PWM driving principle of white light LED

This paper designs a PWM-based dimmable LED drive circuit, which can provide the voltage and current required by the LED, and has the characteristics of high color temperature, economical and practical, and long life. The electrical characteristics of white light LEDs are highly discrete, and white light LEDs are a homogeneous electric light source and a semiconductor lighting device. 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 type of light source product with great development prospects. However, due to the very steep forward volt-ampere characteristics of white light LEDs, it is difficult to power them. Small fluctuations in the working voltage of white LEDs will cause a sharp change in the working current, and may even burn out the LED. In order to keep the working current of the LED stable and ensure that the LED can work normally and reliably, the design of the drive circuit is crucial.

1 Electrical characteristics of white light LEDs

1.1 Relationship between LED luminous intensity and current

The luminous intensity of LED devices increases with the increase of forward current within the limit operating current range, but the relationship between the luminous intensity and forward current varies for LED devices made of different semiconductor materials. In general, the luminous intensity Ir increases with the increase of forward current If.

The relationship curve between Ir and If describes how much current should be used to drive the LED to achieve the required luminous intensity. The relationship between the LED luminous intensity and the forward current is shown in Figure 1. The curve in Figure 1 takes the red LED as an example. When the forward current is about 40 mA, the luminous intensity of the red LED hardly changes. In other words, as long as the forward current of the red LED array is controlled to a certain value, its luminous intensity will tend to saturation.

1.2 Effect of temperature on forward current of white light LED

The forward current of a white light LED also changes with temperature. Figure 2 is a curve showing the change in the allowable forward current of a commonly used white light LED with temperature.

2 LED PWM drive method

2.1 Principle and Formation of PWM Signal

PWM dimming 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 dim. If the frequency of brightness and dimming exceeds 100 Hz, the human eye sees the average brightness instead of the LED flicker. PWM dimming adjusts the brightness by adjusting the time ratio of brightness and darkness. This method can achieve dimming by adding an adjustable duty cycle and a fixed frequency digital signal to the pin that adjusts the time ratio of brightness and darkness, but the range of dimming depends on the speed of soft start or recovery of normal operation of the internal circuit of the device, so the range is not very wide.

The PWM principle is to control the voltage on the LED by passing a fixed DC voltage through a switch K that opens and closes at a certain frequency. Suppose the maximum current when the LED is turned on is Imax. The switch opening and closing cycle is T, and the closing time each time is t. When the duty cycle is D=t/T, the average current of the LED is:

From formula (1), we can see that when T remains unchanged (i.e. the switching frequency of the switch remains constant), the average current across the LED can be changed by simply changing the on-time t, thereby changing the brightness of the LED.

There are three types of pulse width modulation signal formation circuits: 1) can be generated by voltage-to-pulse width converter, that is, hardware generates pulse width modulation signal; 2) generated by software timing, timed by timer, the timing time is controlled by software, and the pulse width adjustable signal is output from the output port P1.0 or other ports of the pulse width signal; 3) the single-chip microcomputer controls the external timer/counter (such as 8253) hardware circuit to generate pulse width modulation signal, only need to use two counters to work in mode 1 and mode 2 respectively, and the pulse width modulation signal can be generated through hardware connection. Among them, the first type is hardware circuit implementation, and the circuit is complex. The second type uses timer TO, but due to insufficient system counters, it must be expanded. The third type uses 8253, which is very convenient and takes up less software time.

Atmega 16 microcontroller has 4-channel PWM, and has multiple working modes such as fast PWM mode and phase correction PWM mode. Considering the cost and simplification of the entire system, this design directly uses Atmega 16 microcontroller to generate pulse width modulation signal.

2.2 PWM driving white LED

The luminous intensity of LED is basically proportional to the current passing through the LED device, which means that the luminous brightness of LED is the same under the same conditions of average current of pulse current and DC current. In addition, driving LED with high-amplitude pulse current and then adjusting the duty cycle of pulse to obtain a more appropriate average current can reduce power consumption. Because when LED works in pulse state, the brightness value of LED perceived by human eye is between peak brightness and average brightness value. Therefore, pulse current driven LED can be brighter than direct constant current driven LED, that is, to obtain the same luminous brightness, the average current value required by pulse current driving method is smaller than that of DC current driving method.

Secondly, for LED, if the pulse circuit is used to drive, the control part adopts the pulse width modulation method. Compared with the constant current control method, the control efficiency of the control part will be greatly improved, and the current limiting resistor can be removed or its value can be reduced. Therefore, from the perspective of energy saving, it is better to use the pulse power drive method.

The pulse drive method uses the visual inertia of the human eye to repeatedly turn the power on and off to light up the LED device. However, this drive method usually requires consideration of the determination of the pulse current amplitude and the selection of the repetition frequency. To obtain a luminous intensity equivalent to that of the DC drive method, the average value Ia of the pulse drive current should be the same as the current value of the DC drive. As shown in Figure 3, the average current is the time integral of the instantaneous current i.

For a rectangular wave, we have

Where Ic is the DC drive current value, Ia is the average value of the pulse drive current, IF is the pulse current amplitude, and ton/T is the duty cycle.

In order to make the average current Ia in the pulse drive mode the same as the DC drive current Ic, the pulse current amplitude IF must satisfy:

It can be seen that when the pulse is driven, the amplitude of the pulse current is T/ton times the current amplitude of the DC drive circuit. It is necessary to pay attention to the operating frequency of the drive device. When the frequency exceeds a certain range, the device will not work properly because the device cannot be turned on and off normally. The operating frequency of the LED is in the range of 10 MHz to several hundred MHz.

3 LED lighting circuit design

3.1 White LED Power Supply

The LM317 is an adjustable three-terminal positive voltage regulator that can provide more than 1.5 A of current with an output voltage range of 1.2 to 37 V. This regulator is easy to use and only requires two external resistors to set the output voltage. When operating, the LM317 establishes and maintains a nominal reference voltage (Vref) of 1.25 V between the output and the regulation terminal. This reference voltage is converted by R1 into a programming current that is fed to ground through R2, as shown in Figure 4.

The output voltage is obtained from the regulated output voltage formula:

Because the current IADj at the regulating end is controlled to be less than 100μA, this error can be ignored. A white LED requires a 3.3 V DC voltage. This design is for three white LEDs in series, and the LM317 output is required to output a 9.6 V DC voltage. Since the LED is connected in series with the field effect tube, the voltage drop of the field effect tube is removed, and the voltage output by the LM317 is about 10V. First determine that the resistance of R1 is 220Ω, and the resistance value of the potentiometer R2 is 1.4 kΩ.

3.2 White LED drive circuit

In this design, the PWM signal is connected to the gate of the P-channel power MOSFET IRF9540 through the base of the transistor VQ1. The gate drive of the P-channel power MOSFET adopts a simple NPN transistor drive amplifier circuit to improve the conduction process of the MOSFET and reduce the power of the driving power supply. When the driving circuit directly drives the power MOSFET, it will cause the driven power MOSFET to turn on and off quickly, which may cause the voltage between the drain and source of the driven power MOSFET to oscillate. On the one hand, it will cause radio frequency interference; on the other hand, it may cause the power MOSFET to be damaged by excessive voltage. To solve this problem, a non-inductive resistor R1 needs to be connected in series between the gate of the driven power MOSFET and the output of the driving circuit. When the PWM wave outputs a high level, the transistor VQ1 is turned on, so that the gate voltage of the MOSFET is lower than the source voltage, the source and drain of the MOSFET are turned on, and the LED is lit; when the PWM wave outputs a low level, VQ1 is cut off and the LED is off. When the PWM frequency exceeds 100 Hz, the human eye can see the average LED on and off time, creating a sense of LED brightness change, which is proportional to the LED on period, as shown in Figure 5.

4 Conclusion

White LED has excellent characteristics such as long life, low voltage drive, safety and stability, so it has become a new light source with great development potential. However, the semiconductor characteristics of LED make it difficult to design the power supply system. In order to obtain higher luminous efficiency and dimming effect, a PWM driven LED lighting circuit is designed. The designed LED driving circuit not only facilitates the control of LED brightness, but also greatly increases the color temperature compared with the ordinary driving method.