LED drive circuit working principle and LED drive protection

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. Usually, the LED is driven by a dedicated constant current source or a driver chip, which is easily limited by factors such as volume and cost. The most economical and practical method is to use a capacitor buck power supply. It uses it to drive low-power LEDs, has the advantages of not being short-circuited by load, simple circuit, etc., and a circuit can drive 1 to 70 low-power LEDs (however, the current impact when starting this power circuit, especially frequent startup, will give LED Cause damage. Of course, proper protection can avoid this kind of impact).

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 bleeder resistor of capacitor C2, and its resistance should be increased as the number of LEDs increases.

It should be noted that the circuit must select the appropriate capacitor according to the current of the load, instead of the voltage and power of the load. Generally, the relationship between the capacity C of the step-down capacitor C1 and the load current Io can be approximated as: C=14.5Io, The unit of capacity of C is uF, and the unit of Io is A. The current limiting capacitor must use a non-polar capacitor, and the capacitor's withstand voltage must be above 630V.

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 lightning strikes), the grid system will invade various surge signals, and some surges will cause LED damage. LEDs are less resistant to surge currents and reverse voltages, and it is important to enhance protection in this area, especially when some LEDs are installed outdoors (such as LED street lights). Therefore, the LED driving power supply must have the ability to suppress the intrusion of surges and protect the LED from damage. This circuit uses a negative temperature coefficient thermistor to limit the sudden change of current. The positive temperature coefficient thermistor is used to automatically adjust the current to a certain range of variation, and there is a transient voltage suppressor at the input of the power supply. To avoid voltage overload.

(1) Negative temperature coefficient thermistor protection

Negative temperature coefficient thermistor is abbreviated as NTC thermistor. NTC is the abbreviation of NegaTIve Temperature Coefficient, which means negative temperature coefficient. It refers to semiconductor materials or components with large negative temperature coefficient. It is the simplest and most effective way to limit the inrush current. The method is to connect an NTC thermistor in series with the line input, as shown by R2 in Figure 1. Since the NTC thermistor exhibits high impedance during cold start, the inrush current is limited. 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 occurs, and of course, the loss is small.

(2) Positive temperature coefficient thermistor protection

PosiTIve Temperature Coefficient (PTC) is a thermistor with a sharp increase in resistance at a certain temperature and 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. When the current passes through the PTC thermistor, the temperature rises, that is, the temperature of the heating element rises. When the temperature exceeds the Curie point, the resistance increases, thereby limiting the current increase, so that the drop of the current causes the temperature of the component to decrease, and the resistance value decreases. By increasing the circuit current, the component temperature rises again and again, so it has the function of keeping the temperature within a specific range.

Under normal circumstances, the PTC component is connected in series in the circuit, which 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 the fault that generates an overcurrent is removed, the PTC component automatically returns to the 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.

(3) Transient voltage suppressor 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 into a very low impedance at a speed of the order of 10-12s, absorb high-energy surges, and clamp the voltage between the two poles. Located at a predetermined value, the components in the protection circuit are protected from the impact of various surge pulses.

For the aspect of overvoltage protection, this circuit is a TVS connected in parallel with the power input terminal, as shown in Figure 1 of VD3, so that the voltage can be maintained within the maximum TVS tolerance range. When the voltage is higher than the TVS breakdown point. When the phenomenon is pressed, the current can flow through the TVS, thereby protecting the LED lighting fixture.

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. In terms of device selection, it is also possible to replace the NTC with a 1W metal film resistor or a wire wound 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 as far as possible from the rear load circuit, and the distance between the high-voltage input parts should be guaranteed to be 1 when allowed. Mm or more.

12V high efficiency white LED driver

Using LM3578 integrated circuit

Working voltage: 10 - 18V DC

Working current: 70mA (8 LEDs)