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  1. #1
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    Current source for high current LED driver- 24Vdc/max:16A

    I am working on designing a current source for a LED driver, which can work up to max:16A, as long as the driver stays on less than 500msec. Actually, the leds circutry is a Dome-light which is divided in 4 sectors. The 4 sectors have a common ground and they are trigger in PNP mode(24Vdc) individually.

    DOME light, 4 sectors
    Click image for larger version. 

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    I am using a current sense-resistor of 10mOhm resistance, between a n-channel mosfet`s source terminal and ground. The voltage measured on the current sense-resistor is feed to a differencial OpAmp which output controls the mosfetīs gate voltage. Nothing new here, it is the simplest current source circuit. I am using a digital rheostat to create a voltage divider which sets the OpAmps reference voltage on the non-inverting terminal:
    Click image for larger version. 

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    Components:
    - TLV2372IP (Op Amp)
    - MCP4162 (Digital Rheostat)
    - STP55NF06L (Mosfet 60V,55A,Rds-On 18mOhm)
    - Current sense-resistor 10mOhm/0.1% tolerance.

    Basic schematic:
    Click image for larger version. 

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    Questions:

    *The circuit drives the current to the leds array with stability, but I know that this low-side driver will be unstabled if there is noise in the power supply or noisy enviroment like in a factory.It works steady only by chance. The power supply is a Meanwell DR-120-24/5A. Why can I get 4,6,8 or 15 Amps currents?If the power supply is rated 5 Amps max? (I didnīt have a higher current rated power supply at hand).
    *I found in internet that a N-Channel or P-Channel MOSFET High-Side Drive should be more stable and worth to try. But How? I donīt know how to go about it. Could you give me some ideas, please?

    This circuit it is for using the Dome_light for a photometric stereo machine vision application. Actually, I am a machine vision programmer. I am not an electronics engineer, sorry for not giving more information. Thanks in advanced

  2. #2
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    Re: Current source for high current LED driver- 24Vdc/max:16A

    I found in internet that a N-Channel or P-Channel MOSFET High-Side Drive should be more stable and worth to try.
    Why particularly? I don't recognize any advantage if you are free connect the LED string as you want.



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  3. #3
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    Re: Current source for high current LED driver- 24Vdc/max:16A

    Hi,
    I have read this article from maxim:
    https://www.maximintegrated.com/en/a...dex.mvp/id/746
    Apparently using a high-side configuration is better because. "The low-side resistor adds undesirable extraneous resistance in the ground path" and "an accidental connection that bypasses the monitor resistor, canīt be detected".
    Do you think that it worth the effort to design a high-side circuitry? It is more complex especially for high currents like 16A.



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    Re: Current source for high current LED driver- 24Vdc/max:16A

    Ground short is a problem to consider if you can't guarantee the quality of external wiring. Don't know how it is in your design?

    If the driver is placed next to the LEDs, e.g. on a common PCB, I don't see a particular ground short risk.


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  5. #5
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    Re: Current source for high current LED driver- 24Vdc/max:16A

    What about "The low-side resistor adds undesirable extraneous resistance in the ground path"?
    Actually, during the test I carried out on the circuit, I have seen in my scope that for a duty cycle of 100msec, the differences between voltage peaks are 330uV at maximun.
    I going to sustitute the opamp with a better one, that has higher Slew rate, smaller voltage input offset and a mosfet with smaller Rds resistance, plus I am going to use a voltage reference IC for the voltage divider on the opampīs non-inverting terminal:
    - Opamp LT1055CN8#PBF; SR - Slew Rate:12 V/us, Ib - Input Bias Current:0.00001 uA , Vos - Input Offset Voltage:120 uV
    - Voltage reference IC MAX6250ACPA+ ; Initial Accuracy:0.02 % ,Output: 5V
    - Mosfet STP140N6F7; N-Channel, Rds On - Drain-Source Resistance:3 mOhms , Vgs th - Gate-Source Threshold Voltage:2 V, Id - Continuous Drain Current:80A

    I am quite sure that substituting this components will benefit the stability of the circuitry.
    Could you give some tips in order to improve the circuitry? Any important aspects to consider? Like putting a filter (capacitors) here or there?
    Thanks



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    Re: Current source for high current LED driver- 24Vdc/max:16A

    Hi,

    To the function:
    I see no problem with low side regulation, as long as wiring is correct.
    On the other side you donīt need to feel responsible for all mistakes a user may make.

    *The circuit drives the current to the leds array with stability, but I know that this low-side driver will be unstabled if there is noise in the power supply or noisy enviroment like in a factory.It works steady only by chance.
    I have done this many times in industrial designs without problems. But you really should use some filtering.

    *I found in internet that a N-Channel or P-Channel MOSFET High-Side Drive should be more stable and worth to try. But How? I donīt know how to go about it. Could you give me some ideas, please?
    I canīt see why a high side drive should be more stable. It all depends on the complete regulation loop, not only on the MOSFET.

    I am quite sure that substituting this components will benefit the stability of the circuitry.
    I doubt it.
    ***

    To your circuit:
    Wow, your complete circuit uses not a single capacitor. Not a bulk capacitor, not a decoupling capacitor, not a filter capacitor....
    Believe me: Capacitors are very useful and increase the stability and reliability of electronic circuits.

    Using a 10mOhms sense resistor calles for a kelvin wired PCB layout.
    Are you sure: 10mOhms with 0.1% tolerance? Do you really need this tight tolerance and are you able to get an almost equal overall performance?

    Using an optocoupler ... when both sides refer to the same GND ... makes no sense.

    ***
    I recommend to use some simulation software, like LTspice.

    Klaus
    Please donīt contact me via PM, because there is no time to respond to them. No friend requests. Thank you.


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    Re: Current source for high current LED driver- 24Vdc/max:16A

    Using a 10mOhms sense resistor calles for a kelvin wired PCB layout.
    Are you sure: 10mOhms with 0.1% tolerance? Do you really need this tight tolerance and are you able to get an almost equal overall performance?
    I will have a look at it. Vishay WSK Serieshttps://www.mouser.es/new/vishay/vis...ing-resistors/( Kelvin Sensing).
    To your circuit:
    Wow, your complete circuit uses not a single capacitor. Not a bulk capacitor, not a decoupling capacitor, not a filter capacitor....
    Believe me: Capacitors are very useful and increase the stability and reliability of electronic circuits.
    I know, I came with the simplest circuit and left it there. I am not a electrical engineer but I can understand all that you have told me and it has been very helpful.
    Another question. If I use a power supply rated of 24Vdc and 5A, How can i get pulses with currents above 5A,like 13A, if I drive the leds with my circuit?
    But if I don't use a mosfet with a current source, just a SSR(solid state relay) for the same period of time directly connected to the power supply, I only get 4.5A? The period of time being 100msec.



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    Re: Current source for high current LED driver- 24Vdc/max:16A

    The circuit in post #1 is no correct constant current source. The gate voltage, at least the DC value, should be fed to the current controller.


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    Re: Current source for high current LED driver- 24Vdc/max:16A

    Hi,

    I will have a look at it. Vishay WSK Series
    I didnīt say you need resistors with true kelvin connections.... but if you want to spend the money ... it may be a further improvement.

    Another question. If I use a power supply rated of 24Vdc and 5A, How can i get pulses with currents above 5A,like 13A, if I drive the leds with my circuit?
    If a power supply is rated for 5A ... this doesnīt mean it will limit it at this value.
    In opposite: you have to take care about the limit.

    Now the 5A rating usually is the DC or average value rating. Thus you safely may draw higher pulses than 5A.

    Here comes into play what I meant about the bulk capacitors: If you use bulk capacitors (in combination with any series impedance like wiring or extra parts) they act like a low pass filter. Correctly calculted they can deliver about any high current pulse you want .... whil they draw just the avrage current from the supply. This is the safe way. .. for your application as well as for the power supply.

    MOSFET vs SSR:
    Donīt compare apples with oranges.
    Only if the timing is equal and if the ON resistance is equal you will get equal current.

    Klaus
    Please donīt contact me via PM, because there is no time to respond to them. No friend requests. Thank you.


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  10. #10
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    Re: Current source for high current LED driver- 24Vdc/max:16A

    Bulk capacitors are fine to source pulse currents, but the OP is planning 100 ms pulse duration if I understand right. You'll need huge (100 mF range) capacitors for it. Providing a 15 or 20 A rated power supply seems more appropriate.

    - - - Updated - - -

    Here's the basic topology of a current source with Kelvin connected shunt.
    Additional loop compensation may be required depending on the OP speed and FET gate capacitance.

    Click image for larger version. 

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  11. #11
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    Re: Current source for high current LED driver- 24Vdc/max:16A

    Thanks guys. Good information.
    With this information at hand, I will make another few test and ask an external electrical engineer to design me the actual and final pcb, with the power supply, filters etc.
    You are the best.



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