Information driver

hi

I have lithium battery (18650) 4S3P. I would like to use a xhp 70.

I saw this led driver

https://web.archive.org/web/2018062...3-Mode-Drive-Circuit-Board-for-1-x-Cree-XHP70

I don't really understand how it work.

Web site said

Operating voltage: 4-18V, maximum 20V
LED current: 4.5-5A



xhp 70 specification said

Maximum Drive Current: 4.8 A (6 V)
Typical Forward Voltage: 5.8 V White @ 2100 mA (6 V)


if i undersdand correctly this driver can be used with 12v xhp 70 otherwise led with burn.
My battery will be ok... but surely output voltage to the led will be reduce to 6v to be able to get 4.5-5A.


just to confirm... i begin


thanks

That driver is designed for the 6V configuration of the XHP70.

Yes, it's the job of the driver to reduce the input voltage as necessary to get the desired output current. Since the driver wants to put out about 4.8A, and the XHP70 (6V) will draw 4.8A at around 6V (it will vary a little with temperature, and from one LED to another), the driver should put out around 6V.

In the 12V configuration, the max drive current of the LED would be 2.4A. Since the driver is designed for about 4.8A, It would destroy the 12V LED.

DIWdiver said:

That driver is designed for the 6V configuration of the XHP70.

Yes, it's the job of the driver to reduce the input voltage as necessary to get the desired output current. Since the driver wants to put out about 4.8A, and the XHP70 (6V) will draw 4.8A at around 6V (it will vary a little with temperature, and from one LED to another), the driver should put out around 6V.

In the 12V configuration, the max drive current of the LED would be 2.4A. Since the driver is designed for about 4.8A, It would destroy the 12V LED.

Click to expand...

ok so i could use it with a 4S3P OR 2S6P 18650 battery pack

thanks a lot

Yes, it should work with 4S or 3S packs. It will also work with 2S pack, but you may not have enough voltage as the batteries discharge. I'll explain.

Despite the driver's ratings, which include 4V input and 6V output, I'm 99% sure that is a buck type driver. If so, it means that the output voltage can never be higher than the input voltage. In fact, it means that the output voltage must always be lower than the input voltage. The important question is "by how much?". Unfortunately, without specs, testing, or construction details, it's hard to know. It could be as little as a few tenths of a volt at full power, even less at low power. But it could be as much as a volt or more. This is called the 'overhead' voltage, and it often varies with load (less load, less overhead).

So when your batteries have dropped to 3V per cell, you only have 6V input. If you need 6V output, you aren't going to get it. What will happen is that the output current will drop. This causes the LED voltage to drop, and you will reach the overhead voltage again. As the cells continue to discharge, the output will continue to drop until either a protection circuit cuts out or you give up and turn it off.

In some cases this behavior is desirable - you get a lot of warning that your batteries are running low, but you get that warning earlier than you would with a higher voltage pack. If you'd rather have full output until the protection circuit suddenly cuts out, use the 3S or 4S configuration. I would guess that this driver would be a bit more efficient with a 3S4P pack than 4S3P, but only testing could verify that. The difference isn't likely to be huge either way.

What if I'm wrong, and this isn't a buck type driver? Well then it would have to be a buck-boost type. That would allow it to produce a 6V output with 4-18V input. But these are more complex and thus more expensive. If I were selling one, I'd make darn sure to tell you that's what it was. They don't. They hint at it with the 4V minimum input spec, but I'm not buying it.

Also, the buck-boost configuration is inherently less efficient that the buck configuration. This can be minimized with more complex designs, but I'll promise you that this driver doesn't incorporate such.

So overall, I'd be comfortable matching this driver to work with a 6V XHP70, and either 2S, 3S, or 4S pack, depending on which behavior you prefer.

DIWdiver said:

What if I'm wrong, and this isn't a buck type driver? Well then it would have to be a buck-boost type. That would allow it to produce a 6V output with 4-18V input. But these are more complex and thus more expensive. If I were selling one, I'd make darn sure to tell you that's what it was. They don't. They hint at it with the 4V minimum input spec, but I'm not buying it.

Also, the buck-boost configuration is inherently less efficient that the buck configuration. This can be minimized with more complex designs, but I'll promise you that this driver doesn't incorporate such.

Click to expand...

Buck-boost may not be common but not necessarily too expensive for some cases.

I opened up a fairly low-cost automotive-type spotlight using 4 LEDs. In this arrangement, it would either have two control circuits and two inductors, or one controller and two inductors (SEPIC?); turned out to be the latter. I buzzed out connectivity of the 4 LEDs and they are all in series. Light is spec'ed from ~10-30vdc. Total LED drop would be ~12-14v which is similar range as supply in a 12v vehicle, therefore design must be buck-boost.

Common configuration for some of these lights is LEDs in groups of three, with buck converter for each group.

Some of these lights have plastic lens glued in place, can't open with screws as there were none, but I managed to get it open.

I'll dig up the light and have another look (as to the control IC).

Dave

Dave_H said:

I'll dig up the light and have another look (as to the control IC).

Dave

Click to expand...

My 4-LED light uses Diodes Inc. PAM2842 which does buck, boost, or buck/boost depending
on circuit. It's older chip in 20-pin TSSOP. Interesting that buck-boost can use two separate
inductors, or a single "coupled" inductor, and the PCB appears to have an option for it.

OP's driver shows only one inductor with single winding so looks like buck. 4v minimum input could still work with
one series LED, but not two or more in series.

Dave