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Registered Member #16903
Joined: Thu May 09 2013, 08:00PM
Location: Sumter, SC
Posts: 23
Hello I was wondering since a cfl ballast can power a flyback but since they have low current transistors, the one i've come across is the 4124DL which is a high frequency switching transistor, and when i build the driver by having a capacitor on one pair of pins to the filament and the remaining pair to the primary then power it, after 20 seconds of running they pop and the board is useless. so my question is could i replace the 4124DL with a mosfet like a STW88N65M5 or STW78N65M5 with gate would be where the base would be connected, drain to the collector, and source to the emitter , and the board will still work?
Registered Member #4074
Joined: Mon Aug 29 2011, 06:58AM
Location: Australia
Posts: 335
Correct me if I'm wrong, but I'm not sure the base driver would work properly with a MOSFET, since a transistor is current driven, while the gate of a FET is charged and discharged.
The 4124DL datasheet states a max base-emitter voltage of 7V and a max base current 1A, while the STW78N65M5 datasheet states a gate threshold voltage of up to 5V, although for nice low-loss switching you would want the gate to reach something like 12V. So the existing base driver may not supply enough voltage to fully switch the MOSFET.
It may be possible, but it doesn't seem like you can just stick the MOSFET where the NPN was without modifications.
Registered Member #16903
Joined: Thu May 09 2013, 08:00PM
Location: Sumter, SC
Posts: 23
GrantX wrote ...
Correct me if I'm wrong, but I'm not sure the base driver would work properly with a MOSFET, since a transistor is current driven, while the gate of a FET is charged and discharged.
The 4124DL datasheet states a max base-emitter voltage of 7V and a max base current 1A, while the STW78N65M5 datasheet states a gate threshold voltage of up to 5V, although for nice low-loss switching you would want the gate to reach something like 12V. So the existing base driver may not supply enough voltage to fully switch the MOSFET.
It may be possible, but it doesn't seem like you can just stick the MOSFET where the NPN was without modifications.
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cfl flyback driver modification
