Design of current mode control auxiliary power sup

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Design of current mode control auxiliary power supply for maglev train

switching power supply needs to adopt closed-loop control for the controlled output variables, so that the system can adjust in time to the change of input voltage or load current, and has the desired dynamic response. The traditional switching power supply mostly adopts the voltage type control used by Mr. Tan dunsong, senior manager of Shanghai Volkswagen, that is, only the output voltage is sampled and used as a feedback signal. Through experiments, it is found that there is a closed-loop control to stabilize the output voltage. In its control process, the inductance current does not participate in the control and is an independent variable. The switching converter is a second-order system with two state variables, namely, the voltage of the output filter capacitor and the current of the output filter inductor. The second-order system is a conditionally stable system. Slat and his team cannot help dragging the device back to the shore for inspection and maintenance. Only by carefully designing and calculating the error amplifier compensation network can the system work stably. Since the current of the switching power supply flows through the inductor, the voltage signal on the filter capacitor will delay the current signal by 90 °. Therefore, only using the method of sampling the output voltage, its voltage stabilization response speed is slow, the stability is poor, and even oscillation will occur when the large signal changes, thus damaging the power device

the switching power supply with current mode control is a double closed-loop control system. In view of the shortcomings of voltage mode control, current mode control not only retains the output voltage feedback of voltage mode control, but also increases the inductance current feedback. Moreover, this current feedback is used as the slope function of PWM control converter, so there is no need for sawtooth wave generator, so that the performance of the system price rises again and again has obvious advantages. Since the change rate of the feedback inductance current directly follows the change of the input voltage and the output voltage, the average value of the inductance current is proportional to the load current. In the voltage feedback loop, the output of the error amplifier is used as the current given signal. Compared with the feedback inductance current, it directly controls the on-off duty cycle of the power switch, so that the peak current of the power switch is controlled by the current given signal

The characteristics of current mode control method are as follows:

(1) the system has fast input and output dynamic response and high stability

(2) high output voltage accuracy

(3) it has the internal ability to control the power switching current

(4) good parallel operation ability

at present, with the emergence of current mode control integrated controller, current mode control technology is more and more used in practical design

although current mode control has many advantages over voltage mode control, there are still some deficiencies in current mode control for converters that detect fixed frequency and peak current, mainly in the following points:

(1) there is a problem of poor anti-interference ability under certain conditions. If the inductance current rise rate is not large enough, the distributed capacitance of the circuit will cause the opening current spike, which may cause the switch to suddenly turn off and cause subharmonic IO scillat ion. Without slope compensation, when the duty cycle is greater than 50%, the subharmonic oscillation will diverge, causing system instability

(2) the control signal is related to the switch current. Therefore, the oscillation of the power stage circuit will bring noise to the control loop

(3) there is an error between the peak current of the inductance and the average output current. After the current control mode adopts slope compensation, some of the above problems can be satisfactorily solved without affecting its advantages

2 slope compensation of current mode control converter [2]

for any current mode control converter with fixed frequency, when the duty cycle exceeds 50%, regardless of the state of the voltage feedback loop, the current inner loop is unstable. Because the duty cycle can not be achieved very much, the input voltage becomes very strict. After adding slope compensation, the system can work stably for any duty cycle

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