control LED with MOSFET

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yassin.kraouch

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i have a string of LED of 150V DC, the current is 20mA, i want to connect the LED in the bottom with a MOSFET ( drain connected to cathode, and source connected to GND),
do i need to have Vds=150V ?
 

how many LEDs? what is their Vd?
 

Your Vds has to accommodate (VIN-Vstring) but when off,
Vstring can be well less than your 150V net (Vf@If*N). You
may not have good data for what the string voltage relaxes
to, at "off" range currents. The FET probably does not like
being avalanched even at low currents, so I'd rate the FET
for VIN, not Vstring or VIN-Vstring, and you should add some
margin there - most likely, according to your customer's set
of safety and reliability derating guidelines / preferences, if
you have not established your own scientifically enough to
supersede.
 

yes, i think you should select the MOS with Vds=150v,cause when you disable the MOS,the voltage of the Vds will be closed to 150V.
 

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