added 39 characters in body

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edited Oct 26, 2021 at 13:25

7E10FC9A

429
3
14

Background:

I need a microcontroller adjustable high voltage (300V) constant current.
Source: High voltage boost converter.

Load: Gas discharge lamp (20mA max). Can only be switch from high side.
There is no digital constant current IC at that voltage.
There is no (affordable) op-amp at that voltage.
Linear regulator can operate at constant current mode, and there is (only) one linear regulator that can operate at 300V:LR8
Digital potentiometer can be used to adjust current.

Problem:

The adjusting resistor is in series with the load. The load current is too much (20mA max) for a digital potentiometer.

I found a constant current topology in the LM350 datasheet: 2 linear regulators without having R_set in series to the load.

I don't quite understand how this works, but I assume the lower linear regulator serves as constant current reference.

Could someone please explain how this works and how to calculate output current ?

Update:

Following advice from @SimonFitch. A DAC-controlled current sensing opamp seems to be a better choice.

As suggested, this circuit uses current feedback to drive a MOSFET, a bit like current mode SMPS. The comparator is protected by opto-isolated gate driver.

Background:

I need a microcontroller adjustable high voltage (300V) constant current.
Source: High voltage boost converter. Load: Gas discharge lamp (20mA max).
There is no digital constant current IC at that voltage.
There is no (affordable) op-amp at that voltage.
Linear regulator can operate at constant current mode, and there is (only) one linear regulator that can operate at 300V:LR8
Digital potentiometer can be used to adjust current.

Problem:

The adjusting resistor is in series with the load. The load current is too much (20mA max) for a digital potentiometer.

I found a constant current topology in the LM350 datasheet: 2 linear regulators without having R_set in series to the load.

I don't quite understand how this works, but I assume the lower linear regulator serves as constant current reference.

Could someone please explain how this works and how to calculate output current ?

Update:

Following advice from @SimonFitch. A DAC-controlled current sensing opamp seems to be a better choice.

As suggested, this circuit uses current feedback to drive a MOSFET, a bit like current mode SMPS. The comparator is protected by opto-isolated gate driver.

Background:

I need a microcontroller adjustable high voltage (300V) constant current.
Source: High voltage boost converter.

Load: Gas discharge lamp (20mA max). Can only be switch from high side.
There is no digital constant current IC at that voltage.
There is no (affordable) op-amp at that voltage.
Linear regulator can operate at constant current mode, and there is (only) one linear regulator that can operate at 300V:LR8
Digital potentiometer can be used to adjust current.

Problem:

The adjusting resistor is in series with the load. The load current is too much (20mA max) for a digital potentiometer.

I found a constant current topology in the LM350 datasheet: 2 linear regulators without having R_set in series to the load.

I don't quite understand how this works, but I assume the lower linear regulator serves as constant current reference.

Could someone please explain how this works and how to calculate output current ?

Update:

Following advice from @SimonFitch. A DAC-controlled current sensing opamp seems to be a better choice.

As suggested, this circuit uses current feedback to drive a MOSFET, a bit like current mode SMPS. The comparator is protected by opto-isolated gate driver.

added 4 characters in body

Source Link

edited Oct 25, 2021 at 13:21

7E10FC9A

429
3
14

Background:

I need a microcontroller adjustable high voltage (300V) constant current.
Source: High voltage boost converter. Load: Gas discharge lamp (20mA max).
There is no digital constant current IC at that voltage.
There is no (affordable) op-amp at that voltage.
Linear regulator can operate at constant current mode, and there is (only) one linear regulator that can operate at 300V:LR8
Digital potentiometer can be used to adjust current.

Problem:

The adjusting resistor is in series with the load. The load current is too much (20mA max) for a digital potentiometer.

I found a constant current topology in the LM350 datasheet: 2 linear regulators without having R_set in series to the load.

I don't quite understand how this works, but I assume the lower linear regulator serves as constant current reference.

Could someone please explain how this works and how to calculate output current ?

Update:Update:

Following advice from @SimonFitch. A DAC-controlled current sensing opamp seems to be a better choice.

As suggested, this circuit uses current feedback to drive a MOSFET, a bit like current mode SMPS. The comparator is protected by opto-isolated gate driver.

Background:

I need a microcontroller adjustable high voltage (300V) constant current.
Source: High voltage boost converter. Load: Gas discharge lamp (20mA max).
There is no digital constant current IC at that voltage.
There is no (affordable) op-amp at that voltage.
Linear regulator can operate at constant current mode, and there is (only) one linear regulator that can operate at 300V:LR8
Digital potentiometer can be used to adjust current.

Problem:

The adjusting resistor is in series with the load. The load current is too much (20mA max) for a digital potentiometer.

I found a constant current topology in the LM350 datasheet: 2 linear regulators without having R_set in series to the load.

I don't quite understand how this works, but I assume the lower linear regulator serves as constant current reference.

Could someone please explain how this works and how to calculate output current ?

Update:

As suggested, this circuit uses current feedback to drive a MOSFET, a bit like current mode SMPS. The comparator is protected by opto-isolated gate driver.

Background:

I need a microcontroller adjustable high voltage (300V) constant current.
Source: High voltage boost converter. Load: Gas discharge lamp (20mA max).
There is no digital constant current IC at that voltage.
There is no (affordable) op-amp at that voltage.
Linear regulator can operate at constant current mode, and there is (only) one linear regulator that can operate at 300V:LR8
Digital potentiometer can be used to adjust current.

Problem:

The adjusting resistor is in series with the load. The load current is too much (20mA max) for a digital potentiometer.

I found a constant current topology in the LM350 datasheet: 2 linear regulators without having R_set in series to the load.

I don't quite understand how this works, but I assume the lower linear regulator serves as constant current reference.

Could someone please explain how this works and how to calculate output current ?

Update:

Following advice from @SimonFitch. A DAC-controlled current sensing opamp seems to be a better choice.

As suggested, this circuit uses current feedback to drive a MOSFET, a bit like current mode SMPS. The comparator is protected by opto-isolated gate driver.

deleted 13 characters in body; edited title

Source Link

edited Oct 25, 2021 at 10:46

JRE

75.1k
10
115
197

Hight High voltage adjustable constant current souurcesource controlled by MCU

Background:

I need a microcontroller adjustable high voltage (300V) constant current.
Source: High voltage boost converter. Load: Gas discharge lamp (20mA max).
There is no digital constant current IC at that voltage.
There is no (affordable) op-amp at that voltage.
linearLinear regulator can operate at constant current mode, and there is (only) one linear regulator that can operate at 300V:LR8
Digital potpotentiometer can be used to adjust current.

Problem:

The adjusting resistor is in series with the load. The load current is too much The load current is too much (20mA max) for digital pot.(20mA max) for a digital potentiometer.

I fondfound a constant current topology atin the LM350 datasheet: 2 linear regulators without having R_set in series to the load.

I don't quite understand how this works, but I assume the lower linear regulator serves as constant current reference.

Could someone please explain how this works and how to calculate output current ?

Update:

As suggested, this circuit uses current feedback to drive a MOSFET, a bit like current mode SMPS. The comparator is protected by opto-isolated gate driver.

Any comment is greatly appreciated !

Hight voltage adjustable constant current souurce controlled by MCU

Background:

I need a microcontroller adjustable high voltage (300V) constant current.
Source: High voltage boost converter. Load: Gas discharge lamp (20mA max).
There is no digital constant current IC at that voltage.
There is no (affordable) op-amp at that voltage.
linear regulator can operate at constant current mode, and there is (only) one linear regulator that can operate at 300V:LR8
Digital pot can be used to adjust current.

Problem:

The adjusting resistor is in series with the load. The load current is too much (20mA max) for digital pot.

I fond a constant current topology at LM350 datasheet: 2 linear regulators without having R_set in series to the load.

I don't quite understand how this works, but I assume the lower linear regulator serves as constant current reference.

Could someone please explain how this works and how to calculate output current ?

Update:

As suggested, this circuit uses current feedback to drive MOSFET, a bit like current mode SMPS. The comparator is protected by opto-isolated gate driver.

Any comment is greatly appreciated !

High voltage adjustable constant current source controlled by MCU

Background:

I need a microcontroller adjustable high voltage (300V) constant current.
Source: High voltage boost converter. Load: Gas discharge lamp (20mA max).
There is no digital constant current IC at that voltage.
There is no (affordable) op-amp at that voltage.
Linear regulator can operate at constant current mode, and there is (only) one linear regulator that can operate at 300V:LR8
Digital potentiometer can be used to adjust current.

Problem:

The adjusting resistor is in series with the load. The load current is too much (20mA max) for a digital potentiometer.

I found a constant current topology in the LM350 datasheet: 2 linear regulators without having R_set in series to the load.

I don't quite understand how this works, but I assume the lower linear regulator serves as constant current reference.

Could someone please explain how this works and how to calculate output current ?

Update:

As suggested, this circuit uses current feedback to drive a MOSFET, a bit like current mode SMPS. The comparator is protected by opto-isolated gate driver.