A constant voltage and constant current model for driving LED array lighting and its simulation

Introduction

Due to the advantages of LED, such as environmental protection, long life and high luminous efficiency, it has been recognized by various countries as the next generation of ideal light source. However, there are still many technical bottlenecks for LED to eventually replace traditional light sources, among which LED driving technology is one of the key bottlenecks to be broken through.

At present, the power of a single high-power LED chip is about 1 W. To build a higher-power LED lamp, multiple LED chips must be combined. The luminous flux of LED light is approximately linear with the forward current, and its luminous flux and color temperature are controlled by the current flowing through it. An effective way to ensure that each LED light-emitting chip has the same brightness is to use multiple LEDs in a string. However, a major problem with this is that their accumulated voltage drop limits the number of series connections and the expansion of the lamp power. On the other hand, due to the exponential relationship of the V/I characteristics of LEDs and the negative temperature characteristics of the LED forward voltage drop, multiple LEDs in parallel will encounter the problem of current sharing of each LED string.

Another factor that affects the luminous flux and color temperature is temperature. In use, once the LED lamp is lit, the system temperature will rise, and the forward current of the LED will increase accordingly. The increase in current will make the temperature higher. If this cycle continues, the LED lamp will eventually burn out. Therefore, the current control and temperature control of the LED light string are very important in the LED lighting system.

1 Difference in forward voltage of LED package chips

Due to the complexity of LED packaging technology, there are large differences in the optimal working forward voltage (Vf) of LED chips. Table 1 lists the Vf value ranges of products from three different manufacturers. It can be seen from the table that the maximum deviation of the Vf value of the LED chip can reach 40%, and the average deviation is also around 20%. The forward current of the LED is exponentially positively related to the forward voltage. A small difference in Vf will cause a large difference in the forward current of the LED, thereby affecting the normal lighting of the LED. The data in the table shows such a large error, so this issue has to be seriously considered in the lighting driver. Compared with the development of driving technology, improvements in LED chip manufacturing and packaging are often more difficult. Therefore, it is more meaningful to control the current flowing through the LED chip.

2 Driving model of high-power LED array lamps

There are several methods to drive multiple LED strings in parallel arrays. A direct method is to directly use the bus power supply mode to power multiple strings of LED chips. According to the reference, such a mode does not solve the problem of equal current of LED strings, which has an adverse effect on the life and luminous effect of LED lamps. The model shown in Figure 1 connects a current regulator to each LED string on the basis of regulating the bus voltage. Generally, the current regulator can be a linear mode or a switching mode. Figure 1 adopts the switching mode. The LED string bus voltage is provided by the front-stage switch conversion module. Each LED string is connected to a switch tube, so that the current of the LED string is subjected to phase shift pulse width modulation (PS-PWM). Its function is: through phase control, ensure that the current pulses of each string do not jump at the same time; through feedback control, adjust the duty cycle of the switch pulse to ensure that the current of each string of LEDs is equal. A PS-PWM modulation circuit can be implemented as a reference. In the model shown in Figure 1,
it would be better to use a digital control chip to implement PS-PWM.

The sampling of the LED string current can be realized by the circuit shown in Figure 2. The proportional relationship between the sampled output value and the string current is shown in the equation in the figure.

3 Digital Implementation of Constant Voltage and Constant Current Drive Model for High-Power LED Array
The working performance of digital control chips is rapidly improving, while their cost is also decreasing rapidly. For example, the dsPIC33FJXXGSDSC series chips of Mieroehip and the TMS320F280xx series chips of TI have operating frequencies of tens of MHz. For a switching frequency of 100KHz, a control cycle can be completed within a single switching cycle. In terms of cost, the official website price of TI's TMS320F2 802-7DAT (September 2010) is $2.85. It integrates a 7-channel 12-bit ADC, 8-way PWM output module, an analog comparator, and various commonly used communication interfaces. One chip can realize most of the functions of drive control, which has a great advantage in cost.

Figure 3 is a digital control logic diagram of the LED array drive structure in Figure 1. Its phase setting module, protection module and ADC module can be completed by simple settings of digital chips; PID control, constant voltage current limiting and PWM duty cycle adjustment modules are implemented through software programming. By receiving dimming commands through the communication interface, the brightness of LED lighting can be easily adjusted. Since the control structure is implemented by software, it brings great flexibility to the working state detection and other function expansion of the LED driver.

4 Matlab simulation of digital control logic
As shown in Figure 4, it is a simulation model diagram of digital control logic. Among them: RL=105 Ω, RL1=100 Ω, RL2=95 Ω.

According to the V/I characteristics of LED, when LED works near Vf, LED string can be replaced by pure resistance. Different resistance values ​​represent the difference of total Vf of each LED string.
4.1 PID control and PWM phase shift
The PID control of LED string current mainly includes:
(1) PID operation of output error signal to ensure the stability of the system;
(2) Normalization operation of output error adjustment value;
(3) Comparing the normalized adjustment value with the sawtooth wave with amplitude of 1, adjusting the duty cycle to output PWM pulse;
(4) Controlling the output pulse phase. Among them, constant voltage and current limiting PID control completes the dual-loop control of DC-DC output voltage and current.
The PWM waveforms driving two adjacent LED strings differ by 360°/N, where N is the total number of parallel LED strings. As shown in Figure 4, in the simulation model, N=3, then the initial phases of the PWM waveforms driving the 1st to 3rd strings are 0°, 120°, and 240° respectively.

4.2 Simulation results
As shown in Figure 5, the current of each string is well controlled. The duty cycle of the output current pulse decreases from the first string to the third string, but its peak value increases successively, ensuring that the mean current of each string is consistent.

The PID regulation outputs of the DC-DC module voltage and LED string current are shown in Figure 6. The overshoots are 2.4% and 2.1% respectively, which are quite good.

Figure 7 shows the comparison of bus current without PS-PWM control (dashed line) and with PS-PWM control (solid line). It can be seen from Figure 7 that the bus current pulse jump amplitude without PS-PWM control is 3 times that with PS-PWM control. That is, when N strings of LEDs are connected in parallel, the bus current pulse jump amplitude can be reduced to 1/N of the original through PS-PWM modulation.

The system efficiency is shown in Figure 8. Iset is the input comparison value of the PID module. By modifying it, it is equivalent to dimming the LED string. In most dimming ranges, its efficiency is greater than 91%.

5 Conclusion
It can be seen from the simulation results that the bus current jump amplitude is reduced by 2/3 compared with that without PS-PWM control through PS-PWM modulation, reducing the EMI of the system. By adjusting the PWM duty cycle of each LED string current, the problem caused by the difference in LED chip Vf is better overcome, while maintaining a higher efficiency. The lamp can be easily dimmed by adjusting the current reference value. The digital control chip can enable the model to be better implemented in terms of high performance and low cost.