Five Feedback Control Modes of Switching Power Supply PWM

1、 Introduction

The basic working principle of PWM switching voltage stabilizing or current stabilizing power supply is that when the input voltage changes, the internal parameters change and the external load changes, the control circuit carries out closed-loop feedback through the difference between the controlled signal and the reference signal to adjust the conduction pulse width of the switching device of the main circuit, so as to stabilize the output voltage or current of the switching power supply. The switching frequency of PWM is generally constant, and the control sampling signals include: output voltage, input voltage, output current, output inductance voltage and switching device peak current. These signals can form a single loop, double loop or multi loop feedback system to achieve the purpose of voltage stabilization, current stabilization and constant power. At the same time, some attached functions such as overcurrent protection, anti magnetic bias, current sharing and so on can be realized. For the PWM closed-loop feedback control system with constant frequency and width adjustment, there are mainly five PWM feedback control modes. The following takes the voltage stabilizing forward buck chopper composed of VDMOS switching devices as an example to illustrate the development process, basic working principle, detailed circuit schematic diagram, waveform, characteristics and application points of five PWM feedback control modes, so as to facilitate the selection, application and simulation modeling research.

2、 Five feedback control modes of switching power supply PWM

1. Voltage-mode control PWM:

Figure 1 shows the schematic diagram of voltage mode control PWM feedback system of Buck step-down chopper. Voltage mode control PWM is the first control method adopted since the development of switching regulated power supply in the late 1960s. Combined with some necessary overcurrent protection circuits, this method is still widely used in industry. The voltage mode control has only one voltage feedback closed loop. The pulse width modulation method is adopted, that is, the slow changing DC signal sampled and amplified by the voltage error amplifier is compared with the oblique wave on the triangular wave with constant frequency. Through the pulse width modulation principle, the current pulse width is obtained, as shown in the waveform in Fig. 1a. A pulse by pulse current limiting protection circuit must be added. The main disadvantage is the slow transient response. When the input voltage suddenly decreases or the load impedance suddenly decreases, because there is a large phase shift delay of output capacitor C and inductance L, the reduction of output voltage also lags behind. The information of reduction of output voltage can be transmitted to PWM comparator to widen the pulse width only after the delay of compensation circuit of voltage error amplifier. These two delays are the main reasons for the slow transient response.

Figure 1A the voltage error operational amplifier (E / a) has three functions: â‘  amplify and feed back the difference between the output voltage and the given voltage to ensure the steady-state voltage stabilization accuracy. The DC amplification gain of the operational amplifier is theoretically infinite, but it is actually the open-loop amplification gain of the operational amplifier. â‘¡ The DC voltage signal with a wide band switching noise component at the output of the main circuit of the switching power supply is transformed into a relatively "clean" DC feedback control signal (VE) with a certain amplitude. That is, the DC low-frequency component is retained and the AC high-frequency component is attenuated. Because the frequency and amplitude of switching noise are high, if the attenuation of high-frequency switching noise is not enough, the steady-state feedback is unstable; If the attenuation of high-frequency switching noise is too large, the dynamic response is slow. Although they are contradictory, the basic design principle of voltage error operational amplifier is still "high low-frequency gain and low high-frequency gain". â‘¢ The whole closed-loop system is corrected to make the closed-loop system work stably.

Advantages of voltage mode control: ① the PWM triangular amplitude is large, and it has a good anti noise margin when adjusting the pulse width. ② Duty cycle adjustment is not limited. ③ For multi-channel output power supply, the interactive regulation effect between them is better. ④ Single feedback voltage closed-loop design and debugging are relatively easy. ⑤ It has a good response to the change of output load. Disadvantages: ① the dynamic response to the change of input voltage is slow. ② The design of compensation network is complex, and the closed-loop gain changes with the input voltage, which makes it more complex. ③ The output LC filter adds bipolar points to the control loop. When compensating and designing the error amplifier, it is necessary to attenuate the main pole at low frequency or add a zero point for compensation. ④ It is troublesome and complex in sensing and controlling the saturation fault state of magnetic core.

There are two ways to improve and speed up the transient response speed of voltage mode control: one is to increase the bandwidth of voltage error amplifier to ensure a certain high-frequency gain. However, it is easy to be affected by high-frequency switching noise, so measures need to be taken in the main circuit and feedback control circuit to suppress or smooth in-phase attenuation. Another method is to use voltage feedforward mode control PWM technology, as shown in Figure 1b. The triangular wave with variable up ramp generated by charging the resistance capacitor (RFF, CFF) with the input voltage replaces the fixed triangular wave generated by the oscillator in the traditional voltage mode control PWM. Because the change of input voltage can be immediately reflected in the change of pulse width, the transient response speed caused by the change of input voltage is significantly improved. The feedforward control of input voltage is an open-loop control in order to increase the dynamic response speed to the change of input voltage. The control of output voltage is closed-loop control. Therefore, this is a double loop control system composed of open loop and closed loop.

2. Peak current-mode control PWM:

Peak current mode control is called current mode control for short. Its concept came from the single ended self-excited flyback switching power supply with primary side current protection function in the late 1960s. It was only in the late 1970s that he made in-depth academic research. Until the early 1980s, the emergence of the first current mode control PWM integrated circuits made the current mode control widely used. It is mainly used in single ended and push-pull circuits. In recent years, current mode control is facing the challenge of voltage mode control after improving the performance due to the difficulty in the implementation of synchronous undistorted slope compensation technology and poor anti noise performance when the duty cycle is large. Because this improved voltage mode control has input voltage feedforward function and perfect multiple current protection functions, it already has most of the advantages of current mode control in control function, but it is not difficult to realize and the technology is relatively mature.

As shown in Fig. 2, after the error voltage signal ve obtained from the difference between the output voltage Vout and the reference signal VREF is amplified by the operational amplifier (E / a) and sent to the PWM comparator, it is not compared with the fixed triangular wave voltage ramp generated by the oscillation circuit like the voltage mode, Instead, it is combined with a triangular waveform or trapezoidal sharp angle waveform signal V whose peak value represents the peak value of output inductor current Σ Then the PWM pulse off time is obtained. Therefore, the (peak) current mode control does not directly control the PWM pulse width with the voltage error signal, but directly controls the inductance current on the peak output side, and then indirectly controls the PWM pulse width. Current mode control is a control method with fixed clock on and peak current off. Because the peak inductance current is easy to sense and logically consistent with the change of average inductance current. However, the peak inductance current cannot correspond to the average inductance current one by one, because when the duty cycle is different, the same peak inductance current can correspond to different average inductance current. The average inductance current is the only factor that determines the output voltage. Mathematically, it can be proved that adding at least half of the slope of the lower ramp of the inductance current to the upper ramp of the actual detection current can remove the disturbance of the average inductance current with different duty ratios, so that the controlled peak inductance current finally converges to the average inductance current. Thus, the waveform signal V is synthesized Σ It should be composed of slope compensation signal and actual inductance current signal. When the slope of the external compensation ramp signal increases to a certain extent, the peak current mode control will be transformed into voltage mode control. Because if the slope compensation signal is completely replaced by the triangular wave of the oscillation circuit, it becomes voltage mode control, but the current signal at this time can be regarded as a current feedforward signal, as shown in Figure 2. When the output current decreases, the peak current mode control tends to change to voltage mode control in principle.

When it is in no-load state, the output current is zero and the amplitude of slope compensation signal is relatively large, the peak current mode control actually becomes voltage mode control. Peak current mode control PWM is a double closed-loop control system, and the outer voltage loop controls the inner current loop. The current inner loop is instantaneous and fast, and works pulse by pulse.

The power stage is a current source controlled by the inner current loop, while the outer voltage loop controls the current source of the power stage. In the dual loop control, the current inner loop is only responsible for the dynamic change of the output inductance, so the voltage outer loop only needs to control the output capacitance, not the LC energy storage circuit. Because of these, the peak current mode control PWM has a much larger bandwidth than the voltage mode control. The advantages of peak current mode control PWM are: ① fast transient closed-loop response, fast transient response to the change of input voltage and output load. ② The control loop is easy to design ③ the adjustment of input voltage can be compared with the input voltage feedforward technology controlled by voltage mode ④ simple and automatic flux balance function ⑤ instantaneous peak current limiting function and inherent pulse by pulse current limiting function. ⑥ Automatic current sharing parallel function. The disadvantages are ① open-loop instability with duty cycle greater than 50%, and there is an error between peak current and average current that is difficult to correct. ② The closed-loop response is not as ideal as the average current mode control. ③ It is prone to subharmonic oscillation. Even if the duty cycle is less than 50%, it is possible to have high-frequency subharmonic oscillation. Therefore, slope compensation is required. ④ It is sensitive to noise and has poor noise resistance. Because the inductor is in the continuous energy storage current state, compared with the current level determined by the control voltage programming, the up ramp wave of the current signal of the switching device is usually small, and the small noise on the current signal can easily make the switching device change the off time and make the system enter subharmonic oscillation. ⑤ The circuit topology is limited. ⑥ The interactive regulation performance of multi-channel output power supply is not good. The main application obstacles of peak current mode control PWM are easy oscillation and poor anti noise. The oscillation can come from: current spike caused by reverse recovery when the device is turned on, noise interference, insufficient transient amplitude of ramp compensation, etc. The switching power supply controlled by peak current mode is easy to oscillate when starting up and when the voltage or load changes suddenly.

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