This circuit is wrong. It won't oscillate as drawn. You would have to reverse the inverting and non-inverting inputs to the comparator to make it oscillate. It also has an awful start-up transient until C1 charges which causes the current through the load to be very high initially.

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Mike ML.

 

I think SoftStart was ommitted for clarity of illustration. I'd say the LED would tolerate the high starting current anyway, going on the longievity of my "10 W LEDs setup" in my earlier posting.

The 'Comparator' doesn't have to be a comparator, it can be a transistor. It doesn't need to compare accurately, just fast.

Could the posted circuit have its flaws fixed enough to become a practical circuit?

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Yes you're right, another option is to use a N-channel MOSFET and move it to the low side.

That shouldn't happen, the current ramps up until the voltage across the sense resistor exceeds the reference slightly, the MOSFET is turned off and the current falls. This makes the start up time slow which is only a problem if you want to PWM it which would be a rubbish way of varying the brightness, a more sensible way is to vary the reference voltage.

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Hysteretic converters like this are good but they do have their limitations, namely that they can be unstable under certain loading conditions. Not a problem if you just want to drive a couple of LEDs though.

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This 3 transistor constant current smps oscillates/regulates around load current and could be adapted for 12v-3v or 12v-6v use;
2-transistor Black Regulator

You could use a logic level PFET for the main switch device to get currents up to a few amps with no problems.

 

Here is a constant-current switch-mode regulator derived from Hero's reference. I show how it handles start-up, turn-off, and variation of input voltage from 12 to 15V. I set the output current to 1.7A. The current is set by the 0.33Ω shunt resistor and the Vbe of the 2N3904.

The inductor is 10mH, and must handle 2A without saturation. The dissipation in the FET is miniscule, in fact the only significant dissipation occurs in the shunt resistor (~1W) and the LEDs (~4.5W each).

Now all we need is someone to build it

I see one potential problem with a switcher vs the linear regulator I presented earlier. It will be a bitch to keep it out of any audio in your car. It switches at an audible rate, and varies frequency as the input voltage changes. I tried one like this (for incandescent lamp dimming) in an airplane once, and could not get it out of the headphone audio. I gave up and went to a linear transistorized dimmer instead. There is about 2000 square feet of aluminum to heatsink transistors to in an airplane.

Attached Thumbnails

 

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Mike ML.

 

Simple, design it to switch at an ultrasonic rate and interference is less of a problem.

I would be tempted to use a much smaller inductor but the problem with that circuit is that R4 and R6 will slow the switching of the MOSFET down, making it unsuitable for high frequency operation.

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Here's my idea, LTSpice says it's 83% efficient.

The resistance of the inductor was 100mΩ.

Q1 and D1 convert the LM311 open collector comparator output to a proper push-pull output. I found adding R5 improves the turn on time of the M1 no end.

Using a low capacitance MOSFET helps to improve the efficiency and seems to as important than on resistance.

Using a high speed comparator with a push-pull output will probably reduce the switching losses as well as cut down on components.

EDIT:
The LED current is just under 1.5A and the switching frequency is 40kHz.

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Even at 40kHz, I still see the a noise problem. Since the OP started out wanting 5 LEDs per headlight, the circuits posted drive them three at a time, which means like four strings of three each. If you just build four switchers, the beat frequencies between them would be as bad as running at audible frequencies. You would have to some how synchronize the four switchers so they run at the same frequency. At that, it would be preferable to interlace the PWM so as to reduce the ratio of peak-to-average current drawn by the four strings.

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Mike ML.

 

You're right about the beat frequency issue.

An easy option would be to just stick them all in series and use a boost converter.

If should be fairly easy to add other synchronised switchers, providing they have the same number of LED with the same voltage drop. Just add more inductor, Schotky, LED and MOSFET units. The wavefrom can be easily tapped from M1's source and used to drive another MOSFET using a driver similar to Q1 and D1 (but using a PNP). I can post a schematic if you're interested.

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It's not great. The load current has a lot of ripple and the drive to the switching FET is not as sharp as it could be.

The 3 transistor circuit I posted has less parts, has DC load current (for less noise and better LED life) and (i believe) faster switching of the main switch due to better pos feedback from the inductor itself. And it has separate current regulation and voltage regulation, so the open circuit voltage can be set a fraction above the LED voltage to ensure that LEDs are not killed when connected to a live supply.

 

Don't get me wrong, the Black regulator is truly great, I just wouldn't recommend it in this application.

I can't see how it would be able to achieve fast switching speed from a MOSFET, not without adding another transistor, so I think it'd be better to stick to bipolar transistors for the Black regulator.

Another issue is the inductor; tell me exactly where you are going to find a 2A, 470µH inductor from and how much it'll cost? It'll be almost five times the size as the 100µH inductor and have a proportionally higher ESR which will hamper the efficiency.

Wow, I've just looked back and noticed that Mike's circuit uses a 10mH 2A inductor which will be huge, the size of a small mains transformer.

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Well the black regulator can use a lower value inductor, as it oscillates around the inductor properties. The only problem with a lower L is the switching freq increases so switching losses increase.

2A and 3A pre-wound toroids can be bought from most of the hobby electronics suppliers now, they are about 1" to 1.25" across and available in the 100uH to 470uH range.

Mike's circuit is cool because it doesn't use an output cap and oscillates around load making it fairly failsafe and simple, but my circuit uses similar or less parts count and includes DC filtered output and regulates both voltage and current which can be set independantly.

As for driving the FET its always a problem, Mike's circuit uses a 1k pull down resistor to drive the gate, a weak point there. My circuit needs a logic level driven PFET and uses a pull-up resistor to turn the gate off, again a weak point. It would probably be limited to 2 LEDs as well, not 3.

The main thing I don't like about Mike's is the feedback is not inductor driven. I think feedback is best from the inductor as it helps tune the oscillation to the inductor properties AND provides a lot more oomph in the feedback to ensure fast switching. It woul dbe hard to add inductor feedback to his circuit as its polarity is inverted to the regulator stage.

 

Here's a MOSFET version of the Black regulator.

I've added an extra transistor and diode to make the drive push-pull instead of open collector. I've also put C3 before R1 which helps.

Efficiency is just under 90%, the switching frequency is 30kHz and the current is just under 1.6A.

This circuit only works because I used a low threshold, low Ron MOSFET, the AO6047, which is being pushed to its limits as the output voltage is nearly 8.5V.

The ESR of the inductor is 25mΩ and the peak current is just under 3.2A.

The disadvantage of the Black regulator is that the peak inductor current is high but this is smoothed out by a filter capacitor so there's very little ripply in the load.

The black regulator also has the disadvantage of relying on a low threshold MOSFET to work properly, when the load voltage gets neart the supply voltage.

Attached Thumbnails

 

Attached Files

MOS Black Reg.asc (3.0 KB, 24 views)

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Very nice Hero.

I have a couple of questions;
1. Did you really need Q3 / D2? Was there a problem getting the fet to turn off quick enough? I would have tried just reducing R2 to a couple of hundred ohms or less, and seen if it turned off fast enough. The current used for R2 only ends up going through the load anyway.

2. The inductor current ripple is much larger than I would like or expect, I think you should change L1 to 100uH or 220uH, there is no benefit to a very high switching speed and it causes more switching losses and more I2R losses on the inductor peak current.
The inductor current is best in a range of +/- 0.3 Iout, so from 1 amp to 2 amp would be much better, 1.2 amp to 1.8 amp better still.

3. did you try putting another cap on the output? Putting the main cap there and reducing C3 should reduce current ripple quite a lot. The way you have it set up now slows the current feedback down a lot (hence larger current ripple).

PS. If you like I can put your circuit up on the web page after you fix the inductor current.