Using diodes to increase voltage drop in LED string ?

Thread Starter

4lch_

Joined Nov 16, 2023
22
Hi all and thanks in advance for your time !

I am currently designing an LED light sources that has 4 individual strings of LEDs, to be controlled by a commercial constant current machine vision driver (Smartek HPSC4). One of the strings has significantly less LEDs than the others (3 vs 8, 9 and 14). The driver works best when the voltage drop across all strings is similar, and this discrepancy is causing issues, so I need to increase the voltage drop on this line, but can't add more LEDs because of space constraints and to avoid wasting extra (expensive) LEDs.

For this purpose, I am considering adding diodes in series to this string, as a way to increase the voltage drop, without limiting the current that could go through it. However, I am unsure of the side effects that this could bring, especially as the LEDs are strobed at high-current and high-frequency with very short on-times (1500mA and about 20µs ON vs 200µs OFF) so I would not want to introduce too much extra rise time etc.

EDIT : The HPSC4 driver effectively acts as 4 independent drivers, and each string here is attached to one channel of the driver. All strings are completely independent and each driven by their own constant-current sub-driver.

Please, feel free to let me know how this sounds or if there is a better solution out there :)

Best regards,
Adrien
 
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Thread Starter

4lch_

Joined Nov 16, 2023
22
Rather than have lots of added diodes you might want to consider using a Vbe multiplier. This could save space and simplify any heat-sinking needed.
Thanks, Alec !
I did not know about Vbe multipliers, and it indeed sounds nice for this use, as it would otherwise require quite a few diodes in series. If I understand it correctly, the voltage across it is adjusted by the resistors and it seems to behave like a current mirror too, which is great as the driver is effectively controlling the current, and the string should not limit it. Are there any specific considerations when picking components for this circuit ? Namely for the high-speed and high-current aspects of this application I imagine.

Thanks again ! :)
 

BobTPH

Joined Jun 5, 2013
9,276
If you are driving multiple stings in parallel with a constant current driver, you have defeated its purpose, there is no guarantee the current will divide equally. You would be better off to use a voltage source with a resistor in each string.
 

Thread Starter

4lch_

Joined Nov 16, 2023
22
Are the strings in parallel. 9 parallel with 14 LEDs?

Do you have an inductor to average this out? Like a PWM + inductor?
How is your CC LED driver built?
Sorry this was not very clear from the original post, especially as the driver is quite specific. The driver is a commercial unit that can drive 4 independent "channels", each representing a string of series LEDs. Each string is connected to a channel of the driver and the strings are completely independent.

The driver (Smartek HPSC4) is COTS so I do not know exactly how it is designed, but it is designed for high-speed strobing in machine-vision applications. Its job is to send very short and high intensity pulses to freeze movement when inspecting fast moving objects, so while this is a sort of PWM, it is not used for dimming like traditionnal drivers. In fact, this driver somehow manages to be able to output up to 40A @48V in pulsed operation from a regular 24V 6A power supply, pretty neat stuff :)
 

Thread Starter

4lch_

Joined Nov 16, 2023
22
If you are driving multiple stings in parallel with a constant current driver, you have defeated its purpose, there is no guarantee the current will divide equally. You would be better off to use a voltage source with a resistor in each string.
Sorry I should have been more explicit on the original post, the driver has 4 independent channels, each with controllable constant current and each string of series LEDs is connected to its own channel :)
 

BobTPH

Joined Jun 5, 2013
9,276
So what is the voltage range that each channel will operate within? If the short string has Vf within range, you don’t need to add anything.
 

Thread Starter

4lch_

Joined Nov 16, 2023
22
So what is the voltage range that each channel will operate within? If the short string has Vf within range, you don’t need to add anything.
It is already within the operating range at around 6V, but the driver is supposed to perform best when the forward voltage of all enabled channels is similar (according to Smartek's tech support and experimentation), as otherwise it has to do some extra balancing on top of driving them.
In fact, the "problematic" string is the one with the smallest forward voltage of the 4 so it is the easiest to drive, but this is why I am looking to increase it to get a closer match to other channels.
 

MisterBill2

Joined Jan 23, 2018
19,453
INDEED!! adding a resistor so that the current is divided equally among the strings should work.
( This advice is based on my guess that all of the LEDs are the same type and color, and have very similar current to brightness characteristics.)
 

LesJones

Joined Jan 8, 2017
4,227
Until I looked at the manual on the HPSC4 I thought the the power supply was just a normal single channel constant current supply and I thought the TS was trying to share the output current between 4 strings in parallel. Am I correct in assuming that each string is on it's own channel ?

Les.
 

Thread Starter

4lch_

Joined Nov 16, 2023
22
Until I looked at the manual on the HPSC4 I thought the the power supply was just a normal single channel constant current supply and I thought the TS was trying to share the output current between 4 strings in parallel. Am I correct in assuming that each string is on it's own channel ?

Les.
Absolutely, I have just edited the original post with this information since it is quite a specific bit of hardware:)
 
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Thread Starter

4lch_

Joined Nov 16, 2023
22
Okay. Just add a resistor to bring the voltage of the short string nearer to the others.
I have edited the original post to make it clearer that allstrings are completely independent, being driven each by their own "sub-driver". It is as if there were effectively 4 drivers for the 4 strings. I am looking to increase the voltage drop of the short string to make it a closer match to the other ones, since the HPSC4 behaves better when the load is balanced between its 4 channels. I assume that your reply assumed that they all shared a single current source, which is fair and should hopefully be less vague now :)
 

Thread Starter

4lch_

Joined Nov 16, 2023
22
INDEED!! adding a resistor so that the current is divided equally among the strings should work.
( This advice is based on my guess that all of the LEDs are the same type and color, and have very similar current to brightness characteristics.)
Thanks for your input ! I have edited the original post to explain the setup better. All strings are independent, the HPSC4 driver contains 4 independent constant current sources that are each tied to a single string. All channels/strings are independent, but the driver behaves better when the load is balanced between channels, hence why I am looking to increase the voltage drop of the short one.
 

LesJones

Joined Jan 8, 2017
4,227
I think suitable value resistors should work unless you need to adjust the current after it's value is initially chosen.
I don't understand why there is interaction between the channels.
Les.
 
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