High PF Stroboscopic Efficient Linear LED Driving Scheme

LED is a kind of semiconductor electronic component that can emit light. In the early stage, this electronic component can only emit low luminosity red light. With the continuous progress of technology, it has developed to the extent that it can emit visible light, infrared and ultraviolet light, and the luminosity has also been greatly improved. LED has the advantages of high efficiency, long service life, not easy to be damaged, high switching speed and high reliability. It has been widely used in the field of indicator light, display and lighting.

In the linear LED driving scheme, the high PF and high efficiency linear LED driving technology has become more and more mature. However, with the release of the latest EU ERP directive, the requirements for stroboscopic (SVM ≤ 0.4, PST LM ≤ 1.0) have been increased. There is no check and balance among stroboscopic, high PF and high efficiency. Therefore, how to give consideration to the three is a new research direction at present, as shown in Figure 1.

Figure 2

Generally, in order to realize the de stroboscopic, a de stroboscopic circuit is usually added on the basis of high-efficiency LED driving, but this de stroboscopic circuit will require a control loop, with complex structure and large loss. Pt4519 changes the method. Instead of series de stroboscopic circuit, it uses a constant current source (Q2, op2) as a stroboscopic free drive, which is connected in parallel in the main circuit to reduce the loss caused by series application.

During the trough of AC voltage, the input voltage will be lower than the LED on voltage, so an energy storage capacitor CO is required to maintain the LED output. In conventional applications, the drive of CO (Q1, OP1) is a fixed charging control, and the peak current is fixed after external setting. Considering the efficient realization and adding an over-voltage drop current control, the current drop amplitude is generally set by external resistance. However, this will bring a problem. Once the external setting is completed, the charging current will be fixed and cannot be dynamically adjusted according to the changes of input voltage and output load. Only the passive input voltage can be turned on after being higher than the LED voltage. Therefore, there will be two consequences. First, if the charging current is too much, the input voltage of Q2 will be too high during CO discharge, resulting in increased Q2 loss and reduced efficiency; The other is that if the charging current is too low, resulting in too low CO discharge, the LED output cannot maintain constant current, resulting in power frequency flashing.

In order to solve this problem, pt4519 detects the drain voltage of Q2 as the charging control feedback of Q1, and adds a loop compensation circuit, so that Q2 can maintain a relatively low input voltage while maintaining constant current output under various input and output conditions, so as to minimize the loss of Q2, so as to improve the overall efficiency of the system.

When the drain voltage of Q2 is relatively low, the discharge voltage of capacitor co will also be relatively low, so some conduction angles can be increased during charging to improve PF. In addition, Q1 simultaneously detects the bus voltage to control the charging current, and reduces the charging current when the peak high voltage is input, so as to reduce Q1 loss and improve system efficiency. At the same time, the decrease of Q1 charging current can be adjusted through the external resistance of PF pin, so as to balance the PF and efficiency of the system.

Due to the linear LED drive, when the input voltage is relatively low, due to the limitation of Q2 charging current capacity and the reduction of conduction time, the average current of the output LED will inevitably decrease. If the rated current of the LED is still to be maintained, the output will inevitably flicker at power frequency. In order to solve the problem of low voltage, pt4519 creatively improves the function of loop compensation comp voltage, When the input voltage is relatively low, the comp voltage will rise. Therefore, when the comp voltage rises to a certain extent, it will be used as a control quantity to reduce the reference, so as to reduce the LED output current, so that the average charging current of Q2 is equal to the LED output current, so as to realize the LED output without stroboscopic at low input voltage.

The following table shows some comparisons between pt4519 and competitors. It can be seen that the performance of pt4519 is fully superior after using the above technologies.

Figure 3

Summary of this paper:

This paper introduces the implementation process of high PF stroboscopic efficient linear LED driving scheme pt4519 and the comparative data of some competitors. While meeting the requirements of high PF without stroboscopic, the efficiency of pt4519 is improved at high input voltage, and the linear adjustment rate is also good. It can meet the application of wide input voltage range and approach the performance of switching LED drive, but the system cost and EMI performance are better than switching LED drive.

Editing: hfy