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voltage controlled current source

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jstamour802

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I need a scale-able high-current (5A) source at 48V supply voltage. I'm looking at this circuit from Linear Tech and it looks like a good starting place. Being able to scale the current by adjusting one resistor, and having the ground connected load, etc..

2016-05-25 12_53_52-an105fa.pdf.png

http://cds.linear.com/docs/en/application-note/an105fa.pdf

Constrains:
- current source must be scale-able, 100mA to 5A (or potentially more!)
- VCC can swing from ~1.5VDC to 48VDC, depending on the load
- Load is always grounded


My questions are:
- with some changes to component values and opamp/fet choice, is it realistic to scale this circuit to up to 5A?
- with a 48V VCC, is it correct in assuming I would need a rail-to-rail opamp capable of handling 48V supply?
- are there any other potential problems I'm not seeing?

Thanks in advance!
 

Current scaling is only a matter of FET and shunt selection. FETs with large Cgs and Cds might require some compensation and load capacitance isolation with fast OPs, but should work in any case.

For higher supply voltage, I would use two OPs with individual supply voltages, the I/V level shifter can bridge even higher supply levels like 100 or 200 V, but OPs should be limited to 15 or 30 V each.
 

Load is always grounded

A PNP transistor could be suitable. It permits you to position the load near 0V ground.

2523612800_1464199970.png


Perhaps you intend to control it with a voltage at the bias? Then it needs to be able to sink several mA of current. You may need to make a darlington pair. Or, consider using a mosfet.

If your control voltage is just a few volts then feed it to an N-device, which then turns on a P-device.
 

A word of caution...thermal dissipation.

Short circuiting or driving a low impedance load, a 5 amp, 48 volt source will result in close to 240 watts dissipation in the series-pass device.

You will have to parallel several pass devices, and provide very generous heatsinking. Most likely, a fan will be also required for continuous duty and/or high ambient temperatures.
Lastly, I would include a thermistor to sense temperature and be able to cut out power in case of excessive temperatures.
 

I have a heavy duty linear power supply that automatically reduces the voltage going into the linear regulator under high dissipation conditions.

Its a type of voltage auto ranging circuit, that selects secondary tappings on the main power transformer via relays.
Crude and simple... But very effective.
 

The load is typically a high current LED string. I will be controlling VCC so that it provides the bare-minimum overhead to drive the string at the desired current. I'll be able to adjust VCC on the fly based on the drive requirements of the LED string. I hope to keep the power dissipation low by keeping that overhead as low as possible.
 

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