NewBie
*Retired*
I remember building a boost supply back in 2003, and tossed it in the box with dozens of other ones I built back then.
Why did I toss it? Well, it's efficiency was really poor.
But it had one unique feature, and some folks have been asking me about a low input voltage boost converter that can run off of a single NiMH AA.
It is one of the super duper simple switcher circuits, which have very few parts, like the old Zetex favorite, and most folks can build.
It is based on the NCP1450A found here:
http://www.onsemi.com/PowerSolutions/product.do?id=NCP1450A
The circuit is super simple to put together and needs very few parts for a voltage boost:
On page 5, Figure 7, the closest to our scenario, they recommend the NTGS3446T1 MOSFET, and some other components/values. Just remember to keep the power paths short and the traces broad, and pay close attention to layout rules, mentioned on page 21.
You can find suggested part values, part numbers, and such on page 19.
Typically, if you use a really great inductor, it can push 500mA at 68% efficiency, if the NiMH can hold up to the current draw and maintain 1.2V.
You also need to use a uber low Vf Luxeon, such as a TWOF or TWOG.
Unfortunately, the NiMH are not that stiff, so the efficiency drops into the 60's. Then if you use a TWOH, you are looking in the 60% efficiency range.
Now, if you are willing to back off on the current, to 200mA (to the Luxeon), you can manage to squeak 80% efficiency out of it. As the cell drops 0.9V, the efficiency manages to still hold 70%.
The efficiencies on 2AA pick up alot, and one can get a little bit over 80%.
I'll leave the constant current portion up to your imagination and skills, if you want to add that, which will drop the efficiencies a bit.
Since this converter can't hold much more than 200mA at 0.9V, the discharge curve looks alot like the voltage plot on a discharging cell under load.
I am now aware of one rather recent converter here on CPF that implements this device and the datasheet's recommended MOSFET, the circuit is pretty much right off the datasheet. Sorry, there is nothing unique or special about this, and I know I'm not the only one that played a bit with this really old chip. You have my apologies ahead of time.
The part I used way back then, is shown below, in the red circle:
Anyhow, by the time 2004 rolled around, these low efficiency non-synchronous converters were all tossed aside as newer synchrous much higher efficiency converters became available. Very late in 2003, I had a chance to skim though alot of vendors websites, and pick out some suitable devices. The rewards were great, with efficiencies in the mid-90s. For those interested, feel free to dig through the archives. Unfortunately, only one of these chips will work down to 0.9V.
Why did I toss it? Well, it's efficiency was really poor.
But it had one unique feature, and some folks have been asking me about a low input voltage boost converter that can run off of a single NiMH AA.
It is one of the super duper simple switcher circuits, which have very few parts, like the old Zetex favorite, and most folks can build.
It is based on the NCP1450A found here:
http://www.onsemi.com/PowerSolutions/product.do?id=NCP1450A
The circuit is super simple to put together and needs very few parts for a voltage boost:
On page 5, Figure 7, the closest to our scenario, they recommend the NTGS3446T1 MOSFET, and some other components/values. Just remember to keep the power paths short and the traces broad, and pay close attention to layout rules, mentioned on page 21.
You can find suggested part values, part numbers, and such on page 19.
Typically, if you use a really great inductor, it can push 500mA at 68% efficiency, if the NiMH can hold up to the current draw and maintain 1.2V.
You also need to use a uber low Vf Luxeon, such as a TWOF or TWOG.
Unfortunately, the NiMH are not that stiff, so the efficiency drops into the 60's. Then if you use a TWOH, you are looking in the 60% efficiency range.
Now, if you are willing to back off on the current, to 200mA (to the Luxeon), you can manage to squeak 80% efficiency out of it. As the cell drops 0.9V, the efficiency manages to still hold 70%.
The efficiencies on 2AA pick up alot, and one can get a little bit over 80%.
I'll leave the constant current portion up to your imagination and skills, if you want to add that, which will drop the efficiencies a bit.
Since this converter can't hold much more than 200mA at 0.9V, the discharge curve looks alot like the voltage plot on a discharging cell under load.
I am now aware of one rather recent converter here on CPF that implements this device and the datasheet's recommended MOSFET, the circuit is pretty much right off the datasheet. Sorry, there is nothing unique or special about this, and I know I'm not the only one that played a bit with this really old chip. You have my apologies ahead of time.
The part I used way back then, is shown below, in the red circle:
Anyhow, by the time 2004 rolled around, these low efficiency non-synchronous converters were all tossed aside as newer synchrous much higher efficiency converters became available. Very late in 2003, I had a chance to skim though alot of vendors websites, and pick out some suitable devices. The rewards were great, with efficiencies in the mid-90s. For those interested, feel free to dig through the archives. Unfortunately, only one of these chips will work down to 0.9V.
