Electronic Circuits and Projects Forum

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.

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.

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.

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.

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.