Powering LED light displays more efficiently.

[onload]

Dear Engineers,

I am working with LED light displays. There is a series chain of 5 LEDs whose brightness must be controllable. This series chain is across a 24V DC supply. There is a series NPN transistor in series with the LEDs which "burns off" excess volts if the LEDs need to be dimmed. This series NPN has its base fed by an op-amp which has a 0-10V input supply. -The brightness of the LEDs is thus controlled by adjusting the 0-10V supply. -However, the series transistor is "burning off" excess volts inefficiently. -Does anyone know if it is feasible to apply volts to the LEDs using a switch mode convertor?

Their are hundreds of these series LED chains and i think it would be saving electricity to use a controllable switch mode convertor to apply volts to the series bank of LEDs.

Any thoughts greatly sppreciated.

 

[RajeevCell]

OnLoad,

Can you post the circuit diagram? Did you design this circuit, or are you debugging someone else's work.

Your description/interpretation of LED driver looks wrong.

Is there a series resistor with the 5 LEDs and the transistor?

 

[MacGyverS2000]

Controlling LED brightness by varying the suply voltage is bad design, particularly since they're heavily non-linear with voltage control. Consider using PWM to control brightness. If you PWM the transistors, your losses will become a (roughly) percentage of the total power used... if you dim the LEDs and total power drops by 50%, your transistor losses will also drop by 50% (within reason). It sounds like with your current method, total power usage remains the same regardless of LED brightness, and that's bad.

A switch-mode power supply could save you some serious power if your supply voltage and LED string voltage are quite different (say, if your supply is at least 25% higher than the LED drop voltage), but it looks like you need to rethink the entire design before heading down that path.


Dan - Owner

http://www.Hi-TecDesigns.com

 

[VEBill]

In the consumer world, I'll bet that the majority of dimmable LEDs are done with PWM (because it is typically cheaper than using a high power transistor and heat-sink). You can sometimes see the high-frequency blinking in a dark environment by turning your head quickly and your vision will pick-up a series of dots.

The simple circuit is a series power FET and feed to gate with an adjustable duty-cycle square wave. Choose the frequency to be appropriately high so you don't see the blinking. Watch out for EMI from overly-square edges.

 

[onload]

Hi,

RajeevCell, -the series chain of LEDs is basically the collector load with the NPN in common emitter arrangement. There is an emitter resistor.
The circuit definetely works. I'll try and post it. I have it in .jpg form and also .png form. Have you any ideas about how i can post it here? I've tried before and it doesn't seem to paste in to these captions. Incidentally, the maximum current through the LEDs is 100mA.

VE1BLL......the NPN transistor is not heat-sinked, but there are fans blowing over it. (there are many of them as there are very many chains of LEDs)

I think that what i need to do for the switch-mode design is use the 0-10V supply to vary the PWM to a MOSFET in series eith the LED chain. Does anybody know any sort of "off-the-shelf" type hardware solutions to this? Alternatively, any links to application notes for this would be greatly appreciated.

 

[onload]

Apologies. Max current through the series chain of five LEDs is about 25mA.

 

[ScottyUK]

The old SG3524 IC is basically a PWM subsystem on one chip. It's not the latest or greatest but there are a lot of 'simple' PWM controllers based on this family of ICs. It's fallen out of favour somewhat now that drives tend to use high speed DSP chips to generate the complex switching patterns required by modern control strategies.

http://focus.ti.com/docs/prod/folders/print/sg3524.html

Google [blue]PWM controller IC[/blue] and there are numerous hits.

----------------------------------

Sometimes I only open my mouth to swap feet...

 

[itsmoked]

Posting pictures;
faq238-1161

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[Jarhead1]

There are quite a number of switching power supply circuits that are very efficient, and can also dim, they work on current regulation. If you are doing white, then the tint may shift a bit while dimming with current control.

Here is one place you can look, click on the links down at the bottom:

http://www.lumiledsfuture.com/products/power.cfm

Other items that sound like they would be of more use for you:

http://www.linear.com/pc/productDetail.do?navId=H0,C1,C1003,C1094,P12833

These switchers only do 3000:1 dimming, not sure how much you need:

http://www.maxim-ic.com/quick_view2.cfm/qv_pk/5001

 

[felixc]

LED driver ICs is a hot topic with chip manufacturers, since white LEDs will soon become integrated into domectic lighting products. National Semiconductor also has such offerings. Like the others are saying, they rely on switcher principles, even stocking energy in inductances to drive and dim the LEDs while keeping a good efficiency ratio. Reading the datasheets of these chips you will learn a lot about how to optimize your power consumption.

 

[onload]

Thankyou for these posts......i will view them as all i want is basically a Buck convertor (non isolated) with the LEDs as the load. The LEDs we have vary brightness linearly with voltage control.......the currently in use linear regulator circuit (which i want to get away from and use SMPS instaed) varies the voltage across the LEDs from 13V (LEDs minimally ON) to 18V (LEDs at full brightness).

I am restricted to keeping the currently installed electrical fixtures in the units.......so basically i have the 24V rail to feed to the Buck convertor and a 0 to 10V variable supply (this uses same ground as the 24V supply)which i will have to somehow use to vary the duty ratio of the PWM.

 

[MacGyverS2000]

I guarantee you those LEDs are not even coming close to dimming linearly if you're using linear voltage control... guarantee.


Dan - Owner

http://www.Hi-TecDesigns.com

 

[itsmoked]

I bet he'll get a lesson in e[sup]x[/sup]!

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[RajeevCell]

OnLoad,

We have not seen your original circuit yet.

However, if I'm not wrong, you have a 24v dc supply (with good current capability) and a variable 0-10v dc signal (not much cerrent capability). Is it an industrial setting?

You should use the 24v supply for powering the LEDs, but should control it using PWM. You already have a transistor, so turning the LED branch ON and OFF is easy. There should be a resistor in series also.

A PWM chip (like TL494) will control the base drive of the transistor; turning it ON/OFF. PWM duty-cycle will be controlled by the 0-10v signal. Brightness of the LEDs will depend on the duty-cycle, which will in turn depend on the variable signal, linearly.

Several characteristics of your LED need to be known for optimizing power usage.

1. Normal LED voltage drop (ususally slightly less than 2 volt),
2. Normal LED current (usually 10 mA).

You mentioned the LED current as 25mA. Is it absolute maximum? If so then do not design for that value.

Your 24v supply will always waste some power in the series resistor. Incidentally, if you used 10-LED series, instead of 5-LED, you'd use smaller resistor and saved quite some power. Check, if that is a possibility. You can replace the NPN trnasistor with a MOSFET also.

Yeasir Rahul

 

[onload]

Hello,

Regards the linear circuit we are trying to get away from....unfortunately its inadequately demonstrated here but i cannot provide further deteil....you get the general idea that its linear and not a switcher:-

http://i16.tinypic.com/47m9eoi.jpg

In a typical display unit, there would be 160 of these series LED chains with the NPN.

I suggested replacing this with a switcher (eg based on a buck with the MAX16802B)...i suggested it would run cooler (giving a lower failure rate) and use less electricity....However, colleagues seem to think i am well off the mark...that there would be big problems with harmonics and also RF interference. Not to mention the extra cost.

Is replacing the linear one with a switcher really such a bad idea?

The linear one wastes half the input power when set for minimal brightness, and wastes 30% of the input power when on full brightness.

Do you think my solution with lots of mini switchers would cause havoc with harmonics and RFI?

 

[RajeevCell]

OnLoad,

From your circuit diagram, it is evident that the NPN transistor will waste a lot of power. Replace the simple OpAmp with a PWM chip that is controlled by the 0-10v signal. I suggest to use TL494. Look at its datasheet.

Using switchers in place of the 24v supply will introduce RFI, indeed. But so will the PWM.

If you plan to vary the supply voltage of the LED series for modulating brightness, then it's an impractical plan. Listen to other posters. Use the same 24v supply, but modulate it with PWM. That will not only give direct control over brightness, it will be more energy efficient also.

Regards
Rahul

 

[richs]

Hiya-

I concur with the other posters that some sort of PWM would
be a good idea. To make the circuit more efficient, you
don't want to waste power in heat.

You might be able to get away with a simple 555 timer (I know,
that circuit has been beat to death). In a simple
astable multivibrator you can change the duty cycle of the
output. Reference:

http://cache.national.com/ds/LM/LM555.pdf

pages 7 and 8 for additional information. This circuit
can be used to drive the NPN transistor into and out
of saturation. By actively modifying the duty cycle,
or rather, either the charge or discharge times of the
timer, you can achieve the necessary brightness change.

Frequency is not an issue with the driving of the LEDs,
however, as mentioned earlier, RFI might be an issue.

Also, be advised that the selection of a capasitor with
a stable temperature coefficient might also be an issue,
depending upon the environment, and "user satisfaction"
issues. But, in general, this should give you a good
first cut at doing some PWM work with the LEDs.

I would, of course, have the 555 drive an external active
device (like the NPN transistor) rather than relying
upon the drive capabilities of the timer itself.

Hope that this helps.

Cheers,

Rich S.

 

[onload]

Hello,

Thankyou for the advice about driving the NPN from a 555 timer. I presume this means that the LEDs are basically turning ON and OFF very quickly. I am wondering if their will be a visible flickering at the lower brightness levels?

It is quite interesting that most posters have suggested using PWM, becuase within the company, it is considerd far too expensive, compared to the current linear solution.

Incidentally, the 24V supply in my (very incomplete) diagram

http://i16.tinypic.com/47m9eoi.jpg

cannot be made variable. This is because there are hundreds of LED chains across it, and each chain must have its own brightness variability, which must obviously be controlled from within the chain itself.

 

[RajeevCell]

I presume this means that the LEDs are basically turning ON and OFF very quickly. I am wondering if their will be a visible flickering at the lower brightness levels?

When everyone suggested PWM, they meant exactly this - "turning ON and OFF very quickly", and doing it with variable duration of the ON state.

If the turning on/off frequency is above 24 Hz, then no flicker will be visible to human eye. I'd suggest to use frequency in the range of several kHz.

Which company do you work for? Is it mainly a software copmpany?

Regards
Rahul

 

[IRstuff]

Flicker is a function of frequency and persistence. Since LEDs have essentially zero presistence, their flicker is more noticeable than that of a standard CRT. Standard display terminals set to 60Hz refresh have noticeable flicker when viewed with peripheral vision, since the rods and cones are not as saturated and therefore have faster response.

I'd recommend running at a minimum of 60 Hz to eliminate most of the flicker.

TTFN