EDN Access -- 06.09.94 Low-cost MOSFET quashes power resisto

-June 09, 1994

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Design Ideas:June 9, 1994

Low-cost MOSFET quashes power resistor

Christophe Basso ,
European Synchrotron Radiation Facility, Grenoble, France

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.

A circuit that provides higher current (Fig 1b) still uses the LND150, but this time as a high-voltage switch. At power-on, the LND150's positive VGS allows the current to flow through R1. The source potential starts to rise, authorizing the PWM IC to oscillate. The auxiliary winding begins to deliver voltage and forces the MOSFET's source voltage to rise until D1 reaches its voltage limit. When the dc rail exceeds the 10V gate voltage, the VGS becomes negative and soon stops MOSFET conduction, thus freeing R1 from dissipating any heat. The heat dissipated by the resistor thus falls to zero, avoiding all the nuisance caused by excessive heat in the circuit and leading to better overall efficiency. A further enhancement to this circuit would be to lock the MOSFET gate to ground, thus inhibiting all oscillations when the auxiliary power supply disappears, such as when the tube is broken in ballast applications. (DI #1442)

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