NPNs need current to turn them on - 1st transistor is used to drive higher current into the 2nd.  Voltage drop across the 2nd transistor collector-emitter is pretty high, 2 to 4V depending on current.  N-channel MOSFET is driven by voltage level instead, needs very little current.  Voltage drop across drain-source is dependent on the Rds of the device and the current flow.  Rds can be very low, like 0.030 ohm, so for 360 LEDs, arranged in 90 parallel strings of 4 LEDs in series, drawing 20mA/string, so 90 * 0.02 = 1.8A,  the voltage drop across the MOSFET would be 1.8A * 0.03ohn = 0.054V  http://aosmd.com/res/data_sheets/AOI516.pdf  This might allow strings of 5 LEDs from 12V even, reducing your current draw even more - 360/5 = 72 * 0.02 = 1.44A.   Power dissipated = Current * Voltage  So for TIP120, 1.8A * 2V = 3.6W, gonna need heatsinking.  For a MOSFET, 1.8 * 0.054 = 0.09W,  heat sink likely not needed.   MOSFET, want to  switch voltage levels quickly, so go from high resi...
 
 
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