# Understanding the difference between Linear, Buck, Boost and Direct Drive drivers

Thank you! In that case I think I’ll put “Understanding” as it fits better what I intended to say.

Nice, simple explanation.

When I was working with buck and boost circuits we simply called them DC-DC inverters which was basically just an oscillator (square or sawtooth) driving a toroidal transformer and operated at 20khz and above if I remember correctly. It’s been many, many years.
Higher frequencies allowed smaller components but usually lower efficiencies. That’s part of the buck/boost drivers used here.
You have to have AC, in one form or another, to transform voltage, so DC-DC converts DC to AC, changes voltage up or down, then converts to DC again.

The L/C portion in these drivers looks like part of the regulation circuit and operates at audible frequencies.
Your explanation is a lot easier to understand.

What we, the end user, care about is:

1. Voltage range usable for input.
2. Usable voltage and current out and stability.
3. Overall efficiencies at various voltage/current levels.
4. What driver to use in which situation.

That you describe very well.

Thanks for an excellent article.

Excellent Resource !
Thanks for this, it’s possibly the most valuable post in the last month.

Thanks, This is all new to me. Maybe this could be the first in a new sub-section of basic information for newbies. Now that I have a super nice light it might be good to understand more.

Thanks for sharing this lagman. I know how to swap drivers but I really don’t understand how these drivers differ from one another - until I read this.

I was also somewhat ‘relieved’ because the driver of the BLF Starry Light flashlight I bought through group buy failed and I replaced it with a Nanjg 105c driver. I now feel safer using the light with 4 AA NiMH because its total voltage of 4.8 volts when fully charged is way below the 6 volt limit of the 7135 chip.

There is also a power and temperature limit on the 7135 chip and this is usual reached way before the 6 volt limit. Luckily the chip will protect itself and just reduce the current when it gets to hot.

Basically you are wasting the power of one AA battery into the driver. Not the most efficient solution but it should work as long as the driver doesn’t overheat.
If you want to reduce heat losses you could use a dummy AA cell to reduce the voltage a bit. That would make your light a 3xAA light.
The light should be as bright. Maybe that it will start reducing current a bit earlier at the end of discharge though. 7135 chips start reducing current about 0.2V above the LED voltage.

Same sorta issue here, referring to disadvantages of linear drivers:

You said above that the 7135 can’t take over 6v, but if the cells in series add up to 6v or less, couldn’t you use them? I’m thinking of Cr123, NimH, and alkaline cells, and there are lots of others. I suggest adding the designation Li-Ion thusly: …multiple Li-Ion cells in series. (I would also change “batteries” to “cells” to be correct in terminology.)

Yes you can (Note CR123 are 3.2 volt).

Another thing to note when using the 7135 chip is that it is the chip that is limited to 6 volt, not the supply voltage for the circuit. This is used in some clever circuits that runs at higher voltage but uses a 7135 chip.

Just an FYI 4s NiMH is 5.3v fully charged.

Thank you HKJ. The more I read the posts here in BLF, the more I realize that there are so much more to learn.

Noted lagman and thanks for this info. If only we can buy a similar driver (I tried but they’re not selling) then it would be much simpler.

Oops! So its still safe on the 4 NiMH, right? Or should I follow lagman’s suggestion of using 3x NiMH instead.

But if the chip automatically reduces the current when it gets hot then there should be no problem right?

4xNiMH starts at about 5.8 volt, but will very soon drop to around 5 volt (Depending on load).

Here is my test of some NiMH batteries:

HKJ I’m not at all doubting any info you post but I have a light (pukelight v1) that uses 2 parallel strings of 4s NiMH and it runs at 5.3v fresh off the charger, dead they’re around 5.1v, why the difference?

I have multiple 4s AA lights (and the one 2s4s) running 7135 based drivers and they all work perfectly and have yet to overheat.

That sound like the charger is undercharging the batteries or maybe they rest a long time before you measure them.

I assume you measure the final voltage without any load on the batteries, that will be higher than loaded voltage.

You might not see that they overheat, it is very difficult to see if the brightness drops 30% and when you add a ammeter you add some extra resistance, i.e. the voltage to the 7135 will be lower with less risk of overheating.

The risk of overheating depends on the number of 7135 places close to each other and their contact to a heatsink (I.e. flashlight body).

what i would like to know is the difficulty in creating a single li-ion boost driver for single high Vf emitters (xp-g2, xm-l2 etc). i know the discussions with lightmalls fell through. what could have been the technical causes? large drop out, low efficiency?

Something like this: http://lygte-info.dk/review/DriverTest%201A%202.7-5.5V%20Buck-Boost%20UK.html