(1) Resistance current limiting circuit
As shown in Figure 1, the resistance current limiting drive circuit is the simplest LED drive circuit. It uses a series current limiting resistor in the loop to control the current flowing through the LED. The size of the resistance is determined according to the volt-ampere characteristics of the LED. However, the resistance current limiting circuit cannot achieve constant current control, and the resistance itself will consume a certain amount of power, especially when the LED power is high, the loss on the current limiting resistance will increase significantly, resulting in low efficiency of the LED driving power supply. When the input voltage fluctuates, the current through the LED will also change, so the regulation performance is poor. Such drive circuits are mainly used in low-power, low-cost LED lighting systems.
(2) Linear regulator circuit
The core of the linear regulator is to use the power transistor or MOSFFET working in the linear region as a dynamically adjustable resistor to control the load. There are two types of linear regulators: parallel type and series type (see Figure 2). Figure 2 (a) shows a parallel linear regulator, also called a shunt regulator. It is connected in parallel with the LED. When the input voltage increases or the LED decreases, the current through the shunt regulator will increase, which will Increase the voltage drop across the current limiting resistor to keep the current through the LED constant. Since the shunt regulator needs a resistor in series, the efficiency is not high, and it is difficult to achieve constant regulation when the input voltage varies widely. Figure 2(b) shows a series regulator. When the input voltage increases, the dynamic resistance of the adjustment increases to keep the voltage (current) on the LED constant.
Since power transistors or MOSFET tubes have a saturated turn-on voltage, the minimum input voltage must be greater than the sum of the saturation voltage and the load voltage for the circuit to work correctly. Although the design of the linear regulator power supply is relatively simple, the overall power supply efficiency is not very high.
(3) Switching regulator circuit
The above-mentioned driving technology is not only limited by the input voltage range, but also has low efficiency. When used for low-power ordinary LED drive, since the current is only a few milliamps, the loss is not obvious. When used to drive high-brightness LEDs with a current of several hundred milliamps or higher, the loss of the power circuit becomes A more serious problem. Switching power supply is currently the most efficient in energy conversion, which can reach more than 90%. Power converters such as Buck, Boost and Buck-Boost can all be used to drive LEDs, but to satisfy the constant current drive of LEDs, the output current is detected instead of the output voltage for feedback control.
Figure 3(a) shows the LED drive circuit using Buck converter. Unlike the traditional Buck converter, the switch S is moved behind the inductor L to ground the source of S, which facilitates the drive of S. LED and L are connected in series. , And the freewheeling diode D is anti-parallel to the series circuit, the driving circuit is not only simple, but also does not require an output filter capacitor, which reduces the cost. However, the Buck converter is a step-down converter and is not suitable for applications where the input voltage is low or where multiple LEDs are connected in series.
Figure 3(b) is an LED drive circuit using a Boost converter, which pumps the output voltage to a higher expected value than the input voltage through inductive energy storage, so as to drive the LED under low input voltage. The advantage is that the output of such driver ICs can be used in parallel, effectively increasing the power of a single LED.
Figure 3 (c) shows the LED drive circuit using Buck-Boost converters. Similar to the Buck circuit, the source of the circuit S can be directly grounded, thus facilitating the driving of s. Although Boost and Buck-Boost converters have one more capacitance than Buck converters, they can both increase the absolute value of the output voltage. Therefore, they are more commonly used when the input voltage is low and multiple LEDs need to be driven.