High Power LED Controller

[Mr RB]

I modified my circuit per these suggestions. When simulated, it took a few tries to get it to work. The biggest problem with it is that the gate pull-up resistor, even when it was 680Ω could not source enough current to turn on the NFET fast enough. The power dissipation in the NFET during the first few tries was a Watt or two. I borrowed Hero's bootstrapped gate driver, and that fixed that. Now even with a crummy NFET, the dissipation is < 500mW.

Looks good. You used my circuit and improved the gate drive. I wasn't sure that the FET would turn on quick enough, but then I would have used a larger value inductor and run it about 18kHz so the FET switching losses are reduced a lot and may have been ok with the resistor.

How did you go with voltage regulation?? I see you used a compensating resistor as I mentioned (R2 110K). Did you simulate for the full range of 12v battery input voltage and check current regulation?

Next issue, which always bothered me with most of RB's circuits, is that the base of Q1 was driven far enough below ground so as to exceed the Vbe reverse breakdown, so I added the diode clamp D3, and limited the current with R6. I played with R2 to equalize the output current both at 12V and 15V input.

Yep, that's what Ctime is for (which you left out). The energy from Cfb won't drive the base voltage lower than about 2v if you use the cap from base to ground (which is normally much larger than Cfb). This is a fundamental part of the black reg, the other thing it does is produce a timed off-period which is basically the way all the SMPS IC's work using a cap to give a timed off-period after the switching event. With Ctime you will get a lower switching frequency and no need for the diode.

Added: I changed the inductor to 33uH; that just raised the switching frequency without changing anything else. When the input is 15V, the average input power (from the battery) is 17W. The LED power is 16W, but somehow I dont believe that this circuit is 94% efficient?

That's about right for a simulator. With the real thing you get milliohms everywhere in the solder joints, parts leads, PCB tracks etc which is usually about 5% lost. So it would be about 90% which is what I expected. That change we made placing the current sense resistor in the input current path, not in the buck loop, is worth 2 to 3%. I still want to see your Vin to Iout regulation chart.

Hero999 said;
RB. Why are you so obsessed with using large inductors?

Because lower switching speeds mean less switching losses, means less fussy gate drivers, less parts, less to go wrong, lower switching speeds mean higher reliability in silicon. An inductor with 20 turns of 1mm wire doing 20kHz is an incredibly reliable part compared to a FET silicon die doing 100kHz... I would also prefer to have heat dissipated in that inductor than in the FET given equal total circuit efficiency.

I understand the modern trend is toward tiny inductors and very high switching frequencies. Like modern cars use tiny engines revving their guts out. But I prefer the casual revs of a big old V8 or Harley that will go forever. Just because you can go smaller and higher revving doesn't mean that it's the best way.

For anything over an amp I just wind my own toroids here with 1mm wire and have no problems getting 330uH with 10-15 milliohms. If it was for a commercial product things would be different though.

 

[Hero999]

Have you tried using high gain transistors in a Black regulator?

You could probably increase the current rating of the circuits on your site by a factor of at least 5 without alterating any other component value apart from using better transistors an a higher current inductor.

The ZTX690 (NPN) has a maximum saturation voltage of 0.5V with a collector current of 1A and base current of just 5mA.

The ZTX790 (PNP) has a maximum saturation voltage of just 0.45V with a collector current of 1A and a base current of just 10mA.

**broken link removed**
**broken link removed**

 

[Mr RB]

Hi Hero, yeah I bought a few types of Zetex PNP transistors from Farnell a few years back and had a play. They never worked as well as I expected! Although they have technically high gains and low saturation voltages, they just wouldn't switch as fast with a low base drive current. It's like they have a "gate capacitance" or something like a FET, for DC they will hold the low sat voltage with just little base current but in active switching they needed a lot more base power than the DC specs would let you believe. I think they were "oversaturating" and hard to turn off without active turnoff devices added to the circuit.

I found the BC337/327 were much more efficient in needing only a little base power to switch them fast, both on and off. One of my doodads around here has a ZTX951 (i think) in a black reg type SMPS, it worked ok but I remember not being that happy with it at the time.

 

[Hero999]

I see what you mean, the input capacitance for the PN2N2907 (I couldn't find any information on the BC327) is only 30pF and the ZTX790A is 225pF, 10 times more.

https://www.electro-tech-online.com/custompdfs/2009/08/MM2FMMBT2907A.pdf

Still, 225pF is much lower than a MOSFET.

You could use a lower value pull-up I suppose.

Still, I think a MOSFET plus push-pull driver will give you the highest efficiency at higher currents.

 

[marcbarker]

I think they were "oversaturating" and hard to turn off without active turnoff devices added to the circuit.

Storage charge? It's those pesky minority carriers that do it.

You aggravate that if you drive too much saturation. I get round the problem by not saturating more than I need to, or do what you do and use BJTs less affected. (i.e. replace a 2N2222 switching with a 2N2369 which is optimised as a saturated switch)

 

[Hero999]

Thanks, I didn't know about that.

Well I think I probably do remember covering it at college but we weren't told of the practical implications of minority carriers.

 

[Mr RB]

Ok, since the thread seems to have stabilised and there are no new designs I have added Hero's regulator and my own constant-current design to my SMPS web page;
2-transistor Black Regulator

I also added Mike's last 2 designs are they are relevant and good designs too for people who want to make SMPS LED drivers.

If anyone wants me to change anything on that page please say, otherwise I will leave it like that until i get some time to solder up some actual designs and chart the performance etc.

 

[Hero999]

I'm flattered that my design is on your site.

I hope you get round to testing the designs suggested here. I wonder who's will be the most efficient.

 

[audioguru]

Hey!
Mr. RB is Roman Black.

Hi dude,
You have a nice SMPS.

 

[muvideo]

and a buck-boost?

Hello, I've just registered to post a comment in this interesting thread:
what do you say about a buck-boost?
I'm not expert in smps, and I'm only trying to drive decently some high
power leds. I was considering the ZXSC310 from zetex as driver, the problem
is that the Vf of the leds I'm using may be higher or lower
of the 12V battery voltage.
Tomorrow I try to build a prototype of a zetex-based driver and
one circuit like the one in image, based on last MikeMi driver.

**broken link removed**

What do you think?
Bye, Fabio form Italy...

 

[Maia]

Hi Fabio,
Did you build your circuit? It works? I tryied to simulate your circuit in Proteus and neither your nor the MikeMi´s works.
Thank you for any help.
Regards,
Maia

 

[muvideo]

Hello Maia, I have built the circuits, and both work. The problem with my circuit
is low efficiency (less than 70%), probably the problem comes from poor pcb and
components, the mosfets I had available, have high gate charge, and the diode is
an MBR1645 that is huge for the task.
The MikeMi one is better but I have only led strings with 3 or 4 series chips,
as soon as I'll have SSC P7 I'll test with 2 leds, I tried the circuit only with 4.7 ohm resistor from 12V supply.
As for the simulation, try LTSpice IV from linear.com, is free and the simulations i made
of these circuits agree with the real behaviour.
Bye, Fabio.

 

[Mr RB]

Use a much larger inductor. That will slow the switching frequency right down and reduce the FET switching losses caused by your high gate charge.

You are also powering the FET gate from a reduced voltage because you have a zener voltage regulator before your regulator transistor. Get rid of the zener.

Also your LEDs/diode/inductor are all wrong. The LEDs appear to be operating in flyback only?? Why have you changed it so much from Mike's design?

 

[muvideo]

Thanks for your suggestion,
I try to explain... (apologize for my english
I had leds with 3-series chips (Vf about 9-11V) and 4-series chips (12-15V).
I was exploring the possibilities of driving these leds form 2 kind of power sources:
8-16V (1 12V lead battery), and 16-32V (2 12V series lead battery).
I was searching for a circuit that could drive also 2 pieces of each of the leds I had
in series (Vf 20 to 26V). All this at about 1A. So I tought that an inverting buck-boost
could do the job. I was wrong, or at least I was unable to realize a good circuit.
The zener net was there to limit the voltage on mosfet gate at input voltages > 20V,
and I tough that could reduce the variation of output current with wide excursion of input voltage.
I didn't mind of the high gate charge of mosfet because I planned to use other mosfets with reasonable gate charge. As for inductors, I had plenty 27-33uH hi current so I used these.

In the meantime I built also a driver with ZXSC310, the buck-boost version suffered
form the same problems so there is a problem that I don't understand, the boost-only version (8-16V in 20-26V out) is good.

Now I have ordered SSC P7, the circuit of mikemi is fine for the job of
driving 2 leds from 8-16V input.

Fabio.

 

[gbbickerton]

Hi does anyone have a circuit to drive a 1W or 3W led from a 3V or 6V battery supply

 

[Hero999]

Have you read the whole thread and looked at all the attachments?

There's enough information provided for you to design your own.

 

[gbbickerton]

Yes I have read the whole thread, while I could design one using an ic I don't think I could design a transistor circuit to operate at 3V. I just wondered if anyone had already designed one as there is no point in reinventing the wheel as it were.

 

[Hero999]

For 3V you need a boost converter and a 3W LED won't last for very long when run off a couple of AA cells.

6V is easier because you can use a similar circuit to the one described in this thread.

 

[nrannalli]

Dimming

I am trying to get my head around buck regulators and dimming. Normally I could put an N-Channel MOSFET like a FQP50N06L between the LEDs and ground and then use a micro controllers PWM @ the gate to turn it off and on. Does that same concept work on buck regulation? or can I just apply the PWM on one of the other transistor? It seems that pulsing another FET would mess with the regulation am I missing something?

 

[kjennejohn]

Does something like this board help anyone for power control situations? It's only $25.
"Introducing the low cost Basic ATOM Nano Driver board. The Basic ATOM Nano Driver board features a L293 H bridge, 4 IRF520 MOSFETS, ULN2803 Darlington driver and screw terminals for easy access to outputs. The Basic ATOM Nano Driver board also includes 7 headers for driving servos or reading analog peripherals such as potentiometers, accelerometers, temperature sensors and more."
They claim each FET can handle 9 Amps, up to 24V! The processor on board has a free BASIC compiler for it, and does PWM, so you can literally dim lights to different degrees. The blurb is at
**broken link removed**
You can get the data sheet from there.
Have a nice project!
kenjj