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Part 3: Semiconductors Switches
Chapter 10: Silicon Power Management Power Semiconductors (Preview)

Silicon power semiconductors are employed in power management systems. They are found in two different forms: 

• Discrete Power Semiconductors (single type housed in a package.)
• Integrated Power Semiconductors 

MOSFETs and BJTs (bipolar junction transistors) can be integrated with other circuits in a single package. In addition, various types of power semiconductors may housed in a hybrid (multi-chip module, or MCM) package, that is, interconnected with other monolithic discrete devices in the same package.

Power switches are actually the electronic equivalent of a mechanical switch, except for much faster switching speed. Figure 10-1 is the representation of a mechanical switch. The individual power semiconductor switch applies power to a load when a control signal tells it to do so. The control signal also tells it to turn
off. Ideally, the power semiconductor switch should turn on and off in zero time. It should have an infinite impedance when turned off so zero current flows to the load. It should also have zero impedance when turned on so that the on-state voltage drop is zero. Another idealistic characteristic would be that the switch input consumes zero power when the control signal is applied. However, these idealistic characteristics are unachievable with the present state of the art.

In the real world, actual power semiconductor switches do not meet the ideal switching characteristics. For example, Figure 10-2.1 shows a control signal applied to an ideal power semiconductor switch whose output exhibits zero transition time when turning on and off (Figure 10-2.2). When the transistor is off (not conducting current) power dissipation is very low because current is very low. When the transistor is on (conducting maximum current) power
dissipation is low because the conducting resistance is low. In contrast, an actual power switch exhibits some delay when turning on and off, as shown in Figure 10-2.3. Therefore, some power dissipation occurs when the switch goes through the linear region between on and off. This means that the most power dissipation depends on the time spent going from the off to on and vice versa, that is, going through the linear region. Thus, the faster the device goes through the linear region, the lower the power dissipation and losses.

Power MOSFETS (Metal-Oxide Semiconductor Field Effect Transistors) are among the most widely used power switch semiconductors. Power MOSFETs are three-terminal silicon devices that function by applying a signal to the gate that controls current conduction between source and drain (Figure 10-3). They are available in n-channel versions that require a positive gate turn-on voltage and also p-channel devices that require a negative gate voltage to turn on. Their current conduction capabilities are up to several tens of amperes, with breakdown voltage ratings (BVDSS) of 10V to 1000V.

MOSFETs used in integrated circuits are lateral devices with gate,
source and drain all on the top of the device, with current flow taking
place in a path parallel to the surface. The Vertical Double diffused
MOSFET (VDMOS) uses the device substrate as the drain terminal. MOSFETs used in integrated circuits exhibit a higher on-resistance than those of discrete MOSFETs.

The fabrication processes used to manufacture power MOSFETs are the same as those used in today's VLSI circuits, although the device geometry, voltage and current levels are significantly different. Discrete monolithic MOSFETs have tens or hundreds of thousands of individual cells paralleled together in order to reduce their on-resistance.

The gate turns the MOSFET on when its gate-to-source voltage is above a specific threshold. Typical gate thresholds range from 1 to 4 V. For an n-channel MOSFET a positive bias greater than the gate-to-source threshold voltage (VGS(th) ) is applied to the gate, a current flows between source and drain. For gate voltages less than VGS(th) the device remains in the off-state. P-channel MOSFETs use a negative gate drive signal to turn on.

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When power semiconductor switches first found wide use, discrete
transistors, pulse transformers, opto-couplers, among other components were used to drive the power MOSFET on and off. Now, specially designedgate driver ICs are used in many applications. This minimizes the drive requirements from a low power circuit, such as a microprocessor, and also acts as a buffer between the controlling signal and the power semiconductor switch. The gate driver supplies enough drive to ensure that the power switch turns on properly. Some gate drivers also have protection circuits to prevent failure of the power semiconductor switch and also its load.

MOSFET characteristics include several parameters critical to their performance:

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