Introduction to Improving Standby Efficiency of Switching Power Supplies
Cutting off startup
For a flyback power supply, the control chip is powered by the auxiliary winding after startup, and the voltage drop across the startup resistor is about 300V. To improve the standby efficiency, the resistor channel must be cut off after startup. TOpSWITCH, ICE2DS02G has a special startup circuit inside, which can turn off the resistor after startup. If the controller does not have a dedicated start-up circuit, you can also connect a capacitor in series with the start-up resistor, and its loss after start-up can be gradually reduced to zero. The disadvantage is that the power supply cannot restart itself, and the circuit can only be started again after disconnecting the input voltage and discharging the capacitor.
Reduce the clock frequency
The clock frequency can be decreased smoothly or abruptly. Smooth decrease is when the feedback exceeds a certain threshold value, through a specific module, to achieve a linear decrease in clock frequency.
Switching Operating Modes
QR→pWM For switching power supplies operating in high-frequency mode, switching to low-frequency mode during standby reduces standby losses. For example, for a quasi-resonant switching power supply (with an operating frequency of several hundred kHz to several MHz), it can be switched to the low-frequency pulse-width modulation control mode pWM (several 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, the auxiliary winding voltage is small, Q1 is off, the resonant signal can not be transmitted to the FB terminal, the FB voltage is less than a threshold voltage inside the chip, which can not trigger the quasi-resonance mode, and the circuit operates in the lower-frequency pulse-width modulation control mode. pWM → pFM For the switching power supply that operates in the pWM mode when the power rating is rated, the efficiency of standby power supply can be improved by switching to pFM mode, i.e., the fixed turn-on mode is used to improve the standby efficiency. Improve the standby efficiency, i.e., fix the turn-on time and adjust the turn-off time, the lower the load, the longer the turn-off time and the lower the operating frequency. The standby signal is added to its pW/ pin, which is high under rated load conditions and the circuit operates in pWM mode, and when the load is below a certain threshold, the pin is pulled low and the circuit operates in pFM mode. Achieving the switching between pWM and pFM also improves the efficiency of the power supply during light load and standby states. Reducing the standby operating frequency and improving standby efficiency is achieved by reducing the clock frequency and switching the operating mode, which keeps the controller operating all the time and the output properly regulated throughout the load range. Even when the load surges from zero to full load, the response is fast and vice versa. Output voltage drop and overshoot values are kept within permissible limits.
Controlled Burst Mode
(BurstMode) Controlled Pulse Mode, also known as SkipCycleMode, refers to the control of a certain part of the circuit by a signal with a period larger than the clock period of the pWM controller to make the output pulse of the pWM periodically active or inactive when it is in the condition of light load or standby, so that the output pulse of the pWM can be made active or inactive at a constant frequency by reducing the number of switching times and increasing the duty cycle to improve the performance of light load and standby. to improve the efficiency of light load and standby. The signal can be added to the feedback channel, the pWM signal output channel, the enable pin of the pWM chip (e.g., LM2618, L6565) or the internal module of the chip (e.g., NCp1200, FSD200, L6565, and TinySwitch series chips).
