could have sensing at the output of the protection (on the load) and not on the input to the protection.https://youtu.be/7Tk5ghH_U2shttps://community.keysight.com/community/keysight-blogs/general-electronics-measurement/blog/2018/01/25/remote-sensing-is-important-for-your-power-supply

author: bitman
author_rank: Unknown
timestamp: September 29, 2020, 10:32:20 pm
content: Quote from: ledtester on September 29, 2020, 09:04:29 pmIt's been my (perhaps false) understanding that the 100K resistor takes care of lowering the voltage applied to the gate to an acceptable level. That said, as it's connected the GND I'm not so sure that actually has an effect. Although I haven't seen it not work or get hot yet even under at 1.5A load from the DC Load.

author: ledtester
author_rank: Super Contributor
timestamp: September 29, 2020, 11:49:58 pm
content: The zener and resistor work the same as a zener voltage regulator circuit and you need both to limit the Vgs voltage. For instance, a 4V Zener will limit Vgs to 4V. The remaining voltage will be bled off by the resistor.However, for this to happen the Zener has to conduct current -- that is, the voltage across it has to be greater than Vz.When you applied the load I suspect the gate was already damaged and so the MOSFET didn't turn on. Current only ran through the body diode which dropped the source voltage to 10.5V -- not enough to force conduction through the Zener. Thus no current ran through the 100K resistor which means the gate was at ground and Vgs was -10.5V.If you use, for instance, a 4V Zener under load, I would suspect the source to be near 12V (or 12V - I*RdsON), the gate to be near 8V, the current through the Zener to be 8V/100K and the MOSFET to be turned on. In this case almost no current would run through the body diode because there is a lower resistance path through the MOSFET.

author: bitman
author_rank: Unknown
timestamp: September 30, 2020, 06:19:08 pm
content: Quote from: ArthurDent on September 29, 2020, 10:04:38 pm@greatscott is one of my favorite youtuber with great explanations and I've definitely viewed this video before. Since I'm not handling batteries I didn't see the sense line as important, but now that everyone is pointing out the Vgs max that seems to be "the" solution to make the MOSFET get in compliance. So I've put this into "version 2" things to do.Thanks for the sense-line link - there was a few things there I'd forgotten about and it actually now explains what I am seeing. I see a drop if I connect the DC load directly to the bench supply of about 0.4-0.5V and it's well explained using the keysight link you provided. I now see the same drop on my circuit under test so the circuit doesn't seem to drop anything in addition, and that's what I wanted.Btw. I never realized I should twist the cables for better signals. So that will definitely be a change on my bench. And my DP832 Rigol doesn't have a sense line -only my DC load does, so I'll just need to keep adding a few decivolts to my power input to make up for the wire-loss. Luckily most of what I do on the bench is below 1A - but there are exceptions like this one.Thanks again for the links.

author: bitman
author_rank: Unknown
timestamp: September 30, 2020, 06:24:25 pm
content: As I just wrote in another reply - the drop I see now corresponds to the drop I see when I hook the DC load straight to the bench supply, so I don't see a drop that could only be explained as coming from the circuit - it drops to about 11.5V under 1.4A with or without my circuit. I may need to consider better (shorter) wires for power/test on the bench but I do like the flexibility 1m gives me.I did order better p-channel mosfets that would fit my 12V input - but for now, this seems to work. Perhaps I'm just lucky with my 2305 - not sure. But as I move on I'll definitely keep an eye on Vgs - won't make that mistake again. Thank you for valuable feedback and suggestions here.

author: Jwillis
author_rank: Super Contributor
timestamp: September 30, 2020, 09:30:51 pm
content: I agree that my knowledge is limited but with that circuit you would be running that FET in linear mode . which would mean that at 12V the maximum Drain current would be only around 40 to 50 mA . Even a 10S pulse wouldn't get to 1 amp . You would only get above 1 amp if the gate was pulsed at 10mS . But I'm probably not looking at it right . Maybe some one could help me understand it correctly.

author: eblc1388
author_rank: Frequent Contributor
timestamp: October 01, 2020, 07:07:22 am
content: Quote from: bitman on September 30, 2020, 06:24:25 pmThat voltage drop 0f 500mV seems too much.Your post prompted me to check how much my bench supply leads drop would be. Under 2A load, the total drop is 50mV. Its length is 1.5 meter for both positive and negative, so 25mV each under 2A load. I used 12AWG silicone wire to make them. I always use thick wire to make supply cables and never ever have to worry about voltage drops in doing my other experiments.

author: bitman
author_rank: Unknown
timestamp: October 01, 2020, 02:20:33 pm
content: Quote from: eblc1388 on October 01, 2020, 07:07:22 amI have about 1m on both the PSU and the DC Load so the total length of wire is about 2m - it may be closer to 2.5 since I haven't gotten the measuring tape out. I've used 14 AWG stranded wire - not using silicone yet but I've considered it as the cables a a lot more flexible. Doing a bit of calculation I see you're right - with `14AWG I should only expect 29mV but at this point it has nothing to do with the circuit I posted here. So I need to go back and see how precise my relatively new Rigol DC Load is (DL3021).Thanks - your post prompted me to reach out to my trusted calculator app and attempt to verify my numbers. And you're absolutely right, they're off by magnitudes. So "back to the bench" to figure out what is going on.

author: bitman
author_rank: Unknown
timestamp: October 02, 2020, 03:11:01 am
content: Last update - just another thanks for the input on this thread. Had I not read all the observations here I would be rather clueless to why things go from "working" to absolutely not working with very high current draws within a minute. I cannot wait till I get a chance to use my replacement p-channel and with the sensor line (voltage divider) to keep Vgs within the limits. Unfortunately it will have to wait a week or so.Anyway - thank you! I wouldn't have been able to figure out why things did what it did without this forum.

author: slburris
author_rank: Frequent Contributor
timestamp: October 02, 2020, 03:38:53 am
content: If you want some high end protection, look at the LTC4367 from Analog Devices. Overvoltage, undervoltage, and reverse polarity protection in one. Pricey because it’s AD, but I’ve done them in a few of my circuits and I’m happy with them.