Does this idea to remove the mosfet gate drive in a load tester make sense? - Page 1
Forum section: electronics / projects

author: Davidwasalreadytaken
author_rank: Contributor
timestamp: March 13, 2025, 06:05:44 pm
content: In a DC load tester, with the mosfet operated in its linear region as the load for an external DC power supply (as the DUT.)As a way to limit the current through the drain/source of the mosfet: have the voltage across the sense resistor act as the base drive for a baby NPN. something like 6 or 7 amps across a 0.1 ohms resistor should start the transistor flowing current. The collector is tied to the gate drive of the MOSFET, and conducts the gate's drive signal to ground-ish levels, or at least low enough for the FET to stop passing current. The transistor has to dissipate whatever current the opamp can supply but that's ok.I thought of this as a way for me to stop blowing up MOSFETs in my load, as a hopefully-instantaneous and brute-force way to make sure each load MOSFET (many, in parallel) doesn't pass too much current. No matter how high Istupidlyaccidentally manage to set the current requirement for the opamp driving the load.Is this practicable? Is there a name for this sort of thing? All resistor values are off the top of my head, please mention if they ought to be different! I take it for granted I have missed something, what else do you see here?

author: youngda9
author_rank: Regular Contributor
timestamp: March 13, 2025, 08:39:47 pm
content: In your circuit, what you're calling VSET is really ISET. You already have a circuit that will control the current based on the commanded input voltage.The added transistor is an additional Band-Aid that isn't needed. Also, to develop 0.6V at 6A requires 3.6W of power in the current sense resistor. That's quite a lot of power and completely unnecessary in this type of circuit.Having said that, there's no harm in having it there if your sense resistor and MOSFET can handle the power dissipated with 6A flowing through them if linearly clamped.

author: Davidwasalreadytaken
author_rank: Contributor
timestamp: March 13, 2025, 08:54:54 pm
content: I had to call it something, and as you note it is commanding based on a voltage, so VSET works fine.If you have something cheaper and simpler that will act as an instantaneous current limit faster than the existing fuses that are failing to protect my circuit, please suggest it. "Be more careful with the big knob on front of your load" is (obviously) not working out for me.I have plenty of Dale RH-25 0R1 resistors, and the entire point of this system is to blow off energy as heat so ... I'm not really worrying about that!

author: youngda9
author_rank: Regular Contributor
timestamp: March 13, 2025, 08:57:53 pm
content: Not sure why you're blowing stuff up. Please post the exact schematic (with part numbers) of your circuit.Have you checked for stability of the amplifier loop? There aren't an compensation components in the feedback.What is the input voltage that you are pulling current from?The amplifier loop should be very fast...start at 0v input and ramp up to the desired load current.

author: langwadt
author_rank: Super Contributor
timestamp: March 13, 2025, 08:59:10 pm
content: just limit VSET ...

author: Davidwasalreadytaken
author_rank: Contributor
timestamp: March 14, 2025, 06:07:50 pm
content: If I could just gently nudge everyone back in the direction ofdoes this idea make sensethat would be spiffy. Let's not get sidetrackedredesigning my entire shitover here k thanks

author: langwadt
author_rank: Super Contributor
timestamp: March 14, 2025, 06:10:20 pm
content: Quote from: Davidwasalreadytaken on March 14, 2025, 06:07:50 pmNo ...

author: schmitt trigger
author_rank: Super Contributor
timestamp: March 14, 2025, 07:32:55 pm
content: It makes sense, in limited circumstances where the main control loop “loses it”.A scenario I can think of is: if the Vset is enabled, but no load is connected, the opamp will wind-up because there is no voltage developed across the sense resistor. If a device is now suddenly connected, it will find a full-on Mosfet, essentially a short circuit.Of course the opamp will attempt to correct the situation but the loop compensation capacitor (not shown in your circuit) will slow down the response significantly.In this instance the NPN transistor will clamp down the gate drive much faster than the opamp.

author: Ian.M
author_rank: Super Contributor
timestamp: March 14, 2025, 07:43:26 pm
content: +1However 'fighting' the OPAMP output to clamp the MOSFET drive is not a good way to solve this problem. You need to prevent the OPAMP railing in the first place. See my comments here about other ways to avoid this fatal flaw of simple OPAMP + MOSFET constant current loads as described by 'schmitt trigger'.https://www.eevblog.com/forum/projects/solid-state-current-limit-for-my-active-load-maybe-a-simple-question/You may also wish to limit the maximum Vset according to Vload, and maybe even heatsink temperature. to provide SOA protection.