Key technologies in primary-side feedback AC/DC control chips

The AC/DC control technology of primary feedback is a new AC/DC control technology developed in the past 10 years. Compared with the traditional structure of optocoupler plus 431 of secondary feedback, its biggest advantage is that it saves these two chips and a group of components that work with them, thus saving space on the system board, reducing costs and improving system reliability. It has broad application prospects in markets with high cost pressures such as mobile phone chargers, and markets with high volume requirements such as LED drivers.

After eliminating these components, in order to achieve high-precision constant current/constant voltage (CC/CV) characteristics, new technologies must be used to monitor real-time changes in load, power supply and temperature as well as the tolerance of components in the same batch. This involves primary (primary side) regulation technology, transformer tolerance compensation, cable compensation and EMI optimization technology.

The principle of primary regulation is to detect load change information by accurately sampling the voltage change of the auxiliary winding (NAUX). When the controller turns on the MOS tube, the transformer primary winding current ip rises linearly from 0 to ipeak, and the formula is

At this time, the energy is stored in the primary winding. When the controller turns off the MOS tube, the energy is transferred to the secondary winding through the transformer and sent to the output terminal VO after rectification and filtering. During this period, the output voltage VO and the forward voltage VF of the diode are reflected to the auxiliary winding NAUX. The voltage on the auxiliary winding NAUX at the beginning of demagnetization can be expressed by the formula

In other words, VF is the forward conduction voltage drop of the output rectifier diode, which can be expressed by the formula at the end of demagnetization.

It can be seen that at the end of demagnetization, the secondary winding output voltage has a linear relationship with the auxiliary winding. As long as the voltage of the auxiliary winding at this point is sampled and a loop feedback of the error amplifier clamped by a precise reference voltage is formed, the output voltage VO can be stabilized. The output current IO at this time is given by the formula

Indicates, where VCS is the voltage on the CS pin, and the meanings of other parameters are shown in Figure 1. This is the working principle of the constant voltage (CV) mode.

When the load current exceeds the current limit, the load current will be clamped at the limit current value. At this time, the system enters the constant current (CC) mode. Here, a limiting condition needs to be added to the IO formula, that is,

, that is, the ratio of the demagnetization time to the switching period remains constant, so the output current formula in CC mode becomes

, where C1 is a constant less than 0.5 and VCSLMT is the CS pin voltage limit.

After keeping the ratio of demagnetization time to switching cycle constant, the output voltage and current are independent of the inductance of the transformer. Therefore, at a practical level, the application solution's requirements for the consistency of inductance values ​​in the same batch are reduced, thereby reducing the cost of large-scale production and processing.

[page]At the same time, the primary feedback system will also face the problem of cable voltage drop. Because the system does not directly sample the voltage at the output end (after rectification of the secondary winding), but controls the loop feedback by sampling the voltage at the demagnetization end point of the auxiliary winding, when the output line is long or the wire diameter is thin, there will be a large internal resistance on the load line (for example, in the charger solution). When the load current changes greatly, the end voltage of the output line will also change greatly. In CV mode, this change is unacceptable in some occasions. Therefore, the primary feedback driver chip should also provide the function of compensating for the cable voltage drop, which is usually achieved by pulling a small current on the INV pin. The total resistance of the voltage divider connected to the INV pin is adjusted by estimating the compensation value (the voltage divider ratio remains unchanged), thereby compensating for the cable voltage drop caused by different load line types and load sizes to maintain the horizontality of the CV curve (as shown in the CV curve in Figure 2).

In addition, a good primary feedback AC-DC controller should also have excellent EMI characteristics, and the interference of both conduction and radiation should be reduced as much as possible. The common practice at present is to use frequency jitter technology and drive signal softening technology. Frequency jitter technology refers to introducing a small frequency change value based on the base frequency of the switching frequency to reduce the spectrum energy intensity at the switching frequency point and optimize the EMI characteristics. The drive signal softening technology refers to making the opening edge (rising edge) of the drive signal that drives the gate of the MOS tube smoother, so as to reduce the energy conduction and radiation at the moment when the MOS tube is turned on, thereby further optimizing the EMI characteristics.

The CL1100 launched by Xinlian Semiconductor is a primary-side feedback AC-DC controller with primary (primary side) regulation technology, transformer tolerance compensation, cable compensation and EMI optimization technology, and has multiple protection functions, such as soft start, cycle-by-cycle overcurrent protection (OCP), CS sampling end leading edge blanking (LEB), and overvoltage protection (OVP) and undervoltage protection (UVLO). The measured constant voltage/constant current characteristic curve of CL1100 is shown in Figure 3. The chip can control the constant voltage/constant current accuracy within ±3%.

Conclusion

As low-power isolated AC-DC applications develop towards lower costs and smaller volumes, primary-side feedback AC-DC control chips have emerged. In order to meet the requirements of high-precision constant current and constant voltage applications, primary-side feedback control chips use primary (primary) regulation technology, transformer tolerance compensation, cable compensation, and EMI optimization technology. The use of these technologies ensures that primary-side feedback AC-DC control chips have strong adaptability to the application power range, loads with different characteristics, and component batch tolerances, making it a control technology that can be widely used in different occasions.