In parallel? In series? LED drive circuit selection

After the advent of LED era, we can see its presence in all aspects of life, whether it is the automotive field, the intelligent field or the industrial field. Due to its characteristics of high efficiency, energy saving, long life, environmental protection, etc., it has become an optional solution of today's lighting technology, and is gradually applied in lighting. One of the key factors driving attention to LED lighting technology is that it significantly reduces energy consumption and enables reliable, long-term performance. Today we will from a practical LED circuit to give you the extended introduction of LED lighting drive circuit.

This article first from the beginning of the adoption of constant current source circuit, the circuit of the main components triode, to more than 400 v voltage, power to more than 10 w high power tube, such as MJE13003, MJE13005 etc, and to add heat sink, filter capacitance C capacity of 4.7 uF, want to have more than 400 v voltage, the size of the luminous tube current is determined by R2 adjustment, for the convenience of adjustment available variable resistor and then changed into the same value of the fixed resistor and the circuit can be luminous tube number is less a dozen, up to more than 90, while increased some cost, However, the application effect is much better than that of resistance current limiting circuit only. Even if the voltage fluctuates greatly, the circuit can still keep the current constant, which is very beneficial to the life of luminous tube. In this range, the current can basically remain constant. The number of light-emitting tubes used in this circuit should not be too small, and the less the less efficient it is. The total power consumption of this circuit is about 6W.

Constant current source circuit

By the way, I would like to tell you whether it is better to use and connect LED or series connect LED.

LED USES and or series connection method, mainly should be decided according to the power box circuit form and requirements.

Using series connection circuit, when one of the LED circuit broken when the whole string of LED is not bright; But when one of the leds short-circuits, the others light up. When one LED is broken, the other leds will still be bright. However, when one of the leds shortens, the power supply of the whole circuit will be short-circuited. In this way, the other leds will not work normally, and the power supply may be damaged. So in comparison or series connection circuit has advantages.

And connection method only needs to apply a lower voltage at both ends of each LED, but needs to use ballast resistance or current source to ensure the same brightness of each LED. If the magnitude of the bias current flowing through each LED varies, so does its brightness, resulting in an uneven overall light source. However, using ballast resistors or current sources to ensure consistent LED brightness will shorten battery life. In essence, the series connection method can ensure the consistency of the current flowing through each LED, but it requires a high power supply voltage. When LED is adopted and connected, the total current of the circuit is the sum of the current of each LED, so the power supply is required to supply large enough current.

In fact, strictly speaking, parallel or series connection has its own advantages and disadvantages. You need to consider many factors in practical terms. In practical application, LED arrays formed by series and parallel are often adopted, which can overcome or reduce the failure of the whole series of LED or the influence on the whole circuit and power supply caused by the single LED circuit breaking or short circuit mentioned above. The so-called series parallel is a small number of LED series and then string ballast resistance to form a branch, and then a number of branches in parallel form "branch group". In addition, still can adopt and chain form, is in the form of "branch group, on the basis of" again "branch group" series will make up the whole circuit of lamps and lanterns, the connection not only reduces the influence of a LED failure, division and the ballast resistance, resistance to several large power become dozens of small power resistor, installed by concentrated into scattered, both conducive to heat resistance, and can be more compact design of lamps and lanterns.

First of all, we must consider the power drive of any circuit. Usually, special constant current source or driver chip is used to drive LED, which is easily limited by factors such as volume and cost. The most economical and practical method is to use capacitor step-down power supply. Using it to drive small power LED, has not afraid of load short circuit, circuit simple and other advantages, and a circuit can drive 1 ~ 70 small power LED(however, this power supply circuit when the current shock, especially frequent start, will cause damage to the LED. Of course, this impact can be avoided with proper protection. In this regard, ansemei semiconductor's NUD4700 LED shunt protection solution can be applied. When the LED is working normally, the leakage current is only about 100 microns. When encountering transient or surge condition, LED will open circuit, then NUD4700 shunt protector's shunt channel is activated, bringing voltage drop of only 1.0v, thus reducing the influence on circuit as much as possible. This device USES A small space-saving package and is designed for A 1 W LED(rated current 350 mA@ 3v), which also supports operations over 1 A current if properly disposed of.

Typical circuit of capacitor step-down power supply

To examine the drive circuit, should according to the circuit diagram carefully check whether the circuit is wrong, pay special attention to check the rectifier bridge (long feet is the positive output, the diagonal is negative, the other two feet are ac input) or rectifier diode and the polarity of the zener diode is correct (with black or white line is at the end of the cathode), and check the transistor or voltage regulator integrated circuit of three electrodes is wrong answer, and so on.

C1 is a step-down capacitor (metallized polypropylene capacitor), and R1 provides discharge circuit for C1. Capacitor C1 provides a constant operating current for the entire circuit. Capacitor C2 is an electrolytic capacitor, and its voltage withstand value depends on the number of leds connected in series (about 1.5 times of its total voltage). Its main function is to inhibit voltage mutation caused by electrification at the moment, so as to reduce the impact of voltage shock on LED life. R4 is the discharge resistance of capacitor C2, whose resistance value should increase appropriately with the increase of the number of leds.

Since the capacitor step-down power supply is a kind of non-isolated power supply, a large current will be generated at the moment of electrification, which is called surge current. In addition, due to the influence of external environment (such as lightning strike), the power grid system will invade various surge signals, and some surges will lead to the damage of LED. Therefore, to provide thermistor protection, it is mainly composed of Negative Temperature Coefficient thermistor protection (NTC thermistor Coefficient), Positive Temperature Coefficient thermistor protection (PTC(Positive Temperature Coefficient), and Transient Voltage Suppressor protection (TVS).

Negative temperature coefficient means negative temperature coefficient, generally referring to the semiconductor material or component with a large negative temperature coefficient. The simplest and most effective way to limit the surge current is to series an NTC thermistor at the input end of the line

The positive temperature coefficient current passing through PTC thermistor causes the temperature to rise, that is, the temperature of the heating element rises. When the temperature exceeds the Curie point, the resistance increases, thus limiting the current to increase.

The transient voltage suppressor is mainly used to protect the circuit elements quickly. When TVS is hit by a reverse transient high energy shock at the poles, it can change the high impedance between the poles into a very low impedance at a speed of 10-12s, absorb the surge of high energy, clamp the voltage between the poles to a predetermined value, and protect the components in the electronic circuit from being damaged by various surge pulses.