How to ADD a Output ON/OFF function to low cost power supply - SOLVED - Page 1
Forum section: electronics / general

author: --Oz--
author_rank: Regular Contributor
timestamp: August 13, 2021, 01:52:44 am
content: I have a 30v 10a 300w CV/CC power supply (~$50 on ebay) that I want to add a output on/off control function. These low cost supplies do not have a output ON/OFF function built in, so you need to either turn the entire power supply off or disconnect the banana connection and vise versa for turning the output back on.Goals:No bounce when output turns on or off (so no mechanical contacts for output current, IE: relay/switches/etc).EDIT: Control the positive output (not the ground connection).Simple KISS design (Keep It Super Simple and low cost, few dollars).Be able to control 0.1V to 30V and upto 10A.A toggle or push button switch is fine to activate the circuit.I am looking for a add on circuit to control (on/off) the main power supply + output and no switch debounce (clean ramp up/down).TIA!

author: station240
author_rank: Unknown
timestamp: August 13, 2021, 02:02:44 am
content: Just type the search term "hotbed mosfet" into ebay (or the like)Intended for switching a 3D printer's heated bed, you get a high current mosfet, heatsink, terminal block, drive circuit all on a PCB.Easily found for <US$5

author: NiHaoMike
author_rank: Super Contributor
timestamp: August 13, 2021, 02:29:31 am
content: Quote from: --Oz-- on August 13, 2021, 01:52:44 amThat sort of lab power supply is "floating" and there should be no difference between switching the positive and switching the negative.

author: --Oz--
author_rank: Regular Contributor
timestamp: August 13, 2021, 02:30:36 am
content: Thanks for the reply, but, those hotbed controllers are controlling the negative side of the supply to the hotbed, that is fine for a simple load. But if you do this with a supply powering a circuit with scope probes and other supplies with common ground connection, then you have ground loops. No regular supply controls the negative output, they all control the positive output.I added to my first post to clarify that requirement.

author: NiHaoMike
author_rank: Super Contributor
timestamp: August 13, 2021, 03:25:53 am
content: The negative of the output and ground can be detached from each other. In fact, you probably don't want it grounded by default since that will create ground loops.One simple high side switch that doesn't bounce would be a MOSFET with 2 or 3 lithium coin cells supplying the gate drive voltage. Use a small SPDT switch to switch the gate between the coin cells and shorting it to source. The coin cells will basically last their shelf life since the steady state gate current is virtually zero.

author: --Oz--
author_rank: Regular Contributor
timestamp: August 13, 2021, 04:59:54 am
content: @ NiHaoMikeWhat kind of switch does not bounce? (link)Or add a RC (~10K and 0.1uF) or?I am thinking n-fet on the high side, or are you thinking p-fet?PS, lets not get into the ground loop discussion, haha, thanks.

author: NiHaoMike
author_rank: Super Contributor
timestamp: August 13, 2021, 05:11:53 am
content: The gate capacitance of the MOSFET debounces the switch. When the switch moves over, the gate capacitance gets charged on the first contact so bouncing has no effect.

author: julian1
author_rank: Frequent Contributor
timestamp: August 13, 2021, 09:17:57 pm
content: This article discusses implementing SSRs using discrete back-to-back fets for clean audio output switching. For non bipolar, one could simplify to single high-side n channel fets.https://sound-au.com/articles/mosfet-relay.htmIt's a pity that opto-isolators with stacked photovoltaic outputs (that can drive fet gates directly) are not higher current - for better on/off gate switch times.

author: --Oz--
author_rank: Regular Contributor
timestamp: August 14, 2021, 06:43:58 am
content: @NiHaoMikeHere is the super KISS battery/switch/fet, it does the debounce well, better than I thought, good call, thanks for the info.The first pic is your setup, rise time is fast (not that it matters) into a resistive load.So it works as advertised for the this 5Vout and 2A setting (see first picture). Then I started to up the output voltage/current, when I got to 18v and 9A it blew the fet (new IRFZ48N 64A 55V 2nF gate to-220), drain-source=110Ω, gate-source=71Ω. The resistive load is 5Ω 100W resistors in a parallel bank of 10 with switches to enable each one (5Ω to 0.5Ω) and about 3 feet of speaker cable (inductance).I started again with a fresh fet and added a 16v zener across gate-source (I have blown fets from gate voltage spikes and drain avalanche in my 3.8Mrpm project), and again it blew the fet again around 18v 9A, so I am ruling out thats its over volting the gate (with the battery I didn't think so). I am not sure if its happening during turn on or turn off but I am leaning towards the fast turn off. I did not have scope probe on the drain-source to see what's happening, next time.Next I wasjust messing aroundafter replacing the fet, I played with a gate resistor, ended up with a 1K in line gate resistor, this cleaned up the wiggles on the turn on Vout rise and of course slowed down the rise time (not a problem), but it randomly did not debounce (it would charge a bit, then stay at that level then abit, then start to rise again a few times), I added a 0.1uF across gate/source and that helped quite a bit with the limited range I tested in my lunch hour and the output looks much more professional smooth ramp up and clean at the limited testing I did.I have another fet I want to try (STW80NE06-10 60V 80A 7.6nF gate TO-247) it takes a beating the in the avalanche area, haha.What other methods to protect the fet (diode DtoS, zener GtoD)?Blu=gate, red=Vout, green=Iout

author: --Oz--
author_rank: Regular Contributor
timestamp: August 14, 2021, 07:12:14 am
content: @julian1Will read your link soon, thanks!

author: --Oz--
author_rank: Regular Contributor
timestamp: August 19, 2021, 05:04:25 am
content: @julian1I read that IC and its a very interesting option, specially since the input is isolated from the output, so this can be driven off a supply voltage in the main supply, but this has to be found (easy for some) and depending on the supply, it maybe hit or miss. I ordered the part, but the slow boat may take awhile, thanks again for your input.

author: --Oz--
author_rank: Regular Contributor
timestamp: August 19, 2021, 06:15:28 am
content: @NiHaoMikeIrandomlystill got bounce issue with your circuit, but let me clarify, it was not shutting the fet on/off/on like normal switch contact bounce, it was just holding the last charged voltage. This was with a brand new C+K toggle switch. It seemed to get worse when I switched from the to-220 fet to a larger to-247 fet (with larger gate capacitance, 7.6nF). As you can see in this capture, its more like a peak hold (voltage) when the switch is bouncing, then resumes on contact, I did not like this random issue as it directly influences the output voltage and current.Early testing I was using 4V for the gate supply, but I quickly learned this is no good for higher current and after some testing at 10A, 7 to 8V was good to get the full output voltage at 10A. Raising it to 12V decreased rise time and increased output voltage overshoot which I did not like.I went back to my circuit and lowered the gate pull down resistor from 200K (was draining 21uA) to 5 or 10MΩ, this reduced the current draw so low I think it does not matter, but in the end i used a SPDT switch and used the other contact to discharge to gate, I ended up with 2K.I played with RC, CR, CRC (plus Gcap or Ciss) to control bounce and it does work but I was never happy with the output turn on voltage rise waveform, maybe I never hit the right combinations. Then that night I got the idea in bed to try acap across the switch contacts, then in the morning I thought thats not going to do anything, then reversed my thoughts, back and forth again, haha. Well it works very well to control bounce! But it had the fast rise time and not a very smooth waveform.To solve this I then tried R, RC, CR after the cap across the switch and testing different values. I then settled on 8K gate resistor and no cap after it (besides the Gate cap) because it was a good balance of turn on and minimum Vout overshoot. Well until I started testing above 20V output, at 30V there was overshoot with 8K, after some tuning I settled on 10K for best compromise from 0.1V to 30V at 20mA to 10A output with gate voltage of 8V.I left the 16V zener in there with the new fet, not sure if it really needs it. And last I added a output discharge resistor on the second set of contacts on the DPDT switch.Then I let it sit ON at 30V 10A for 12 minutes, the fet Vds=84mV, so it was burning 840mW, it was on a heatsink at the time and slightly got warm.Soon I will post my semi final schematic and some pics/results, thanks for your help.

author: --Oz--
author_rank: Regular Contributor
timestamp: August 28, 2021, 08:12:09 pm
content: Here is my final schematic.Pic# 280= 0.1v 180mAPic# 286= 5v 4.7APic# 294= 20v 4.7APic# 302= 30v 9.9A5v 9A Discharge30v 9.9A DischargeYellow=gate,Red=Vout,Green=Iout (current probe)

author: --Oz--
author_rank: Regular Contributor
timestamp: August 28, 2021, 08:44:37 pm
content: Couple internal pics and tuning setup, thanks for the help guys.