Overview of Switching Devices
A switching function has a value of either 1 or 0; either a switch is on or it is off. When on, it will carry any amount of current for any length of time and in any direction. When off, it will never carry current, no mattery how much voltage is applied. This combination of conditions describes an ideal switch. It is entirely lossless. An ideal switch changes from its on state to its off state instantaneously.
A real switch is some available device that approximates an ideal switch in one way or another. Real switches as the devices shown below differ from the idea in characteristics such as:
- Limits on the amount or direction of on-state current.
- A nonzero on-state voltage drop, such as a diode forward voltage.
- Some level of leakage current when the device is supposed to be off.
- Limitations on the voltage that can be applied when off.
- Operating speed. The time of transition between the on and off states becomes important at high frequencies.
The degree to which properties of an ideal switch must be met by a real switch depends on the application. For example, a diode can easily be used to conduct DC current; the fact that it conducts in only one direction is often an advantage, not a weakness. Some characteristics of the most common power semiconductors are listed in another post. A wide variety of speeds and rating levels are available for these semiconductor devices. As a rule, faster speeds apply to lower ratings. For each device type, cost tends to increase both for faster devices and for devices with higher power handling capacity.
Conducting direction and blocking behavior are fundamentally tied to the device type, and these basic characteristics constrain the choice of device for a given conversion function. Consider again a diode. This part carries current in only one direction and always blocks current in the other. Ideally, it exhibits no forward voltage drop of off-state leakage current. The ideal diode is an important switching device, yet it does not have all the characteristics of an ideal switch. It is convenient to define a special type of switch to represent this behavior; the restricted switch.
A restricted switch is an ideal switch with the addition of restrictions on the direction of current flow and voltage polarity. The ideal diode is one example of a restricted switch.
The diode always permits current flow in one direction, while blocking flow in the other. It therefore represents a forward-conducting reverse-blocking restricted switch. This so-called FCRB function is automatic – the two diode terminals provide all the necessary information for switch action. Other restricted switches require a third gate terminal to determine their state. Consider the polarity possibilities given in the table below. Additional functions such as bidirectional-conducting reverse-blocking (BCRB) can be obtained simply by reverse connection of one of the five types in the table (in this case, the BCFB in reverse). Commercial IGBTs can be built to meet either FCBB or BCFB functions.
Symbols for restricted switches can be built up by interpreting the diode’s triangle as the current-carrying direction, and the bar as the blocking direction. Although the restricted switch symbols are not in common use, they show the polarity behavior of the switching devices, a Circuit drawn with restricted switches represents an idealized power converter.
As an example of the utility of the restricted switch, consider an inverter intended to deliver AC load current from a DC voltage source. A switch matrix built to perform this function must be able to manipulate AC current and DC voltage. Regardless of the physical arrangement of the matrix, we would expect BCFB switches wt be useful for this conversion. This is a correct result: Modern inverters operating from DC voltage sources are built with FETs or IGBTs. An IGBT example is illustrated here to the right. The symbol arrangement shown points out that the transistor-diode combination shows the intended BCFB behavior. The hardware problem for this converter consists of picking a device to meet the necessary ratings while coming as close as possible to a true BCFB. As new devices are introduced to the market, it is straightforward to determine what types of converters will use them.