Compared with ordinary light source, LED lamps are high efficiency, environmental protection and long service life, so they are becoming the main solution to reduce indoor and external lighting energy solutions. Designed for lighting power supply switching power supply should also be highly efficient in order to comply with the energy-saving features of LED lights. In addition to the normal work process with high power conversion efficiency, the switching power supply standby power has become the focus of the LED industry's general concern. In the near future, standby power consumption is expected to adjust to 1W or even 300mW below. However, in LED lighting applications, the auxiliary power level dedicated to the standby power supply is not applicable, mainly because the lighting application does not have a standby condition during operation. However, the switching power supply for the lamp supply is connected to the grid and absorbs energy even in the absence of a lamp or a lamp is damaged. This is the main reason for the standby power level in lighting applications.
In the empty office building, the poor standby power consumption of the lighting system is not environmentally friendly, this paper discusses 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, we designed a two-stage switching power supply with a rated power of 120W and a standby power dissipation of less than 1W over a wide input voltage range.
Two-level configuration
Due to the reasons for the rated power and the need to improve the power factor, LED street lights switching power supply is usually used two-stage configuration, which consists of the first stage of the PFC module and the second stage of the downstream DC-DC converter. In the medium power range of about 100W, the critical conduction mode (CRM) is the appropriate control scheme for the PFC level. In this rated power range, downstream DC-DC converters typically use quasi-resonant flyback topologies. The highly integrated FAN6300 pulse width modulation (PWM) controller has an internal trough voltage detector that ensures that the power supply system operates in a quasi-resonant state over a wide range of line voltages and reduces switching losses so that the power MOSFET drain Switching voltage is minimized. In order to minimize standby power consumption and improve light load efficiency, the proprietary green mode function provides off-time modulation in order to reduce the switching frequency and perform an extended trough voltage switch to ensure that the MOSFET is off The drain-source voltage remains at the lowest level. With this feature, the second DC-DC stage enters the intermittent operating mode under no-load conditions, enabling very good standby power consumption characteristics. Most of the existing PFC controllers have no intermittent work function, primarily because the PFC stage was originally targeted at consumer applications and display applications, and in those applications the auxiliary power supply for the PFC and DC-DC stages was isolated. In LED lighting applications, usually do not use auxiliary power level, therefore, should be off the PFC level, or standby power consumption can not be less than 1W.
PFC level intermittent mode of operation
In the two-stage switching power supply, the PFC level 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 intermittent mode of operation, the PFC stage will operate continuously, even if there is no load. Therefore, for a two-stage switching power supply design with an existing PFC controller, turning off the PFC stage is the only viable method. However, a large inrush current can occur when the PFC stage is restarted and results in an increase in voltage or current stress on a power switch such as a MOSFET. In addition, it will cause the LED lights to flash during constant current operation. The industry needs to find a new way to meet the standby power requirements, while avoiding the above problems. One possible way to solve these side effects of completely shutting down the PFC stage is that the PFC stage uses an intermittent mode of operation.
It is recommended to use a simple auxiliary circuit to synchronize the operation of the PFC with a quasi-resonant flyback DC-DC converter because the PFC stage can also enter the intermittent mode when the DC-DC converter starts intermittent operation. Once the second-stage flyback converter has completed the intermittent mode operation, the PFC level will immediately exit the intermittent mode of operation. Figure 1 shows how the auxiliary circuit works. PFC stage bias power supply is controlled by quasi-resonant flyback DC-DC converter feedback.
In the no-load condition, when the feedback voltage of the flyback converter drops, the PFC stage supply voltage is cut off and the PFC controller is stopped. Figure 2 shows the load from full load to no load, and then to full load process of the working waveform. Once the second stage flyback converter enters the intermittent operation, the PFC stage enters the intermittent operation mode and stops the intermittent operation mode in synchronization with the flyback converter. By performing intermittent work on the PFC stage, it is possible to eliminate large inrush currents that could cause potential problems and significantly reduce standby power consumption. In order to evaluate the intermittent operation of the PFC stage, a 120W (48V) LED street lamp was designed using the FAN7930 critical conduction mode PFC controller, the quasi-resonant flyback controller FAN6300A with intermittent function, and the proposed PFC control circuit. /2.5A) LEB-016 demonstration circuit board. As shown in Figure 2, the proposed circuit works well. Table 1 shows the standby power consumption measurements at various input line voltages. It can be shown that within a wide input range, the standby power consumption can be reduced by more than 80%. It is also possible to achieve standby power consumption of less than 0.3W at high line input voltages.
This is a simple but very effective way to improve the standby power consumption of the lighting switching power supply. This recommended circuit allows the PFC stage to be intermittently synchronized with the second stage DC-DC converter. This method eliminates the influx of current problems associated with closing and restarting the PFC stage. The proposed circuit can effectively reduce standby power consumption. By evaluating the board verification, the standby power consumption can be less than 1W over a wide input voltage range. The proposed method is of great appeal to lighting applications that typically do not have a standby power adjustment module.
