Introduction to Improving Standby Efficiency of Switching Power Supply
Cut off startup
For flyback power supply, the control chip is powered by the auxiliary winding after startup, and the voltage drop across the starting resistor is about 300V. Assuming the starting resistance value is 47k Ω, the power consumption is nearly 2W. To improve standby efficiency, the resistance channel must be cut off after startup. TOpSWITCH, ICE2DS02G has a dedicated startup circuit inside that can turn off the resistor after startup. If the controller does not have a dedicated startup circuit, a capacitor can also be connected in series with the startup resistor, and the losses after startup can gradually decrease to zero. The disadvantage is that the power supply cannot restart automatically, and the circuit can only be restarted after disconnecting the input voltage and discharging the capacitor.
Reduce clock frequency
The clock frequency can smoothly decrease or suddenly decrease. Smooth descent refers to the linear decrease in clock frequency achieved through a specific module when the feedback exceeds a certain threshold.
Switch working mode
For switching power supplies operating in high-frequency mode, switching to low-frequency mode during standby can reduce standby losses. For example, for quasi resonant switching power supplies (operating at frequencies of several hundred kHz to several MHz), they can switch to the low-frequency pulse width modulation control mode pWM (tens of kHz) during standby. The IRIS40xx chip improves standby efficiency by switching between QR and PWM. When the power supply is in light load and standby mode, the auxiliary winding voltage is low, Q1 is turned off, and the resonant signal cannot be transmitted to the FB terminal. The FB voltage is less than a threshold voltage inside the chip, and the quasi resonant mode cannot be triggered. The circuit then operates in a lower frequency pulse width modulation control mode. 2. pWM → pFM For switching power supplies operating in pWM mode at rated power, standby efficiency can also be improved by switching to pFM mode, with a fixed turn-on time and adjusted turn off time. The lower the load, the longer the turn off time, and the lower the operating frequency. Apply the standby signal to its pW/pin. Under rated load conditions, this pin is at a high level, and the circuit operates in pWM mode. When the load is below a certain threshold, this pin is pulled to a low level, and the circuit operates in pFM mode. By switching between pWM and pFM, the power efficiency during light load and standby states is improved. By reducing the clock frequency and switching operating modes, the standby operating frequency can be lowered, standby efficiency can be improved, and the controller can be kept running continuously. The output can be properly adjusted throughout the entire load range. Even when the load surges from zero to full load, it can quickly respond, and vice versa. The output voltage drop and overshoot are both kept within the allowable range.
