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

[HKJ]

@DavidEF

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.

[Cereal_killer]

@Nightbird95

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.

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

[Nightbird95]

@HKJ

@Nightbird95

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.

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

[Nightbird95]

@lagman

@HKJ

@Nightbird95

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.

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.

[Nightbird95]

@Cereal_killer

@Nightbird95

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.

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

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

[Nightbird95]

@Nightbird95

@Cereal_killer

@Nightbird95

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.

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

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?

[HKJ]

@Nightbird95

@Cereal_killer

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

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

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:

[Cereal_killer]

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?

[Cereal_killer]

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

[HKJ]

@Cereal_killer

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?

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.

@Cereal_killer

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

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).

[islisis]

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?

[HKJ]

@islisis

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

[islisis]

thanks hkj. i know buck-boosts like this are available. of course i mean i would like to know the inherent problems of a boost driver for to meet Vf at high current (>4A). i see your reviewed driver regulated well at 1A, and that boost mode was more inefficient than buck mode.

[HKJ]

@islisis

thanks hkj. i know buck-boosts like this are available. of course i mean i would like to know the inherent problems of a boost driver for to meet Vf at high current (>4A). i see your reviewed driver worked well at 1A.

It is no problem to make a boost or buck driver, but combining to a buck/boost is a problem. There exist only a few chips that can do it.

The problem is that you basically need two switchers sharing one inductor, this requires multiple transistors. As long as all the transistors are in a chip it is fine, but a led driver does not have place for external transistors for higher power.

[islisis]

a mass market driver with boost should be buck+boost, and i can understand the complexities of such drivers. good explanation also! but the driver in dicussion with lightmalls would have been boost only (DD at Vin>Vf). in other words, a specialised driver for lights which can withstand DDing an emitter (i.e. 5A+).

at high current would a boost driver never reach the required efficiency to work from a single li-ion? or something else, like lack of commercial ics? i can’t even find any mass produced step-up voltage converters for this range and output.

[lagman]

@DavidEF

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

You can’t use this if you have multiple batteries in series.

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.)

I agree, I changed the OP accordingly. Thanks!

[lagman]

@HKJ

@islisis

thanks hkj. i know buck-boosts like this are available. of course i mean i would like to know the inherent problems of a boost driver for to meet Vf at high current (>4A). i see your reviewed driver worked well at 1A.

It is no problem to make a boost or buck driver, but combining to a buck/boost is a problem. There exist only a few chips that can do it.

The problem is that you basically need two switchers sharing one inductor, this requires multiple transistors. As long as all the transistors are in a chip it is fine, but a led driver does not have place for external transistors for higher power.

Buck-Boost drivers are also less efficient than just a Buck.
If he wants to drive a XM-L2 at 4A or more he should consider having two Li-Ion cells in series to get 7.4V and using a buck converter… However it will be bulky to be able to withstand 4A.

[lagman]

@Nightbird95

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

Again, as long as the input voltage is below 6V even with fully charged cells, the driver won’t fail. But it will be wasteful and run hot.
Let’s do some maths:
Your 4*AA are at about 5V during discharge.
The LED needs about 3.3V
that means that the driver needs to lower the voltage of 1.7V!
3.3/5=66% efficiency! That’s not really good… And that means that 37% of the power is lost in the driver and heats it up.
Use a dummy cell and efficiency will go up, reducing the heat in the driver and you’ll still have the same brightness.

[HKJ]

@islisis

at high current would a boost driver never reach the required efficiency to work from a single li-ion? or something else, like lack of commercial ics? i can’t even find any mass produced step-up voltage converters for this range and output.

The general problem with a boost drivers is that it always need more input current than it delivers as output current.

Efficiency varies with design, but because the current is high it will have high ohmic losses.

[islisis]

@HKJ

The general problem with a boost drivers is that it always need more input current than it delivers as output current.

Efficiency varies with design, but because the current is high it will have high ohmic losses.

i see in tests of some high discharge cells that outputs of 10A @ >3V are sustainable, so i assumed in theory a boost driver could continue to regulate on a single cell. if the loss is primarily ohmic then i presume it is linear. the size constraints make it difficult i guess.