AP3766 high power factor non-isolated LED driver circuit

Non-isolated solution system specification requirements

The light efficiency of LED is constantly moving towards higher indicators. At present, one measure to improve the light efficiency is to use multiple small current LED grains in series to form a high-voltage and small current LED package structure. Such a structure not only improves the light efficiency of LED, but also helps to improve the overall efficiency of the switching power supply. However, due to the relatively high operating voltage of LED, the traditional isolated flyback switching power supply is no longer applicable. Because to achieve a very high output voltage, the output winding of the isolation transformer of the flyback switching power supply requires a relatively large number of turns, the volume of the transformer will be greatly increased, the coupling between the primary and secondary sides will decrease, and the circuit efficiency will also decrease. In response to the system specification requirements of high-voltage and low-current output and high efficiency, this article proposes a new Buck-boost switching power supply circuit with high power factor, simple and novel control method, small number of components, small size, and high cost performance. The following will introduce in detail a high power factor non-isolated LED driver power supply with an output specification of 110V/60mA.

Circuit Schematic

Figure 2. Schematic diagram of high power factor non-isolated LED driver circuit based on AP3766

In Figure 2, F1 is a fuse, VR1 is a lightning protection varistor, and C1, L1, and C2 form a π-type EMI filter. C3, C4, D2, D3, and D4 form a current-following circuit to achieve the power factor correction function. The principle of the current-following circuit to improve the power factor of the rectifier circuit is to increase the conduction angle of the rectifier circuit. When the input AC voltage is greater than half of the peak voltage, the rectifier bridge BD1 can be turned on, avoiding the problem of large current spikes and waveform distortion caused by the traditional uncontrolled rectifier circuit that can only be turned on instantaneously near the peak of the AC voltage, thereby reducing the total harmonic distortion, that is, THD.

After passing through the current-by-current circuit, the Buck-boost switching power supply circuit composed of L1, L2, Q1, D1, and C9 completes the buck-boost and constant current output functions, and the control chip U1 realizes the switch control function of the Buck-boost switching power supply circuit. Inductors L1, L2, and L3 are coupled to each other through the magnetic core T1.

The working principle of the Buck-boost switching power supply circuit using the primary switch control method is: set in a switching cycle, the on-time of the output diode D1 is Tons, the off-time is Toffs, the output current peak is Ipk, the number of turns of the coupled inductor L1 winding is N1, and the number of turns of the coupled inductor L2 winding is N2. The control chip U1 controls the switch duty cycle to keep the ratio of the on-time Tons and the off-time Toffs of the output diode D1 constant. Then, in a switching cycle, the average value of the output current is:

Figure 3 shows the current waveform of the Buck-boost switching power supply circuit passing through diode D1

Figure 3 Current waveform of diode D1

According to Ampere's theorem, the output current peak value Ipks when the output diode D1 is just turned on has the following relationship with the switch Q1 current peak value Ipk:

Therefore, the average value of the output current is:

The control chip U1 detects the primary switch current and controls the primary switch current peak value Ipk to be constant. At the same time, it controls the switch duty cycle to keep the ratio of the on time Tons and the off time Toffs of the output diode D1 constant, thereby achieving a constant output current.

In Figure 2, resistors R1 and R9 are the startup resistors of chip U1, connected to the VCC pin of the chip, and provide a certain amount of startup current to the chip after the circuit is powered on. L3 is an auxiliary winding, which forms the power supply circuit of chip U1 with D5 and C7. At the same time, the voltage of the auxiliary winding of L3 is divided by resistors R6 and R7 and connected to the FB pin of the chip as an output voltage detection and protection circuit. R2 is the current detection resistor of switch Q1, connected to the CS pin of the chip, that is, the current sampling pin of U1. Pin 2 GND of chip U1 is connected to the ground potential, and pin 1 is the output drive pin, which outputs a PWM signal with a certain pulse width to control the opening and closing of switch Q1.

In Figure 2, transformer T1 uses EE16 core and has three windings. The inductance of the primary winding L1 is 1mH, and the turns ratio of L1, L2 and L3 is 100:100:28. The circuit is designed to work at a frequency of 65KHz.

Experimental Results

Based on the above circuit design, the experimental test results of relevant performance indicators are as follows:

The conducted EMI test results are shown in Figure 4:

Figure 4 Conducted EMI test results

The test results show that within a wide input voltage range of 85V to 265V, the circuit has a power factor of approximately 0.8 and an efficiency greater than 85%, meeting various LED driver power supply specifications such as EMI standards.

in conclusion:

This paper proposes a high power factor non-isolated LED driver power supply solution based on AP3766. The control method is simple and novel, and it achieves high power factor, high efficiency and constant current output in the full voltage range. It has the outstanding advantages of small number of components, small size and high cost performance. At the same time, it meets the requirements of high power factor, high efficiency, compliance with electromagnetic compatibility EMC standards, high current control accuracy, high reliability, small size and low cost of LED driver power supply.