The cascade superposition of independent DC power supply is realized by using the unidirectional conductivity of superposition diode. Its principle circuit includes (a) the main circuit and (b) the control circuit. (a) In, E1 ~ en are n independent DC power supplies, S1 ~ Sn are superposition control switches, VD1 ~ vdn are superposition diodes, VD1 ‘~ vdn’ are isolation diodes (also load reactive power feedback diodes).
The conditions for realizing cascade superposition of n independent DC power supplies are as follows:
① The voltage of n independent DC power supplies must be equal, i.e. E1 = E2 =… = en = E;
② In order to ensure that the control switches S1 ~ Sn flow through the same power, the cascade superposition control of independent DC power supply shall adopt carrier triangular wave phase-shifting SPWM control;
③ SPWM control shall be realized only on the superposition control switches S1 ~ Sn, and SPWM control is not carried out on the inverter switch;
④ The number of independent DC power supplies is n = 1 / 2 (m-1), and M is the number of levels.
After the above conditions are met, n (generally n = 3 ~ 5) independent DC power supplies can be cascaded and superimposed. Superposition is realized by using the unidirectional conductivity of superposition diodes VD1 ~ vdn: when the positive potential of the diode is higher than the negative potential, it is in a positive bias state and the diode is on; Conversely, when the positive potential of the diode is lower than the negative potential, it is in the reverse bias state and the diode is turned off. Through the on-off control of the superposition control switch S1 ~ Sn, the positive and reverse bias states of the superposition diodes can be controlled to realize the selective superposition of n independent DC power supplies E1 ~ en. When any one or several switches in the superposition control switch S1 ~ Sn are turned on, the superposition diodes corresponding to the on switch will be turned off due to the reverse bias, and the corresponding independent DC power supply will be connected to the output circuit, The superimposed DC output voltage includes those corresponding independent DC power supplies.
- Realization of cascade superposition
The cascade superposition of n independent DC power supplies means that the voltage of n identical independent DC power supplies is E1 = E2 = E3 =… = en = e. the control switches S1 ~ Sn are connected in parallel with n superposition diodes VD1 ~ vdn. According to the unidirectional conductivity of the diodes, the bias state of the superposition diodes can be controlled by controlling the on and off of the control switches S1 ~ Sn, so as to realize the selective series cascade superposition of independent DC power supplies.
When the control switch S1 is on and S2 ~ SN is off, the superposition diode VD1 is reverse biased and VD2 ~ vdn is positive biased. VD1 is off, VD2 ~ vdn is on, the independent DC power supply E1 is connected to the output circuit, and the DC output voltage after cascade superposition is UD = E1 = E.
When the control switches S1 and S2 are on and S1 ~ Sn are off, the superposition diodes VD1, VD2 and VD3 are reverse biased, VD1 ~ VD2 are positive biased, VD3 and vdn are off, VD4 ~ vdn are on, the independent DC power supply E1 = E2 = e is connected to the output circuit, and the DC output voltage after cascade superposition is UD = E1 + E2 = 2E.
When VDE 1 and VDE 2 = 3 are connected, VDE 1 and VDE 2 = 3 are connected, and the output of VDE 1 and VDE 2 = 3 are superimposed. When VDE 1 and VDE 2 = 3 are connected, the output of VDE 1 and VDE 2 = 3 are connected to the independent power supply.
When the control switches S1 ~ Sn are turned on, the superposition diodes VD1 ~ vdn are turned off due to reverse bias, and the independent DC power supply E1 = E2 = E3 =… = en = e is connected to the output circuit. The DC output voltage after cascade superposition is UD = E1 + E2 + E3 +… + en = E.
When the control switches S1 ~ Sn are not turned on, the superposition diodes VD2 ~ VD4 are turned on due to positive bias, the independent DC power supply E1 = E2 = E3 =… = en = e is not connected to the output circuit, and the DC output voltage after cascade superposition is UD = 0.
When the control switches S2 and S4 are turned on and S1, S3 and S5 ~ Sn are turned off, the superimposed diodes VD2 and VD4 are reverse biased, VD1, VD3 and vd5 ~ vdn are positive biased, VD2 and VD4 are turned off, VD1, VD3 and vd5 ~ vdn are turned on, the independent DC power supply E2 = E4 = e is connected to the output circuit, and the DC output voltage after cascade superposition is UD = E2 + E4 = 2E.
In short, in the cascade superposition circuit of independent DC power supply, the control switches S1 ~ Sn can be opened in sequence or selectively at intervals; It can be partially opened or fully opened. Which control switch is turned on, which corresponding superposition diode is turned off, which independent DC power supply is connected to the output circuit, and the DC output voltage UD after cascade superposition outputs the voltage of which independent DC power supply; Which control switch does not turn on, which corresponding independent DC power supply cannot be connected to the output circuit, and the voltage of which independent DC power supply is not output in the DC output voltage u after cascade superposition.
- Realization of SPWM control
If the cascade superposition of n independent DC power supplies does not need SPWM control, only multi-level trapezoidal wave DC voltage output can be obtained.
However, in order to adjust the voltage of the inverter and further improve the output voltage waveform, SPWM control must be added. The SPWM control of cascade superposition of n independent DC power supplies is different from the clamp type or 2H bridge cascade superposition multilevel inverter, that is, the SPWM control is not realized on the inverter switch of the inverter, but on the superposition control switches s, ~ s of independent DC power supplies. This has two purposes: one is to reduce the number of switches; The second is to reduce the total equivalent switching times of the inverter and make the inverter switch work in ZVS state, so as to reduce the cost, reduce the switching loss and improve the inverter efficiency.
3.2 cascade superposition of independent SPWM DC power supply
The principle circuit and working waveform of cascade superposition of two (n = 2) independent SPWM DC power supplies are shown in the figure below. It adopts unipolar carrier triangular wave phase-shifting SPWM control (psc-pwm), the phase-shifting angle of carrier triangular wave, psc-pwm control by two independent DC power supplies E1 and E2 respectively, and the phase-shifting angle of carrier triangular wave α= 2π/2=π=180°。 The carrier triangular wave of independent DC power supply E1 is uc1, and the initial phase angle α 1=0°； The carrier triangular wave of independent DC power supply E2 is UC2, and the initial phase angle is α 2=180°； The phase angle of UC2 ahead of uc1 is 2 π / 2 = 180 °. The two independent DC power supplies E1 and E2 jointly adopt a sine wave voltage us after full wave rectification as the modulation wave, so that their output fundamental wave voltage is the same and easy to stack. For the independent DC power supply E1, the carrier triangular wave uc1 is compared with the sinusoidal modulation wave us, and the SPWM pulse generated in the part of us > uc1 is used to control the on-off of switch S1, so that the SPWM output voltage of independent DC power supply E1 = e is ud1; For the independent DC power supply E2, the carrier triangular wave UC2 is compared with the sinusoidal modulation wave us. The SPWM pulse generated in the part of us > UC2 is used to control the on-off of the switch s, so that the SPWM output voltage of the independent DC power supply E2 = e is ud2. In this way, the output voltage of two independent SPWM DC power supplies cascaded in series is UD = ud1 + ud2. The waveform of this output voltage is a two-level SPWM DC step wave voltage waveform similar to the single-phase full wave rectified voltage waveform, as shown in the lower waveform of figure 2-87. After the voltage waveform is synchronously inverted by the full bridge inverter composed of switches S3 ~ S6, the AC 5-level SPWM step wave voltage can be obtained.
Cascade superposition of 4.5 independent SPWM DC power supplies
The principle circuit and working waveform of cascade superposition of five (n = 5) independent SPWM DC power supplies are shown in the figure below. It also adopts unipolar carrier triangular wave phase-shifting SPWM control (psc-pwm). For the phase shift angle of carrier triangular wave, five independent DC power supplies E1 ~ E5 are controlled by SPWM respectively. The carrier triangular wave of independent DC power supply E1 is uc1, and the initial phase angle is α 1=0°； The carrier triangular wave of independent DC power supply E2 is UC2, and the initial phase angle is α= 2π/N=2π/5=72°；…… The carrier triangular wave of independent DC power supply E5 is UCS, and the initial phase angle is α 5= (5-1) 2 π /5=8 π /5. The five carrier triangular waves share a full wave rectified sine wave voltage us as the modulation wave, so that their output voltage has the same fundamental wave, which is convenient for cascade superposition. For the independent DC power supply E1, the carrier triangular wave uc1 is compared with us. The SPWM pulse generated in the part of us > uc1 controls the on and off of switch S1, so that the SPWM output voltage of independent DC power supply E1 = e is ud1; For the independent DC power supply E2, the carrier triangular wave UA is compared with us, and the SPWM pulse generated in the part of us > UA is used to control the on-off of switch S2, so that the SPWM output voltage of independent DC power supply E2 = e is ud2… For the independent DC power supply E5, the carrier triangular wave UC5 is compared with us, and the SPWM pulse generated in the part of us > UC5 is used to control the on-off of switch s5s, so that the SPWM output voltage of independent DC power supply E5 = e is ud5. Thus, the DC output voltage of five independent SPWM DC power supplies cascaded in series is UD = ud1 + ud2 +… + ud5. The waveform of the output voltage UD is a five level SPWM DC step wave voltage waveform similar to the single-phase full wave rectified voltage waveform, as shown in the lower part of figure 2-88. After the voltage waveform is synchronously rectified by the full bridge inverter composed of switches S6 ~ S9, the AC 11 level SPWM step wave voltage can be obtained.
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