Explain in Detail How to Realize the Standby Low Power Consumption of Switching Power Supply

1. Introduction

With the increasing importance of energy efficiency and environmental protection, people have higher and higher expectations for the standby efficiency of switching power supply. Customers require that the power products provided by power supply manufacturers can meet the green energy standards such as blue angel, energy star and energy 2000. The EU's requirements for switching power supply are: by 2005, the switching power supply with rated power of 0.3w 15W, 15W 50W and 50W 75W, Standby power consumption shall be less than 0.3w, 0.5W and 0.75w respectively. At present, when most switching power supplies change from rated load to light load and standby mode, the power efficiency decreases sharply, and the standby efficiency can not meet the requirements. This presents new challenges to power design engineers.

2. Power consumption analysis of switching power supply

In order to reduce the standby loss of switching power supply and improve the standby efficiency, we must first analyze the composition of switching power supply loss. Taking flyback power supply as an example, its working loss is mainly manifested as MOSFET conduction loss

Parasitic capacitance loss of MOSFET

Switching overlap loss, PWM controller and its starting resistance loss, output rectifier loss, clamping protection circuit loss, feedback circuit loss, etc. The first three losses are proportional to the frequency, that is, they are proportional to the switching times of the device in unit time.

In the standby state, the current of the main circuit is small, the conduction time ton of the MOSFET is very small, and the circuit works in DCM mode, so the relevant conduction loss and secondary rectifier loss are small. At this time, the loss is mainly composed of parasitic capacitance loss, switch overlap loss and start-up resistance loss.

3. Method for improving standby efficiency

According to the loss analysis, cutting off the starting resistance, reducing the switching frequency and reducing the switching times can reduce the standby loss and improve the standby efficiency. The specific methods are: reduce the clock frequency; Switch from high-frequency operating mode to low-frequency operating mode, such as quasi resonant mode (QR) to pulse width modulation (PWM), and pulse width modulation to pulse frequency modulation (PFM); Burst mode.

3.1 cut off the starting resistance

For the flyback power supply, the control chip is powered by the auxiliary winding after starting, and the voltage drop on the starting resistance is about 300V. The starting resistance is set as 47K Ω and the power consumption is nearly 2W. To improve standby efficiency, the resistance channel must be cut off after startup. TOPSwitch and ice2ds02g are internally equipped with a special starting circuit, which can turn off the resistance after starting. If the controller has no special starting circuit, the capacitor can also be connected in series in the starting resistance, and the loss after starting can gradually drop to zero. The disadvantage is that the power supply cannot restart automatically. The circuit can be started again only after the input voltage is disconnected and the capacitor is discharged. The starting circuit shown in Fig. 1 can avoid the above problems, and the power consumption of the circuit is only 0.03W. However, the circuit increases complexity and cost.

Figure 1 UC3842 flyback power supply starting circuit

3.2 reduce clock frequency

The clock frequency can drop smoothly or suddenly. Smooth descent is to realize the linear descent of clock frequency through specific modules when the feedback exceeds a certain threshold. TOPSwitch GX of power company and sg6848 chip of SG Company have built-in such modules, which can adjust the frequency according to the load. Figure 2 shows the relationship between the clock frequency of sg6848 and its feedback current.

Figure 2   Relationship between sg6848 feedback current and clock frequency

The implementation method of sudden drop is shown in Figure 3: taking ucc3895 as an example, when the power supply is in normal load state, Q1 is on, and its clock cycle is:

When the power supply enters the standby mode, Q1 turns off and the clock cycle increases to

That is, the switching frequency decreases. The switching loss is reduced to the value before frequency reduction

(less than 1) times. L5991 and Infineon's coolset F2 series have integrated this function.

Explain in Detail How to Realize the Standby Low Power Consumption of Switching Power Supply 1

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