Research And Design On Loop Stability For Buck Converter

Switching power supply have been widely used in the areas of communicati-ons equipments, computers and automotive electronics with its small size, light weight, high efficiency and high reliability qualities. Due to the stability of the system determines its ability to work in the applied perturbation, how to make use of the limited chip area, in the case of the output voltage, input voltage and load transitions occur can ensure stability of the system is the power engineers are faced with a new challenge. From the above perspective, this paper focuses on the loop stability of Buck converter. The main work and innovations of achievements are as follows:1. Based on the theory of linear control systems, the transfer function of voltage control mode from the control to the output is derived in this paper by large-signal analysis, detailed analysis of this transfer function, there is a double pole voltage control mode at low frequencies, so the loop compensation is more complex; the transfer function of current control mode from the control to the output is derived in this paper by small-signal analysis, through mathematical analysis of this transfer function, verify the current control mode at half the switching frequency under the current control mode The sub-harmonic oscillation characteristics.2. There are two structures of error amplifier used in Buck converter, one is ordinary operational amplifiers (OPA), and the other is open transconductance amplifier (OTA). In this paper, The transfer function and pole-zero of these two error amplifier are derived in detail, including OPA type of Type-Ⅰ、Type-Ⅱ and Type-Ⅱ, OTA type of Type-2A、Type-2B、Type-3A and Type-3B.3. In the frequency compensation of the voltage control mode for OPA compensator, because the Type-Ⅲ provides a more zero in crossing frequency than Type-Ⅱ, and the resistor-capacitor network is adjustable, the system can be very stable when the input voltage or output or load changes. AS for OTA compensator, when output voltage is small, the crossover frequency can exceed the Nyquist sampling frequency for Type-2A, so the compensation effect is poor. Type-3A can overcome this shortcoming by adding a more capacitance out of the chip.4. In the frequency compensation of the current control mode for OPA compensator, when output voltage is small, whether Type-Ⅱ or Type-Ⅲ is used, the crossing frequency is near the Nyquist sampling frequency, therefore the compensation effect is not good, but the improved OPA compenstor by adding a variable resistor out of the chip can sovle this problem proposed in this paper. In view of OTA compensator, if Type-3B is used, the crossing frequency has nothing to do with the input voltage or output voltage or the load. Moreover, In addition, during the frequency compensation, if compensation capacitor inside the chip is greatly over one hundred picofarads, a large chip area will be used. To solve the problem, an on-chip active compensation circuit is designed in the final section of this paper, the large capacitor can be achieved through a small capacitance by the principle of the multiplier.