Single Cell LED boost Converter

kbruin

Newly Enlightened
Joined
Mar 19, 2019
Messages
5
I am planning on powering 10 white LEDs(LUXEON 3014) from a single AA rechargeable battery with Vin min=0.9V, Vin max=1.4v
VLed=2.8v @ 23mA so total Vout for 10 LEDs Vout=28v, iout=23mA
I found this boost driver from Diodes ZXSC310. The issue is the app notes doesn't tell you how to calculate key parameters like the inductor.
This is the schematic I will be using. Instead of D1 I will have 10 LEDs.
Hjza6.jpg


Now as far as values I used this calculator to find the inductor value
https://learn.adafruit.com/diy-boost-calc/the-calculator
Does L1=7.2uH ipeak=0.665A and a duty cycle D=0.975 sounds about right?


Would you recommend having the LEDs in parallel instead?
if that's the case then Vout becomes Vout=2.8V, Iout=23ma*10=230mA.
The downside is the inductor will have to be bigger and the LEDs will need a balast resistor to even their light output.
I would appreciate your input.
 

DIWdiver

Flashlight Enthusiast
Joined
Jan 27, 2010
Messages
2,725
Location
Connecticut, USA
The FMMT617 is only rated for Vce of 15V, so it wouldn't survive the circuit's operating voltage.

Two strings of five LEDs would be nominally 14V, but that's too close for me. I'd use the datasheet recommended FMMT618, which is rated for 20V.

The lower output voltage would reduce the duty cycle, which would probably increase the efficiency.

As for efficiency, it's probably going to be terrible. Notice that none of the reference designs put out anywhere near the power level you are asking for.

You are asking for three times the power output at less than half the input voltage of their highest output reference design. Sorry to rain on your parade, but I doubt you can achieve that at any efficiency.
 

kbruin

Newly Enlightened
Joined
Mar 19, 2019
Messages
5
Thanks for the reply.
I am actually going to use NSS40201LT1G which is rated at 40 v VCE.
I realized that one string in series isn't going to do it but a string of 10 LEDs in parallel might work. Since the transistor along with the inductor is going to
do the heavy lifting, I don't see why it wouldn't work?
The duty cycle is ~0.5
I am planning on using: SRN5040TA-6R8M That's a 6.8uH with 2.5A rating and 43 mohms DC resistance.



The FMMT617 is only rated for Vce of 15V, so it wouldn't survive the circuit's operating voltage.

Two strings of five LEDs would be nominally 14V, but that's too close for me. I'd use the datasheet recommended FMMT618, which is rated for 20V.

The lower output voltage would reduce the duty cycle, which would probably increase the efficiency.

As for efficiency, it's probably going to be terrible. Notice that none of the reference designs put out anywhere near the power level you are asking for.

You are asking for three times the power output at less than half the input voltage of their highest output reference design. Sorry to rain on your parade, but I doubt you can achieve that at any efficiency.
 

DIWdiver

Flashlight Enthusiast
Joined
Jan 27, 2010
Messages
2,725
Location
Connecticut, USA
That looks like a better choice. But look at figure 6 in the data sheet. It shows that to get to 500 mA collector current, you need nearly 3 mA of base current, which you can't get at your low input voltages (see the drive current vs input voltage curve on page 4 of the ZXSC310 data sheet).
 

kbruin

Newly Enlightened
Joined
Mar 19, 2019
Messages
5
That looks like a better choice. But look at figure 6 in the data sheet. It shows that to get to 500 mA collector current, you need nearly 3 mA of base current, which you can't get at your low input voltages (see the drive current vs input voltage curve on page 4 of the ZXSC310 data sheet).

I see so you're comparing Figure 6 of the BJT which says I need at least 3mA at the base to get to 500mA at the collector to page 4 on the driver. Page 4 of the driver shows the base drive current (to the BJT) to be ~2.2mA @ 1.25v input voltage Ta=25C

Therefore the transistor will not reach the peak current required of 0.665A. Is that what you mean?

I am guessing it will still work but the LEDs will be operating at a lower brightness level.
 

DIWdiver

Flashlight Enthusiast
Joined
Jan 27, 2010
Messages
2,725
Location
Connecticut, USA
Yes, that's exactly what I meant. You understand very well!

It may operate, but not at the design parameters.

Welcome to the world of specsmanship.
 
Last edited:

kbruin

Newly Enlightened
Joined
Mar 19, 2019
Messages
5
Thank you for your input. Do you think FMMT617 is the same in regards to the collector vs base current? I don't see a similar to figure 6 in the FMMT617 datasheet.

Also, what do you think of this boost current calculator? I got a 3.8uH for the inductor ( but I will use 6.8uF considering tolerance and part availability) does this sound reasonable? and a peak current of 0.665A
That's for having 10 LEDs in parallel.

Yes, that's exactly what I meant. You understand very well!

It may operate, but not at the design parameters.

Welcome to the world of specsmanship.
 

DIWdiver

Flashlight Enthusiast
Joined
Jan 27, 2010
Messages
2,725
Location
Connecticut, USA
Yes, I think it probably is. I was looking for a chart like that for the '617 and '618, bud didn't see one.

My point was that you don't have to worry about 0.665A, because you'll never get there.

No calculator is going to give you accurate results because they assume the circuit will work, and it won't.
 

DrafterDan

Flashlight Enthusiast
Joined
Jul 28, 2013
Messages
1,317
Location
Phoenix, AZ
I'm glad there are people like you guys in our modding community.
My comfort zone is metal, definitely fall short on the electronics side of things.

Which is kinda sad, as I'm a licensed low-voltage contractor :)
 

Latest posts

Top