Compared to ordinary light sources, LED lamps are characterized by high efficiency, environmental protection and long service life, so they are becoming the main choice for reducing the energy consumption of indoor and external lighting. Switching power supplies designed for lighting power supply should also be highly efficient in order 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 become a common focus of the LED industry. In the near future, standby power consumption is expected to be adjusted to below 1W or even 300mW. However, in LED lighting applications, the auxiliary power stage dedicated to the standby power source is not suitable, primarily because the lighting application does not have a standby condition during operation. However, the switching power supply that supplies the bulb remains connected to the grid and draws energy even when there is no lamp or the lamp is damaged. This is the main reason for concern for standby power levels in lighting applications.
In an empty office building, lighting systems with poor standby power characteristics are not environmentally friendly. This article explores how to introduce simple auxiliary circuits to reduce standby power consumption. The proposed circuit enables intermittent operation of a power factor correction (PFC) stage that 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 rated at 120W to achieve less than 1W of standby power over a wide input voltage range.
Due to the power rating and the need to improve power factor, the switching power supply for LED street lights typically uses a two-stage configuration consisting of a PFC module of the first stage and a downstream DC-DC converter of the second stage. In the medium power range of around 100 W, the critical conduction mode (CRM) is a suitable control scheme for the PFC stage. In this rated power range, downstream DC-DC converters typically employ a quasi-resonant flyback topology. The highly integrated FAN6300 Pulse Width Modulation (PWM) controller features an internal valley voltage detector that ensures the power system operates in a quasi-resonant state over a wide range of line voltages and reduces switching losses on the drain of the power MOSFET. The switching voltage is minimized. To minimize standby power consumption and improve light load efficiency, the proprietary green mode feature provides off-time modulation to reduce switching frequency and perform extended valley voltage switching to ensure MOSFETs are turned off The drain-source voltage is kept to a minimum. Using this feature, the second DC-DC stage enters the intermittent mode of operation under no-load conditions, achieving very desirable standby power consumption characteristics. Most existing PFC controllers do not have intermittent operation, primarily because the PFC stage was originally targeted at consumer and display applications, and the auxiliary power supplies that provide voltage sources for the PFC and DC-DC stages in those applications are separate. In LED lighting applications, the auxiliary power stage is usually not used. Therefore, the PFC level should be turned off, otherwise the standby power consumption cannot be lower than 1W.
Intermittent mode of operation at the PFC level
In a two-stage switching power supply, the PFC stage should be turned off to meet the requirements of the standby power 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 the intermittent mode of operation, the PFC stage will continue to operate, even if no load conditions will draw energy. 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, large inrush currents occur when the PFC stage is restarted, and an increase in voltage or current stress on a power switch such as a MOSFET is caused. In addition, it can cause the LED lamp to flicker during constant current operation. The industry needs to find a new way to meet standby power regulatory requirements while avoiding the above issues. One possible way to address these side effects of completely shutting down the PFC stage is that the PFC stage uses intermittent operation.
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 the PFC stage can also enter the intermittent mode when the DC-DC converter starts to operate intermittently. Once the second stage flyback converter has finished working in the intermittent mode, the PFC stage will immediately exit the intermittent mode of operation. Figure 1 shows the working principle of the auxiliary circuit. The PFC stage bias supply is controlled by the feedback from the quasi-resonant flyback DC-DC converter.
Figure 1: Recommended circuit for implementing PFC-level intermittent operation
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