9.2.2.2.2 Boost Switch Selection
The power switch carries the Boost inductor current during its ON period (TON). It carries no current during its OFF period (TDCH). Equation 28 describes the switch RMS current, over a single switching cycle, at angle θ in the Line half-cycle.
Equation 28.
Equation 29 describes the duty cycle of switch conduction for ideal transition mode (CrM) operation.
Equation 29.
The switch ON time is constant across the Line cycle but the OFF time varies according to the position in the Line cycle. Volt-second balance across the Boost inductor, within each switching cycle, requires that.
Equation 30.
Equation 31 calculates the duty cycle of switch conduction.
Equation 31.
Equation 32 describes the RMS switch current across a complete Line half-cycle.
Equation 32.
Maximum RMS current in the switch occurs at maximum load and minimum Line.
Equation 33.
Use the following guidelines for MOSFET selection for the Boost switch.
- The voltage rating must be greater than the maximum output voltage. Under transient or Line surge testing the output voltage may exceed the normal regulation level. For this design example, the MOSFET voltage rating is 650 V supports a regulated output voltage of 390 V.
- Based upon an acceptable level of conduction loss in the MOSFET, the required on-resistance (rDS(on)) value can be calculated from the maximum RMS current. For this example design an STF24N60DM2 MOSFET, from STMicrolelectronics was selected with an on-resistance of 0.37 Ω, when TJ = 125°C which allows maximum conduction power loss (less than 1.7 W) in the MOSFET.
- For best efficiency, use a MOSFET that incorporates a fast body diode. Operation using discontinuous inductor current (DCM) from a low input voltage incurs additional switching power loss if a MOSFET with slow body diode is used.