How to boost 2V to 12V

[uglyduck]

Hello,

I need to boost a 2 cell rechargeable battery to 12V / 500mA output. This will be used to power a small stepper motor, but for short time periods - few steps, few seconds pause, next few steps, next few seconds pause. The cells will discharge of course, so I need the device to operable with even a little bit lower input voltage. I tried with MAX1771 which is able to boost from 2V and provide current depending on the power switch but didn't like it. Few transistors burned out and I'm not exactly sure why - maybe because with voltages <=2V the MOSFET enters the linear region... And this circuit cannot be fully shut down because of the boost converter topology. Anyway I didn't managed to do the job with this controller... Can you suggest another approach? One stupid question - is it possible to use kind of Joule-thief for this purpose and how to calculate one for 12V / 500mA? Is there another IC solution for my problem or what is the way for calculating a step-up converter for this?

 

[barry]

I'm not sure about this. You'll need to deliver 3 amps from your 2V battery, is your battery capable of that? (Or you'll need a BIG cap). How much time does your 'few steps' take? Also, if you read the spec, the MINIMUM input voltage for the MAX1771 is 2V. You can't run this at less than 2V and then complain that it doesn't work.

 

[uglyduck]

Yes, you are right of course. I do not complain that the MAX1771 is not working, I just didn't expected the transistors to be damaged. The idea is a bit crazy, I know that, but I wanted to be able to move the motor for 1-2 sec and than wait 0.5-1 min before do it again. The motor is a stepper with nominal current of about 350-400mA. I'm not quite sure this task is feasible but I wanted to try to squeeze the maximum of my battery. Is the a way to build a "pump"?

 

[barry]

You need 400 mA at 12 V for 2 seconds. That's a LOT for a capacitor to supply. If you can tolerate, say, 0.5V drop, then your cap would have to be about 1.5 FARADS. Yes, there are countless boost circuits that can step your 2V to 12V(just do a google search). But I still question whether you can get 3+ amps out of your battery.

 

[BradtheRad]

to move the motor for 1-2 sec and than wait 0.5-1 min before do it again. The motor is a stepper with nominal current of about 350-400mA.

This is a very low duty cycle. It is practical to charge a large capacitor to 12V, then switch it On to power your motor briefly.



The 20V comes from your boost converter. It only needs to supply a fraction of an ampere continuously for a minute. This might draw 1/2 A from your batteries.

I believe you'll need a control circuit to make a stepper run.

Is the a way to build a "pump"?

Although there is a sort of pumping action going on, this is not the same as a charge pump.

is it possible to use kind of Joule-thief for this purpose and how to calculate one for 12V / 500mA?

Yes, although the transformer would need to be upwards of 100 H, in order to give the cycle time you wish.



Your battery pack would need to supply almost 1 A at the peaks.

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The scope trace shows a period of 19.88 mHz. This works out to 50 seconds per cycle.