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Low Cost MOSFET Quashes Power Resistor: 06/09/94 EDN-Design Ideas /
(Electronic Circuit diagram added 03/03) At power-on, off-line power supplies use a
resistor to provide start-up current for the PWM IC, current that is necessary to
start driving the power switch. After a few periods, an auxiliary winding delivers a
sufficient voltage to power the IC. Unfortunately, the start-up resistor dissipates
heat and raises the power supply's overall temperature. Manufacturers have recently
introduced high-voltage MOSFETs to replace the power resistors. Wired as current
sources, these MOSFETs provide the few hundred microamps necessary to start the IC.
Fig 1a shows a circuit based on the new Supertex (Sunnyvale, CA) LND150.
Unfortunately, this configuration isn't sufficient for a main IC that requires
start-up current of a few milliamps. For example, this current source cannot start a
half-bridge power MOSFET driven by an International Rectifier (El Segundo, CA) IR2110
in off-line fluorescent ballast applications..... |