20A+ Current sink - Page 1
Forum section: electronics / beginners

author: IvoS
author_rank: Frequent Contributor
timestamp: July 04, 2014, 11:59:35 am
content: I built smaller size current sink based on Dave's design with MTP3055 output transistor that works beautifully, I used LM6132 rail to rail opamp.I am trying to design current sink for about 20A and higher current, but I am not sure about the last stage, especially how to parallel the output MOSFETs. I want to have 2 or 4 output MOSFETs in bigger package (TO247 better heat dissipation..) but I don't know how to drive them properly. These IRFP140N have higher gate capacitance, so I am not sure about oscillation, stability and other issues. Do I need to have R19 and R20? Thanks in advance for any help.

author: mij59
author_rank: Frequent Contributor
timestamp: July 04, 2014, 12:31:21 pm
content: About 20 A and higher.How much higher at what voltage ?Download the service manual for the Agilent 60503B and take a look at the circuit for the mosfets.Each mosfet has its own control circuit.R19 and R20 prevent oscillation and balance the current between the mosfets. ( more or less )The mosfet must be operated within the maximum safe operating area ! check the data sheet.

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: July 04, 2014, 12:45:44 pm
content: Hi,I showed how to use 2 MOSFETs in this thread:https://www.eevblog.com/forum/projects/dynamic-electronic-load-project/You can expand this to 4 or more by copying the MOSFET op-amp circuit. Each MOSFET is controlled separately. This forces sharing between them.Regards,Jay_Diddy_B

author: IvoS
author_rank: Frequent Contributor
timestamp: July 04, 2014, 02:04:43 pm
content: Quote from: Jay_Diddy_B on July 04, 2014, 12:45:44 pmVery nice!Thanks a lot. That's exactly what I was looking for but couldn't design myself.

author: IvoS
author_rank: Frequent Contributor
timestamp: July 04, 2014, 02:08:31 pm
content: Quote from: mij59 on July 04, 2014, 12:31:21 pmVoltage anywhere between 5 to 40V. I don't need to go higher than 40A. I was thinking to start with 20A and have and option for another 20A MOSFET expansion module or something like that. I will look at that Agilent manual as well. Thanks.

author: mij59
author_rank: Frequent Contributor
timestamp: July 04, 2014, 02:38:24 pm
content: Voltage anywhere between 5 to 40V. I don't need to go higher than 40A. I was thinking to start with 20A and have and option for another 20A MOSFET expansion module or something like that. I will look at that Agilent manual as well. Thanks.You need to specify the max power rating you need, 40V at 20A means you need to dissipate 800W ( 11 mosfets ?)

author: Spikee
author_rank: Frequent Contributor
timestamp: July 04, 2014, 03:39:50 pm
content: In a proof of concept dummy load i used the IRF064N before. I had 4 of them mounted on a pc cpu cooler (coolermaster z600). And the maxixum i ever tried was 63A @ ~8V from a lead acid battery. The problem i had was that the wires (2.5mm^2 solid copper) got really hot and the insulation started to melt.To get 5-40V and 0-20A shoulden't be that hard. Take a look at the IXYS extended linear fet's.Something like the: IXTN200N10L2 (in SOT227) package you can mount on a big fan assisted heatsink and use copper bars to connect to the load.

author: IvoS
author_rank: Frequent Contributor
timestamp: July 04, 2014, 07:06:57 pm
content: This MOSFET cost $37 on DigiKey....I think I would rather go with low cost multiple transistors instead.

author: rob77
author_rank: Super Contributor
timestamp: July 04, 2014, 07:32:00 pm
content: are you aware of the 1,6kW heat dissipation in case of the 40V @ 40A ? that would require a serious liquid cooling with a huge radiator

author: IvoS
author_rank: Frequent Contributor
timestamp: July 04, 2014, 08:20:23 pm
content: Quote from: Jay_Diddy_B on July 04, 2014, 12:45:44 pmI looked at your design and can not figure out what's the gain of U1D & U1C opamps driving the MOSFETs in inverting configuration. What resistors determine that?

author: IvoS
author_rank: Frequent Contributor
timestamp: July 04, 2014, 08:21:39 pm
content: Quote from: rob77 on July 04, 2014, 07:32:00 pmI am aware of that. I don't have heat sinks close to that size. I will stick with 20A.

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: July 05, 2014, 12:27:36 am
content: Quote from: IvoS on July 04, 2014, 08:20:23 pmHere is the schematic for the amplifier, U1D and U1C are identical:The op-amp is in equilibrium when the current in R3 and R4 are equal in magnitude.V1, the reference voltage is negative. With the values shown a reference of -1V volt needs 0.1V across the shunt resistor R2.So the complete circuit has a gain of 1A per -1V of reference voltage.The AC gain is determined by BW= 1/ 2 x Pi x C1 x R3 = 220 kHzHere is the AC model:and the frequency response:Does this answer your question?Regards,Jay_Diddy_B

author: IvoS
author_rank: Frequent Contributor
timestamp: July 06, 2014, 12:18:37 pm
content: Yes, thank you.