Design elements of high-power LED driver power supply

LED (Lighty Emitting Diode), also known as light-emitting diode, is a semiconductor light-emitting device that can convert electrical energy into visible light.

In recent years, LED technology has achieved rapid development, with significant improvements in luminous brightness and efficiency, especially high-power LED, which is expected to gradually replace the previous light sources and become a new generation of green light sources in the lighting field. It has the advantages of high light efficiency, low power consumption, long life, high stability, pure light color, good safety, and strong controllability.

However, the driving power supply required for high-power LEDs is a low-voltage DC power supply. Therefore, the power supply traditionally used to drive light bulbs (tungsten filaments), fluorescent lamps, energy-saving lamps, sodium lamps and other light sources is not suitable for directly driving high-power LEDs; and ordinary step-down and voltage-stabilized power supplies must be re-improved before they can be used to drive high-power LEDs.

Based on the working characteristics of high-power LEDs, this article analyzes the common driving power supplies on the market; it proposes that in the application of street lighting, in order to better play the advantages of high-power LEDs, the driving power supply must meet multiple design factors such as constant current output, heat dissipation, efficiency, power factor, over-voltage, over-current and over-temperature protection.

1. Working characteristics of high-power LEDs

The figure below is a nonlinear relationship curve between the forward voltage drop (VF) and the forward current (IF). As can be seen from the curve, when the forward voltage exceeds a certain threshold, which is usually called the on-voltage, it can be roughly considered that IF rises sharply with the increase of VF; as can be seen from the figure, high-power LEDs are low-voltage, high-current light-emitting devices. The highest IF of current high-power LEDs can reach 1A, while VF is generally 2 to 4V.

Since the optical characteristics of high-power LEDs are usually described as a function of current (IF) rather than voltage (VF). In addition, due to the differences in LED production processes and temperatures, the forward voltage drop (VF) of high-power LEDs will fluctuate greatly (up to 1V or more). As can be seen from the VF-IF curve in the figure above, a small change in VF will cause a large change in IF; the intensity of the LED light is determined by the current flowing through the LED. Too strong a current will cause the LED to attenuate, and too weak a current will affect the LED's light intensity. Therefore, the use of a constant voltage source to drive cannot guarantee the consistency of the LED's brightness, and affects the reliability and life of the LED. Therefore, high-power LEDs are usually driven by a constant current source to ensure the safety of high-power LEDs and achieve the ideal light intensity.

The figure below is the relationship curve between the luminous flux (φv/lm) and temperature (℃) of the LED. It can be seen from the figure that the luminous flux is inversely proportional to the temperature. The change in temperature also has a certain impact on the wavelength of the LED. Therefore, good heat dissipation is the guarantee for the LED to maintain constant brightness.

2. Constant Voltage Source Mode

2.1. Power frequency transformer linear voltage regulator

2.1.1 Circuit composition and working principle

As shown in Figure (3), the common characteristics of this type of power supply are that it is composed of an industrial frequency transformer, a rectifier filter circuit, and a constant voltage circuit;

The primary and secondary coils of the power frequency transformer in this type of circuit are completely isolated. By adjusting the transformation ratio of the primary and secondary coils, the required AC voltage can be obtained on the secondary side. The power frequency transformer plays the role of isolation and voltage reduction. Through the subsequent rectification and voltage stabilization circuit, the expected DC voltage can be obtained.

2.1.2 Advantages: simple circuit and low cost

2.1.3 Disadvantages: large loss, low efficiency, large volume, high temperature rise, unstable brightness

While the power frequency transformer of this type of circuit plays a role in reducing the voltage, its own loss will also increase significantly with the increase of load, resulting in serious temperature rise; at the same time, the efficiency of energy conversion will also become lower; in addition, due to the physical structure of the transformer, the miniaturization of the power supply is limited.

When driving a high-power white light LED, this type of circuit must be connected in series with a RES to limit and stabilize the current flowing through the high-power white light LED. Otherwise, once the operating current is too large and exceeds the maximum operating current of the high-power white light LED, it will cause permanent damage to the high-power white light LED. However, the method of adding a current-limiting resistor not only increases excessive energy loss, but also cannot solve the problem of unstable luminous brightness caused by the fluctuation of the power frequency voltage.

The coil ratio of the power frequency transformer of this type of circuit determines the number of high-power white light LEDs in series. Once the power frequency transformer is selected, it is difficult to increase the number of high-power white light LEDs in series. In order to meet different luminous brightness requirements, the number of high-power white light LEDs can only be increased in parallel. However, due to the different properties (on-state voltage, temperature) of high-power white light LEDs, it is difficult to achieve the same current flowing through each parallel LED branch, resulting in inconsistent luminous brightness of each parallel branch LED.

This type of circuit has low power conversion efficiency and cannot achieve the purpose of energy saving; the fluctuation of the power frequency voltage and the temperature rise of the circuit cause the brightness of the high-power white light LED to be unstable, making this circuit unsuitable for the field of street lighting.

2.2 High-frequency transformer switching voltage regulator

2.2.1 Circuit composition and working principle

As shown in Figure (IV), the common feature of this type of power supply is that it has a high-frequency transformer, and the DC voltage is rectified and filtered from the high-frequency pulse voltage of the transformer secondary winding. The primary and secondary coils of the transformer are isolated, and the input voltage is a high-voltage DC directly rectified from the AC mains.

2.2.2 Advantages: low power consumption, high efficiency, small size, light weight, wide voltage regulation range

2.2.3 Disadvantages: The control circuit is relatively complex, the requirements for components are high, and the brightness is unstable

This type of circuit uses high-power switching transistors, fast recovery diodes and high-frequency transformers, and the switching speed is very fast, which makes the power consumption of the high-power switching transistor very small and the efficiency of the power supply can be greatly improved; since no bulky power frequency transformer is used, the power consumption of the high-power switching transistor is very small, and a large heat sink is eliminated, so the power supply is small in size and light in weight; the key is that the output voltage of this type of circuit is adjusted by the duty cycle of the excitation signal, and the change of the power frequency input signal voltage can be compensated by pulse modulation. When the grid voltage changes greatly, a relatively stable output voltage can still be guaranteed.

This type of circuit uses the same principle as the power frequency transformer linear voltage regulator in driving high-power white light LEDs, and adopts the constant voltage and current limiting method. It does not conform to the nonlinear VI curve characteristics of high-power white light LEDs, so in terms of performance, there are still problems such as unstable and inconsistent luminous brightness in series and parallel connections.

Compared with the industrial frequency transformer linear voltage regulator, this type of circuit has significant improvements in efficiency, volume, and voltage regulation range, so it is more commonly used in other lighting occasions, but this type of circuit is still not suitable for the field of street lighting.