0-30A 50V Eload - Mcp6v27@IRFP064N (300W) - Page 1
Forum section: electronics / projects

author: eevblogfan
author_rank: Frequent Contributor
timestamp: January 02, 2013, 07:49:15 pm
content: good evening any oneI decided to build some 300W electronic load .the specs are :1)220V powered2) 0-30A range3) some were at the 50V level ..4) UVP (variable)5) OVP6) accuracy .... I thought about somewere in between 1~ 0.1% (CC mode) and If I'll go a head and do CV ... then ... 0.5~0.1% for the CV modenow ... I have built this following circuit :

author: eevblogfan
author_rank: Frequent Contributor
timestamp: January 02, 2013, 08:03:29 pm
content: and cannot understand ... I know ther's a problem ... But I just can't think ...let me tell you ... when the current exceeds 3A .... something goes wrong ... and both mosfets will fried out ..can someone please tell me . either draw me . the correct schema ?BTW .. notice the R1/ R2 / and VR1 ... they have no spec on the schema ... that because I don't know what will fit correct ..there's gonna be . 3 unit's of mcpv627 ( Ie 6 op-amps ) hence 6 mosfets ..I figure 2 CPU heatsinks will gonna make it ( I ren it 24Hr ... with 75W load . and with 150W .. I think it was like 82C or so ... But I may be wrong ... )the Vout from the trim pot will gonna be 0-50mV / 0-16.66mV ( set by closing one switch )now I don't understand why ... but when shorting the trim pot to the GND and then connecting the V+ through resistor ( to limit max Vout/-Ie Cout) the Vout becomes noisy ...thank you in advance

author: Spikee
author_rank: Frequent Contributor
timestamp: January 02, 2013, 08:07:02 pm
content: You wont get 300W out of that mosfet. I use the same fet and a 0.21 deg C/W heatsink (very good) and the max you can get out of it safely is like 110W . I did the thermal calculations before but i dont want to type that here. You could make the circuit three times and parallel it and you are done.You need to decide what kind of range you want to test . If you use a 1 ohm resistor it really limits high currents and low voltage ( < 10V ) .So for low voltage high current you need a 0.1 ohm resistor or lower.So a ballpark of the parts you need:3x artic freezer 7 pro rev 2 (or similar) heatsink total: 90 dollar ( you can insulate them and use 2 heatsinks but that also has an thermal effect)tube of thermal adhesive ~ 10 dollar3X 100W 1ohm resistor ~ 20 dollar10 turn pot ~ 15 dollarmosfetsetc ...ballpark cost: 135-160 dollarI use the l2722 1A power opamp because i had it laying around and the LM324N would die after a while.when i replaced it with the L2722 the problem was fixed and it is working perfectly since than.My topic: you can get alot info therehttps://www.eevblog.com/forum/projects/did-i-forget-anything-constant-current-load/

author: eevblogfan
author_rank: Frequent Contributor
timestamp: January 02, 2013, 08:13:00 pm
content: hello spikeeI thank you for the postattention to details :ther's gonna be 6 Nmos .. Ie 33.333W per device ..ther's problem with this circuit ...the shunt is 10mohm ... so no problem with low VoltageI have already 8+ heatsinks ... for freeand just for fun . let me tell you ... It maneg to work 24Hrs with 200W load ( 11.5V*17.3A )

author: Spikee
author_rank: Frequent Contributor
timestamp: January 02, 2013, 08:22:26 pm
content: Why use more mosfets , opamps ? it only makes it more difficult and expensive.I dont know what kind of setup you are currently running but thermal calculations are always handy and not difficult.Dave has a video about it or just use an Android app like ElectroDroid (free) .

author: eevblogfan
author_rank: Frequent Contributor
timestamp: January 02, 2013, 08:30:56 pm
content: The setup now . is 1 op-amp ... 1 Nmos .. 1 heatsink . and one 10mohm Multimeter shunt ( the 10$ stuff ... )with only one from each .... It worked extremely wellbut when added the second one ... Boom ...I decided to build it robust ... 33W per mosfet is overkillOVP and UVP and maybe OCP ... well I forgot OTP ... But all of these ... will be attached When I'll finish the very basic circuit ..I guess I'll go asleep .. Good day/night any one

author: Spikee
author_rank: Frequent Contributor
timestamp: January 02, 2013, 08:39:57 pm
content: Did you isolate the two mosfets ?else the drains will be connected via the heatsink and give you all sorts of problems.

author: eevblogfan
author_rank: Frequent Contributor
timestamp: January 02, 2013, 08:49:08 pm
content: Why shall I sleepLols ><"umm , nope . I did notanyhow ... the Drains are gonna be shorted on the output - Am I wrong ?BTW ... does someone have the BK Precision 8500 schema ?

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: January 02, 2013, 09:20:38 pm
content: Hi,To the original poster.See this thread:https://www.eevblog.com/forum/projects/did-i-forget-anything-constant-current-load/msg170808/#msg170808You need to add some components around the op-amp to control loop gain.Jay_Diddy_B

author: baoshi
author_rank: Regular Contributor
timestamp: January 04, 2013, 06:10:36 pm
content: Quote from: Spikee on January 02, 2013, 08:39:57 pmHi Spikee, can you please explain what sort of problem? Aren't drain supposed to be connect? I'm worried because I'm building something similar.

author: Niklas
author_rank: Frequent Contributor
timestamp: January 04, 2013, 09:29:51 pm
content: It would be difficult not to connect the drain pins/tabs on these N-channel MOSFETs... However, the source pins must be separated. I first misread the schematics and thought the MOSFETs were P-channel ones, mainly because of the high side positioning. If they were only low loss load switches that would have made sense. Assuming P-channel, then the drains would be isolated instead.

author: CarlG
author_rank: Regular Contributor
timestamp: January 04, 2013, 10:14:45 pm
content: Quote from: eevblogfan on January 02, 2013, 08:03:29 pmIs the OPA capable of driving the MOSFET gate capacitance? You'll need to choose you OPA carefully, otherwise it will oscillate.It might well be that that kills your MOFETs.Edit: I've checked; you'll need at least 200 ohm at the OPA output.Where's the OPA's decap?Where's the V+ going? (I would guess +5V)Where's the V- going? It looks like the V- of the OPA and the low end of the sense resistor is floating. (But I assume V- is connected to GND.)How is V- connected to the GND at the 390pF? (Make sure the 390 pF GND is close to the OPA V-)How is you're DUT's GND connected? Make sure that there's RI drop from Rsense bottom to the 390pf and V- of the OPA.I would also advice separate caps on each of the OPA non-inv inputs to reduce the risk of oscillation since the "driver" resistance (i.e. the pot) is high (I'd guess).//C

author: CarlG
author_rank: Regular Contributor
timestamp: January 04, 2013, 10:59:56 pm
content: Quote from: eevblogfan on January 02, 2013, 07:49:15 pmHow is that spec related to the schematic?Also, please try to keep the component type numbers correct (e.g. IRFP064N, not IFP064N, etc). It makes it easier to help you. And if you're going into the business you'll have to get it right, the sooner the better.//C

author: eevblogfan
author_rank: Frequent Contributor
timestamp: January 05, 2013, 12:57:26 pm
content: Quote from: CarlG on January 04, 2013, 10:14:45 pmQuote from: CarlG on January 04, 2013, 10:59:56 pm

author: CarlG
author_rank: Regular Contributor
timestamp: January 05, 2013, 01:57:48 pm
content: Quote from: eevblogfan on January 05, 2013, 12:57:26 pm[some answers inclued in the quote above]I write a lot about possible ways that your circuit may oscillate, even though it may not be the problem at all. But as we all know, an amplifier circuit have a tendency to oscillate...while an oscillator circuit have a tendency to notTo continue on the oscillation track, you might need to decouple the drain of the MOSFETs, to avoid the reactance of the DUT leads to react with the MOSFETs Miller capacitance.I'm a bit sceptic to the use of micropower OPA to control a power MOSFET, though I admit I haven't tested it myself. I'd use an OPA specified to drive heavy cap load, maybe something like LM8272 etc. Have you simulated the circuit, especially with transients on the DUT connection?//C

author: eevblogfan
author_rank: Frequent Contributor
timestamp: January 05, 2013, 02:14:52 pm
content: thank you Carlgnope , I haven't simulated .. good point . unless any one els can do it for me . I guess I'll start tomorrowI hope I'll be able to buy Oscope soon . I suppose It will help me a-lot .