Energy conversion systems must have energy consumption. Although 100% conversion efficiency cannot be obtained in practical applications, a high-quality power supply can reach a very high level of efficiency, close to 95%. The operating efficiency of most power ICs can be measured under specific operating conditions, and these parameters are given in the data sheet.
Losses of switching devices MOSFET conduction loss MOSFET and diode are the main factors causing power consumption. The related losses mainly include two parts: conduction loss and switching loss. MOSFET and diode are switching elements, and current flows through the loop when turned on.
When the device is turned on, the conduction loss is determined by the on-resistance (RDS(ON)) of the MOSFET and the forward conduction voltage of the diode. The conduction loss (PCOND(MOSFET)) of the MOSFET is approximately equal to the product of the on-resistance RDS(ON), the duty cycle (D) and the average current (IMOSFET(AVG)) of the MOSFET when it is turned on.PCOND(MOSFET) (using average current) = IMOSFET(AVG)² × RDS(ON) × D The above equation gives an approximation of the MOSFET conduction losses in an SMPS, but it is only an estimate of the circuit losses because the power dissipated when the current is linearly ramped is greater than the power dissipated calculated from the average current. For "peak" currents, a more accurate calculation method is to integrate the square of the current waveform between the peak and valley of the current to get an estimate. The following equation gives a more accurate estimate of the losses, replacing the simple I² term with the integral of the current waveform I² between IP and IV. PCOND(MOSFET) = [(IP3 - IV3)/3] × RDS(ON) × D = [(IP3 - IV3)/3] × RDS(ON) × VOUT/VIN Where IP and IV correspond to the peak and valley of the current waveform, respectively, as shown in Figure 3.The MOSFET current rises linearly from IV to IP. For example, if IV is 0.25A, IP is 1.75A, RDS(ON) is 0.1Ω, and VOUT is VIN/2 (D = 0.5), the calculation based on the average current (1A) is: PCOND(MOSFET) (using average current) = 12 × 0.1 × 0.5 = 0.050W A more accurate calculation using waveform integration is: PCOND(MOSFET) (calculated using current waveform integration) = [(1.753 - 0.253)/3] × 0.1 × 0.5 = 0.089W or approximately 78%, which is higher than the result calculated based on the average current.
For current waveforms with a small peak-to-average ratio, the difference between the two calculations is small, and the calculation using the average current can meet the requirements.
