Application of PWM dimming method in LED brightness adjustment

LED is a solid-state electric light source and a semiconductor lighting device. Its electrical characteristics are highly discrete. It has the characteristics of small size, high mechanical strength, low power consumption, long life, easy adjustment and control, and no pollution. It is a new light source product with great development prospects. There are two ways to implement LED dimming: analog dimming and digital dimming. Analog dimming is achieved by changing the current in the LED circuit; digital dimming is also called PWM dimming. It changes the conduction time of the forward current by turning on and off the LED through the PWM wave to achieve the effect of brightness adjustment. Analog dimming adjusts the brightness of the LED by changing the current in the LED circuit. The disadvantage is that the adjustable gear is limited within the adjustable current range; PWM wave dimming can arbitrarily change the LED on time by changing the duty cycle of the high and low levels, thereby increasing the gears of brightness adjustment. This paper intends to use the two methods together to achieve the effect of adjusting the brightness of the LED.

1 LED dimming method

Analog dimming is achieved by changing the current in the LED circuit. The power supply voltage remains unchanged. By changing the resistance value of R, the current in the circuit is changed, thereby changing the LED brightness. Many other analog dimming methods are extensions of this method. Its advantage is that the current can be continuous, but the adjustable current range is often limited by hardware, and there are not many adjustment gears. This method is not ideal for high-precision lighting equipment that requires brightness sensitivity.

Digital dimming, also known as PWM dimming, uses PWM waves to turn LEDs on and off to change the conduction time of the forward current to achieve the effect of brightness adjustment. This method is based on the fact that the human eye is not sensitive enough to brightness flicker, so the load LED is sometimes bright and sometimes dark. If the frequency of brightness and darkness exceeds 100 Hz, the human eye sees the average brightness, not the LED flickering. PWM adjusts the brightness by adjusting the time ratio of brightness and darkness. In a PWM cycle, because the human eye is sensitive to light flickering greater than 100 Hz, the perceived brightness is a cumulative process, that is, the greater the proportion of the bright time in the entire cycle, the brighter the human eye feels. However, for some high-frequency sampling devices, such as high-frequency sampling cameras, it is possible to capture images of the LED when it is dark during sampling. Therefore, this paper combines analog and digital to design an LED drive circuit.

2 PWM Regulation Method Using Inductor

2.1 Driving Circuit

In the circuit, when current flows through the inductor, the inductor will generate a magnetic field, that is, part of the current is converted into magnetic energy and "stored" in the inductor; when the current is no longer flowing through the inductor, the inductor will release the magnetic energy in the loop through the current. This is also the reason why the current on the inductor cannot change suddenly. Based on this "charging and discharging" principle of the inductor, it can be used to average the discontinuous current generated in PWM wave dimming. Equation (1) and Equation (2) are the charging and discharging process of the LR circuit and the relationship between current and time.

Among them, If is the final stable current, I0 is the initial discharge current, and τ (τ=L/R, L is the inductance value, R is the loop resistance) is the time constant of the LR circuit.

Figure 1 shows the driving circuit. The selection of inductance value and PWM wave frequency is very important in this driving circuit. The C8051330 chip is selected as the output of the PWM wave, and the timer is used to flip the high and low level time to control the duty cycle of the PWM wave.

Figure 1 Driving circuit

It is necessary to ensure that the PWM cycle is less than the τ time of the inductor, because if the PWM cycle is greater than τ, it is very likely that when the PWM duty cycle changes, the current in the circuit can reach the saturated DC current of the inductor, affecting the LED current regulation. When the clock frequency of C8051330 is 25 MHz, the selection of the PWM cycle has a great influence on the current change gear. If the cycle is larger, the PWM duty cycle has more gears, and vice versa. It is planned to use a duty cycle of 256 gears, so the frequency of the PWM wave should be selected below 100kHz, that is, the cycle is more than 10 μs, and the DC inductor is 10Ω. At this time, the inductance value should be selected to be greater than 0.1 mH. Figure 2 shows the simulation results of the circuit current value when the PWM frequency is 100kHz, the duty cycle is 90%, and the inductance is 0.1 mH, 1 mH and 40 mH.

(a) Current versus time when the inductance is 0.1mH

(b) Current versus time when the inductance is 1mH

(c) Current versus time when the inductance is 40mH

(d) Partially enlarged view of curve c

Figure 2. Current variation over time for different inductance values.

Through simulation, a 40 mH inductor can be preliminarily selected as the driving circuit. Figure 3 is a voltage waveform captured by an oscilloscope. This voltage is the voltage on a 20 Ω resistor in series in the circuit. After stabilization, the voltage is 340 mV, that is, the current in the circuit is 17 mA. Because there is current loss in the actual circuit, the actual current value is smaller than the simulated current value, but the overall current change trend is basically consistent with the simulation.

Figure 3 Voltage change of series resistor in a circuit with an inductance of 40mH

2.2 Relationship between current and PWM duty cycle

Figure 4 shows the charging and discharging curves of the LED driving circuit, where Imax is the maximum current of the circuit under DC conditions. Assuming that the current value in the circuit fluctuates around the current value at time t1 on the charging curve when the PWM duty cycle is m, the following conditions should be met: the slope of the charging curve at point t is k1, the slope of the discharging curve at point a is k2, and k1mT=|k2|(1-m)T should be met. Therefore, the current in the driving circuit is maintained at a constant value with slight fluctuations.

Figure 4 Schematic diagram of RL circuit charge and discharge curve

From the analysis, it can be seen that after starting the driving circuit, the current reaches a relatively stable value after several charge and discharge cycles, and then the current fluctuates around this stable value. As shown in Figure 5, for each cycle, the slope of the current curve is constantly decreasing during charging; the absolute value of the slope of the current curve is constantly increasing during discharging; when the conditions of Figure 4 are met, the current is relatively stable. It can be concluded that when the LR circuit time constant τ is constant, the relationship between the inductor current and the PWM duty cycle is:

Where m is the PWM duty cycle.

Figure 5 is the experimental result curve of the inductor current changing with the PWM duty cycle. This curve is measured when the inductance value is 40 mH and a 22 Ω resistor is connected in series in the circuit. By analyzing the theoretical formula and experimental results, it can be found that when the PWM duty cycle is 36%~86%, the current value on the inductor changes linearly with the PWM wave duty cycle, and the change trend is consistent with the theoretical derivation.

For the high duty cycle section, since the slope of the charging curve has become almost constant, the current value also tends to the maximum value. In the low duty cycle section, due to the short charging time and large losses in the circuit, the current value on the inductor also tends to zero.

Figure 5 Experimental results of inductor current changing with PWM duty cycle

2.3 PWM duty cycle adjustment method

The computer is used to control the change of PWM duty cycle online through RS-485, and 256 gears are selected according to needs. Each time, a two-byte hexadecimal command is sent to RS-485 by the computer to change the duty cycle generated by C8051, thereby changing the brightness of the LED.

The main functions of the RS-485 interface circuit are: to convert the transmission signal TX from the microprocessor into a differential signal in the communication network through the "transmitter", and to convert the differential signal in the communication network into an RX signal received by the microprocessor through the "receiver". At any time, the RS-485 transceiver can only work in one of the two modes of "receiving" or "transmitting". Therefore, the circuit shown in Figure 6 is used, and the R/D signal output by the microprocessor directly controls the transmitter/receiver enable of the SN75LBC184 chip: when the R/D signal is "1", the transmitter of the SN75LBC184 chip is valid and the receiver is disabled. At this time, the microprocessor can send data bytes to the SN75LBC184 bus; when the R/D signal is "0", the transmitter of the SN75LBC184 chip is disabled and the receiver is valid. At this time, the microprocessor can process data bytes from the RS-485 bus. In this circuit, only one of the "receiver" and "transmitter" in the SN75LBC184 chip can be in working state at any time.

Figure 6 RS-485 circuit

Whether from the perspective of simulation or experiment, adding an inductor to the PWM dimming drive circuit can successfully "average" the current that varies widely in the circuit and stabilize it near a value that can be calculated theoretically. This article combines the common advantages of analog dimming and digital dimming, and can use RS-485 to change the duty cycle of the PWM wave through the functional relationship between the PWM wave and the current on the LED in the drive circuit, so that the LED has an ideal current value, and the brightness of the LED can be changed in real time and meticulously using a computer.