source:Industry News Popular:adapter release time:2022-05-06 10:19:22 Article author:yu
In high-power power supplies, the consumption of MOS devices is critical. It is most likely related to the overall efficiency of the power supply.
Dissipation of Synchronous Rectifiers
For all but the maximum load, the drain-source voltage of the MOSFETs of the synchronous rectifier is clamped by the capture diode during turn-on and turn-off. Therefore, the synchronous rectifier does not incur switching losses, making its power dissipation easy to calculate. Only resistive dissipation needs to be considered.
Worst-case losses occur when the synchronous rectifier load factor is at its maximum, ie when the input voltage is at its maximum value. By using the RDS(ON) HOT and load factor of the synchronous rectifier and Ohm's law, an approximation of the power dissipation can be calculated:
PDSYNCHRONOUSRECTIFIER=[ILOAD2×RDS(ON)HOT]×[1>-)]
Dissipation of switching MOSFETs
The calculation of the switching MOSFET resistive losses is similar to that of the synchronous rectifier, using its (different) load factor and RDS(ON)HOT: PDRESISTIVE=[ILOAD2×RDS(ON)HOT]×(VOUT/VIN)
Calculating switching losses for switching MOSFETs is difficult because it relies on many factors that are difficult to quantify and often outside of specification parameters that affect switching. A rough approximation is used in the following formula as a first step in evaluating a MOSFET and later verifying its performance in the laboratory: PDSWITCHING=(CRSS×VIN2×fSW×ILOAD)/IGATE.
where CRSS is the reverse switching capacitance of the MOSFET (a performance parameter), fSW is the switching frequency, and IGATE is the source of the MOSFET gate drive sink current and the extreme current.
Once the selection is narrowed down to a specific generation of MOSFETs based on cost (the cost of a MOSFET is a very important feature of the generation it belongs to), the model with the least power dissipation in that generation is the model with equal resistive and switching losses . With smaller (faster) devices, the increase in resistive losses is greater than the decrease in switching losses.
With larger [RDS(ON) low] devices, the increase in switching losses is greater than the decrease in resistive losses. If VIN is varied, the power dissipation of the switching MOSFET at both VIN(MAX) and VIN(MIN) must be calculated. The worst case power dissipation of the MOSFET will be at the minimum or maximum input voltage. Dissipation is the sum of two functions: maximum resistive dissipation at VIN(MIN) (higher load factor), and maximum switching dissipation at VIN(MAX) (due to the effect of VIN2). The ideal choice is somewhat equal to the dissipation at the extremes of VIN, which balances resistive and switching dissipation across the VIN range.
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