Unlike the three-level DC converter trying to replace the power switch in the ordinary two-level converter with a three-level switch unit, it is actually a series superposition of two ordinary two-level DC converters.
Since the output current of the series circuit is the same, as long as the output voltages of the two converters are equal, the power of the converters can be divided to ensure that the input voltages of the two converters are equal.
The switching stress of each power device is the same, which is half of the input voltage on the DC side.
Half-bridge cascade converter
Zero-voltage and zero-current composite full-bridge cascade converter
1. Each converter adopts independent PWM (or phase-shifted PWM) control, and the voltage setting adopts the same setting value.
2. Only one set of control circuit is used, and each output signal of the control circuit is sent to the corresponding power tube of two different converters. In this control mode, as long as the parameters of each converter are basically the same, it can ensure that each The output voltages of the two converters are basically equal.
Features of cascaded DC converterâ‘ The circuit structure is simple, no clamping diode is needed.
â‘¡The control is simple, no need to consider the switching sequence and capacitor voltage balance. â‘¢The voltage stress of the secondary rectifier diode is half of that of the three-level converter.
â‘£ It is a modular structure consisting of two identical circuits. A high-power converter is divided into two smaller-capacity converters, making the design simpler.
⑤ N converters can be cascaded, and the voltage stress of the power device is 1/N of the input voltage.
ExampleA certain type of 6KW aircraft power converter, the input is 400Hz, 115V three-phase four-wire, the output is 360V DC, and two parallel interleaved double-tube forward converters with an output of 180V are cascaded. The withstand voltage of the rectifier tube is only 600V, and the total harmonic content of the input current is less than 15% without any other measures, which has little impact on the aircraft power grid.
An 80KVA subway auxiliary inverter (the difference between a cascaded inverter and a cascaded DC converter is only that the control law of the power tube is different), the input voltage is 1500V ± 20%, and two three-phase bridges are used at the input The side series are superimposed, and the voltage rating of the power tube only needs to be 1700V.
The cascaded inverter scheme using discrete DC power supplies has been applied in engineering in the fields of high-voltage and high-power transmission and active filters.
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