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  1. #1
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    Why is anybody using High Frequency Switched Mode LED drivers for LED streetlights?

    Hello,
    My question is why is anybody using High Frequency Switched Mode LED drivers to power LED streetlights?

    As the attached LTspice simulation and schematic shows, simple ‘Switched Linear Regulator’ based LED streetlights are very good and have admirable efficiencies...

    They also need no electrolytic capacitors. Also, they are far easier to protect from mains transients (the biggest streetlight ‘killer’ out there) than High Frequency Switched Mode LED drivers. ‘Switched Linear Regulator’ based LED streetlights are easier to protect from mains transients than High Frequency Switched Mode LED drivers because the FETs can be very high voltage rated, without this badly affecting the efficiency. If a High Frequency Switched Mode LED driver uses a high voltage FET with a low enough Rds(on), then its switching losses become excessive. Therefore, a High Frequency Switched Mode LED driver based LED streetlight will use a lower voltage rated FET and be more susceptible to mains transient related failure.
    Also, the ‘Switched Linear Regulator based LED streetlight’ needs no custom wound transformer and uses few different components.
    The only disadvantage is the larger number of LEDs that are needed to make the ‘Switched Linear Regulator’ based LED streetlight. However, the cost of this is totally offset by the reduction in susceptibility to mains transients…..which means a dramatically lower failure rate and massively less replacement costs.
    The power factor of the “Switched Linear Regulator based LED streetlight”, is not as good as the High Frequency Switched Mode LED drivers can be, however, it is good enough to pass regulations. After all, streetlights have only low Power Factor and Mains Harmonic regulatory requirements because they are only on at night, when the electricity grid is under-utilised, and so power factor does not matter anywhere near so much.
    So why on Earth is anybody using High Frequency Switched Mode LED drivers for LED streetlighting?
    The attached is a Streetlight with 63.5W of LED power. It is 76% efficient. It uses only switched Linear current regulators (which at certain times are “swamped” and have little dissipation during those times).
    The linear regulators are switched ON/OFF at around the twice mains frequency.
    The efficiency actually varies with the mains input level, and in fact at 265VAC input, it is actually 84% efficient, and gives 77.5W of LED power.
    If you wanted to add a microcontroller and change the reference voltage of the linear regulators as the mains input voltage changed, then you could make it >84% efficient over the whole 206 to 265 VAC range. It wouldn’t be workable over the full 100-265VAC mains range however.
    Attached is the LTspice simulation, pdf schematic and mains input voltage and current waveform scope shot.
    So indeed, why on Earth is anybody using High Frequency Switched Mode LED drivers for LED streetlighting?

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  2. #2
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    Re: Why is anybody using High Frequency Switched Mode LED drivers for LED streetlight

    This Switched Linear regulator LED streetlight (attached) is over 96% efficient at 240VAC input.
    It has a power factor of 0.89
    I really am wondering why anybody is doing LED streetlights driven by High Frequency Switched Mode LED drivers. It is surely madness?
    The “Switched Linear regulator” type is far more resilient against mains transients, which are the biggest killer of LED streetlights by far. And it costs a fortune to replace failed streetlights.
    Attached is pdf schematic, LTspice simulation and Mains input voltage and current waveforms of an even better Switched Linear regulator LED streetlight. It uses far less LEDs than the one in the top post.

    As discussed, streetlights are Government property. Your Government is ultimately responsible for Electrical Regulations. Your Government is going to prefer streetlights which have a slightly lower power factor (0.89 in this case) but save money due to the fact that they need replacing less.
    Streetlights that are used in large company premises such as dockyards can have any low power factor that is wished, because big companys pay for their reactive power, so therefore they can pollute the mains as much as they like.
    I can see a worldwide revolution towards “Switched Linear regulator” LED streetlights. Do you agree?



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  3. #3
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    Re: Why is anybody using High Frequency Switched Mode LED drivers for LED streetlight

    The “Switched Linear Regulator” method that I show does have some drawbacks…..The dissipation in the low side FET can be very high at high mains voltage, unless extra circuitry discussed below is added. Also, the LED current can vary with mains voltage and also the LED current varies with the forward voltage variation of the LEDs.
    The solution to these problems would be to have a microcontroller to switch in/out various strings of LEDs, at various points in the overall LED string in order to match the LED string length and current to the mains level, and to the overall forward voltage of the LEDs. This would be like an adaptive control. The problem is, that each switch would not be ground referenced, and so would need its own high side supply and digital isolator to interface to it. This gets a little messy.

    Does anyone know of any FET drivers which pass through a little power to the high side without needing a pulse transformer or high side supply? As you know, hardly any power is needed to power the high side fet drives....if some kind of RF chip were available which could drive the fet and pass through enough power to do so?



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  4. #4
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    Re: Why is anybody using High Frequency Switched Mode LED drivers for LED streetlight

    Does anyone know if there are off-the-shelf modules for the ferrite pulse transformer gate drive circuits which exist in the attached version of the “Switched Linear Regulator” LED streetlight? (LTspice sim and pdf schem attached)
    Otherwise its going to be a lot of circuitry…..because more series FETs will be needed than shown in order to handle the variation in LED Vf and Mains voltage.



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