Study of peak current mode subharmonic oscillations in switching power supplies
DC-DC switching power supply has been widely used in the field of electronics, electrical equipment, home appliances and entered a period of rapid development due to the advantages of small size, light weight, high efficiency, stable performance, etc. DC-DC switching power supply adopts power semiconductors as switches, and adjusts the output voltage by controlling the duty cycle of the switches. Its control circuit topology is divided into current mode and voltage mode, current mode control is widely used due to the advantages of fast dynamic response, simplified compensation circuit, large gain bandwidth, small output inductance and easy equalisation. Current mode control is further divided into peak current control and average current control, and the advantages of peak current are:
1) The transient closed-loop response is relatively fast, and the transient response to changes in the input voltage and changes in the output load is also relatively fast; 2) The control loop is easy to design.
2) the control loop is easy to design; and
3) has a simple and automatic magnetic balancing function; 4) has a transient peak current limiting function and so on. However, the peak inductor current may cause the system to appear subharmonic oscillation, many literature, although this is introduced to some extent, but there is no systematic study of subharmonic oscillation, especially its causes and specific circuit implementation, this paper will be a systematic study of subharmonic oscillation.
1 The cause of subharmonic oscillation
Taking the PWM modulated peak current mode switching power supply as an example (as shown in Fig. 1, and the down-slope compensation structure is given), the causes of subharmonic oscillations are analysed in detail from different perspectives.
For the current inner-loop control mode, Fig. 2 gives the inductor current variation when the system duty cycle is greater than 50% and the inductor current undergoes a small step ∆Script, where the solid line is the waveform of the inductor current when the system is working normally, and the dashed line is the actual operating waveform of the inductor current. It can be seen that: 1) the inductor current error of the latter clock cycle is larger than that of the previous cycle, i.e., the inductor current error signal oscillates and spreads out, and the system is unstable; 2) the oscillation period is two times the switching period, i.e., the oscillation frequency is 1/2 of the switching frequency, which is where the name of the subharmonic oscillation comes from. Figure 3 gives when the system duty cycle is greater than 50% and the duty cycle occurs a small step AD when the inductor current changes, it can be seen that the system will also appear subharmonic oscillation. And when the system duty cycle is less than 50%, although the perturbation of the inductor current or duty cycle will also cause the inductor current error signal to oscillate, this oscillation belongs to the attenuation oscillation. The system is stable.
