Fully digital PFC thyristor dimming driver high brightness LED solution based on C8051F3xx

1. Introduction

The European Union, the United States, Canada, and Taiwan, China, and many other countries and regions have issued policies and schedules to ban incandescent lamps. By 2012, almost all developed countries will ban the sale of incandescent lamps. As a new type of high-efficiency solid light source, high-brightness LED has the advantages of long life, high energy efficiency, high reliability, rich colors, miniaturization, small size, light weight, low voltage, low current, high brightness and fast light response speed. It will gradually replace fluorescent lamps and incandescent lamps and become the most promising energy-saving and environmentally friendly lighting product in the 21st century. It will trigger the third lighting revolution. After a detailed analysis of the market, we, Shiqiang Telecom, hereby launch a solution based on C8051F3xx full digital PFC thyristor dimming driver high-brightness LED.

2. Basic characteristics of high brightness LED

The basic characteristics of high-brightness LEDs are shown in Figures 1 and 2. From the voltage-current curve of the high-brightness LED in Figure 1, it can be concluded that the high-brightness LED is not conducting when the voltage is less than 2.5V; when the voltage is greater than 2.5V, the current increases exponentially with the increase of voltage, indicating that the high-brightness LED is a voltage-sensitive component. From the current-brightness curve of the high-brightness LED in Figure 2, it can be concluded that the relationship between current and brightness is basically a linear relationship. Therefore, to achieve stepless dimming, the linearity of the current must be well controlled.

Figure 1 High brightness LED voltage-current curve

Figure 2 High-brightness LED current-brightness curve

3. PFC principle and LED driving technology

3.1 PFC Principle

PFC stands for Power Factor Correction. Its working principle is as follows: after the output current (or voltage) of the main circuit is compared with the reference current (or voltage), it is input to the current (or voltage) error amplifier CA1. The rectified voltage detection value and the basic current (or voltage) signal of VA are added to the input of the multiplier M. The output of the multiplier M is used as the reference signal for current feedback control. After being compared with the switch current detection value, it is added to the PWM and the driver through the current error amplifier CA2 to control the on and off of the switch, so that the waveform of the input current is basically consistent with the waveform of the rectified voltage, which greatly reduces the current harmonics and improves the power factor at the input end.

There are basically three commonly used methods for controlling AC-DC switching converters to achieve PFC: current peak control, current hysteresis control, and average current control. Table 1 shows the three commonly used PFC control methods.

Table 1 Three commonly used PFC control methods

Since most LEDs on the market require a DC power supply to drive, a good driving power supply is necessary to bring out the good characteristics of LEDs. There are two types of LED driving methods: constant current and constant voltage. These two methods have their own advantages and disadvantages: (1) The output current of constant current driving is constant, which is easy to ensure the uniformity of LED brightness, but the output DC voltage varies within a certain range with the load size, while the output current of constant voltage driving varies with the increase or decrease of the load, which is not easy to ensure the uniformity of LED brightness; (2) Since constant current driving has a maximum current and voltage value, it is necessary to limit the number of LEDs used, while in constant voltage driving, in order to make each string of voltage stabilizing circuits drive the LED to display uniform brightness, it is necessary to add a suitable resistor; (3) Constant current is not afraid of load short circuit but strictly prohibits load complete open circuit, while constant voltage is not afraid of load open circuit but strictly prohibits load complete short circuit; (4) Constant current driving LED is very ideal, but the price is relatively high. After comparing these two methods, in this solution, we use constant current to drive LEDs.

4. Common dimming methods, thyristor characteristics and dimming difficulties

4.1 Common dimming methods

The commonly used dimming methods on the market are:

①. Pulse width modulation (PWM) dimming method: This dimming control method uses the pulse duty cycle of the power switch in the high-frequency inverter to adjust the output power of the lamp.

②. Dimming method by changing the supply voltage of the half-bridge inverter: By changing the supply voltage of the half-bridge inverter, the output power is changed to achieve the dimming effect.

③ Pulse frequency modulation dimming method: If the switching operating frequency of the high-frequency AC electronic ballast increases, the impedance of the ballast inductor will increase, so that the current flowing through the ballast inductor will decrease, resulting in a decrease in the current flowing through the lamp load, thereby achieving dimming;

④. Pulse phase modulation dimming method: The output power is adjusted by adjusting the conduction phase of the two power switch tubes in the half-bridge inverter to achieve the purpose of dimming.

⑤. Thyristor phase-controlled dimming method: Since thyristor phase-controlled (chopping method) dimming has the advantages of small size, light equipment weight, reasonable price and dimming power control range, thyristor phase-controlled dimming method is currently the most widely used dimming method. Applying the working principle of thyristor phase control, by controlling the conduction angle of the thyristor, a part of the sinusoidal wave voltage input from the power grid is chopped off to reduce the average value of the output voltage, so as to control the power supply voltage of the lamp circuit, thereby realizing dimming. Thyristor phase-controlled dimming has a fast voltage regulation speed for the lighting system and high dimming accuracy. The dimming parameters can be adjusted in real time in different time periods. Therefore, in this solution, we use thyristors for dimming, which can accurately control the output power.

4.2 Principle and characteristics of thyristor

Principle and characteristics of thyristor: Standard bidirectional thyristor can be triggered by both the forward current of the gate and the reverse current of the gate, and it can be turned on in four quadrants. When the gate voltage reaches the threshold value VGT and the gate current reaches the threshold value IGT, the thyristor is triggered to turn on. When the pulse width of the trigger current is narrow, the trigger level should be increased. When the load current exceeds the latch current IL of the thyristor, the thyristor can still maintain the on state even if the gate current is reduced to zero at this time. When the load current is zero, it is best to use an inverted DC or unipolar pulse (gate) current trigger.

The principle of thyristor phase-controlled chopping is shown in Figure 3: At a certain phase after the sine wave AC passes through zero, a positive trigger pulse is added to the gate of the thyristor to trigger the thyristor to conduct energy. According to the switching characteristics of the thyristor, this conduction will last until the end of the positive half cycle of the sine wave. Therefore, in the positive half cycle of the sine wave, the thyristor is not conducting within a range, and this range is called the control angle of the thyristor; while the thyristor is conducting within the phase interval, and this range is the conduction angle of the thyristor, which is usually expressed as. Similarly, in the negative half cycle of the sinusoidal AC, a trigger pulse is applied to the other thyristor in reverse connection at the phase angle to make it conduct. This cycle repeats, and its conduction is controlled for each half cycle of the sine wave to obtain the same conduction angle. If the trigger phase of the trigger pulse is changed, the size of the thyristor conduction angle (or control angle) is changed.

4.3 Difficulties of thyristor dimming and our coping strategies

The current difficulties of thyristor dimming include: inaccurate conduction angle detection, slow and unstable dimming, inability to start at low voltage, low power factor, and unstable power factor.

Our response strategy is: use a combination of software and hardware to detect the conduction angle, which can accurately detect the conduction angle, and the error of the conduction angle can be within 0.5%; use fast PWM update speed and accuracy to make the input and output correspond well, so as to achieve fast dimming and accuracy, that is, stability; stable and reliable auxiliary power supply design can make the system start directly when the conduction angle is the smallest; power factor stability is achieved by optimizing the design software, and the power factor is high.

5. Solution Implementation

5.1 Hardware Description

Figure 4 Principle block diagram

The principle block diagram of this solution is shown in Figure 4: First, the 220V AC input is chopped using a thyristor, and then after passing through an EMI filter and a rectifier bridge, the input voltage is divided and input to the MCU. The output of the transformer, that is, the output voltage and the LED current, is controlled by controlling the duty cycle of the MOSFET. The LED voltage and current are isolated and sampled using an optocoupler. The MCU adjusts the duty cycle of the MOSFET by detecting the conduction angle, input voltage, input current, and the sampling feedback signals of the LED voltage and current, thereby achieving stepless dimming of the LED lamp. The circuit structure topology block diagram is shown in Figure 5.

Figure 5 Circuit structure

5.2 Software Process

Since Silabs' MCU is compatible with traditional 8051 microcontrollers, the assembly instructions are the same as those of traditional 8051 microcontrollers, and it also supports the most widely used Keil C simulation software in China. Anyone who has experience in 51 microcontroller programming or has used Keil C can easily get started with the programming of the C8051F3xx series without having to invest a lot of time in learning beforehand.

Figure 6 Software flow chart

This solution is programmed in C language, and the program is highly portable. The software flow chart is shown in Figure 6: First, the variables, MCU clock, I/O port and ADC are initialized; second, soft start is performed to reduce the impact current during startup and effectively increase the life of the high-brightness LED. Then, loop control is performed to control the PWM duty cycle of the main power tube by sampling the input voltage, input current, conduction angle, output current and output voltage, and achieve a fast and stable dimming effect through optimized control. The software interrupt flow chart of the solution is shown in Figure 7.

Figure 7 Interrupt flow chart

6. Performance characteristics

This solution achieves driving capability at an input voltage of 230ACV±15%, 50Hz, and a power of 8~12W; fast, uniform, and stable dimming of 1%~100% within a conduction angle range of 15%~80%; a power factor of more than 0.95 at full load; and a stable and reliable auxiliary power supply design that can directly start the system when the conduction angle is minimum.

7. Conclusion

This article introduces in detail the C8051F3xx all-digital PFC thyristor dimming driver high-brightness LED solution launched by Shiqiang Telecom. This solution uses a single chip control to achieve dimming and PFC functions, with fast and effective over-current and over-voltage protection, isolated drive, good EMI characteristics, and high cost performance.