Design elements for high power LED driver

However, the driving power required for high-power LEDs is a low voltage of DC, so the power source traditionally used to drive light sources such as bulbs, fluorescent lamps, energy-saving lamps, sodium lamps, etc. is not suitable for directly driving high-power LEDs; The power supply must be reworked before it can be used to drive high power LEDs.

According to the working characteristics of high-power LEDs, this paper analyzes the common driving power sources on the market. In the application of street lighting, in order to better utilize the advantages of high-power LEDs, the driving power supply must meet the constant current output and heat dissipation. Multiple design factors such as efficiency, power factor, overvoltage and overcurrent protection.

First, the working characteristics of high-power LED

Since the optical characteristics of high power LEDs are generally described as a function of current, not as a function of voltage. In addition to the difference in LED production process and temperature, the forward voltage drop range of high-power LEDs will show relatively large fluctuations. From the VF-IF curve in the above figure, small changes in VF will cause large IF changes. The intensity of the LED light is determined by the current flowing through the LED. If the current is too strong, the LED will be attenuated. If the current is too weak, the LED's luminous intensity will be affected. Therefore, the use of constant voltage source drive can not guarantee the consistency of LED brightness, and affect 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 desired luminous intensity.

Second, constant voltage source mode

2.1, power frequency transformer linear regulator source

2.1.1, circuit composition and working principle

The common feature of this type of power supply is composed of power frequency transformer, rectifier filter circuit and constant voltage circuit;

The primary and secondary coils of the power frequency transformer of this type are completely isolated. By adjusting the ratio of the primary and secondary coils, the required AC voltage can be obtained on the secondary side, and the power frequency transformer functions as isolation and voltage reduction. The expected DC voltage can be obtained by subsequent rectification and voltage stabilization circuits.

2.1.2 Advantages: Simple circuit and low cost

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

The power frequency transformer of this type of circuit acts as a step-down transformer, and its own loss will also increase significantly with the increase of the load, resulting in a serious temperature rise phenomenon; at the same time, the efficiency of energy conversion will also become lower; The physical structure limits the size of the power supply.

When driving such high-power white LEDs, this type of circuit must be connected in series with a RES to limit and stabilize the current flowing through the high-power white LED. Otherwise, if the operating current is too large, the maximum operating current of the high-power white LED will be exceeded. The permanent damage of high-power white LEDs, however, the method of increasing the current-limiting resistor not only increases the excessive loss of energy, but also cannot solve the problem of unstable illumination 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 series of high power white LEDs. Once the power frequency transformer is selected, it is difficult to increase the number of series of high power white LEDs. In order to meet different lighting brightness requirements, the number of high-power white LEDs can only be increased in parallel, but because of the different properties of high-power white LEDs, it is difficult to achieve the same current flowing through each parallel LED branch. There is a phenomenon in which the luminances of the LEDs connected in parallel are inconsistent.

The power conversion efficiency of this type of circuit is low, and the purpose of energy saving cannot be achieved; the fluctuation of the power frequency voltage and the brightness of the high power white LED caused by the temperature rise of the circuit are unstable, making the circuit unsuitable for the field of street lighting.

2.2, high frequency transformer switching regulator source

2.2.1, circuit composition and working principle

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 secondary winding of the transformer. The original secondary winding of the transformer is isolated, and the input voltage is high voltage direct current obtained directly from the AC mains rectification.

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

2.2.3. Disadvantages: The control circuit is more complicated, requires high components, and is unstable in brightness.

This type of circuit uses high-power switching transistors, fast recovery diodes, and high-frequency transformers. The switching speed is fast, which makes the power consumption of high-power switching transistors small, and the efficiency of the power supply can be greatly improved. Power frequency transformer, high power switching transistor power consumption is very small, eliminating the need for a large heat sink, 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 derived from the duty cycle of the excitation signal. Adjusted, the change of the power frequency input signal voltage can be compensated by pulse modulation, and a stable output voltage can be guaranteed when the grid voltage changes greatly.

This kind of circuit is the same as the linear voltage regulator of the power frequency transformer in the principle of driving high-power white LED. It adopts the method of constant voltage current limiting, and does not meet the nonlinear VI curve characteristic of high-power white LED, so in terms of performance, There is still a problem that the serial brightness of the series is unstable and the brightness of the light is inconsistent;

Compared with the linear voltage regulator of power frequency transformer, this kind of circuit has significant improvement in efficiency, volume and voltage regulation range. Therefore, it is used in other lighting occasions, but this type of circuit is still not suitable for street lighting.