Analysis of constant current control technology of AC drive free LED
Date:2020-02-25Visits:
AC free-drive LED failed to avoid the essential constant current control technology analysis
1. AC-free LED driving is not a fundamental change of the device.
That is to say, in fact, there is no LED chip that works in the AC electric field. The AC free drive LED is a device with a special arrangement of internal chip sets. It is only a change in the internal structure of the LED device. Of course, this is done. The craft level is not easy.
Most of the introduction of AC drive-free LEDs now refer to the information published by Seoul Semiconductor. It can be seen from this that it uses the traditional rectifier bridge circuit to solve the problem of AC power supply to the so-called DC LED. It seems that the benefit is to eliminate the rectifier diode, but the reverse voltage of the LED is limited. If it is damaged when the grid surge peak is large, it is not necessarily a benefit.
Because it is an imitated rectifier bridge circuit, only half of the current flows through the four bridge arms, and the entire current flows through the DC load terminal, causing the current distribution on each group of LEDs to be quite uneven. If undercurrent will affect the luminous lumen value And luminous efficiency, such as overcurrent, will cause light decay and affect life.
To solve it is also simple, just remove the LED on the DC side and short-circuit directly, and the current on the remaining four bridge arms can be consistent. Take a closer look at the circuit, and now it has become a group of forward and reverse LEDs connected in parallel. In fact, there is no need to make it too complicated at first, as long as the forward and reverse are connected in parallel, the positive and negative half of the AC can pass.
Second, the AC drive-free LED failed to avoid the indispensable constant current control technology.
From the introduction, the luminous efficiency of AC LEDs is not high enough compared with ordinary LEDs. It is said that it is in the development stage and will be higher in the future. The author believes that the basis of the same LED chip itself is the same, and the main restriction on the amount of light emitted per watt is the rectifier bridge circuit type. Imagine that only one of the five groups of LEDs can work normally and give full play to the light effect. The remaining four groups, which account for 80%, work in an undercurrent state with poor light efficiency. No matter how it develops together, the luminous efficiency is always lower than that of ordinary LEDs. The method of improvement is to abandon the unpractical rectifier bridge, and the direct forward and reverse parallel connections can all work in a state of good luminous efficiency.
There is no constant current protection function in AC free-driving LEDs. When using it, an external current limiting resistor must be connected. However, when the current is limited to the upper limit of the power supply voltage, it will work in a low light efficiency state of undercurrent at normal or low voltage. Resistor current limiting is a poor protection method, which not only has defects in function, but also depends on energy consumption to work, so that the power utilization rate is also reduced.
If you want to add a PTC to solve the LED constant current problem, the performance of the device is not fully understood. PTC is mainly used in overcurrent protection and thermal protection. When the current in the circuit is too large and the PTC reaches the Curie point temperature after heating, the resistance value rises sharply and closes to the open circuit. The degaussing circuit in the color TV uses it to generate large current attenuation amplitude Degauss.
When the PTC resistor is affected by the surrounding heat, and the temperature becomes more than the Curie point, it will be in an open circuit state, which can play a switch-type thermal protection role.
If you want to rely on the positive temperature coefficient characteristic of PTC resistor to play a constant current protection role for LED, you should also think of:
One is that the PTC resistor passes through the current, and it takes a heating time to heat up. When the resistance has not increased enough, the current that may be exceeded has already damaged the LED.
The second is that the self-heating temperature of the PTC resistor should be able to control the constant current, then the influence of the ambient temperature high and low may change by tens of degrees, so that the "constant current value" changes meaningless.
Can it be generalized? Isn't it true that the usual LED devices can also be constant current in such a simple way? Unfortunately, it can't be done. And usually AC constant current is more troublesome than DC constant current.
In fact, there can be various types of high-quality commercial LED adaptive constant current drivers such as circuit simplification, energy saving, constant current precision, surge resistance, high reliability, small size, and low cost, and many high-quality AC free drive LED driver manufacturers. Product support. Rather than focusing on non-professional circuit technology, LED manufacturers should focus on developing low-cost, high-efficiency LED lighting products that will better meet market demands.
