Introducing several isolated LED drive power supply solutions

In the context of global energy shortage and increasing environmental protection requirements, countries around the world are vigorously developing green energy-saving lighting. As a revolutionary energy-saving lighting technology, LED lighting is developing rapidly. However, the requirements for LED driver power supplies are also increasing. High efficiency, high power factor, safe isolation, compliance with EMI standards, high current control accuracy, high reliability, small size, and low cost are becoming key evaluation indicators for LED driver power supplies.

1 Specific requirements for LED driver power supply

LED is a low-voltage light-emitting device with the advantages of long life, high light efficiency, safety, environmental protection, and convenience. For AC input power drive, isolated output is based on safety regulations. The higher the efficiency of the LED driver power supply, the more it can play the advantages of high light efficiency and energy saving. At the same time, the high switching operating frequency and high efficiency make the entire LED driver power supply easy to install in compact LED lamps. High constant current accuracy ensures the brightness and light color consistency when LED lighting is used in large quantities.

2 Application solutions for LED lights with power below 10W

At present, power LEDs below 10W are widely used, and many integrated products have been launched, that is, LED driver power supply and LED lamp are integrated into one lamp, which is convenient for users to use directly. Typical lamp specifications include GU10, E27, PAR30, etc.

For this application, we designed the following solution (see Figure 1)

Figure 1 Schematic diagram of LED driver circuit based on AP3766

The program features are as follows:

1. Based on the latest LED dedicated driver chip AP3766, the primary side control method is adopted, and no optocoupler and secondary side current control circuit are required to achieve isolated constant current output, and the circuit structure is simple. The primary side current is detected by resistor R5, and the primary side current peak is controlled to be constant. At the same time, the switch duty cycle is controlled to keep the conduction time of the output diode D1 constant and the ratio of the entire switch cycle time, so as to achieve constant output current.

2. AP3766 adopts proprietary "sub-microampere startup current" technology, requiring only 0.6μA startup current, thus reducing the power consumption on the startup resistors R1 and R2 and improving system efficiency. The typical 5W application efficiency is greater than 80%, and the no-load power consumption is less than 30mW.

3. AP3766 uses constant current tightening technology to achieve vertical constant current characteristics with high constant current accuracy.

4. The number of circuit components is small. AP3766 adopts SOT-23-5 package, which is small in size. The whole circuit can be installed in a lamp cup of common specifications.

5. Safe and reliable, isolated output, with output open circuit protection, overvoltage protection and short circuit protection functions.

6. The power switch tube adopts triode, eliminating the high-voltage field effect tube and reducing the system cost.

Figure 2 is a photo of the 5W application circuit prototype of the solution. Figure 3 is a photo of the 5W LED driver based on AP3766. Figure 4 is a curve showing the full-load efficiency of the 5W LED driver circuit based on AP3766 versus AC input voltage. Figure 5 is a full-load output IV characteristic curve of the 5W LED driver circuit based on AP3766.

3 10~60W power LED street lights, LED straight tube lamp application solutions

The IEC International Electrotechnical Commission has put forward clear harmonic requirements for lighting fixtures, namely the IEC61000-3-2 standard. Therefore, for higher power LED lighting applications, it is necessary to use power factor correction (PFC) control technology. For applications below 60W, there is a cost-effective single-stage PFC control solution, the circuit schematic of which is shown in Figure 6.

Figure 6 Schematic diagram of LED driver circuit based on AP166+AP4313

The program features include:

1 Single-stage PFC solution, using only one-stage flyback circuit topology, achieving power factor correction and isolated constant current output at the same time. Fewer components, small size, and high cost performance.

2 High power factor, using active power factor correction control chip AP1661, power factor PF>0.9, meeting IEC61000-3-2 harmonic standard.

3. The secondary side constant voltage and constant current control chip AP4313 is adopted, with high constant current accuracy and output current error <3%.

4 High efficiency, full load efficiency up to 90%.

5. Safe and reliable, isolated output, with output open circuit protection, overvoltage protection and short circuit protection functions.

AP1661 is an active power factor correction control chip working in critical conduction mode, which can be widely used in pre-adjustment converter control of switching power supplies, LED drivers and electronic ballasts. The chip includes a start timer to meet the requirements of standby applications, a single-quadrant multiplier that can achieve a power factor close to 1, and a zero current detection module to ensure critical discontinuous operation. The totem pole drive output of AP1661 can provide a maximum drive current of 600mA and a shutdown current of 800mA for external MOSFET or IGBT. It adopts advanced bipolar complementary metal oxide semiconductor design and manufacturing process, with the characteristics of low start-up current, low operating current and low power consumption. AP1661 also has complete protection functions, including overvoltage protection, input undervoltage lockout with hysteresis, and multiplier output clamping to limit the maximum peak current of the switch. Figure 7 is a real photo of the 60W application circuit prototype of the solution. Figure 8 is a real photo of the LED straight tube lamp using this solution.

4 60～150W high power LED street light application solution

In the outdoor lighting sector, China's annual street lamp market demand is more than 20 million lamps. China's urban reconstruction, town construction, residential construction and road construction have extremely huge market demands for high-power, high-brightness, energy-saving LED street lamp products.

Under higher power application conditions, it is difficult for a single-stage PFC circuit to take into account both the PFC pre-regulation and the output constant current regulation performance of the flyback circuit at the same time. In this case, a two-stage circuit architecture is required to achieve this goal. Therefore, a new PFC+PWM two-stage architecture circuit topology solution is introduced, and the circuit schematic diagram of this solution is shown in Figure 9.

Figure 9 Schematic diagram of LED driver circuit based on AP1661+AP3102

The circuit adopts the Boost PFC+Flyback two-stage topology, and the control chip uses the PFC control chip AP1661, the PWM control chip AP3102 and the secondary constant current control chip AP4310. The advantages of this circuit solution are small input/output capacitance, small output ripple, isolated output, high power factor, current accuracy controlled within 2%, and high reliability. Figure 10 is a photo of the 90W application circuit prototype of this solution. Figure 11 is a photo of a high-power LED street lamp using this solution.

Figure 11 Photo of high-power LED street lamp

Figure 12 90W LED power supply full load efficiency curve