Basic Circuit Theory(基本电路理论)

Basic Definitions

Electron: an indivisible particle of negative charge. The amount of charge is

measured in coulombs (C). The magnitude of the charge associated with an

electron is 1.602x10-l9 C.

Current: charge in motion (electrons). Current is measured in units of amperes,

or more simply amp.

Voltage: an electric potential difference that causes electron flow. It is also

called electromotive force (EMF). An analogy often used to describe current and

voltage is water in a pipe. Current is analogous to the flow of water, while

voltage is analogous to the pressure.

Conductor: a material that allows a continuous current to pass through it under

the action of a fixed voltage. An example of a good conductor is copper or

aluminum which is used in homes and offices for all electrical connections.

Insulator: the opposite of a conductor, it does not allow a continuous current

to pass though it under the action of a fixed voltage. An example of an insulator

is the plastic on electrical cords. Using our water analogy, a conductor can be

envisioned as the region inside a pipe, while an insulator can be envisioned as

the actual material of the pipe which contains the water flow.

Switch: used to control the flow of electrons, or current as it is commonly called.

Ideally, a switch turns on or off instantly, and has no voltage across it while it is

conducting. In our water analogy, an ideal switch would cut the flow

immediately, from completely on to completely off in an instant.

Common Passive Circuit Elements

All circuit elements can be separated into two groups: active and passive. The

electrical definition is very similar to the common definition: active circuit

elements are capable of delivering power, while passive elements are capable of

receiving, and possibly storing, power. In our water analogy, a pump would be

an active element. A narrow section of pipe that restricts the flow, a tank, and a

water wheel would all be examples of passive elements.

Resistors: circuit elements that literally "resist" current flow. Voltage is higher

on the end of the resistor that sees the current first. Figure 1 shows two

schematic representations of a resistor. In our water analogy, a resistor would

be a narrow section of pipe that restricts the flow.

Figure 1. Schematic representations of a resistor

The on-resistance (RDS(on)) of our HEXFET® power MOSFETs is usually one of two

parameters critical to the designer. The other is breakdown voltage (V(BR)DSS) or

how much voltage the device can block when it is off. On-resistance is merely

the resistance from drain to source of the power MOSFET in the "on" state. In

the "off" state, the resistance is extremely high, but instead of RDS(off), we

measure it as leakage current, or IDSS.

Capacitors: circuit elements that store electrons. In many instances, they are

used as a rechargeable battery, providing a stable voltage reference far from

the input power point. They have many different uses in electrical circuits in

addition to simply storing electrons. There are many different types of

capacitors, including aluminum electrolytic, tantalum electrolytic, ceramic disk,

mica, polycarbonate, polypropylene, and polystyrene.

Two important considerations in the selection of capacitors are equivalent series

inductance (ESL), and equivalent series resistance (ESR). Ideally, these two

parameters should be as close to zero as possible, especially as frequency

increases. The capacitors above are mentioned approximately in order of

decreasing ESL and ESR. Aluminum electrolytic capacitors have extremely high

capacitive values, but also high ESL and ESR. This makes them good for dc

applications, such as the capacitors on the output of a bridge rectifier, to provide

the dc supply to the rest of the circuit. Polypropylene and polystyrene capacitors

have very low capacitive values, but also extremely low ESR and ESL values

making them good for extremely high frequency applications.

Stray capacitance exists in all circuits to some extent. While usually to ground,

it can occur between any two points with different potentials. All semiconductor

devices have capacitance between their external terminals, and are specified on

the data sheets. Figure 2 shows several different schematic representations of

capacitors. In our water analogy, a capacitor would be a tank storing water for

later use.

Figure 2.Schematic Representations of Capacitors

Stray capacitance is also responsible for electro-static discharge (ESD). ESD is

responsible for the shock you receive in the winter after walking across a carpeted room