High current drive - Page 1
Forum section: electronics / projects

author: ricko_uk
author_rank: Super Contributor
timestamp: August 10, 2021, 07:18:53 pm
content: Hi,I need to drive a high current load and set the current through a voltage coming from a DAC.I am thinking of using the classic op-amp driving a mosfet/bjt like in the attached schematic(consider the resistor that says load is just a current sensing resistor and the load is between the rail and the collector - that was just the first random image I found just to give a quick visual).The specs are as follows:- current can vary from 10A to 35A- frequency from DC to 50KHz- max load voltage in the system 28V (of little relevance)Initially I was thinking of using just a power mosfet so I can use any general low-current op-amp to drive directly, but than thought that perhaps a BJT/IGBT might work best. Any feedback on pros and cons of each given those design parameters?Thank you

author: TimFox
author_rank: Super Contributor
timestamp: August 10, 2021, 07:54:27 pm
content: That circuit will regulate the voltage across the load, not the current through the load.Also, the op amp needs to tolerate the load voltage.To stabilize the current, you need to compare the voltage across a “sense resistor” in series with the load.If you connected the load in the collector of your BJT it would be close to a stabilized current (except for the base current of the BJT), so a MOSFET with zero gate current would be preferred. The voltage across the sense resistor (source to ground) is less than the load voltage and should be within the op amp capability.Your requirements are somewhat severe, and need careful design, especially heat-sinking on the power device and frequency compensation/response.

author: ricko_uk
author_rank: Super Contributor
timestamp: August 10, 2021, 08:00:40 pm
content: Thank you Tim,I just took the first screenshot I found of a buffered transistor just to provide a visual and did not notice that the collector is connected to rail. It would obviously be connected to the load and the resistor shown in that image would be the current sensing resistor.The thermal is ok, I have plenty of heat sinking. The frequency response is something I was wondering about more...Thank you

author: TimFox
author_rank: Super Contributor
timestamp: August 10, 2021, 08:20:13 pm
content: With your high frequency requirements and the capacitance of high-power MOSFETs, the feedback stability will be a difficult (but possible) design problem.You might use a BJT darlington pair for lower base current (load on op amp and error in load current), but MOS is the obvious answer at DC.

author: TurboTom
author_rank: Super Contributor
timestamp: August 10, 2021, 08:39:14 pm
content: Have a good look at the DC SOA (safe operating area) of your power semiconductors. You may need several such current sources / sinks in parallel to cope with the gate threshold voltage tolerance of your power devices (if you are using voltage controlled semiconductors). Many power semiconductors nowadays are made for switching applications, and high, long-term power loads in the "resistive" area of their traces aren't permitted by the manufacturer. This is especially the case for most IGBTs.

author: ricko_uk
author_rank: Super Contributor
timestamp: August 10, 2021, 10:02:19 pm
content: Thank you Tom,do you mean multiple MOSFETS in parallel or multiple of the entire current source? I think that putting multiple MOSFETS in parallel would introduce less issues than making multiple sources in parallel... Or is that not correct?Thank you

author: TurboTom
author_rank: Super Contributor
timestamp: August 11, 2021, 07:19:26 am
content: Usually you cannot parallel several mosfets unless you use them in switching applications. Since every individual mosfet (actually even within a single mosfet does this happen since they are made up of a huge quantity of individual "cells") has got a slightly different gate threshold voltage, when in parallel operated in the "analog" region, there will be always one or a few transistors that take "all the beating", and they will fail eventually if overloaded. That's also the reason that nowadays many powerful mosfets that at first sight appear very capable, fail miserably if operated in that way.You may want to have alook at this topic, lots of valuable information IIRC.

author: ricko_uk
author_rank: Super Contributor
timestamp: August 11, 2021, 03:23:42 pm
content: Thank you Tom

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 11, 2021, 04:15:07 pm
content: Sure, BJTs are quite capable in this range. Audio amps can easily go higher in both frequency and voltage. You do need a strong enough driver; additional transistors are usually required, say in a biased Darlington configuration (or a full discrete circuit, because audio is just like that..).Note that, if your outline conditions are true, then maximum power is dissipated around half load, 245W. You'll need a couple transistors in parallel for that, and a fair sized heatsink, give or take if it's continuous or intermittent. If the load can take any impedance however, the worst case is a short circuit and the maximum dissipation is four times that or just shy of 1kW, a much bigger challenge!Note also that, for 50kHz at that current, and reasonable drive voltages (say <10% of total supply), you need less than 0.25uH in the output ground return path. This suggests noninductive resistors and short wiring, though obsessively short shouldn't be required. (This is equivalent to about 20cm of loose hookup wire.)Likewise, the maximum load inductance must be below 2.5uH, else the output will be clipping.Don't forget some kind of transient overvoltage protection. This can be as simple as a plain old TVS across the output, or a zener from input (gate/base) to output, etc. (Probably with a ferrite bead so it doesn't cause oscillation.)MOSFETs are easy to parallel if you have some voltage to spare: just put a relatively large source resistor under each, such that Vs > ΔVgs(th). The same is true of BJTs, but BJTs have a smaller ΔVbe between each other, so Re is just about inconsequential. For MOSFETs, this is reasonable practice at high voltages; at this voltage, it might be better to use independent servos (one amp per transistor) and only wire the outputs in parallel. The price is saturation voltage, in turn requiring higher supply voltage for the same load range, and bigger Rs's (more watts).Tim

author: David Hess
author_rank: Super Contributor
timestamp: August 11, 2021, 07:03:38 pm
content: Quote from: ricko_uk on August 10, 2021, 07:18:53 pmAt higher frequencies, either way will likely require more output drive than an operational amplifier can provide, but that just means adding some type of buffer between the operational amplifier output and the power transistor or transistors.