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clever led driver

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jonny5

Thu Jun 18 2009, 06:56PM

Registered Member #1807 Joined: Tue Nov 11 2008, 07:36AM
Location: San Luis Obispo, California
Posts: 19

The exact inductance isn't critical. More inductance reduces the current ripple, less inductance allows the use of physically smaller inductors. Less inductance (Buck inductor) increases the magnitude of the rate of current ramp (V = L * di/dt). Due to the constant off-time control, the circuit switches at a higher frequency for a smaller Buck inductance! Here's a sim of a 33uH inductor with 9 VDC IN.

Just a heads up: I have never built this circuit! The little experience I have had in electronics has taught me to be wary of simulations...they can't tell you everything. Real life is full of surprises...which makes it so much more interesting!

If you do build a variation of the 555-comparator circuit, let us know how it works!

Before I forget, the inductor shouldn't be allowed to saturate! The toroid power supply cores work nicely. Just remember that they decrease in inductance as the inductor current increases.

polop

Thu Jun 18 2009, 07:07PM

Registered Member #1537 Joined: Thu Jun 12 2008, 06:44PM
Location:
Posts: 51

does the inductor need to take the full current of the led? if smaller inductance is better, and large is ok, can i use an arbetry amount there fore 20 or so 0.8mm diameter windings on the ~1cm toriod, a reasonable MOSFET, a reasonable schokkty diode, would that be a reasonable set of components?
. the other componants are non-changable and i have them already.
what components are the ones that sets the current output?

ConKbot of Doom

Thu Jun 18 2009, 07:21PM

Registered Member #509 Joined: Sat Feb 10 2007, 07:02AM
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Posts: 329

I have 5 of these badboys sitting next to me at the moment. I'm contemplating putting them into a maglight or something similar.(not all of them, though that would be awesome) I'd like to make my own driver, and I was too busy looking around at fancy switchmode converters and the like to think about a simple inductor and chopper circuit.

Given the fixed load of the circuit an inductor/chopper should be able to get a fairly high efficiency, correct?

I cant see the images here at work, but I'll come back to this at home, and maybe I can start messing with a driver circuit for one of these again.

jonny5

Thu Jun 18 2009, 07:29PM

Registered Member #1807 Joined: Tue Nov 11 2008, 07:36AM
Location: San Luis Obispo, California
Posts: 19

Indeed, the inductor carries the full LED current. As for using an arbitrary inductor...depends on how risky you wish to be! More turns on the core will increase inductance and simultaneously lower the peak saturation current. After all, only so much energy can be stored in the core before it saturates.
Since the 555 can't drive the PFET very hard, make sure that the PFET gate charge isn't too high! Otherwise, the switching losses may be significant.
The LED current is set by the reference voltage into the comparator and the voltage dropped across the current sense resistor. BY adjusting the voltage into the noninverting comparator input, the LED current can be changed. Good luck!

polop

Thu Jun 18 2009, 08:25PM

Registered Member #1537 Joined: Thu Jun 12 2008, 06:44PM
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Posts: 51

ok ill go to maplin tommorow buy me a schottky diode and a decent mosfet. ill try this over the weekend

edit: that is a 50 milli ohm resistor yes, not a Megohm

and thats a 10uF electrolytic cap (there is no sign but large capacitance)

polop

Tue Jun 23 2009, 08:30PM

Registered Member #1537 Joined: Thu Jun 12 2008, 06:44PM
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Posts: 51

hello there
i have put the circuit together now but i cannot change the current, it is always 1 amp. i am varying the referance voltage from 0 to .5 and nothing changes.

test notes: if i take out the shottky diode it continues to work the same

inductor is 100uH, current is flowing through it to the led.

as far as i can se the circuit is wired as diagramed.

one thought, if the trig on the 555 is triggered by a low voltage, my comparitor when saturated low is still roughly 1.5 volts out, is that low engh?.

jonny5

Tue Jun 23 2009, 09:20PM

Registered Member #1807 Joined: Tue Nov 11 2008, 07:36AM
Location: San Luis Obispo, California
Posts: 19

The 1.5V minimum comparator voltage is probably OK. The following sim circuit

suggested that the 1.5V trigger voltage did indeed activate the 555 one-shot. Below is the simulated LED current at 9 VIN.

Unfortunately, the Schottky is an integral part of the Buck topology. As the P-FET turns OFF, the Schottly diode turns ON, allowing the inductor current to "free-wheel" during the switch OFF-time. Without the free-wheeling diode, the inductor current has no where to go! As such, the P-FET will experience a massive negative voltage spike! Unless the PFET is exceptionally rugged (dissipating the stored inductor energy via avalanche breakdown), the switch will fail. The following sim shows voltage peaking around -550V...which one would never see across an actual low-voltage PFET...yet one more example of simulation limitations.

Good luck!

polop

Tue Jun 23 2009, 10:10PM

Registered Member #1537 Joined: Thu Jun 12 2008, 06:44PM
Location:
Posts: 51

well i tested the mosfet it hafnt blow, i dont think it has turned off so there fore would not blow as current is just running through it and the inductor. therfore it is the current sencing that is not interuptinging th flow of current. pulling the trigger low with a wire turns the led off.

is there any simplifications that can be made to the circuit, like a simple transistor version or somthing

jonny5

Wed Jun 24 2009, 07:07AM

Registered Member #1807 Joined: Tue Nov 11 2008, 07:36AM
Location: San Luis Obispo, California
Posts: 19

Hi polop!

Simpler is almost always better. Here's an idea that uses a single comparator and the control loop phase-shifts to make a high powered LED driver:

The buck topology keeps relatively constant current through the high-powered LED (simulated by two smaller LEDs in parallel)by comparing the voltage dropped across the current-sense resistor to a regulated reference voltage. Since the rail-to-rail comparator output cannot drive the PFET gate directly, the totem-pole 2n3904/2n3906 pair source and sink the necessary gate currents to turn the switch on and off very quickly. Be careful with the phase shifts adding up around the control loop - the buck inductor, "output" 4.7uF capacitor, and low-pass filter off of the current-sense resistor all contribute to the total phase shifts. You might want to play around with a dummy load before risking the awesome LEDs!

Here's the simulated LED current:

I'm excited to try this circuit (have to find good LED's first)!

polop

Wed Jun 24 2009, 04:48PM

Registered Member #1537 Joined: Thu Jun 12 2008, 06:44PM
Location:
Posts: 51

is there a chip that youguys would reccomend that does all the timing and things for me, ie the only components are the chip the inductor and the diode, that will regulate anywere between 0 and 3amps ? these solutions although they inerest me more, get me so fustrated because i do not have any equipment to dest them (apart from a hardly working ddm)

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