AC drive LED cannot avoid the essential constant current control technology analysis

1. AC LED is not a fundamental change in the device.

In other words, there actually does not exist an LED chip that works with an alternating electric field mechanism. The AC LED that is now available is a device with a special arrangement of internal chipsets. It is merely a change in the internal structure of the LED device. Of course, it is not easy to achieve such a technological level.

Most of the introductions to AC LEDs now cite data published by Seoul Semiconductor. From this, we can see that it uses the traditional rectifier bridge circuit to solve the problem of AC powering the so-called DC LED. The advantage seems to be that the rectifier diode is omitted, but the reverse withstand voltage of LEDs is limited. If they are damaged when encountering a large surge peak in the power grid, it is not necessarily a benefit.

Because it is an emulated rectifier bridge circuit, only half of the current flows through the four bridge arms, while the entire current flows through the DC load end, resulting in a very uneven current distribution on each group of LEDs. If there is undercurrent, it will affect the luminous lumen value and luminous efficiency. If there is overcurrent, it will cause light decay and affect the lifespan.

The solution is also simple. Just remove the LED at the DC end and short-circuit it directly, and the currents on the remaining four bridge arms will be consistent. Take a closer look at the circuit. Now each group of forward and reverse LEDs are connected in parallel. In fact, it doesn't need to be too complicated at the beginning. As long as the forward and reverse are connected in parallel, both the positive and negative half cycles of the AC can pass.

Second, AC LED cannot avoid the essential constant current control technology.

From the introduction, the current luminous efficiency of AC LED is not as high as that of ordinary LED. 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 type of LED chip itself is the same, and the main factor that restricts the luminous amount per watt is the type of rectifier bridge circuit. Imagine that only one of the five groups of LEDs can work normally and give full play to the luminous efficiency, and the remaining four groups, accounting for 80%, are working in an under-current state with very poor luminous efficiency. No matter how they develop together, they are naturally much lower than the luminous efficiency of ordinary LEDs. The way to improve it is to abandon the impractical rectifier bridge circuit and directly connect the forward and reverse parallel connections so that all can work in the state of best luminous efficiency.

AC LEDs do not have a constant current protection function, so they must be connected to an external current limiting resistor when in use. However, when the current is limited to the upper limit of the power supply voltage, it will work in a low-light efficiency state with low current at normal or low voltage. Resistor current limiting is a poor protection method. It not only has functional defects, but also consumes energy to work, which reduces the power utilization rate, that is, the energy efficiency.

If you want to add a PTC to solve the LED constant current problem, you don’t have a comprehensive understanding of the performance of this component. 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 approaches the circuit break. The demagnetization circuit in the color TV uses it to generate a large current attenuation amplitude demagnetization.

When the PTC resistor is affected by the surrounding heat and its temperature exceeds the Curie point, it will be in an open circuit state, which can play a switching type thermal protection role.

If you want to rely on the positive temperature coefficient characteristics of PTC resistors to provide constant current protection for LEDs, you should also consider:

First, when current passes through the PTC resistor, it takes some time to heat up. When the resistance has not increased high enough, the current that may exceed the limit may have already damaged the LED.

Secondly, if the self-heating temperature of the PTC resistor is to be able to control the constant current, then the influence of the high or low ambient temperature that may vary by dozens of degrees will make the change of this "constant current value" meaningless.

If this can be extended to other applications, wouldn't it be possible to simply control the constant current of common LED devices? Unfortunately, this is not possible. Moreover, AC constant current is usually more troublesome than DC constant current.

In fact, there are many excellent products from many LED driver manufacturers, such as the mains LED adaptive constant current driver with simple circuit, low energy consumption, accurate constant current, surge resistance, high reliability, small size and low cost. Instead of focusing on non-professional circuit technology, LED manufacturers should focus on developing low-cost and high-efficiency LED lighting products to better meet the market's needs.