quick question:  what power device for 3A 75W const current sink? - Page 1
Forum section: electronics / projects

author: max_torque
author_rank: Super Contributor
timestamp: September 27, 2015, 07:38:21 pm
content: Anyone got a recommendation for which power device to use for a 3A 75W low side constant current sink?I need to power it with 24Vdc, and have it constantly sink 3A, so there's going to be around 75W being dissipated in the power silicon!Can i do this with one device? I suspect not. Size isn't an issue (it's for a large industrial rig) so i can use a massive heatsink and fan etc. It also won't have to run in any ambient air temp more than room temp (say 30degC max).I guess at these currents a FET based device means i don't have to compensate for the gate current like in a NPN style transistor, but this is a LOT of power to linearly dissipate in FET!Could just go "old skool" with a row of 3 or 4 T0220 NPN's ;-)Suggestions anyone?

author: max_torque
author_rank: Super Contributor
timestamp: September 27, 2015, 07:40:42 pm
content: Actually, come to think about it, perhaps i could dissipate most of the power in a large fixed wire wound resistor, and then just use the power silicon to "trim" the current, and hence only get a share of the power dissipation??

author: Fungus
author_rank: Super Contributor
timestamp: September 27, 2015, 07:44:49 pm
content: Quote from: max_torque on September 27, 2015, 07:38:21 pmDatasheet says a bog-standardIRF540can dissipate 150W.Quote from: max_torque on September 27, 2015, 07:38:21 pmWould also work.

author: Fungus
author_rank: Super Contributor
timestamp: September 27, 2015, 07:48:32 pm
content: Quote from: max_torque on September 27, 2015, 07:40:42 pmOnly if the input voltage is constant (I got the impression it wasn't - usually the input voltage is unknown when people say "constant current sink").

author: mzzj
author_rank: Super Contributor
timestamp: September 27, 2015, 07:57:29 pm
content: IRF540: 150W Not in real life.This has been discussed more than once, but to make it short:-Preferably use mosfet specified for linear operation ie:http://www.ixys.com/Documents/Articles/Article_Linear_Power_MOSFETs.pdfIf cost or availability is a concern select:Older generation mosfet with huge headroom for voltage rating. IE something like IRFP460http://www.infineon.com/dgdl/Infineon+-+Application+Note+-+PowerMOSFETs+-+OptiMOS%E2%84%A2+-+Linear+Mode+Operation+and+SOA+Power+MOSFETs.pdf?fileId=db3a30433e30e4bf013e3646e9381200

author: microbug
author_rank: Frequent Contributor
timestamp: September 27, 2015, 08:03:29 pm
content: As mzzj said, IXYS Linear L2 MOSFETs. They are designed to continuously run in the DC range. Stick a CPU heatsink / fan on one and you're good to go.

author: Fungus
author_rank: Super Contributor
timestamp: September 27, 2015, 08:08:13 pm
content: Quote from: mzzj on September 27, 2015, 07:57:29 pmThat's in ideal laboratory conditions, obviously.Point is: This isn't a really difficult thing to do as he was implying.He only wants half that and he can probably use two of them.

author: max_torque
author_rank: Super Contributor
timestamp: September 27, 2015, 08:21:31 pm
content: Supply will always be 24Vdc +-1v and i'm just trying to maintain a constant 3A current. If i use a 7R fixed resistor, that'll take the bulk of the power, meaning the pass device will be much less thermally loaded!The existing rig just uses a fixed 8ohm resistor, but as you would expect, the current changes with the temperature of that resistor

author: Siwastaja
author_rank: Super Contributor
timestamp: September 28, 2015, 07:22:53 am
content: Of course use the resistor if the voltage is fixed and known. It will take the silicon power dissipation down, and it'll also stabilize the control loop.

author: Fungus
author_rank: Super Contributor
timestamp: September 28, 2015, 10:16:30 am
content: Quote from: max_torque on September 27, 2015, 08:21:31 pmIn that case just get yourself some big resistors.eg. Three of these in series bolted to a big block of aluminum:http://www.ebay.com/sch/i.html?_nkw=2.2-ohm+100+watt+power+resistorIt doesn't get much more reliable than that. They go down tofractions of Ohmsso you can tune it quite well. Maybe you only need 50W size, it depends on how many you use and how much space you have.You only need MOSFETs and stuff if you wantprecisely30A.

author: max_torque
author_rank: Super Contributor
timestamp: September 28, 2015, 11:59:04 am
content: I need some level of control to trim the current to as close to 3A as possible, because i am measuring the voltage drop on across a switch under that load to determine the switches resistance. With the current setup, that is just using fixed resistors, the current changes as those load resistors warmup, and as the PSU fluctuates a bit.

author: Fungus
author_rank: Super Contributor
timestamp: September 28, 2015, 02:32:45 pm
content: Quote from: max_torque on September 28, 2015, 11:59:04 amOK, so send most of the current through a couple of those big resistors and use a MOSFET/potentiometer to trim the other 200mA (or whatever). nb. This assumes you can see the current on a meter.If you want to automate it you could put a current sensor on the wire and use an Arduino to PWM control the MOSFET (put a resistor and inductor in series with the MOSFET to smooth out the current flow through that path). You should be able to get 1% accuracy even with a cheapo sensor.

author: Kleinstein
author_rank: Super Contributor
timestamp: September 28, 2015, 06:22:19 pm
content: If one dissipaes something like half in resistors, one could get a way with single MOSFET like IRFP250. If one prefers BJT for most of the power one can use a MOSFET to drive the BJT in a kind IGBT like circuit. For 75 W even a 2N3055 might be good enough.

author: Fungus
author_rank: Super Contributor
timestamp: September 28, 2015, 06:37:34 pm
content: Quote from: Kleinstein on September 28, 2015, 06:22:19 pmIn theory an ordinary $0.15 BJT is enough for the trimming... they can vary a current by 0.5A, no problem.