What we need...

[Saaby]

What we need is a super-efficient step down AND step up converter. I think something similar is at work on the Techass verilu thing. Say you wanted to run 5 Nichias* at 3 volts. You could run it off 4 AAs (6V) at which time the Super Circut would run as a highly efficient step down. At that 3V point it would run direct drive--no step up or step down. Below 3V it would operate as a step up.

The beauty is that you would get extremely long runtime (6 Volt to 3 Volt swing) and, if it was designed right, constant brightness.

Maybe the circut in the Opalec already does this to some extent? Had I read of people using it in 2 123 torches or was it only 1 123?

*The goal for Luxeon always seems to be bright, brighter, brightest--but there's no reason you couldn't use a Luxeon instead...just when people think Nichia they think "Efficient.

 

[jeff1500]

I saw a step-up and step-down chip on the maxim web site.

Maybe this one:
http://dbserv.maxim-ic.com/quick_view2.cfm?qv_pk=1807

 

[Jonathan]

Originally posted by star882:
A SEPIC circuit is both step up and step down.

<font size="2" face="Verdana, Arial">Quite correct. However for an isolated load such as an LED, which doesn't care about polarity, a simple inverting converter will be simpler and as effective for doing the step up/step down job.

When the input voltage is nearly equal to the output voltage, a SEPIC circuit will not be as efficient as a matched cost buck or boost circuit.

Thanks for pointing out the SEPIC design, I knew of its existence, but your comment made me take a second look. A particular attractive aspect (which is not true with reference to the inverting designs) is that the SEPIC design might be made to work with available current regulating boost converter chips, for example, I might be able to use the LT1618, which I am very familiar with, to make a step down driver for a Luxeon.

-Jon

 

[star882]

"However for an isolated load such as an LED, which doesn't care about polarity"
LEDs do care about polarity.
They are just a special type of diode.

 

[Jonathan]

With Nichia's, you can get roughly the same effect by putting the LEDs in series, making the series string voltage greater than the supply voltage. Then you are simply operating as a boost converter _all_ of the time.

The cost is that direct drive will always be more efficient that a boost converter, and a step down converter will _usually_ be more efficient than the boost converter. However, the simplest approach is to use a 'buck-boost' converter, which isn't direct drive even when the battery voltage and the LED voltage are exactly the same, and to have a device which is actually a separate boost, direct, buck system with switching would be pretty darn complex.

I've been using a boost converter circuit which will produce up to 36V output, with an input voltage range of 2V to 18V, although there are some power restrictions that mean you can't get the full output voltage at high current with the minimum input voltage. With 2V in, the circuit would happily run 5 Nichia LEDs in series at 20mA.

-Jon

 

[star882]

A SEPIC circuit is both step up and step down.

 

[Scoper]

star882: could you please direct me to more info on SEPIC circuits?

 

[CNC Dan]

Jonathan said"However for an isolated load such as an LED, which doesn't care about polarity"

Originally posted by star882:
LEDs do care about polarity.
They are just a special type of diode.

<font size="2" face="Verdana, Arial">I think what they mean by polarity when talking about DC/DC converters is that the NEG IN and the NEG OUT arn't the same. Other times they are the same and you can have + and ground input and - and ground output. This would be a negative supply. Most would be + and ground input and + and ground output. This would be a positive supply. If your input ground and output ground don't have to be connected, then the polarity of the supply doesn't matter. I think that is what Jonathan meant by "isolated load".

 

[Jonathan]

Originally posted by star882:
"However for an isolated load such as an LED, which doesn't care about polarity"
LEDs do care about polarity.
They are just a special type of diode.

<font size="2" face="Verdana, Arial">CNCDan got it right. LEDs of course care about the polarity applied to the LED terminals, but they don't care which of those terminals is _ground_ in your circuit.

The three classic single inductor non-isolated switching topologies are the buck, the boost, and the inverting converter. The buck takes a higher voltage and steps it down to a lower voltage with the same polarity relative to ground. The boost takes a lower voltage and steps it up to a higher voltage with the same polarity relative to ground. The inverting topology takes its input and produces an output with the opposite polarity relative to ground, but the voltage may be higher or lower.

Since the LED doesn't care if its positive or negative terminal is grounded, you could connect the positive terminal to ground, and use an inverting converter producing a negative output to supply the negative terminal. The input supply would ground the negative terminal of the battery, and the input voltage could be higher or lower than the LED forward voltage.

-Jon