Vincent Ko, System Engineer, Freescale Semiconductor
Light-emitting diode ( LED ) technology has been available for about half a century. A light-emitting diode is a semiconductor device that emits light when a voltage deviation occurs. Due to its low power and low voltage operation, this technology is quickly applied to the status indication of various electronic devices. The lifetime of LED technology is usually very long, typically up to 10 years, far longer than other traditional lighting technologies (such as incandescent tubes and fluorescent tubes). This makes it very desirable to apply LED technology to a wider range of lighting applications.
New technologies that have recently emerged have enabled LEDs to achieve higher power levels. LEDs can reach a level of one watt, and some even up to 5 watts, with 18-44 lumens per watt. This type of LED device is called a high-brightness LED (HB-LED). Due to significant improvements in efficiency, HB-LEDs are rapidly being used in a variety of lighting applications.
Here are some examples of these applications:
• Traffic lights • Background lighting for flat panel displays • Flash • Home lighting
HB-LEDs have a non-linear IV characteristic, very similar to a diode. HB-LEDs can only be illuminated when the DC is unidirectionally delivered to the device, commonly referred to as forward current IF. The voltage drop across the HB-LED is called the forward voltage VF. For the HB-LED to achieve maximum brightness, the forward current through the HB-LED must be kept constant. In a typical 1W HB-LED, the forward current needs to be kept at about 350mA, and the corresponding forward voltage is about 3.4V, and the HB-LED can reach its maximum brightness.
The forward current IF has a very close relationship with the forward voltage VF, and a small change in the VF causes a large change in the IF. The ideal power source for HB-LED drives is a constant power source. In practice, constant current is usually achieved by a closed loop current controlled DC-DC converter. There are many DC-DC converter solutions based on independent analog components and relatively low cost. However, microcontroller-based (MCU)-based solutions offer greater flexibility in system design. In addition to general lighting, this controller provides enough processing power for the final application to support additional features. Therefore it still has a great appeal.
Some of the advantages of MCU-based design are as follows:
(1) Light brightness adjustment and flicker can be easily achieved with the MCU software without adding additional components to the system.
(2) Different power or different brands of HB-LEDs have different characteristics, and the MCU can be programmed by software to meet different driving requirements. In this case, lighting manufacturers can reduce the type of inventory and simplify logistics processing.
(3) Many MCUs have chip flash memory that can be used for data storage in applications. For example, when implementing the light brightness control function, the chip flash memory can be used to save the brightness level. The last brightness level is automatically restored each time the light is turned on.
(4) In addition to lighting, the MCU can handle several functions. For example, different types of connection standards (such as Zigbee, RS232, and LIN) can also be easily implemented through the MCU chip module.
Topology
The HB-LED driver requires a constant power supply. It usually requires closed loop control. Sometimes the system is battery powered and the battery voltage drops over time. A feedback control loop is required to maintain a constant drive current before the battery is fully drained. In addition, the forward voltage VF of the HB-LED varies with the ambient temperature, so closed-loop control is required to compensate for the VF change in order to maintain the forward current IF and the brightness of the HB-LED.
The conversion mode adjustment method is generally used instead of the linear adjustment method to drive the HB-LED. Switching regulators have higher functional conversion efficiency and are more suitable for digital design.
Assuming that the supply voltage is higher than the desired HB-LED forward voltage, the switching regulator will be rectified by the supply voltage ripple, and the duty cycle of the chopping control can control the average current of the output. The chopper mechanism is simple to implement, using only one power FET (MOSFET) to act as a switch to disconnect the current between the power supply and the powered device. The MOSFET is controlled by a pulse width modulated (PWM) output, where the chopping frequency is also equal to the frequency of the PWM output.
Normally, if the supply voltage and the required load current are constant, no feedback control loop is required (as shown in Figure 1). The switching regulator can control the average current of the device by adjusting the chopping frequency or its duty cycle. However, in some cases this topology does not apply. If the required device current is large, cutting off the current will generate a large current spike, which may affect the system's electromagnetic interference (EMI) performance.
Figure 1: Direct chopping topology
If you do not let the current on the device be cut off, you must use an energy storage device to ensure that the current is not cut off immediately when the power is turned off. A sensible option is to add an inductor to the circuit path of the device. During the PWM cycle, energy is held in the inductor. When the power is cut off, the stored energy is released and the device continues to be powered. This topology is called a buck converter. Figure 2 is a schematic diagram of a common buck converter.
Figure 2: Buck Converter Topology
Buck converter
The buck converter can only be used to perform a buck operation when the supply voltage is higher than the required device voltage. As shown in Figure 2, when the power switch SW1 is closed, the input voltage VIN is connected to the input of the inductor L. A reverse biased diode ensures that the device current is transmitted in one direction. At the same time, the energy stored in the inductor is increasing. When the power switch is turned off, the energy stored in the inductor is released and current is continuously supplied to the device through the diode. The energy stored in the inductor is gradually reduced, and the device current begins to drop. The main energy storage device of a Buck converter is an inductor. The inductor must be designed to ensure adequate storage of electrical energy to meet equipment power requirements during power-off (SW1 open). For HB-LED applications, the HB-LED needs to operate at a constant current, and the buck converter is also considered to operate only in a continuous conduction mode.
The inductor current has two states: a through current state (SW1 closed) and a current interrupted state (SW1 open). When in the through state, the current of the inductor begins to rise linearly, and the maximum change of current can be calculated by the following formula:
Where tON is the time when SW1 is closed. VOUT is the voltage on device RL. Similarly, when in the off-state, the inductor current drops during the open period of SW1, and the maximum change in current can be calculated using the following formula:
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