Working Principle and Characteristics of LED Driving Circuit

In addition to meeting safety requirements, the LED drive circuit should have two other basic functions. First, it should maintain constant current characteristics as much as possible, especially when the power supply voltage changes by ±15%, the output current should still be able to change within the range of ±10%. Second, the drive circuit should maintain low power consumption, so that the LED system efficiency can be maintained at a high level.

Traditional low efficiency circuit:

Figure 1

Figure 1 is a traditional low-efficiency circuit. The grid power supply is stepped down by a step-down transformer. After bridge rectification and filtering, the three LEDs are stabilized by resistor current limiting. The fatal disadvantage of this circuit is that the existence of resistor R is necessary. The active power loss on R directly affects the efficiency of the system. When the R voltage is small, the voltage drop of R accounts for 40% of the total output voltage. The active power loss of the output circuit on R already accounts for 40%. Adding the transformer loss, the system efficiency is less than 50%. When the power supply voltage changes within the range of ±10%, the current flowing through the LED will change by ≥25%, and the power change on the LED will reach 30%. When the R voltage is large, when the power supply voltage changes within the range of ±10%, although the power change output to the LED can be reduced, the system efficiency will be lower.

Figure 2

Figure 2 adds an integrated voltage regulator MC7809 to Figure 1, which makes the output voltage basically stable at 9V. The current limiting resistor R can be very small and the LED will not be overloaded due to the instability of the power supply voltage. However, in addition to ensuring the basic constant output of the LED, the efficiency of this circuit is still very low. Because the voltage drop on MC7809 and R1 still accounts for a large proportion, its efficiency is only about 40%.

The total lumen output per watt of the above-mentioned circuit is only 20lm/W~25lm/W, which cannot be called an energy-saving lighting product. In order to achieve stable operation of LED and maintain high efficiency, low-power current limiting components and circuits should be used to improve system efficiency.

Figure 3

Figure 3 is an LED driver circuit using the integrated constant current source NUD4001. The notable feature of this circuit is that when the power supply voltage varies within the range of ±15%, the output fluctuation is ≤1%, which can be called a constant power driver circuit. In addition, this IC circuit can work under very low series voltage division (that is, the voltage between pin 1 and each output pin can still work when it is ≥2.8V), so it can be guaranteed that under almost constant power output, keeping the voltage between pin 1 and the output pin at about 2.8V can make the system efficiency reach about 70%. The input power supply of this IC circuit can use industrial frequency AC, but it is best to use a halogen tungsten lamp electronic transformer as the front stage, so as to ensure that the harmonics and power terminal interference meet the requirements of the standard. When the electronic transformer realizes the isolated insulation output of Class II electrical appliances, the circuit in Figure 3 can be used in Class II lamps, and the output end can be made accessible.

Figure 4

The circuit directly uses capacitors as current limiting elements. In this circuit, since the voltage division on the capacitor almost reaches the entire power supply voltage, it has good current limiting characteristics. When the power supply voltage fluctuates within ±10%, the output current also fluctuates within ≤±10%. As long as a certain margin is left for the rated value of the LED in the design, it can be ensured that the LED is still in good working condition when the power supply voltage fluctuates. Since the dielectric loss of the capacitor is extremely small, the loss of the circuit is very small. The role of the resistor R is to ensure that the voltage on the capacitor can be discharged in time when the power is off. Its resistance can be ≥3MΩ. An IN4007 diode can be added to each group of LEDs in series. When one of the two groups of LEDs in series is open, the other group may be broken down by the reverse voltage. If an IN4007 diode is connected in series, the remaining LEDs can be protected from damage. Of course, the addition of IN4007 also slightly reduces the efficiency (when the output current is 30mA, the power consumption on IN4007 is about 0.02W). For integrated night lights, IN4007 can be omitted, and the efficiency of this driving circuit is ≥90%. The LED night light made with this driving circuit has higher efficiency than the night light with gas discharge light source, and its service life is much longer than the night light with other light sources. This circuit can still work stably when 30 LEDs are connected in series. However, the light output by this circuit has a certain flicker (100Hz flicker at 50Hz), which is not suitable for lighting occasions for moving objects, and the LED should be made inaccessible when used, otherwise it will affect safety.

Figure 5

The circuit is based on the original halogen tungsten lamp electronic transformer, and uses high-frequency inductor current limiting to achieve stable operation of LED. The characteristics of this circuit are that the load can carry several groups of LEDs according to the power of the electronic transformer, and can achieve a safe extra-low voltage output with complete secondary isolation, with an output voltage of 12V (at this time, each group of LEDs is 3), (the maximum output voltage Figure 5 can reach 25V/no-load output voltage can reach 33V). Since high-frequency current is used to light up the LED, the flicker phenomenon of the output light can be basically eliminated. The output current limiting inductor can be made very small, and the inductance of each inductor is only 0.05mH~0.2mH

(Different inductors are used according to the current of the LED). As long as the wire diameter of the inductor is not too thin and the debugging level of the electronic transformer is high, the overall efficiency of this circuit can reach 80% to 92% when the output power is 8W to 70W. This circuit can also fully meet the requirements of harmonics and EMI when the line power is ≥25W. When the power supply voltage of this circuit changes by ±10%, the power output to the LED changes by ±20%, so it should be ensured that the power output to the LED is appropriately less than the rated value under the rated power supply voltage to prevent overheating caused by overvoltage when the LED is overloaded and affects the service life.

Figure 6

The circuit is an LED drive circuit using two dedicated IC circuits. The circuit uses IC1 model VIPEr22A, which is an intelligent power switch integrated module produced by ST. It has a PWM control circuit and a 0.7A/730V VDMOS field effect power tube. The model of IC2 is TSM1101. The IC has a 2.5V reference voltage and two comparators composed of op amps. The working current signal of the LED is obtained from R6 and input to the CC comparator in IC2. After comparison and amplification, it is fed back to the previous stage. The working voltage signal of the LED is obtained from the R4 and R5 voltage divider, and input to the CV comparator in IC2. After comparison and amplification, it is also fed back to the previous stage. The feedback signals of the two comparators are coupled to the control pole of IC1 through the photoelectric coupler (model SFH610A). IC1 itself generates high-frequency signals to make its own VDMOS tube continuously work between on and off. When the power supply voltage changes, the voltage on N3 changes, and when the LED current and voltage change, these signals are fed back to the IC1 control electrode, causing the duty cycle (or pulse width) of the high-frequency signal generated by IC1 to change, and the on/off time ratio of its own VDMOS field effect power tube to change, thereby achieving the purpose of achieving constant output of voltage and current at the output of N2. In Figure 6, transformer N1 is the primary winding, N2 is the secondary power output winding, N3 is the bias (working) winding of IC1, and N4 is the bias (working) winding of IC2. From the circuit analysis, it can be seen that N2 is attached to the isolated safety voltage output (28V) winding, and N4 and N2 are directly connected to the output circuit. In order to ensure the requirements of safety standards, the output circuit and the circuit directly connected to the power grid must be completely isolated. Therefore, the feedback signal in the circuit is fed back to the previous stage through a high-voltage optocoupler. The structure between N2 and N4 in the transformer and NI and N3 must meet the requirements of a safety isolation transformer.

The biggest features of the circuit in Figure 6 are:

1. When the power supply voltage works in a wide range (about 180V ~ 265V), it can ensure the constant power output of the LED, and the LED can achieve flicker-free output.

2. Achieve safe voltage output with safety isolation, or even safe ultra-low voltage output.

3. IC2 If TSM104 is used, continuous adjustment of light output from 0 to 100% can be achieved.

It should be noted that N1 and N2 as well as the high-frequency transformer core are channels for power input and output. The efficiency of the entire circuit mainly depends on these three factors. There is almost no potential to be tapped in other aspects. A core with sufficient core cross-section should be used and the current density of N1 and N2 should be kept low, so that the conversion efficiency of the circuit can reach a high level. When the LED output power is 8W, the efficiency is about 80% to 85%, and when the LED output power is 20W to 40W, the efficiency is about 85% to 90%.

From the above, it can be seen that LED needs to have current-stabilizing and voltage-stabilizing components when working, but such components should have the characteristics of high partial pressure but low power consumption. Otherwise, the efficiency of the overall system will be greatly reduced due to the high power consumption of the driving circuit of the LED with high efficiency, which is contrary to the purpose of energy saving and high efficiency. Therefore, it is necessary not to use resistors or series voltage-stabilizing circuits as the main current-limiting circuit of the LED driver as much as possible, but to use high-efficiency circuits such as capacitors, inductors or active switching circuits, so as to ensure the high efficiency of the LED system. The use of series integrated constant power output circuit can keep the light output of the LED constant within a wide power supply range, but the general IC circuit will reduce the efficiency. The use of active switching circuit can ensure constant power output when the power supply voltage changes greatly under high conversion efficiency. At the current stage, the light efficiency of LED is far from being able to replace the three-primary color fluorescent lamp, but with its unique advantages, it can work efficiently under safe special voltage conditions (swimming pools, underwater lamps in paddling pools, mining lamps). In addition, it also has its unique advantages in directly using green electricity (solar energy, wind energy, etc.) and emergency lighting. Especially in dimming, LED can not only achieve 0-100% dimming, but also ensure that the light efficiency is maintained at a high level during the entire dimming process without damaging the life of the LED, which is difficult for gas discharge lamps to do.