LEDs turning off causes EMC problem?

[treez]

hello,

i am doing a lamp with 8 300mA LEDs powered by SMPS.
Source is 13.5V

The LEDs must come on one-by-one in a line then stay ON.

The simple way is to put the 8 in series then switch out FETs which short out each of the LEDs.

However, this means that just as the next LED is switched ON, -all the LEDs (that are ON) go off then on again.
(since initially there is not enough forward voltage to light the LED string with an extra LED in it -until the voltage builds up).

.......this cannot be seen as they only go off for 1ms but the sudden OFF-going of LED current will cause a bad EMC problem.......is this true?

...how may this be mitigated?

 

[treez]

This circuit is an alternative to the above disclosed Sepic idea.

Could readers possibly kindly confirm that this is indeed the lowest-component-count method of achieving my aims, set out in the first post ?

 

[Skogsgurra]

Switching off series connected loads by shorting them out is not as unusual as some of the gentlemen here seem to think.

It even has its merits. You can use one SMPS operating in constant current mode and with enough voltage range to supply anything from one to eight LEDs. That is what would try. Losses in the SMPS can be kept low and you only have one EMI source to worry about.

The shorting of the LEDs can easily be done with a parallel FET that may or may not be controlled via an optocoupler.

The beauty with this arrangement is that you need but one SMPS and that you can optimize your SMPS for the constant current drawn by the LEDs and you do not have any losses in series resistors that would be needed in a parallel arrangement (or eight little lossy switchers that would be needed if you go that way).

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[MacGyverS2000]

Could readers possibly kindly confirm that this is indeed the lowest-component-count method of achieving my aims, set out in the first post ?

Not by a long shot, but that doesn't appear to matter...

Dan - Owner

http://www.Hi-TecDesigns.com

 

[treez]

Thank you.