Design and implementation of white light LED driving circuit

With the rapid development of LED. Compared with traditional light sources, white light LED light sources now have the advantages of long life, solid lighting is not easy to damage, high light efficiency, mercury-free environmental protection, and earthquake resistance. In the future, it will become the third generation of light sources and will bring another revolution in the field of lighting. When LED is used in the field of lighting, a driving power supply suitable for LED is required. In this paper, a high-power driving circuit of white light LED is designed and implemented using PWM switch control.

1 LED electrical characteristics and driving requirements
1.1 Electrical characteristics of LED
The IV characteristics of white light LED are similar to those of ordinary diodes, except that the turn-on voltage is different. The turn-on voltage of LEDs made of different materials is generally between 1.5 and 3.0 V. When in the forward working area, the working current IF is exponentially related to the applied voltage

In the formula, Is is the reverse saturation current; VF is the applied voltage across the diode; q is the electron charge; k is the Boltzmann constant; T is the thermodynamic temperature. The DC current when the LED can work stably for a long time is called the rated working current, and the LED voltage drop at this time is called the rated voltage. A 1 W white light LED has a rated working current of 350 mA and a rated voltage of 3.3 V. The maximum value of the product of the forward voltage applied to the two ends of the LED and the current flowing through the LED is its limit power consumption. When the actual power consumption exceeds this value, the LED luminous characteristics deteriorate, and in severe cases, the LED structure will be damaged.
1.2 LED drive requirements
From the IV characteristics of the LED, it can be seen that when the voltage applied to the two ends of the LED fluctuates slightly, it will cause a drastic change in the current. At this time, it is easy to make the current too large and the input power exceed its limit power consumption, thus causing irreversible damage to the LED. When the working current value of the LED is different, its luminous intensity is also different. If a constant voltage drive is used, the LED array should be connected in parallel. However, due to the parameter error between individual LEDs, the current of each branch will be different, resulting in uneven array luminous intensity. Therefore, the LED drive circuit generally selects a constant current drive mode, and the corresponding LED array is also connected in series. The drive current is generally set to 70% to 85% of the rated current of the LED to protect the LED and achieve the purpose of extending the service life. At the same time, it also makes the luminous intensity of each LED uniform.
In the design of the LED drive circuit, the following basic indicators need to be considered:
(1) Improve the conversion efficiency of the drive circuit and reduce the power consumption in the circuit. (2) Improve the reliability of the circuit, be able to withstand high voltage, and have overcurrent detection function. (3) The circuit is as simple as possible, with a smaller circuit volume and lower manufacturing cost.

2 PWM mode switch circuit design
2.1 PWM principle
PWM is pulse width modulation. By using pulses to control the switching time of the switch circuit, the average voltage or current output of the circuit can be controlled to achieve the output power of the control circuit. The basic
working principle of PWM switch voltage regulation or constant current is that when the input voltage, system parameters and external load change, the control circuit performs closed-loop feedback through the difference between the controlled signal and the reference signal at a fixed operating frequency, adjusts the pulse width of the main circuit switch device, and stabilizes the output voltage or current of the switching power supply. Since the control device has low power consumption and the circuit efficiency working in the switching state is high, the power supply efficiency can generally reach 80% to 90%. This type of circuit has perfect protection measures and is a high-reliability power supply. The PWM switch circuit consists of 4 parts, namely input rectification and filtering, PWM control, switch device and output filtering. The block diagram of the LED drive circuit designed according to the PWM mode switch circuit is shown in Figure 1.

Most of the common PWM switch control signal generation parts have been integrated, which further simplifies the design of PWM switch power supply. The following introduces the design of a typical PWM switch drive circuit suitable for high-power LEDs using the chip HV9910B.
2.2 Circuit Design
HV9910B is a universal LED drive controller with strong adaptability. It can be powered by internationally common mains electricity, batteries or solar energy, and can accept a wide range of input voltages. The output constant current drive current range is extremely wide, from tens of mA to more than 1 A. The driver built using HV9910B uses fewer devices, the circuit is simple, and the production cost will also be reduced. The LED constant current drive circuit designed by HV9910B is shown in Figure 2. The input is AC 220 V mains electricity, and the load is an array of 10 LEDs with a power of 1 W connected in series.

The input stage of the circuit is composed of a full-wave rectifier bridge and a filter capacitor to complete the rectification and filtering of the AC power. The control stage is built by the HV9910B chip. The voltage after the input stage filtering is input to the chip's Vin as the input voltage VI of the circuit, with a peak value of 310 V and an average value of 190 V. The VDD, LD, and PWMD terminals are connected to the GND terminal through a capacitor to maintain the on-chip voltage of the corresponding pin. The square wave pulse signal with a certain frequency output by the GATE terminal is used as a switching signal to control the switch tube. Its frequency is set by the resistor connected to the RT terminal, and the pulse width is controlled by the LED current signal fed back by the sampling resistor RCS at the CS terminal. Inductor L1 plays a vital role in the circuit, providing filtering, energy storage, and freewheeling power supply for the drive circuit to maintain the balance of the current in the load, and the recovery diode completes the role of building a freewheeling path. In the half cycle when the switch signal is turned on, the potential after the pre-stage filter directly supplies power to the LED load and charges L1; in the half cycle when the switch signal is turned off, the full-energy L1 supplies energy to the loop composed of the fast recovery diode and LED, so as to achieve continuous driving of the LED in one cycle.
2.2.1 Circuit parameter calculation and device selection
The peripheral device parameters of the chip can be determined by referring to the chip manual and specific circuit requirements. First, the operating frequency of the circuit must be determined. The RT pin is connected to a resistor with a resistance value of 226 kΩ to 1 MΩ to set the switching signal frequency output by the GATE pin. The selection of this frequency is related to the inductance L value and the performance of the switch tube. Generally, under the condition of AC power supply, the frequency is selected between 25 and 150kHz. When the frequency is too high, the required inductance value is small, but the requirements for the switch tube are very high. At this time, the power consumption of the switch tube is much greater than that of low-frequency operation. In the experiment, the switching frequency was first set to 100 kHz. In the absence of heat dissipation, the MOSFET generates a lot of heat and is very easy to burn. When the frequency is set to 26 kHz, the calculated inductance is very large. In the working state, the inductance consumes too much energy and is not suitable for high-efficiency operation of the circuit. Therefore, the switching operating frequency is selected as 50 kHz.
The LED driving current is set to 0.35 A. According to the calculation formula provided in the chip manual, the RT value is 478 kΩ. Within the design allowable range, a 470 kΩ resistor can be used as RT. The sampling resistor RCS = 0.62 Ω.
The value of the inductor L1 is related to the ripple value of the LED current. Generally, the maximum ripple coefficient is limited to 0.3. The calculation formula of the inductance value is

The circuit drives 10 LEDs, whose VLEDS is 33 V. Vin is the peak voltage after full-wave rectification and filtering, and its value is 310 V. The values ​​of ILED and fs are the same as before. Substituting them into formula (2) to calculate L1 = 5.6 mH, a 6.8 mH inductor is selected in the circuit.
The MOS tube is selected with excellent performance, with a maximum withstand voltage of 500 V, a maximum drain current of 5 A, and an on-resistance of 0.6 Ω. The diode is selected as the fast recovery diode BYV26B, which has a reverse withstand voltage of VD = 500 V, a forward average current of 1 A, and a forward conduction voltage drop of 1.2 V. Capacitor C2 is used as the output filter circuit to achieve voltage filtering. C2 is selected from 4.7 to 33 μF capacitors. The front-stage filter capacitor C0 selects a polar capacitor of 4.7 to 33 μF, and capacitor C1 uses a 22 μF non-polar capacitor. The full-wave rectifier bridge requires high withstand voltage and large overcurrent. DB206S is selected in the circuit, which can withstand pulse high voltage 800V and surge current 2 A, meeting the circuit design requirements.
2.2.2 Circuit efficiency theoretical calculation reference
The main losses in the entire circuit are generated by the power MOS tube, sampling resistor, inductor L1 connected to the load LED, fast diode, and chip HV9910B. According to the relevant formulas provided by the literature and the original parameters of specific models, the overall power consumption of the circuit can be calculated as PLOSS=PMOS+PDIODE+PINDUCTOR+PIC+PRS=0.032+0.389+0.613+0.31+0.008=1.352 W.
The output power of the circuit is Po=33 x0.35=11.6 W, and the overall conversion efficiency of the circuit is η=11.6/(1.35+11.6)×100%=89.57%. From the results of the efficiency theory calculation, the performance of the designed circuit is excellent.

3 Circuit test The
designed PWM switch drive circuit is constructed, and the working state of the actual circuit is tested using experimental instruments such as digital voltmeter, AC power meter, and oscilloscope. Under normal working conditions of the circuit, the voltage waveforms of the two key points in the circuit are tested.
Figure 3 shows the waveform of the PWM switch control signal applied to the gate of the switch device. Its period is 14μs, amplitude is 8V, and duty cycle is 8.3%. There is a certain gap between the period and the preset value. This is mainly due to the frequency setting deviation caused by the resistance error of the resistor RT. Figure 4 shows the waveform of the current in the LED load. The measurement process is to insert a 0.5Ω resistor in series in the LED load loop to measure the voltage waveform at both ends, and use the linear characteristics of the resistor to reflect the current characteristics. From the waveform, the current changes periodically according to the sawtooth waveform, with a peak-to-peak value of 40 mV. The current ripple is calculated to be 80 mA, and the output current average is 350 mA. After calculation, the ripple factor is 22.9%.

The input power PI of the circuit is measured to be 9.9 W, and the load power consumption Pout is 8.7 W. The conversion efficiency of the circuit is 87.8%, which is close to the value obtained by theoretical calculation of the circuit efficiency.
After measuring the waveform of the key points of the circuit and measuring the power of the circuit, it is found that the circuit works in a frequency switching state of 71 kHz, with stable working state, large output power and high efficiency. However, the output ripple coefficient of the circuit is high, which causes the luminous illumination of the LED to not reach its optimal value during safe operation, and the output filtering part of the circuit needs to be further improved.

4 Conclusion
By analyzing and understanding the luminous performance and electrical characteristics of LEDs, the driving requirements of using a constant current power supply to drive and connect the LED array in series are obtained. Based on the principle of the PWM switching circuit, a typical PWM switching circuit based on the HV9910B chip is designed. Its optimal operating frequency is determined by experimental measurement, and the lighting drive of the white light high-power LED is completed well. Through theoretical calculation and actual measurement, it is found that the switching LED driver power supply has a relatively superior circuit conversion efficiency, a wide operating voltage range, constant current output and conversion efficiency exceeding 85%. However, to drive white light LEDs more safely for daylight lighting, it is necessary to perform better filtering on the output of the switching circuit to make the output ripple of the circuit smaller and the current more stable.