Қуат электроникасындағы коммутациялық орталықтың негізгі құрылымы

Basic structure in power electronics

The commutation cell is the basic structure in power electronics. It is composed of two electronic switches (today, a high power semiconductor, not a mechanical switch). It was traditionally referred to as a chopper, but since switching power supplies became a major form of power conversion, this new term has become more popular. The purpose of the commutation cell is to "chop" DC power into square wave alternating current. This is done so that an inductor and a capacitor can be used in an LC circuit to change the voltage. This is, in theory, a lossless process; in practice, efficiencies above 80 90% are routinely achieved. The output is usually run through a filter to produce clean DC power. By controlling the on and off times (the duty cycle) of the switch in the commutation cell, the output voltage can be regulated. This basic principle is the core of most modern power supplies, from tiny DC DC converters in portable devices to massive switching stations for high voltage DC power transmission.

As mentioned above, a commutation cell must be placed between a voltage and current sources. Depending on the state of the cell, both sources are either connected, or isolated. When isolated, the current source must be shorted, as it is impossible for a current to be created in an open circuit. The basic schematic of a commutation cell is therefore given in figure 3 (top). It uses two switches with opposite states: In the configuration depicted in figure 3, both sources are isolated, and the current source is shorted. Both sources are connected when the top switch is on (and the bottom switch is off). It is impossible to have a perfect synchronization between the switches. At one point during the commutation, they would be either on (thus shorting the voltage source) or off (thus leaving the current source in an open circuit). This is why one of the switches has to be replaced by a diode. A diode is a natural commutation device, i. e., its state is controlled by the circuit itself. It will turn on or off at the exact moment it has to. The consequence of using a diode in a commutation cell is that it makes it unidirectional (see figure 3). A bidirectional cell can be built, but it is equivalent to two unidirectional cells connected in parallel.

The commutation cell can be found in any power electronic converter. Some examples are given in figure 4. As can be seen, a "current source" (actually a loop that contains an inductance) is always connected between the middle point and one of the external connections of the commutation cell, while a voltage source (or a capacitor, or a connection in series of voltage source and capacitor) is always connected to the two external connections.