Implementation of the buck structure of LED lighting technology knowledge [chart]

In the first part of the article, we studied the basic LED light source and driving method. Some simple driving methods are introduced, such as voltage source/current limiting resistors and linear regulated power supply. However, the power requirements and complexity of LED light sources are increasing, making these simple driving methods no longer satisfactory. This requires a more advanced switch mode LED driver . So what kind of switching method do we choose? In the second part, we will discuss why the switching mode LED driver uses the constant current step-down circuit to perform best, or in other words, why use the buck mode?

With the widespread use of LEDs, the simple structure of linear power supplies in many places is no longer sufficient. In general, when the forward current required by the LED is set by means of a resistor, this simple driving method can continuously supply energy to the load from the power source. Since the current of the LED is the same as that of the resistor, the power dissipation generated by the resistor increases as the input voltage increases. For example, an LED driven by a linear power supply, with an efficiency of 70%, supplies 1A current to a typical white InGaN LED (VF = 3.5V) with a 5V linear supply. Under the same working conditions, when the input voltage rises to 12V, its efficiency will drop to 30%. It can't be applied in such an inefficient situation.

Switching power supply

The switching power supply improves the problem of a large change in efficiency due to input variations. This method is to control the duty cycle to meet the voltage or current required for the output. Since the switching power supply generates pulsed voltages and currents, it is necessary to shape these pulse waveforms with some energy storage devices (inductors or capacitors). In contrast to linear power supplies, switching power supplies can be implemented with different settings to achieve current or voltage drop, rise, or simultaneous lift. Switching power supplies can also achieve high efficiency over a wide range of input or output. In the previous example, after replacing the linear power supply with a step-down switching power supply, the efficiency of the circuit was changed from 95% to 98% when the input voltage was changed from 5V to 12V.

The switching power supply has been greatly improved in efficiency and structural flexibility, but the noise is increased due to the periodic switching, and the reliability of the circuit is lowered and the cost is increased due to the complexity of the structure. The constant current type LED circuit can be simply considered to be a constant current source. The choice of topology should be based on the minimum of external components and the best performance, which can improve circuit stability and reduce costs. In view of the dynamic dimming characteristics of LEDs, it is necessary to consider this feature in the design. Fortunately, the basic buck switching circuit performs very well when implementing these features, so the LED driver generally selects a step-down switching power supply.

Constant current output stage

The most common switch regulator is the voltage regulator. Figure 1a shows a basic constant voltage buck regulator. The buck controller keeps the output voltage constant by controlling the change in duty cycle or frequency with varying input voltages. The voltage required for the output is calculated by the following formula (Eq. 1)