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    Is "Direct drive LED" more efficient than using a switch mode LED driver?

    Hello,
    Is this so?
    ("direct drive led" means the leds are connected direct to rectified mains with no switch mode driver used...instead some special switchs are used to simply switch in (and then out) more leds as the mains half sine rises and falls at 100Hz)

    The below article says that "centralised" direct led drive is less efficient than using a switch mode driver, but does not state efficiency for the "distributed" direct drive led approach.

    http://www.electronicsweekly.com/new...tring-2016-04/

    •   Alt16th February 2017, 23:22

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    Re: Is "Direct drive LED" more efficient than using a switch mode LED driver?

    Is "Direct drive LED" more efficient than using a switch mode LED driver?
    Strictly electrically speaking, No. SMPS are to this day the most efficient because the main principle of operation is timed controlled On Off as a mean of determining how much power goes to the load. In between the voltage source and the load there is a switch and some coils and capacitors. Switch do not absorb energy, where as coils and capacitors will absorb energy only to eject it at a further moment. In theory, no power is lost during the transformation cycle. In practice today's SMPS reach 95% efficiency.

    In a Direct Drive Mode, to power some LEDs, first those leds are not the typical 3V type (or if they are, there are a multitude in series such that each LED receive no more then it's maximum rated voltage) Direct Drive mode will exploit the LED range from minimum Light emission voltage to the Maximum rated voltage. LEDs can receive a little more voltage then the typical max without burning immediately. This feature is exploited in Direct Drive mode. Simply put, Direct Drive Mode is an arrangement for which the LEDs receive a range of voltage from 0 to beyond max and will emit light in a not so efficient way compared to a pure DC power. This method will result in some power absorption that will not necessarily transform into light as efficiently as DC.

    It's pretty much the same as if you were driving a 3V LED with a 5V supply with a resistor in series to avoid overheating the LED. Direct Mode is not 100% direct. It is a kind of Direct Switched mode that will happen many times during the 1/100 second cycle. The Direct Mode circuit that monitor the LED power is powering each LED gang in such a way that each gang do not receive more then the absolute tolerable maximum voltage. Such slack method has it's disadvantages, the first being that you are not using the LEDs in the most efficient way possible. That inefficiency is reflected in lost heat instead of the desired light. The LEDs are going to emit light and Infrared as a form of heat energy as well as heat by convection to the ambient air or heat dissipating material to which it is fixed. In such case the LED will act as a light emitting diode as well as the resistor in series stated in the above hypothetical circuit. In Direct Mode Drive it is the engineer responsibility to make sure all the LEDs are not receiving more then the critical maximum current for life expectancy.

    So, electrically speaking, Direct Mode, which should rather be called Direct Switched Mode, will not exhibit 95% efficiency like an SMPS but will end-up costing much less then a bulky Coil,Electrolytic Capacitor, Switcher SMPS unit and will be much smaller and much lighter.


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    Re: Is "Direct drive LED" more efficient than using a switch mode LED driver?

    Thanks, I agree with you when you say that the leds of a direct drive luminaire will run hotter than a lumens_equivalent LED bank driven by a SMPS driver.
    This is surely bad because hot leds deteriorate quicker in terms of their light output.
    Also, you say SMPS can be 95% efficient, however, with an offline SMPS preceeded by a PFC stage, the efficiency would be more like 85%.
    So , I wonder what is the efficiency comparison between direct drive leds and SMPS driven leds for offline applications where a PFC stage is needed?
    It doesn’t seem to be possible to find this out anywhere on the web.

    I am presuming that direct drive LED systems must be more inefficient than SMPS driven LED systems, because otherwise the entire market for switch mode led driver controllers would disappear overnight....but the question is how much less efficient is direct drive compared to SMPS method?
    Last edited by treez; 17th February 2017 at 11:04.



    •   Alt17th February 2017, 10:52

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    Re: Is "Direct drive LED" more efficient than using a switch mode LED driver?

    To be honest with your I cannot quote any research on that question. I would have to guess an answer on that.
    First, when I mentioned that SMPS can achieve 95% efficiency I was referring to a best world situation. For example, one can build an SMPS powered by a preliminary power conditionner made of a full bridge rectifier output going into a big value inductor capable of handling at least double the current consumption of the SMPS. At the output of the inductor you would measure the average voltage rather than the peak voltage from a standard Bridge Capacitor common arrangement that we see in almost all configuration. The advantage with this big coil configuration is that the current going into the SMPS would be constant (no peak) which would continually draw current from the AC source even when the voltage reached near zero. This would eliminate the big Peak current surge that common full bridge power supply induce on the power source when the voltage reach peak. That in itself is inefficiency built-in. You don't see such configuration because big inductors are much more expensive then electrolytic capacitors.

    Back to real world situation. My guess is that $ being what it is, building efficient circuitry is not the most important manufacturer priority. The priority is cost over return at the sale. How much profit will the manufacturer will pocket is the way it goes. And that is true with SMPS as well as Direct Drive configuration. This would certainly contribute to SMPS being more in the range of 85% in the real world instead of my 95% statement. That I agree with you.

    Now, as far as which is the most efficient between the two is prone to lab experimentation and would have to be considered on a case by case evaluation. There is a reason why the author of this article didn't show any figure. Probably because it is, at the moment, impossible to pullout other then by experimentation.

    I would bet on SMPS being more efficient. There is no free lunch, goes the saying. If you save on engineering and quality parts you loose on overall efficiency.

    Which brings me to my closing statement. I predict that in a near future you will see a combination of Direct Pulsed Drive coupled with some sort of very quick starting SMPS in order to profit of both worlds. Short Pulsed SMPS would not require as bulky coils and capacitors and a combination of both technologies could be implemented with IC and a few passive parts. Only the future will tell. Cheers



    •   Alt17th February 2017, 14:20

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    Re: Is "Direct drive LED" more efficient than using a switch mode LED driver?

    No LED switcher uses a separate PFC stage. They are implementing a single buck or boost PFC switcher driving the LEDs with sin² current waveform.


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    Re: Is "Direct drive LED" more efficient than using a switch mode LED driver?

    No LED switcher uses a separate PFC stage.
    Sorry but surely eg 120W LED lamps use a PFC stage then downstream LED driver stage. The LED driver stage doing the isolation and LED current regulation.



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    Re: Is Direct drive LED more efficient than using a switch mode LED driver?

    I presume we are talking about general purpose LED lamps, e.g. 1 to 15 W power range, either LV (12 - 24 V) or HV (115/230V supply), like the LED bulb shown in the "Electronics Weekly" article. The usual power supplies that I see implement a single switcher stage which does the current regulation and PFC control in one.

    - - - Updated - - -

    I also wonder how you would drive the 8 chip HV lamp shown in article (about 25 V total Vf) by a "direct switcher" with 115 or 230 V?


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    •   Alt17th February 2017, 15:08

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    Re: Is "Direct drive LED" more efficient than using a switch mode LED driver?

    When I see these types of wonderful new topologies and devices, my stance is to wait until an external lab has performed actual real world measurements and evaluations.

    For instance, the article briefly touches the issue with flicker. How annoying would it be? Fluorescent light bulbs had phosphors with high persistence to remove flicker in powerline ballasts, and even then some people would find it annoying.
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    Re: Is "Direct drive LED" more efficient than using a switch mode LED driver?

    Just noticed that the article mentions multi-chip "HV" LEDs like LUXEON 3535 HV, available with 12, 24 or 48 V Vf per package. So it it can work.



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    Re: Is "Direct drive LED" more efficient than using a switch mode LED driver?

    Well i am guessing that "direct drive LED" is much more inefficient than SMPS LED drive, otherwise, Switching Mode LED drivers would have been obseleted. Direct drive seems to be for street lights.........a country's street lights burns a lot of power, altogether so you would think they'd be under law to use the most efficient method?
    Nowhere on the web can any efficiency figure be found for "direct drive led".



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