EC-12.4 Power Devices and Switching
You can say why a power device is used as a switch rather than an amplifier, compare the main device types by voltage, current and speed, account for conduction and switching loss, and size a heat sink from a thermal model.
Before:EC-2. Electromagnetics and Transmission LinesEC-6. Semiconductor Devices and Device Models
The device physics module already covers the diode and the MOSFET as amplifying and rectifying devices. This topic asks a different question of the same parts: what happens when they are asked to carry tens of amperes and are switched on and off thousands of times a second. The answers, switching loss and thermal limits, are what power electronics is actually about.
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Why a power device is a switch and never an amplifier
A device half on drops voltage while carrying current, and the product is heat. Fully on or fully off, one of the two is nearly zero, which is the only way to control kilowatts without dissipating them.
NPTEL: Power Electronics (IIT Bombay) · CourseThe device family, compared on what actually matters
The diode, the thyristor, the power MOSFET and the insulated gate bipolar transistor differ in voltage, current, switching speed and whether they can be turned off. Those four figures decide the choice.
NPTEL: Power Electronics (IIT Bombay) · CourseConduction loss and switching loss
Conduction loss is the on-state drop times the current and does not depend on frequency. Switching loss is the energy lost in each transition times the number of transitions, and dominates at high frequency.
NPTEL: Power Electronics (IIT Bombay) · CourseDriving a gate, and why it is harder than it looks
Turning a large device on quickly means moving charge into its gate capacitance quickly, which needs amperes for a short time. Slow switching is a common cause of a device running hot for no obvious reason.
NPTEL: Power Electronics (IIT Bombay) · CourseReverse recovery, and the diode that is not instant
A diode carrying current does not stop conducting the instant it is reversed, and the recovery current flows through the device switching on. This is a real loss and a real source of interference.
NPTEL: Power Electronics (IIT Bombay) · CourseThe thermal model, and sizing a heat sink
Junction to case, case to sink and sink to air behave like resistances in series with power as the current and temperature rise as the voltage. Sizing a heat sink is solving that one circuit.
NPTEL: Power Electronics (IIT Bombay) · Course
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