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What are the PWM feedback control modes of high-power DC regulated power supply?
Edit: Yangzhou Kaihong Power Technology Co., Ltd.Release time: 2019-05-29

The following describes the development process, basic working principle, detailed schematic circuit diagram, waveforms, characteristics, and application points of the five PWM feedback control modes with the example of a voltage-regulated forward buck chopper composed of VDMOS switching devices to facilitate the selection of applications. And simulation modeling research.

Different PWM feedback control modes have their own advantages and disadvantages. When designing and selecting a high-power DC stabilized power supply, select a suitable PWM control mode according to the specific situation.

The choice of various control mode PWM feedback methods must be considered in consideration of the specific input and output voltage requirements of the high-power DC stabilized power supply, the main circuit topology and device selection, the high-frequency noise level of the output voltage, and the range of the duty cycle. The PWM control mode is evolving and interrelated, and can be converted to each other under certain conditions.

1. Voltage-mode control PWM (VOLTAGE-MODE CONTROL PWM):

What are the PWM feedback control modes of high-power DC regulated power supply?

Figure 1 shows the schematic diagram of the voltage mode control PWM feedback system of the buck buck chopper. Voltage mode control PWM is the first control method adopted in the switched-mode power supply in the late 1960s. This method is combined with some necessary overcurrent protection circuits, and is still widely used in industry today. Voltage mode control only has a closed loop of voltage feedback. It adopts pulse width modulation method, which compares the slowly changing DC signal sampled and amplified by the voltage error amplifier with a constant frequency triangle wave ramp. Using the principle of pulse width modulation, the current pulse width See the waveform in Figure 1A. Pulse-by-pulse current-limit protection circuits must be added separately. The main disadvantage is the slow transient response. When the input voltage suddenly becomes smaller or the load impedance becomes smaller suddenly, because of the larger phase delay of the output capacitor C and inductor L, the delay of the output voltage decreases and the delay lags. The delay time of the compensation circuit of the error amplifier is delayed before it can be passed to the PWM comparator to stretch the pulse width. These two delays are the main reason for the slow transient response. Figure 1A The role of the voltage error operational amplifier (E / A) is


Three: ① Amplify and feedback the difference between the output voltage and the given voltage to ensure the accuracy of voltage stabilization in steady state. The DC amplifier gain of this op amp is theoretically infinite, but it is actually the open loop amplifier gain of the op amp. ② Convert the DC voltage signal with a wide-band switching noise component at the output of the main circuit of the high-power DC stabilized power supply 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 of switching noise is high and the amplitude is large, 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 contradict each other, the basic design principle of the voltage error operational amplifier is still "low-frequency gain must be high, and high-frequency gain must be low." ③ Correct the entire closed loop system to make the closed loop system work stably. Advantages of voltage mode control: ① PWM triangle wave amplitude is larger, and it has better anti-noise margin when adjusting the pulse width. ② Duty cycle adjustment is not limited. ③ For multiple output power sources, the mutual adjustment effect between them is better. ④ Single feedback voltage closed-loop design and debugging are relatively easy. ⑤ Have better response adjustment to the change of output load.

Disadvantages:

① The dynamic response to input voltage changes is slow.

② The design of the compensation network is more complicated, and the closed-loop gain changes with the input voltage to make it more complicated. ③ The output LC filter adds two poles to the control loop. When designing the error amplifier for compensation, it is necessary to attenuate the low frequency of the main pole or add a zero point to compensate.

④ It is troublesome and complicated to sense and control the magnetic core saturation fault state. There are two ways to improve the speed response of the voltage mode control transient: First, increase the bandwidth of the voltage error amplifier to ensure a certain high-frequency gain. However, it is relatively easy to be affected by high-frequency switching noise interference, and measures need to be taken on the main circuit and feedback control circuit to suppress or smooth the same phase attenuation. Another method is to use the voltage feed-forward mode control PWM technology, as shown in Figure 1B. The triangular wave with variable up-slope 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 in input voltage can be immediately reflected in the change in pulse width at this time, the method responds significantly to the transient response speed caused by the change in input voltage. The feed-forward control of the input voltage is an open-loop control, in order to increase the dynamic response speed to the input voltage change. The control of the output voltage is closed-loop control. Therefore, this is a double-loop control system with an open loop and a closed loop.



2. PEAK CURRENT-MODE CONTROL PWM:

Peak current mode control is abbreviated as current mode control. Its concept originated from a single-ended self-excitation flyback high-power DC regulated power supply with primary-side current protection in the late 1960s. Only in the late 1970s did in-depth academic research. Until the early 1980s, the emergence of the first batch of current-mode control PWM integrated circuits made current-mode control rapidly popularized and applied. Mainly used in single-ended and push-pull circuits. In recent years, due to the difficulty in realizing the synchronous non-distortion slope compensation technology necessary for large duty cycles and poor anti-noise performance, current mode control faces the challenge of voltage mode control after improved performance. Because this improved voltage mode control has input voltage feed-forward function and perfect multiple current protection functions, it already has most of the advantages of current mode control in the control function, but it is not difficult to implement. More mature.

What are the PWM feedback control modes of high-power DC regulated power supply?

As shown in Figure 2, the error voltage signal VE obtained by the difference between the output voltage VOUT and the reference signal VREF is amplified by an op amp (E / A) and sent to the PWM comparator. It is not generated by the oscillator circuit as in voltage mode. The comparison of the fixed triangle wave voltage ramp wave is compared with a triangular waveform or trapezoidal spike-shaped composite waveform signal VΣ whose peak value represents the peak value of the output inductor current, and then the PWM pulse off time is obtained. Therefore, (peak) current mode control does not directly control the PWM pulse width with the voltage error signal, but directly controls the magnitude of the inductor 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 inductor current is easy to sense, and it is logically consistent with the change in the average inductor current. However, the magnitude of the peak inductor current cannot correspond to the magnitude of the average inductor current, because the same magnitude of the peak inductor current can correspond to different magnitudes of the average inductor current when the duty cycle is different. The average inductor current is the only factor that determines the output voltage. It can be proved mathematically that adding at least half the slope of the slope of the slope of the inductor current to the upper slope of the actual detection current can remove the disturbing effect of the different duty ratios on the average inductor current and make the peak inductance controlled The current eventually converges to the average inductor current. Therefore, the synthesized waveform signal VΣ should be composed of two parts: the slope compensation signal and the actual inductor current signal. When the slope of the externally compensated ramp signal is increased to a certain degree, the peak current mode control is converted to voltage mode control. Because if the slope compensation signal is completely replaced by the triangular wave of the oscillating circuit, it becomes voltage mode control, but the current signal at this time can be considered as a current feedforward signal, as shown in Figure 2. When the output current decreases, peak current mode control tends to change to voltage mode control in principle.


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

The power stage is a current source controlled by a current inner loop, and a voltage outer loop controls this power stage current source. In this double-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, and it is not necessary to control the LC energy storage circuit. Because of this, 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 Transient closed-loop response is fast, and the transient response to changes in input voltage and output load is fast. ② Control loop is easy to design

③ The adjustment of input voltage can be compared with the input voltage feed-forward technology of voltage mode control.

④ Simple and automatic magnetic flux balance function

⑤ Instantaneous peak current limiting function, inherent inherent pulse-by-pulse current limiting function.

⑥ Automatic current sharing parallel function.

weakness is:

① The open-loop instability is greater than 50%, and there is an error between the peak current and the average current that is difficult to correct.

② The closed-loop response is not as ideal as the average current mode control.

③ Subharmonic oscillation is prone to occur. Even if the duty cycle is less than 50%, there is a possibility of high frequency subharmonic oscillation. Therefore slope compensation is needed.

④ Sensitive to noise and poor anti-noise. Because the inductor is in a state of continuous energy storage current, compared with the current level determined by the control voltage programming, the up-slope of the current signal of the switching device is usually small, and the small noise on the current signal can easily cause the switching device to change the off-state. At the moment of break, the system enters the sub-harmonic oscillation.

⑤ The circuit topology is restricted.

⑥ The performance of interactive regulation of multiple output power sources is not good. The main application obstacles of peak current mode control PWM are easy oscillation and poor noise resistance. Oscillation can come from: current spikes caused by reverse recovery when the device is turned on, noise interference, insufficient transient amplitude of ramp compensation, and so on. The high-power DC stabilized power supply controlled by the peak current mode easily oscillates when the power is turned on and the voltage or load changes suddenly.


3. AVERAGE CURRENT-MODE CONTROL PWM:

What are the PWM feedback control modes of high-power DC regulated power supply?

The concept of average current mode control was introduced in the late 1970s. The average current mode control PWM integrated circuit appeared in the early 1990's and was maturely applied in the late 1990's. There are three driving forces for the development of average current mode control: First, the peak current mode control PWM encountered many serious problems in the application and promotion; second, the high-speed CPU integrated circuit of INTEL company needs low voltage and high current with high DI / DT dynamic response power supply capability. High-power DC stabilized power supply; the third is the progress in the theoretical research of average current mode control in the late 1980s. Figure 3.A shows the principle of average current mode control PWM. The difference between the output voltage signal VOUT and the reference given voltage VREF is amplified by the voltage error amplifier E / A to obtain the error voltage VE, which is connected to the non-inverting terminal of the current error signal amplifier CA as the control programming voltage signal VCP for the output inductor current ( V CURRENT- PROGRAM).

The output inductor current signal VI with a sawtooth ripple component is connected to the inverting terminal of the current error signal amplifier CA, and represents the actual average inductor current of the tracking current programming signal VCP. After the difference between VI and VCP is amplified by the current amplifier CA, the average current tracking error signal VCA is obtained. Then the VCA and the triangular sawtooth wave signal VT or VS are compared by a comparator to obtain the PWM off time. The waveform of VCA is opposite to the current waveform VI. Therefore, the turn-off signal is generated by comparing the lower ramp of VCA (corresponding to the on-time of the switching device) with the upper ramp of triangle wave VT or VS. Obviously, this means that some slope compensation is added. In order to avoid sub-harmonic oscillation, the upward slope of the VCA cannot exceed the upward slope of the triangular sawtooth wave signal VT or VS. The advantage of average current mode control is that the average inductor current can track the current programming signal with high accuracy. ② No slope compensation is required. ③ The debugged circuit has superior anti-noise performance. ④ Suitable for the control of input or output current in any circuit topology. ⑤ Easy to achieve current sharing. Disadvantages are: ① the gain of the current amplifier at the switching frequency has a maximum limit; ② the double-closed-loop amplifier bandwidth, gain, and other parameters are complicated to design and debug.

3.B is the average current mode control to increase the input voltage feed-forward function, which is very suitable for the situation of China's power grid with large input voltage changes and fast changes. The 48V / 100A half-bridge circuit communication high-power DC regulated power supply module of the Australian RT company actually adopts the control method of Fig. 3.B.

4. Hysteretic current mode control PWM (HYSTERETIC CURRENT-MODE CONTROL PWM):

What are the PWM feedback control modes of high-power DC regulated power supply?

Hysteresis current mode control PWM is variable frequency modulation or fixed frequency modulation. As shown in Figure 4, it is a hysteresis current mode control PWM for variable frequency modulation. The inductor current signal is compared with two voltage values. The first higher control voltage value VC is obtained by amplifying the difference between the output voltage and the reference voltage. It controls the switch-off time of the switching device; the second lower voltage value VCH It is obtained by subtracting a fixed voltage value VH from the control voltage VC. VH is called a hysteresis band, and VCH controls the turn-on time of the switching device. Hysteresis current mode control is determined by the output voltage value VOUT, control voltage value VC and VCH three stable voltage values, one more control voltage value VCH than current mode control, eliminating the possibility of sub-harmonic oscillation, see Figure 4 is a schematic diagram at the bottom right. Because VCH1 = VCH2, the situation in the lower right diagram of Figure 4 does not occur. Its advantages: ① No slope compensation is needed. ② Good stability, not easy to cause unstable oscillation due to noise. Disadvantages: ① Detection and control of the entire period of the inductor current is required. ② Frequency conversion control is prone to frequency conversion noise.

5. SUMMING-MODE CONTROL PWM:

What are the PWM feedback control modes of high-power DC regulated power supply?

Fig. 5 shows the principle diagram of PWM in the addition mode control. It is similar to the voltage mode control shown in Figure 1.A, but there are two differences: The first is that the amplifier (E / A) is a proportional amplifier and there is no reactive compensation component. Capacitor C1 in the control circuit has a smaller function of filtering high-frequency switching clutter. The smaller LF and CF filter circuits in the main circuit (as shown by the dashed lines in the figure, which can also be omitted) also play a role in reducing the output high frequency clutter. If the output high-frequency noise is small, it can be omitted. Therefore, there is no time delay in voltage error amplification and no large time delay in current amplification. The second is that the filtered inductor current signal VI is also added to the voltage error signal VE to form a sum signal VΣ to compare with the triangular sawtooth wave to obtain the PWM control pulse width. Addition mode control PWM is single-loop control, but it has two input parameters: output voltage and output current. If the output voltage or output current changes, the duty cycle will change in the direction that compensates them. Its advantages are: fast dynamic response (3 to 5 times faster than ordinary voltage mode control), small dynamic overshoot voltage, and less output filter capacitors. The VI injection signal in the addition mode control is easily used for current sharing control when the power supplies are connected in parallel. The disadvantage is that high frequency noise suppression during current and voltage sampling needs to be carefully handled.


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