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Input stage to 24W buckboost LED driver is high in component count.

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treez

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Hi ,
The attached is an LTspice sim and pdf schem of the input stage to a 8-136vdc input , 24w buckboost led driver.
The Vin may also go up to 385VDC for one second. (I presume this is some kind of load dump scenario, or some kind of regenerative braking situation).

Anyway, the input stage has a lot of components. I wondered if there are any helpful modular solutions to this, instead of this kind of discrete circuitry?
 

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  • Input stage to 24W LED driver.pdf
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  • Input stage.txt
    10.6 KB · Views: 42

Thought:
As the first stage is only to block reverse polarity, can't you replace it with a diode?
If the second stage is only powering the comparator and the 5V is decided only by D51 and the Vbe drop in Q5, why not replace it with a simple series resistor and Zener. The power dissipation would be the same, you might lose a little stability if the FET is being used as a current source but you also lose the variability in Vbe drop as well.

Brian.
 
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Thanks, yes if we cant find low enough rdson fets (which we probably wont at the high vds) then we will use a standard diode.....
 

At such low current and with a surge limiter after it, do you really need a very low Rds FET? The difference between all that circuit and a simple silicon diode will be ~0.5V at most. I appreciate that is a considerable proportion of the supply at 8V but then the current is also reduced by the lower load power at low voltage. At 2W load, assuming the converter stage is 75% efficient, the current would still only be about 0.3A.

Brian.
 
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