ABSTRACT
The spacecraft power electronics basically control the bus voltage and convert voltage levels to match the operating voltages of various components. These functions are performed by solid-state semiconductor devices used as controlled switches which are turned on and off at high frequency. Capacitors and inductors are used to store energy when the switch is connected to the power source. The stored energy is then discharged to continue powering the load when the switch is off. Transformers are used where needed. Major power electronic components routinely used in the spacecraft are:
Shunt regulator for bus voltage control during sunlight Battery charge converter (buck converter) Battery discharge converter (boost converter)
A variety of solid-state devices are available for use as controlled switches. However, the devices commonly used in space are:
Metal-oxide semiconducting field effect transistor (MOSFET) Bipolar junction transistor (BJT) Insulated gate bipolar transistor (IGBT)
The device selection depends on the required voltage, current and the switching frequency. A common feature among these devices is that all are three-terminal devices. Their generally used circuit symbols are shown in Figure 10.1. The two power terminals 1 and 0 are connected in the main power circuit. The control gate terminal G is connected to the auxiliary control circuit. In normal conducting operation, terminal 1 is generally at higher voltage than terminal 0. Since the device is primarily used for switching power on and off as required, it is functionally represented by a gate-controlled switch. In absence of the gate control signal, the device
resistance between the power terminals is large — the functional equivalence of an open switch. When the control signal is applied at the gate, the device resistance approaches zero, making the device function like a closed switch.
