DC Electronic Load - Page 1
Forum section: electronics / beginners

author: KW
author_rank: Newbie
timestamp: November 06, 2018, 04:05:10 pm
content: Hi...I am trying to construct a DC electronic load. The circuit diagram is attached. The opamp in the circuit is configured as a voltage follower.In this post, the resistor (HSA50R10J) is denoted as RM, whilst the voltage across the resistor is VM. Ideally, VM should track and be identical to Vset. Hence, the load current (ILoad) is given by Vset/RM.When TL081 is used, the current increases linearly with Vset (see first graph in "EEV Blog 2.png"). However, when Vset is greater than 0.45V, the load current starts to saturate.When the opamp in the circuit is replaced with TL071, the current also increases linearly with Vset (see 2nd graph in "EEV Blog 2.png"). In this case, the current starts to saturate at a higher Vset, i.e. 0.6V.Ideally, the voltage at the non-inverting terminal of a negative feedback opamp circuit should be identical to that at the inverting terminal. However, as can be deduced from the experimental results ("EEV Blog 2.png"), the voltage at the inverting terminal can no longer follow up the increasing voltage at the non-inverting terminal after Vset is greater than 0.45V (in the case of TL081) or 0.6V (in the case of TL071). Consequently, the load current starts to saturate after Vset = 0.45V or Vset = 0.6V.Kindly advise why these anomalous behavior (e.g. voltage at opamp input terminals becomes different, current starts to saturate, etc.) happened.

author: t1d
author_rank: Super Contributor
timestamp: November 06, 2018, 04:15:33 pm
content: Search this forum for "Jay-Diddy-B"'s electronic load threads. He does a good job of explaining e-load operations and he offers some great designs.I have just finished building an adaptation of his design. I supplied the schematics and KiCad files.There are other e-load designs, on this forum, if you want to search them.https://www.eevblog.com/forum/projects/electronic-load-project-ltv171-irfp250-with-kicad-files/

author: ArthurDent
author_rank: Super Contributor
timestamp: November 06, 2018, 04:32:37 pm
content: I found this chart on the spec sheet for your MOSFET, which looks a lot like your graph. Try putting another IRF620 in parallel and see if that fixes the problem.

author: Zero999
author_rank: Super Contributor
timestamp: November 06, 2018, 04:39:01 pm
content: Quote from: KW on November 06, 2018, 04:05:10 pmThe figures you've given in your post are1/10those on the graphs.If you're getting between 4A and 5A, then it's because MOSFET isn't turning on enough. Look at figures 1 & 2 on the data sheet. The op-amp won't be able to drive the gate with the full 15V supply and you shouldn't expect more than around 4A with 12V of gate drive and drain voltage of 12V, especially at higher temperatures.http://www.vishay.com/docs/91027/sihf620.pdfUse a MOSFET with a lower on resistance and higher current rating or several in parallel, as mentioned above.Are graphs are the simulated values and the figures given in your post the measured values?If so, then it's probably oscillating and your meter is not giving you the correct readings. An additional RC circuit will be required to stabilise the circuit.See the thread linked below for an example.https://www.eevblog.com/forum/projects/linear-led-driver-with-mosfet/msg1796558/#msg1796558

author: HB9EVI
author_rank: Frequent Contributor
timestamp: November 06, 2018, 06:21:52 pm
content: my guess'd be oscillations as well, if the opamp is not broken.- I don't see any decoupling caps on both opamp rails- I don't see a compensation networkso almost certainly oscillations

author: MarkF
author_rank: Super Contributor
timestamp: November 06, 2018, 07:51:42 pm
content: Here is what I did:

author: KW
author_rank: Newbie
timestamp: November 07, 2018, 10:41:23 am
content: Quote from: Hero999 on November 06, 2018, 04:39:01 pmHi... Thanks for your reply. The graphs show the experimental results.By the way, the original post has been rephrased to describe my problem in more detail.

author: KW
author_rank: Newbie
timestamp: November 07, 2018, 10:43:16 am
content: Quote from: HB9EVI on November 06, 2018, 06:21:52 pmThanks for the suggestion. I will add decoupling caps (0.1uF) on both opamp rails. I will also learn how to add a compensation network to my circuit.

author: capt bullshot
author_rank: Super Contributor
timestamp: November 07, 2018, 10:52:54 am
content: Your circuit just works as it is expected to.The MOSFET reaches its saturation current, this is why the current doesn't follow the setpoint anymore at some value, and the different values for TL071 and TL081 are the result of real life tolerances. One of the OpAmps is able to put out a bit more voltage at the upper end, so the gate voltage is a bit higher shifting the transistors saturation point a bit higher resulting in a somewhat higher saturated current.Anyway, it's good practice to put a series (100R ballpark) resistor into the gate, and use some compensation network and decoupling. Did you check your circuit for oscillations with a scope?

author: KW
author_rank: Newbie
timestamp: November 07, 2018, 01:24:17 pm
content: Quote from: capt bullshot on November 07, 2018, 10:52:54 amThank for the suggestion. I will try to check the circuit for oscillations with a scope.

author: brybot
author_rank: Contributor
timestamp: November 07, 2018, 07:11:41 pm
content: Expanding on 'capt bullshot' post:I also believe you're operating in saturation.The IRF620 is rated for 5.2A continuous, at 25C, or 50W maximum power dissipation. When you're hitting 5A at 12V, that's exceeding the maximum rating of the device, which also assumes a really good heatsink.I'm guessing your FET is heating waaaaay up. Probably over 100C, at which point the continuous current rating goes way down. And peak wattage goes down at 0.4W/C for this chip.Check out Fig. 4 of the datasheet. Even if you can turn your FET on fully and achieve the 0.8ohm Rdson, the heat is going to push it up and limit your maximum current.Fig. 1 shows (at 25C) that you need relatively large Vgs to avoid saturation in your application. The IRF620 is designed for fast switching and low on resistance, which probably means it's not ideal as a load, unless you put quite a few of them in parallel with one another.