Depletion-mode MOSFET kick-starts power supply
Many switch-mode power supplies use “kick-start” circuits to initialize their offline operation. These circuits may be simple resistors, such as International Rectifier’s IRIS4015, or more complicated arrangements built with bipolar transistors or MOSFETs (Reference 1). These transistors provide the initial current for the flyback or PFC (power-factor-correction) IC. When such a power supply starts operating in normal mode, a supply voltage from a dedicated winding keeps supplying the PFC IC, thus reducing power consumption of the kick-start circuitry.
Such schemes reduce—but do not eliminate—the power consumption of the kick-start circuitry, because the active component is usually a high-voltage bipolar transistor or high-voltage enhancement-mode MOSFET. These transistors’ base or gate requires forward-biasing with respect to the emitter or the source for normal operation. Therefore, a power loss always occurs in the circuits that keep the transistors in the off state. Unfortunately, engineers pay too little attention to depletion-mode MOSFETs, which require no forward-biasing for normal operation and, moreover, require gate potentials below the source. These valuable properties of depletion-mode MOSFETs suit them for a role in no-loss kick-start circuits for power supplies.
Figure 1 shows a conventional PFC circuit whose IC initially receives power from the output through a depletion-mode MOSFET, Q2, a DN2470 from Supertex (Reference 2). Q2’s source feeds PFC IC1 with an initial supply current of approximately 10 to 15 mA or less depending on the IC model. A brief power dissipation of approximately 4 to 6W can do no harm to the MOSFET soldered to a copper pour. If you have concerns about the MOSFET’s health, you can use an IXTY02N50D from Ixys (Reference 3). Resistors R3 and R4 set up Q2’s working point to obtain the minimum required current. Zener diode D5 limits voltage across IC1 to approximately 15V for an input voltage of 18V, which is usually necessary for most PFC ICs and is less than the maximum for MOSFET Q2.
When IC1 starts working normally, the secondary winding of the PFC inductor, L, generates the IC’s supply voltage, which diodes D1 and D3 and capacitors C1 and C2 condition. Transistor Q2 keeps feeding zener diode D5 and IC1 for a short interval. Eventually, bipolar transistor Q3 gets its base supply through resistor R5 from diode D2, turning on and clamping Q2’s gate to ground. Q3’s power source is the IC’s positive-supply potential of approximately 15V, which is more than enough to shut off Q2. The residual thermal current of 10 to 20 µA produces no substantial power loss.