Power Electronics 101
Basic Circuit Theory
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 and touching the doorknob. ESD is particularly dangerous to MOS-gated semiconductors. The amount of static required to cause damage is so small, that a person can damage a device without knowing it. This is why anyone who handles MOS-gated semiconductors must follow strict ESD prevention procedures. Following proper procedures is essential as devices can be damaged, reducing their lifetime, with no perceivable effects at the time of damage.
Figure 3. Schematic representations of inductors
Inductors: circuit elements that resist change. If, after a period of current flow, an attempt is made to interrupt the current flow, the inductor will continue to force current. Figure 3 shows the schematic representations of two different inductors. In our water analogy, an inductor would be a water wheel - it is difficult to start spinning, but once it is spinning, it is difficult to stop.
Figure 4.Toroidal Inductor
Inductors are typically manufactured by winding wire in a toroidal (donut) shape shown in Figure 4. If the inductor is wound around a non-ferromagnetic material such as plastic, ceramic, cardboard, or merely air, the inductance per unit volume is considerably less than if the inductor is wound on a ferromagnetic core. The upper inductor in Figure 4 depicts an air-cored inductor, while the lower inductor depicts a ferromagnetic cored inductor. Ferromagnetic refers to magnetic materials, whose characteristics greatly vary.
Figure 5. B-H Characteristics for a Magnetic Material.
Figure 5 shows the B-H characteristics for a ferromagnetic material where B is the magnetic flux density, and H is the magnetic field. Operation follows the line, in the direction indicated by the arrow. Although the explanation of this figure is beyond the scope of this module, some important concepts can be observed without a thorough understanding of the plot. During operation, the operating point slides along the curve in the direction of the arrows. If the positive magnetic flux density (B) is not offset by an equal negative magnetic flux density, the operation curve will slowly creep up, until the material saturates (magnetic flux density (B) is at a maximum and cannot further increase).
At saturation the inductance drops to the value of an equivalent air-cored inductor, and the current through it is merely limited by the core's internal resistance which is usually quite low. This is seen at the top of the above curve where the lines flatten, and further increases in flux density (B) are not allowed. Saturation can be caused by one of two mechanisms. First, if the magnetic material is underdesigned, and the flux generated by the current in the winding is greater than the core can handle, the material will saturate. In the above figure, this would place the operating point at the top of the B-H curve.
The second method applies if the magnetic material is not allowed to reset between consecutive pulses. Sufficient time between pulses is necessary to allow the energy stored in the magnetic element to go to zero, or reset. If the design does not allow this to occur, the flux in the magnetic element will build up, or staircase, with each consecutive pulse until the device saturates. This results in a large current which usually destroys the semiconductors in its path. This phenomenon also affects transformers which are merely special cases of the inductor.
Figure 6. Schematic Representation of a Transformer
The final circuit element is the transformer. Figure 6 shows the schematic representation of a transformer. A transformer could be thought of as a ferromagnetic-cored inductor with two or more sets of wires wound on it. Saturation is also a problem in transformers. Thus transformers and inductors are sometimes lumped together and simply called magnetics.
Transformers are most commonly used for one of two purposes. The first is isolation, which is typically needed between two sections of a system which have different ground levels. The second is to change voltage levels. A familiar example is the large ac adapter wall plug supplied with most portable equipment for home use. The adaptor box contains a transformer which steps the voltage down from the line voltage, usually to around 12V, which is then further conditioned by two diodes, and finally supplied to the equipment.
Leakage inductance is a critical parameter for transformers, generators, and motors. Leakage inductance is the difference between the self-inductance and the mutual inductance of the primary and secondary windings. Its value is typically quite small, but very important in determining the characteristics and operation of the circuit. It is of particular interest as the switching device may be asked to dissipate the energy stored in the leakage inductance. The leakage inductance contributes to a turn-off voltage spike seen by the switching device. If the energy and/or voltage is sufficient, a snubber may need to be added to the circuit to protect the switching device from damage due to this spike. IR specifies the amount of energy HEXFET® power MOSFETs can dissipate in this mode and are tested as shown in Figure 7, the unclamped inductive test circuit.
Figure 7. Unclamped Inductive Test Circuit
Basic Electrical Definitions
Power is defined as current multiplied by voltage:
where P is the power measured in watts (W) (also joules per second), V is the steady state voltage measured in volts (V), and I is the steady state current measured in amps (A).
Energy is defined as current multiplied by voltage, multiplied by time:
where "E" is the energy measured in joules (also watt-seconds), "V" is the instantaneous voltage measured in volts, "i" is the instantaneous current measured in amps, and "T" is the time period measured in seconds.
To calculate power, given energy and frequency, multiply energy by the frequency. For example, if an IGBT has a total switching energy loss of 1.4mJ under a given set of operating conditions, and is operated at 20kHz, the total power loss due to switching will be 28W.
E (1.4mJ) x f (20kHz) = P (28W)
ac versus dc
Direct current (dc) has a constant magnitude. In contrast, alternating current (ac) has a magnitude dependent on time. it follows a sinusoidal waveform, shown below. ac is generated by moving a copper winding through a magnetic field. This causes a voltage to be developed on the winding. Generators in the United States operate at 60Hz, but many places in the world, 50Hz is the standard. Hz is the abbreviation for Hertz, which is the unit of measure for frequency. Frequency is only defined for regular waveforms that repeat indefinitely. Frequency is how many times per second the same position on the waveform occurs. Thus, in the figure below, sixty peaks will pass in one second if the frequency is 60Hz. T is the period, while 1/T is the frequency.
Figure 8. 60Hz Sine Wave.
Nearly all current starts off as ac, which is generated through an electromechanical process, and is then converted to dc. It is difficult to generate dc directly, as it requires either a dynamo or a chemical reaction such as the one within in a solar cell which converts sunlight into dc voltage. In applications where dc is present, there is usually a nearby ac source. For example, in your automobile the battery that drives the lights, all the electronics, and all the motors are typically 12 volts dc. This battery is charged by the alternator which is basically a small generator driven by the engine. A three-phase diode bridge is responsible for converting the ac output of the alternator to be compatible with the dc battery.
The last important concept is the role of frequency on magnetics. It is beyond the scope of this training module to explain why, but as the operating frequency of a circuit increases, the physical size of the magnetics (remember this means both inductors and transformers) shrinks. This is one of the reasons designers are constantly increasing the frequency of their designs. In the power supply world, one of the benchmarks of a design is how many watts per cubic inch the power supply delivers. One way to substantially increase this number is by moving to higher frequency, and hence, physically smaller magnetic components. The tradeoff of higher frequency operation is increased switching losses in the semiconductor devices, whether it be a diode, IGBT, or power MOSFET.