Abstract: According to the characteristics of LED lighting fixtures, a constant current drive circuit is designed. This circuit uses a capacitive buck mode to provide a constant current to the entire circuit. Use TVS (Transient Voltage Suppressor) to over-voltage protect the LED, while using NTC (negative temperature coefficient thermistor) to effectively reduce the impact of inrush current on the LED, and use PTC (Positive Temperature Coefficient Thermistor) Limiting the current through the LED to a certain extent extends the actual life of the LED luminaire, avoids frequent maintenance and replacement, and reduces costs.
0 Preface
With the rise and continuous improvement of the solid-state lighting industry, light-emitting diodes (LEDs) have become an alternative to today's lighting technology due to their high efficiency, energy saving, long life and environmental protection, and are gradually being applied to lighting. A key factor driving attention to LED lighting technology is that it greatly reduces energy consumption and enables long-term, reliable work. There is reason to believe that this new type of solid-state lighting will surely bring about the third revolution in the field of human lighting.
Of course, the first thing to consider when using LED lighting is its brightness, cost and longevity. Since the main cause of LED life is the current surge at the moment of frequent start-up, various surge pulses from the outside, and current limitation during normal operation, this circuit integrates these factors, and avoids large current to LED from the circuit design. The impact of the lighting fixture and its operating current is stabilized within a certain range, which solves the problem of brightness degradation of current LED lighting fixtures and effectively prolongs its service life.
1 drive power
The LEDs are all driven by DC, so a power adapter, LED driver, is required between the mains and the LEDs. Its function is to convert AC mains into DC power suitable for LEDs.
LED drive power can be divided into constant current and voltage regulator according to its driving mode. The current output by the constant current drive circuit is constant, and the output DC voltage varies within a certain range according to the magnitude of the load resistance. The load resistance is small, the output voltage is low, and the load resistance is larger, and the output voltage is also The higher. For a regulated power supply circuit, the output voltage is fixed, and the output current changes as the load increases or decreases.
Since the LED is a device whose current varies significantly with voltage, when the LED is forward-conducting, a small change in its forward voltage can cause a large change in the LED current. For a regulated LED drive power supply, when the load changes, the current fluctuates greatly, and the LED will work for a long time under high current and will be damaged. Experiments have shown that the LED's luminous efficacy is optimal when the actual current flowing through the LED is 70% of its maximum allowable current. At the same time, since the voltage temperature coefficient of the PN junction of the LED is about -2 mVP °C, when the LED heat dissipation causes the temperature to rise, the operating current will also change significantly from the initial stage, which is also the rapid aging of various LED products on the market. The main reason. Obviously, ensuring the stable driving current of the LED is especially important for the anti-aging of the LED. Therefore, the constant current drive power supply is an ideal LED drive method.
Usually, the LEDs are driven by a dedicated constant current source or a driver chip. When limited by factors such as volume and cost, the most economical and practical method is to use a capacitor buck power supply. It can drive low-power LEDs with the advantages of short circuit and simple circuit, and one circuit can drive 1~70 low-power LEDs. However, the current impact when starting this kind of power circuit, especially frequent startup, will cause LED damage. Of course, this shock can be avoided by taking appropriate protection.
A typical circuit for a capacitive buck power supply is shown in Figure 1. C1 is a step-down capacitor (using a metallized polypropylene capacitor) and R1 provides a discharge loop for C1. Capacitor C1 provides a constant operating current for the entire circuit. Capacitor C2 is an electrolytic capacitor, and its withstand voltage depends on the number of LEDs connected in series (about 1.5 times its total voltage). Its main function is to suppress voltage abrupt changes caused by energization moments, thereby reducing voltage shock. The impact on LED life. R4 is the discharge resistance of capacitor C2, and its resistance should be increased as the number of LEDs increases.
Figure 1 Typical circuit of a capacitor buck power supply
It should be noted that the appropriate capacitance must be selected according to the current of the load, not the voltage and power of the load. Generally, the relationship between the capacitance C of the step-down capacitor C1 and the load current Io can be approximated as: C = 14. 5 I , The unit of capacity of C is μF, and the unit of Io is A. The current limiting capacitor must use a non-polar capacitor, and the withstand voltage of the capacitor must be above 630 V.
2 protection circuit
Since the capacitor step-down power supply is a non-isolated power supply, a large current is generated at the moment of power-on, which is called a surge current.
In addition, due to the influence of the external environment, such as the detection of lightning strikes, various surge signals are invaded from the power grid system, and some surges may cause damage to the LEDs. The LED's ability to withstand surge currents and reverse voltage is relatively poor. It is also important to strengthen this aspect of protection. In particular, some LED lights are installed outdoors, such as LED street lights. Therefore, the LED driver power supply must have the ability to suppress the intrusion of surges and protect the LEDs from damage. This circuit uses NTC (negative temperature coefficient thermistor) to limit the sudden change of current. The PTC (Positive Temperature Coefficient Thermistor) is used to automatically adjust the current to a certain range of variation, and at the input of the power supply. TVS (Transient Voltage Suppressor) to avoid voltage overload.
2. 1 NTC protection
NTC is the abbreviation of Negative Temperature Coefficient, which means a negative temperature coefficient. It refers to a semiconductor material or component with a large negative temperature coefficient. The so-called NTC thermistor is a negative temperature coefficient thermistor. The simplest and most effective way to limit the inrush current is to connect an NTC thermistor in series with the line input, as shown in Figure 1 for R2. Since the NTC thermistor exhibits high impedance during cold start, the inrush current is obtained. limit. When the thermal effect of the current causes the temperature of the NTC thermistor to rise and the resistance of the NTC drops sharply, the current limiting effect on the system is less. Since the impedance of the NTC thermistor in the hot state is not zero, power loss is generated. Of course, this loss is small.
2. 2 PTC protection
PTC (Positive Temperature Coeffl Cient) is a thermistor phenomenon or material that has a sharp increase in resistance at a certain temperature and has a positive temperature coefficient. In order to stabilize the current in the circuit under normal operation, the circuit also uses a PTC thermistor, as shown in Figure 1. R3. The current through the PTC thermistor causes the temperature to rise, that is, the temperature of the heating element rises. When the Curie point temperature is exceeded, the resistance increases, thereby limiting the current increase, so that the decrease in current causes the component temperature to decrease, and the decrease in the resistance value causes the circuit current to increase, the component temperature rises, and the cycle is repeated, thus having the temperature maintained at a specific temperature. The scope of the function.
The PTC component is connected in series in the circuit. Under normal conditions, it is in a low-resistance state to ensure the normal operation of the circuit. When the circuit is short-circuited or an abnormally large current is injected, the self-heating of the PTC component increases the impedance and limits the current to be small enough. It acts as an overcurrent protection. When an overcurrent fault is removed, the PTC component automatically returns to a low impedance state.
It not only avoids maintenance and replacement, but also avoids the continuous opening and closing state of the circuit that may cause damage to the circuit.
2. 3 TVS protection
Transient Voltage Suppressor (TVS) is a high-efficiency protection device developed on the basis of voltage regulator tubes. It is mainly used for fast overvoltage protection of circuit components.
When the two poles of the TVS tube are subjected to reverse transient high-energy shock, it can change the high impedance between the two poles to a very low impedance at a speed of 10 to 12 seconds, absorbing high-energy surges, and the voltage between the two poles. The clamp is at a predetermined value that protects the components in the electronic circuit from the impact of various surge pulses.
For overvoltage protection, this circuit is connected to the TVS in the power input terminal, as shown in Figure 1 D2, so that the voltage can be maintained within the maximum TVS tolerance range. When the voltage is higher than the TVS breakdown point. The phenomenon of pressure allows current to flow through the TVS, thereby protecting the LED lighting fixture.
3 Conclusion
Experiments show that after the pointer multimeter is connected into the circuit, the phenomenon that the pointer suddenly deflects at a large angle is obviously improved when the circuit is energized, and the impact of the surge current on the LED is effectively prevented. At the same time, after a period of startup, the current has dropped and gradually stabilized. It is also possible to replace the NTC with a 1W metal film resistor or wirewound resistor. The overvoltage protection can be either TVS or varistor. In terms of circuit board design, it should be noted that the high-voltage input part (ie, the power input end to the rectifier bridge part) should be kept away from the rear load circuit as much as possible, and the distance between the high-voltage input part wires should be guaranteed to be 1 when allowed. Mm or more. Based on these considerations, the LED lighting fixture shown in Figure 2 was designed. The lighting module has been working continuously for about 4 years, and the brightness has not been attenuated without any maintenance. This circuit is easy to popularize and popularize, and can be widely used in public lighting, corridors, warehouses, etc., and has great commercial prospects.
Figure 2 LED lighting
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