How to reduce the standby power consumption of LED lighting switch power supply

Compared with ordinary light sources, LED lamps are becoming the preferred solution for reducing energy consumption in indoor and outdoor lighting due to their high efficiency, environmental friendliness and long service life. Switching power supplies designed for lighting power supply should also have high efficiency to comply with the energy-saving characteristics of LED lamps. In addition to high power conversion efficiency during normal operation, the standby power consumption of switching power supplies has also become a common focus of attention in the LED industry. In the near future, standby power consumption is expected to be adjusted to 1W or even below 300mW. However, in LED lighting applications, auxiliary power levels dedicated to standby power supplies are not applicable, mainly because lighting applications do not have standby conditions during operation. However, the switching power supply that powers the light bulb is still connected to the grid and draws energy even when there is no light or the light is damaged. This is the main reason for caring about the standby power level in lighting applications.

In an empty office building, a lighting system with poor standby power consumption characteristics is not environmentally friendly. This paper explores how to introduce a simple auxiliary circuit to reduce standby power consumption. The proposed circuit enables intermittent operation of the power factor correction (PFC) stage, which is necessary to reduce the standby power consumption of the lighting switching power supply. To evaluate the proposed circuit, a two-stage switching power supply with a rated power of 120W was designed, and standby power consumption of less than 1W can be obtained over a wide input voltage range.

Two-level configuration

Due to the power rating and the need to improve the power factor, the switching power supply for LED street lights usually uses a two-stage configuration, which consists of a PFC module in the first stage and a downstream DC-DC converter in the second stage. In the medium power range around 100W, critical conduction mode (CRM) is a suitable control scheme for the PFC stage. In this rated power range, the downstream DC-DC converter usually adopts a quasi-resonant flyback topology. The highly integrated FAN6300 pulse width modulation (PWM) controller has an internal valley voltage detector that ensures that the power system operates in a quasi-resonant state over a wide line voltage range and reduces switching losses, minimizing the switching voltage on the drain of the power MOSFET. To minimize standby power consumption and improve light load efficiency, the proprietary green mode function provides off-time modulation to reduce the switching frequency and perform extended valley voltage switching to ensure that the drain-source voltage of the MOSFET is kept at a minimum level when it is turned off. Using this feature, the second DC-DC stage enters intermittent operation mode under no-load conditions, which can achieve very ideal standby power consumption characteristics. Most existing PFC controllers do not have intermittent operation function, mainly because the PFC stage was originally targeted at consumer applications and display applications, where the auxiliary power supply that provides voltage source for the PFC and DC-DC stages is separated. In LED lighting applications, the auxiliary power stage is usually not used, so the PFC stage should be turned off, otherwise the standby power consumption cannot be less than 1W.

Intermittent operation mode of the PFC stage

In a two-stage switching power supply, the PFC stage should be turned off to meet the requirements of standby power consumption regulations. The main reason for turning off the PFC stage is that most PFC controllers do not have a burst-operation feature. If the PFC controller does not support burst-operation mode, the PFC stage will operate continuously and absorb energy even under no-load conditions. Therefore, for a two-stage switching power supply design with an existing PFC controller, turning off the PFC stage is the only feasible method. However, a large inrush current will occur when restarting the PFC stage, which will cause an increase in voltage or current stress on power switches such as MOSFETs. In addition, it will cause LED lights to flicker during constant current operation. The industry needs to find a new way to meet the standby power consumption regulations while avoiding the above problems. A feasible way to solve these side effects of completely turning off the PFC stage is to use burst-operation mode for the PFC stage.

It is recommended to use a simple auxiliary circuit to synchronize the operation of the PFC with the quasi-resonant flyback DC-DC converter, because when the DC-DC converter starts intermittent operation, the PFC stage can also enter the intermittent operation mode. Once the second stage flyback converter ends the intermittent mode operation, the PFC stage will immediately exit the intermittent operation mode. Figure 1 shows the working principle of this auxiliary circuit. The bias supply of the PFC stage is controlled by the feedback of the quasi-resonant flyback DC-DC converter.

Figure 1: Proposed circuit for implementing intermittent operation of the PFC stage

Under no-load conditions, when the feedback voltage of the flyback converter drops, the supply voltage of the PFC stage is cut off and the PFC controller stops working. Figure 2 shows the operating waveforms during the load change from full load to no load and then to full load. Once the second-stage flyback converter enters intermittent operation, the PFC stage enters intermittent operation mode and stops intermittent operation mode synchronously with the flyback converter. By performing intermittent operation on the PFC stage, large inrush currents that may cause potential problems can be eliminated and standby power consumption can be greatly reduced. To evaluate the intermittent operation of the PFC stage, a 120W (48V/2.5A) LEB-016 demonstration board for LED street lights was designed using the FAN7930 critical conduction mode PFC controller, the quasi-resonant flyback controller FAN6300A with intermittent operation function, and the proposed PFC control circuit. As shown in Figure 2, the proposed circuit works well. Table 1 shows the measured standby power consumption at various input line voltages. It can be demonstrated that the standby power consumption can be reduced by more than 80% over a wide input range. It is also possible to achieve standby power consumption below 0.3W at high line input voltage.

Figure 2: Entering intermittent operation (left) and returning to full load operation (right)

Table 1: Standby power consumption

in conclusion

This is a simple but very effective way to improve the standby power consumption of lighting switch power supplies. The proposed circuit can synchronize the PFC stage with the second-stage DC-DC converter for intermittent operation. This approach eliminates the inrush current problem associated with shutting down and restarting the PFC stage. The proposed circuit can effectively reduce the standby power consumption. The evaluation circuit board verifies that the standby power consumption can be made less than 1W over a wide input voltage range. The proposed method is very attractive for lighting applications that usually do not have a standby power regulation module.