Design of high-efficiency LED lighting power supply using BP2808 constant current control chip

BP2808 basic working principle

BP2808 is a constant current control chip specially designed to drive LED light sources. BP2808 works in a step-down system in continuous current mode. The chip controls the peak current and ripple current of the LED light source to achieve a constant average current of the LED light source. The chip uses very few external components to achieve constant current control, analog dimming and PWM dimming functions. The system application voltage range is from 12VDC to 600VDC, and the duty cycle can reach up to 100%; it is suitable for AC 85V-265V wide voltage input, and is mainly used in non-isolated LED lamp power drive systems. BP2808 uses patented source drive and constant current compensation technology to make the current driving the LED light source constant, and the change is less than ±3% within the range of AC 85V-265V. Combined with the drive system application circuit of BP2808 patented technology, the 18W LED fluorescent lamp practical solution has a system efficiency of more than 90% within the range of AC 85V-265V. In the AC 85V-265V input range, BP2808 can drive LED light source arrays from 3W to 36W, so it is widely used in E14/E27/PAR30/PAR38/GU10 and other lamp cups and LED fluorescent lamps.

BP2808 has multiple LED protection functions including LED open circuit protection, LED short circuit protection, and over-temperature protection. Once a system fault occurs, the power system automatically enters the protection state until the fault is removed, and the system automatically re-enters the normal working mode. The multiplexed DIM pin can perform LED analog dimming, PWM dimming, and dynamic temperature protection of the lighting system. BP2808 uses SOP8 package.

Typical design of LED fluorescent lamp application

There are many kinds of LED light source light strip power drive solutions for LED fluorescent lamps. Currently, non-isolated solutions are the mainstream due to their high efficiency, small size and low cost, and the majority of them use PWMLED driver controllers to drive LED fluorescent lamps. In fact, traditional fluorescent lamps are all non-isolated solutions.

Taking AC176V-264V full voltage input as an example, when BP2808 is used as the main chip to design a load of low-power LED light sources in multiple strings and parallels, the design block diagram of the entire system solution is shown in Figure 1. The entire circuit consists of surge/lightning protection, EMI filtering, full-bridge rectification, passive power factor correction (PPFC), startup voltage (including feedforward compensation, feed current supply after startup, and drive softening), constant current compensation, PWM control, source drive, LED light source array, as well as sampling resistors, Toff time settings, energy storage inductors, freewheeling diodes and other components.

Figure 1: 18W LED fluorescent lamp system design block diagram.

The LED light source array is designed as a 0.06W white light LED (SMT or straw hat lamp) with 24 series and 12 series in parallel, driving 288 low-power WLEDs with a total power of 18W. The design circuit of the full-voltage 18W LED fluorescent lamp switch constant current source is shown in Figure 2, and the functions of its various parts are marked in red. The lightning protection and EMI filtering in the figure form an EMC circuit, and the feed current power supply is realized by using the rectifier diode already built into the chip.

Figure 2: Practical circuit diagram of 18W LED fluorescent lamp.

From the AC220V point of view, the AC mains inlet is connected with a 1A fuse F1 and a surge/lightning varistor Vz1; followed by an EMI filter, which is composed of Ld1, Lc1, Cx1, and Cx2; DB1 is a full-bridge rectifier with four high-voltage silicon diodes inside; CE1, CE2, R10, D2-D4 form a passive power factor correction circuit; the BP2808 chip is stepped down by the R15 and R16 startup resistors, which are feed-forward compensated by R17 and C3, and the patented constant current compensation circuit composed of Dz1, C2, R18 and the internal circuit of BP2808 stabilizes the voltage and supplies power to the BP2808 control circuit. After the system is started, the static current of the control circuit itself is small, and there is a feed diode from OUT to VCC inside the chip to provide working power to BP2808. At this time, the current passing through the resistors R15-17 will be greatly reduced, so the total system power consumption is also greatly reduced, and the system efficiency is significantly improved. The patented source drive circuit is composed of MOS tube Q1, D6, Rg, Rt, Rcs and the internal circuit of BP2808. Its remarkable features are effective reduction of power consumption and improvement of constant current accuracy. The source drive circuit reduces the system current consumption, especially reduces the current on R15-17 in the traditional high-voltage difference power supply path, thereby reducing power consumption and improving efficiency. D6 and Rg can make the switch turn-on drive softer and keep the turn-off drive strong, which not only improves EMI but also tries not to sacrifice efficiency. The output filter capacitor C0 connected in parallel with the LED light source is used to reduce the current ripple on the LED light source.

The CS terminal of BP2808 collects the peak current on the current sampling resistors Rs1~Rs2, and the internal logic controls the pulse duty cycle of the OUT pin signal in a single cycle for constant current control. The output constant current is combined with the freewheeling circuit of D5 and LM1 to supply constant current to the LED light source. When the LED light source array combination changes, the resistance value of resistors Rs1~Rs2 should also change accordingly, so that the output current of the entire circuit meets the requirements of the LED light source array combination.

The arrangement of PCB is the key to a good product, so the routing of PCB should be designed according to the requirements of power electronics safety regulations. This circuit can be used for T10 and T8 fluorescent tubes. Because the space between the two tubes is different, the width of the two PCBs will be different. The height of all parts should be reduced to fit the T10 and T8 tubes. Figure 3 is a real photo of the T10 constant current source board. 30 components are installed on a 0.8 mm thick epoxy single-sided printed board.

Figure 3: Actual photo of 18W LED fluorescent lamp switch constant current source.

If you are designing for AC85V--264V full voltage input and you need to consider PFC, you can design the LED light source array into a 0.06W white light LED 12 series, 24 series parallel solution. When BP2808 is used as a LED fluorescent lamp power driver, it is recommended that the output voltage is <100VDC and the current is <600mA.

There are several types of LED fluorescent lamp driver ICs available at present, and their performance parameters are different. Table 1 is provided for reference in design selection. It can be seen that the fixed Toff working mode, 100% duty cycle, chip working current of only 0.2mA, efficiency of 92%, constant current compensation and unique source drive mode of BP2808 make it have obvious advantages in LED lighting fixtures.

Table 1: Comparison of performance parameters of LED fluorescent lamp driver IC products.

Key technologies of BP2808

The two patented application circuits of constant current compensation and source drive make the application of BP2808 more convenient and more distinctive. As shown in Figure 4, the internal circuit of BP2808GND and LN and R3, C3, R4, Dz1, C2 form the patented application circuit of constant current compensation; the internal circuit of BP2808Vcc, CS and OUT and Q1, D6, Rg, Rt, Rcs form the patented application circuit of source drive.

Figure 4: Two patented application circuits of constant current compensation and source drive.

Figure 5 is the source drive control circuit diagram, from which it can be seen that the drain of the low-voltage switch MOS tube (700mA) inside BP2808 is connected to the source of the external power switch MOS tube Q1, and its source is connected to one end of the sampling resistor RCS and the input end of the first comparator, and its gate is connected to the output end of the RS trigger. The drain output current of the external power switch MOS tube Q1 directly drives the LED light source through the energy storage inductor. Do in the chip is a feed diode. After BP2808 starts working, the feed current from OUT to VCC is rectified by Do to provide working power to BP2808.

Figure 5: Source drive control circuit diagram.

The use of source drive can effectively reduce the current consumption of the driving circuit, reduce power consumption and improve efficiency; in the traditional high-voltage difference power supply path, in order to reduce the rectified DC high voltage to the low-voltage working voltage required by the PWM chip, a low-resistance and high-power resistor is used to reduce the voltage, which makes the device hot and consumes a lot of power.

Isolation and non-isolation of various LED lamp driver design applications

BP2808 can also be used to design the driving power supply of isolated and non-isolated bulb lamps, PAR lamps, downlights, recessed lamps, garden lamps, explosion-proof lamps, wall washers, table lamps, work lamps, thyristor dimming lamps and other LED light source lamps. The design principle of non-isolated lamps can continue to use the design ideas of the aforementioned typical LED fluorescent lamp application scheme. By changing the arrangement of the LED light source array, it can be transformed into various different types of LED lamps. According to the different requirements of various LED lamps for driving power supplies, the output characteristics of the power supply can be changed to meet different needs. For example, the thyristor dimming control can use brainstorming on the application circuit, add a circuit to extract the conduction angle information in the phase-cut power supply, and control the driving current of the LED light source according to the signal to obtain the dimming effect.

LED fluorescent lamps using BP2808 have been used in the lighting systems of the "Shanghai Eco-home" of the 2010 Shanghai World Expo, the Jiangsu Road Station Hall of Shanghai Metro Line 2, and the Malaysian Palace. The BP2808 has also matured in the thyristor dimming isolation and non-isolation solutions for AC/DC PAR lamps, bulb lamps and other LED lamps. Its application circuits, PCB board drawings, and BOMs that can be used in production are also available to customers.

Conclusion

BP2808 has a fixed Toff working mode, 100% duty cycle, the chip working current is reduced to 0.2mA, the efficiency is 92%, and the constant current accuracy is improved, making it more suitable for the application of LED lighting lamps and lanterns. In addition to inheriting and absorbing the advantages of similar products at home and abroad, BP2808 adopts an innovative topological structure, with significant improvements in chip design and more perfect performance. In particular, the two patented application circuits of constant current compensation and source drive make BP2808 application more convenient and energy-saving.