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18-137VDC on Electric Trains to provide power for 20W LED lamps

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treez

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Hello,
We have a spec for 20W LED lamps to be powered from a train power source which is 18V to 137.5VDC.
In fact, its wider than that, there is an emergency requirement of 4W LED power for times when this supply goes down to 8V. (-but that might be when the emergency battery pack switchs in?)
Do you know what it is on the trains which is providing this DC power source? Is it a big battery bank?
I believe they are electric trains. For example, it’s the trains that will be used in the HS2 project, soon to come in UK.
Do you know what kind of source impedance this 18-137.5VDC power source will present to our 20W LED lamps? We don’t have room for much input capacitance in the LED lamps, so we are hoping there’s not much wiring inductance going back to the power source. Do you know what the wiring inductance is likely to be?
Unfortunately, the client is not coming forward with much information on this for us.
 

And of course, they want to pay for this 2 GBP. Otherwise they'll go to a Chinese supplier.

Turning off my sarcasm mode......No I don't know what kind of power source has such wide voltage range. Definitively a battery bank on its own won't have it. Perhaps it is a hybrid system, whereas in the case of a normal pantograph voltage failure, reverts to an onboard lower-voltage battery bank that allows the train to limp back to safety.

I once experienced such an outage in the Mexico City subway, which has the same design as the Paris Metro. We were humming down the track when suddenly the lights dimmed and the train decelerated but did not stop. We limped to the next station, where the emergency lights were all on. I realized that there had been a major power outage.
 
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Thanks, i was also wondering if its a battery bank, but with regenerative braking feeding into the battery bank and causing the overvoltages up to 137.5V ?
I coudl be wrong, but i think the nominal battery bank voltages for the HS2 trains are to be 24V, 48V and 96VDC ?
The wide voltage range tells me its a really "dirty" power supply and they probably aren't too worried about EMC etc...its probbaly more to do with regenerative braking and "green" stuff etc.....so just a small filter and small capacitor bank should be OK in the converter, since EMC is not the main driver here,.....its being energy saving.
 

Hi Treez, we do 10 - 150VDC routinely on some of our DC/DC converters - at up to 200W, so 20W = trivial.

If you want a solid schematic - let us know
 
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Thanks yes,, but if the source inductance in our case is very large then our buckboost doesnt have enough input capacitance to avoid instability.
Theres not enough room for any more input capacitance. We just have 1uF for each 5W section....
If the source impedance is low then i agree its trivial.
 
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Sepic or Cuk - have as much line L as anything ...
 
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Thanks, i am aware of the coupled sepic where putting "Leakage" inductance in series with the "upstream" coil means the ripple gets referred to the other coil, and its virtually a smooth dc input current, i suppose that is what you refer to?
We have unfortunately done a plain buckboost (with leds referred to vin) and with 1uF input capacitance for each 6W buckboost, just 22uH of source inductance kills it off....makes it go unstable.
I reckon this kind of power source often has at least 22uH of source inductance?
 

the plain old un-coupled sepic works fine too... any line L adding to the input L ...
 
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