introduction
High-power white LEDs are a new generation of semiconductor light sources that are non-linear loads. Since the load characteristics cannot be described in a wide range and accurately, the light-emitting characteristics of the LED cannot be effectively controlled by the voltage type driver. When the load voltage has a slight fluctuation, it can cause a large change in the current, so that the brightness changes greatly. If the load voltage fluctuates too much, the LED may burn out. The LED load current is closely related to the LED's brightness, color temperature, efficiency, luminous flux, and lifetime. Therefore, ultra-high brightness LEDs are usually driven by a constant current source. Although the power efficiency of high-power white LEDs is relatively high, the overall efficiency depends not only on the LED itself, but also on the driver circuit. Therefore, designing a current-mode switching converter is an ideal driving scheme for meeting the high power and high efficiency requirements of LED applications.
2 LED driver design ideas
Since the driver is mainly used for automotive lighting, its power supply is mainly a battery, so a DC/DC converter is needed to accurately adjust the constant current of the LED, thereby obtaining the consistency of the light intensity of the LED and the color integrity. The LED lighting working mode of the in-vehicle system varies widely, so the driver should also be suitable for different application needs. The standard voltage for a car battery is 12 V, and the battery voltage may drop to 8 V when the battery is running out, and the alternator may charge its voltage to 14 V while the engine is running. Since the battery cell voltage varies widely, the required output voltage may be higher or lower than the input voltage. Therefore, the LED driver needs to adopt a step-up and step-down circuit structure to accommodate the constant current requirement of the LED.
Considering the essential characteristics of the LED driver as the current control system, the general-purpose switching power supply chip MC33167 is selected as the core device. It uses 7.5~40 V low-voltage DC power supply. The internal switching current of the chip can reach up to 5 A, and the appropriate peripheral circuit is configured. The up-and-down function can be implemented, so the chip can be used as the core chip of the LED driver for automobiles.
3 Analysis and Research of Switching Power Core Chip
The switching power supply chip of LED special lighting driver adopts MC33167, which can provide various functions. Its internal structure is shown in Figure 1. If the output switch current exceeds 5 A, 5 V output can be provided without external resistors. The internal 2% accuracy reference source can provide under voltage protection and internal thermal protection. Various protection modes can make the circuit work safely. . The protection mode reduces the supply current to 36μA, which greatly reduces the power consumption of the chip. The built-in 72 kHz fixed frequency oscillator allows the switching power supply to output a higher frequency PWM, so a smaller inductor and capacitor can be used outside the chip to achieve filtering, which greatly simplifies external components.
The correspondence between the potential 5 of the chip 5 and the output potential U2 is as shown in FIG. 2 . When the potential of the chip 1 pin U1 changes (when U1 is not equal to the standard voltage of 5.05 V), U5 changes proportionally. U5 and the sawtooth wave generated by the crystal oscillator are compared with the PWM operational amplifier to output high level or low level. U2 is controlled by internal logic circuit operation to control the on or off of the switch. Therefore, with the change of U5, the switching time of the output switching tube is different, that is, the ratio of U2 changes, thereby controlling the magnitude of the output voltage. When U1 decreases, U5 rises, while the duty ratio of control U2 rises, and the average value increases, so input U1 changes inversely with output U2. Therefore, a negative feedback circuit can be added outside the chip to form a stable power supply.
4 LED driver circuit development
4.1 Structural design from voltage source to current source
In the voltage source circuit, the output voltage must be U1=5 V, which is the reference voltage inside the power chip, otherwise the circuit cannot be stabilized. After the actual circuit test, the test results show that the circuit works normally and a stable output voltage Uo=5 V can be obtained. The voltage source can further change the feedback form outside the chip to make it a current feedback, thus designing the current source.
The current source circuit is shown in FIG. 3, and the output current is controlled by externally superimposing the voltage Uc. According to the principle of superposition:
IL is the load current. If R4=R5, R6=R7, Uref=5 V, the control voltage Uc output from the microcontroller is 0~5 V, and the bias voltage Up is 5 V. When Uc=0, IL=0; when Uc=5 V, IL=1 A, sampling resistance Rc=0.25 Ω, can be obtained: IL=0.2Uc. Therefore, by adjusting the size of the control Uc, the value of the output current can be linearly controlled.
4.2 Closed-loop stability analysis of LED driver
From the above, the transfer function of each part of the driver control system can be known. When the PWM output is connected to the LC filter, the overall control structure can be obtained as shown in Fig. 4.
Considering the use of the LED driver, the control system first considers the system stability and control accuracy at the time of design. At the same time, the driver is mainly used in automobiles, the external interference is relatively large, and the dimming speed is not fast, so the system bandwidth does not need to be large. Take the filter inductor 200μH, the capacitance is 20μF. At this time, the filter resonance frequency ωn=15.8 krad·s-1, the driver switching frequency ω=452 krad·s-1, so the output ripple attenuation is about 818 times, which fully meets the filtering requirements; The sum of the internal resistance of the inductor and the power chip is Rin=0.1Ω, the load resistance is 10Ω, and the damping coefficient is 0.4=0.4 73. Selecting a smaller ξ here is also beneficial for filtering the ripple; the current sampling resistor is 0.1 Ω, and the hysteresis correction circuit is used. R2=100 Ω, R3=10 kΩ, C3=1μF, two corner frequencies ω1=100 rad ·s-1, ω2=10 krad·s-1, thus effectively reducing the system high frequency gain and eliminating the ξ pair The influence of system stability; at the same time, the driver PI controller has Rf=2 000Ω, Cf=1μF, its zero turning frequency ω0=500 rad·s-1, and the hysteresis correction pole turning frequency ω1=100 rad·s-1 and system The shear frequency ωc=1150 rad·s-1, thus making the entire system amplitude-frequency characteristic -20 dB/dec through the 0 dB line, which improves the stability of the system.
When the filter internal resistance Rin=0.1 Ω, the load resistance R=10 Ω, L=190μH, C=20μF, the open loop diagram of the system is shown in Figure 5, where Figure 5a shows the wave without the hysteresis correction link. In particular, Figure 5b shows the Bode plot after hysteresis correction. As can be seen from the figure, the system is stabilized by the hysteresis correction system, and the phase angle margin is relatively large.
4.3 LED driver circuit design
The driving current source circuit is based on the switching power supply chip MC33167, and its working principle diagram is shown in FIG. 6.
First consider the voltage source, and fully understand the working principle of the power supply through the experimental circuit of the voltage source. Then design the voltage source, and fully understand the working principle of the power supply through the experimental circuit of the voltage source. Then design an external current feedback loop and an external current control circuit. Since the output voltage range of the MCU is 0~5 V, the control circuit should also include a matching circuit between the control voltage and the output voltage of the MCU. The driver intelligent control module circuit takes the microcontroller PIC18F258 as the core. It uses 16-bit high-performance RISC CPU with 1536 bytes of RAM, 32 k FLASH, on-chip A/D, EEPROM memory, PWM output function, and integrated hardware implementation. USART and CAN serial interface. The peripheral circuit of the MCU control module circuit is equipped with power supply, reset, crystal oscillator, voltage output based on PWM signal, 485-Can communication interface transceiver and other circuits.
4.4 Analysis of test results
The sampling resistance is 0.25 Ω. When the control voltage is changed, the output current should change linearly correspondingly, that is, IL = 0.2 Uc. The corresponding data of the control voltage and the output current measured by the experiment are shown in Fig. 7. The actual correspondence is: IL=0.167Uc+0.043. From the experimental results, the trend of the actual control voltage and output current is basically the same. The result is completely normal considering the actual circuit parameters in the circuit and the error of the bias voltage.
5 Conclusion
The LED driver based on the switch current chip MC331647 is given. The load voltage can be successfully changed in a wide range, and the load with high power can be used. The intelligent control module can be used to control the LED luminous intensity in real time and accurately. Test the steady-state performance of the drive and have the value of being promoted in the car LED.
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