How to improve the efficiency of switching power supply standby 
 The switching power supply standby losses directly affect its service life, to reduce the switching power supply standby loss and improve standby efficiency, we must first analyze the composition of the switching power loss. Loss analysis shows to cut off the startup resistor to reduce the switching frequency to reduce the switching times can be reduced standby losses, improve standby efficiency. This paper mainly introduced to improve the method of switching power supply standby efficiency. 

 I. Introduction 
 With the growing importance of energy efficiency and environmental protection, the expectations of the switching power supply standby efficiency is getting higher and higher, customers require the power provided by power supply manufacturers can to full BLUEANGEL, ENERGYSTAR, ENERGY2000 green energy standards, and the EU switching power supply is: By 2005, the rated power of 0.3W ~ 15W, 15W to 50W and 50W to 75W switching power supply, standby power needs were less than 0.3W, 0.5W and 0.75W. While most of the switching power supply rated load transferred to the light load and standby power efficiency, a sharp decline in the standby efficiency can not meet the requirements. This power supply design engineers with new challenges. 

 Second, the switching power supply power analysis 

 To reduce the switching power supply standby power to improve standby efficiency, we must first analyze the composition of the switching power loss. Flyback power supply, for example, the work loss is mainly as follows: the MOSFET conduction losses, MOSFET parasitic capacitance loss, switch overlap loss, loss of the PWM controller and its startup resistor, the output rectifier losses, the loss of clamp protection circuit feedback circuit loss. Where the first three loss is proportional to the frequency relationship, which is proportional to the number of device switching unit time. In standby mode, the main circuit current is small, the MOSFET turn-on time ton is small, the circuit operates in DCM mode, the conduction losses, the secondary rectifier loss is small, the loss is mainly by the parasitic capacitance losses and switching overlap of loss and start the resistive losses. 

 Methods to improve the efficiency of switching power supply standby 

 Loss analysis shows to cut off the startup resistor to reduce the switching frequency to reduce the switching times can be reduced standby losses, improve standby efficiency. Specific ways: to reduce the clock frequency; high-frequency operating mode switch to low-frequency mode of operation, such as quasi-resonant mode (QuasiResonant QR) switching to the pulse width modulation (PulseWidthModulation, PWM), pulse width modulation switching to pulse frequency modulation (PulseFrequencyModulation , PFM); controllable pulse mode (BurstMode). 

 (A) cut off the startup resistor 

 Control chip is powered by the auxiliary winding flyback power supply, start, startup resistor voltage drop is 300V or so. Set to start the resistance value of 47kΩ, power consumption nearly 2W. To improve standby efficiency, we must channel the resistance starts to cut off. TOPSWITCH, ICE2DS02G internal features a dedicated start-up circuit, and can start after the close of the resistance. If the controller does not have specialized start-up circuit, also available on the startup resistor connected in series capacitor, it starts after the loss can be gradually decreased to zero. The disadvantage is that the power from the restart, only disconnect the input voltage, the capacitor discharges to re-start circuit. 

 (B) reduce the clock frequency 

 Clock frequency can be smoothed down or sags. Smooth decline is When the feedback exceeds a certain threshold, through a specific module, the clock frequency decreases linearly. POWER company's TOPSwitch-GX and SG SG6848 chip module, the frequency can be adjusted depending on the load size. 

 (C) switch operating mode 

 QR → PWM switching power supply for the work in the high-frequency mode of operation in the standby switch to low-frequency operating mode can reduce standby losses. For example, for quasi-resonant switching power supply (operating frequency for a few hundred kHz to several MHz), standby switch to low-frequency pulse-width modulation control mode PWM (tens of kHz). 

 IRIS40xx chip is through QR and PWM switching to improve the efficiency of standby. Power in light load and standby time, the auxiliary winding voltage, Q1 is turned off, the resonance signal can not be transferred to the FB pin FB voltage is less than a threshold voltage of the chip inside the quasi-resonant mode can not be triggered, the circuit is working in a more low-frequency PWM control mode. 




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