Finishing the active load - Page 1
Forum section: electronics / projects

author: Circlotron
author_rank: Super Contributor
timestamp: June 17, 2016, 01:12:52 pm
content: Back in 2004 I stated making an active load for testing power supplies. Never did get it finished. Dragged it out tonight for a look-see. The mosfets are ST Y32NB50. 500V 34A 0.11 ohm TO-247 max. They were a bit of a dog for SMPS PFC usage so I ended up getting a bunch of them free. Each source lead has a pair of paralleled 4R7 2W resistors. 15K gate resistors to make it very docile. I'll run a 1K resistor from each of the sources and join them together and use that common point to measure the current and send it back to an opamp. Each heatsink should be good for 300W continuous, lots more for short bursts. Each mosfet has a rating of 450W @ 25 deg C, derate 3.6 W per deg C.

author: ZeTeX
author_rank: Frequent Contributor
timestamp: June 17, 2016, 01:52:15 pm
content: Where did you got those heatsinks?

author: uncle_bob
author_rank: Unknown
timestamp: June 17, 2016, 04:29:57 pm
content: Quote from: ZeTeX on June 17, 2016, 01:52:15 pmHiThey look a lot like some I have from Heatsink USA.http://www.heatsinkusa.comThere are an almost infinite number of web based outfits that now will sell you short chunks of pretty good extrusion at reasonable prices. For one off type projects they beat the old "buy 50 feet minimum and machine it" approach.Bob

author: ZeTeX
author_rank: Frequent Contributor
timestamp: June 17, 2016, 08:57:29 pm
content: Quote from: uncle_bob on June 17, 2016, 04:29:57 pm42$ shipping for 5$ heatsink.. come on

author: uncle_bob
author_rank: Unknown
timestamp: June 17, 2016, 09:01:27 pm
content: Quote from: ZeTeX on June 17, 2016, 08:57:29 pmHi$6 shipping for $5 heatsink. There are lots of similar places all over the world. Pick one that is close to you.Bob

author: Circlotron
author_rank: Super Contributor
timestamp: June 18, 2016, 02:20:41 am
content: Quote from: EmmanuelFaure on June 17, 2016, 04:11:02 pmI built the entire thing out of unwanted samples and production leftovers at work. Yeah, the mosfet count is a bit over the top, but nothing exceeds like excess.Mosfets are great as a current sink because once you are above a certain current the characteristic curve is almost horizontal so changes in drain voltage mean almost no change in drain current. Of course, having so many mosfets means that it is just that much harder to get up on the horizontal part of the curve...

author: uncle_bob
author_rank: Unknown
timestamp: June 18, 2016, 02:34:01 am
content: Quote from: Circlotron on June 18, 2016, 02:20:41 amHiFor most supply testing, having a load that behaves like a resistor is a handy thing. It's also a nice thing to be able to test with a current sink as a load. The each may / will do different things to supply stability.Bob

author: diyaudio
author_rank: Frequent Contributor
timestamp: June 18, 2016, 10:51:15 am
content: Quote from: Circlotron on June 17, 2016, 01:12:52 pmNice work on the mechanical, hate to be the one to repair it, could have used a PCB with those current bus bars.450W @ 25 deg C, derate 3.6 W per deg C. However it says nothing when using the Mosfet in its linear region.. whats your SOA calculations per FET for a total of 300Watts ?

author: TiN
author_rank: Super Contributor
timestamp: June 18, 2016, 11:31:46 am
content: I have some doubts regarding those heatsinks and fans depicted to handle 300W continuous, based on previous experience with similar DC loads. Should be more like CPU/GFX fansink type or you will need really fast and noisy fans. There is just barely enough surface area.

author: Circlotron
author_rank: Super Contributor
timestamp: June 18, 2016, 03:02:02 pm
content: ^^ The 300 watt estimate was based on an experiment with a single fan and a heat sink section slightly larger than the fan. Also a single mosfet in the centre. It would do 100 watts continuous but it was fairly working, and 150 watts was pretty rude. Got hot enough to smell it. Put the same mosfet on a 150x70x10mm aluminium slab with no fan and put 150 watts into it and eventually the slab got hot enough to melt solder. Ran it for 1 hour and it made a big scorch mark on the bench top but again it survived. Not the sort of thing you would do if you wanted a reliable commercial product though.

author: Circlotron
author_rank: Super Contributor
timestamp: June 19, 2016, 06:12:42 am
content: Okay, here it is running. I didn't end up using the opamp feedback circuit, just stuck the output of a 0-20V psu that has a 20 turn pot into the gates. About 5-7 volts is the approximate working range; the transconductance is mostly set by the paralleled source resistors. From cold to hot it drifts from 39 to 40 amps. Good enough for my application. A bit over 600 watts in the picture. After about 3 minutes at 15 deg C ambient the heatsink got to about 60 deg C.