Static Mode Operation (Power Transistors)

Static Mode Operation

A power device is working in static mode when dc currents and voltages are applied to its terminals.

The above figure shows the schematic symbols, the terminal names, and the specific currents and voltages for BJT, MOSFET and IGBT.
Bipolar transistors are current controlled devices. In contrast, MOSFETs and IGBTs are voltage-driven devices. The performance characteristics of MOSFETs are generally superior to those of bipolar transistors: significantly faster switching times, higher input impedance, the absence of second breakdown, the ability to be paralleled, simpler drive circuitry.

Power BJT

Bipolar-based power devices include high-power bipolar transistors and Darlington transistors.

The current gain β of power bipolar transistors depends on the collector current I_C. When the current increases, the current gain decreases as shown in the above figure.
To raise the current gain, Darlington transistors are used. Darlington transistors consist of two transistors connected in an emitter-follower configuration that share a common collector. The key advantage of the Darlington configuration is that the total current gain of the circuit equals the product of the current gain of both devices.

Power MOS Transistors as Switches

The behavior of enhancement-mode transistors depends on the applied gate-source voltage U_GS. The n-MOS transistor is turned-off when U_GS is smaller than the threshold voltage U_T. There is no current and the device is replaced with an open switch. The inverse diode is inherent to the structure of power MOSFET and ensures a reverse-voltage blocking capability of the device.
When U_GS > U_T, the transistor is now turned-on. The key to MOSFET operation is the creation of the inversion channel beneath the gate; this allows current to flow. The region between the drain and source can be thought of as a resistor R_ON, although it does not behave in the same linear manner as a conventional resistor.

IGBT Characteristics

The above figure shows the equivalent circuit and static output I-V characteristics of an IGBT.
An IGBT is in a turn-off state if the gate-emitter voltage U_GE is smaller than the threshold voltage U_T. When U_GE > U_T the MOS transistor goes to turn-on. Then, the collector current from the IGBT flows through the series connected emitter-base junction of the PNP transistor, and subsequently through the drain-source channel of the MOS transistor. The output characteristics of IGBT devices are similar to those of MOSFET devices, but due to the series connected junction they are shifted approximately 0.7 V to the right. The output characteristics have two regions – the ohmic region (for small U_CE values), and the saturation region (for high U_CE values).

Safe Operating Area for BJT (Power Transistors)

Safe Operating Area (SOA) for BJT

The safe operating area (SOA) defines the current and voltage limitations of power devices.

The above figure shows the typical SOA of a power bipolar transistor. It can be partitioned into four regions. The maximum current limit (section a-b) and the maximum voltage limit (d-e) are determined by the technological features and construction of the particular device. The maximum power dissipation limits the product of the transistor's currents and voltages (section b-c). A secondary breakdown (c-d) occurs when high voltages and high currents appear simultaneously when the device is turned off. When this happens, a hot spot is formed and the device fails due to thermal runaway.

SOA for MOSFET & IGBT

The above figure shows the safe operating area (SOA) of MOSFET transistors. This area is bounded by three limits: current limit (section a-b), maximum power dissipation limit (b-c), and the voltage limits (c-d). The SOA of an IGBT is identical to that of the MOSFET SOA.
Since the drain current decreases when the temperature increases in MOSFET transistors, the possibility of secondary breakdown is almost nonexistent. If local heating occurs, the drain current - and consequently the power dissipation - both diminish. This avoids the creation of local hot spots that can cause thermal runaway.

The above figure demonstrates how the SOA of a device increases when the device is operating in pulse mode.
When the device is operating in DC mode the safe operating area is at its smallest. The SOA grows when pulse mode is used. The shorter the pulse signal, the higher the SOA.

Maximum Power Dissipation (Power Transistors)

Maximum Power Dissipation

High currents and voltages in power devices produce very high internal power loss. This loss occurs in the form of heat that must be dissipated; otherwise, the device can be destroyed as a result of overheating.

 

The maximum power dissipation P_max indicates a device's maximum capability to transfer and conduct this power loss without overheating.
The above figure compares the power dissipation of different devices. As a device type is selected, the circular area changes to show the device's maximum power dissipation. As illustrated in the figure, larger metal cases are more efficient in removing heat from the transistor.

Thermal Resistance

Thermal resistance R_thj-a indicates the ability of the device to conduct and transfer heat from the junction (with temperature T_j) to ambient (with temperature T_a). More resistance means that less heat will dissipate.
The junction-to-ambient thermal resistance R_thj-a consists of two resistances: R_thj-c and R_thc-a in series. The R_thj-c is the junction-to-case thermal resistance. The R_thc-a is the case-to-ambient thermal resistance. The R_thc-a is many times higher than R_thj-c.
The above figure shows the values of R_thj-a for different packages. The particular value of each resistance depends on the type of materials and on the case size. Smaller values are typical for large metal cases.

Maximum Power Dissipation vs. Temperature

 

The maximum power dissipation P_max of the transistor depends on the highest junction temperature that will not destroy the device T_jmax, the ambient temperature T_a, and the thermal resistance R_thj-a according to the equation shown in the above figure.
If the ambient temperature is less than or equal to 25°C the device reaches its maximum specified power rating. When the ambient temperature increases, the power rating decreases. If the ambient temperature T_a reaches the maximum junction temperature T_jmax, maximum power P_max becomes zero.

Heat Sink

One way to increase the power rating of the device is to diminish the thermal resistance R_thj-a. A heat sink, which is usually a metal construction with a large surface area, is used to allow heat to dissipate to ambient more easily.
When a heat sink is present, the global thermal resistance R_thj-a decreases because there are more paths available for heat dissipation. The case-to-heat-sink thermal resistance R_thc-s and the heat-sink-to-ambient thermal resistance R_ths-a both facilitate heat dissipation. As a result, the power rating increases as illustrated in the above figure.

Power Transistors

Introduction

 

Power Transistors are electronics components that are use for the control and regulation of voltages and currents with high values. They are the basic components for the implementation of linear and switched mode power supplies, motor control circuits, automotive and aerospace systems, home appliances, and energy management systems

Why are Power Transistors Necessary?

Power Transistors are used to produce, convert, control and regulate high amounts of power output.Typical headphone amplifiers have a low output value (just a fraction of a watt). They are usually implemented with standard low power transistors.On the other hand, amplifiers with hundred-watt output power are used to ensure quality sound in large rooms or concert halls. These amplifiers operate with high-level currents and voltages (more than dozens of amperes and volts). The output stages of such amplifiers can be implemented only with power transistors.

Power Transistors are capable of providing high currents and high blocking voltages and therefore, high power. They can be classified into BJT (Bipolar Junction Transistors), MOSFET (Metal Oxide Semiconductor Field Effect Transistors), and devices such as IGBT (Insulated Gate Bipolar Transistors) that combine bipolar and MOS technologies.
The principle of operation behind high power transistors is conceptually the same as bipolar or MOS transistors. The main difference is that the active area of the power devices is distinctly higher, resulting in a much higher current handling capacity. For this reason, they have large packages.

Three Major Device Technologies

The above figure depicts the typical structure of BJT, MOSFET and IGBT devices.
For a BJT to maintain conduction, a high continuous current through the base region is required. This imposes the necessity of high power drive circuits.
MOSFETs and IGBTs are voltage-controlled devices. The IGBT has one more junction than the MOSFET, which allows for a higher blocking voltage but limits the switching frequency. In IGBTs, during conduction, the holes from the collector p+ region are injected into the n- region. The accumulated charge reduces IGBT's on-resistance and thus the collector-to-emitter voltage drop is also reduced.

Three-phase motors in single-phase operation

 

Three-phase motors can also be operated with single-phase alternating voltage. A time shift in the current in one winding strand is normally achieved using an operating capacitor. This in turn causes a magnetic field to be generated. However, due to the capacitor the currents flowing in the windings are not all of equal magnitude. The rotating field is no longer circular but has an elliptical shape. This leads to a considerable drop in power and starting torque.

Two winding motor

Two winding motor Design

Most single-phase induction motors are designed as dual-winding machines. In contrast to single-phase motors there are two separate winding phases built into the stator of the two-phase motor – the power winding and the auxiliary winding. The auxiliary winding is normally disconnected after the motor has successfully started. The auxiliary winding coils are laid between the coils of the power winding. A squirrel-cage rotor is used as a rotor.

Direction of magnetic fields

 

If you connect the main winding to an AC voltage, the motor responds like a transformer with a short-circuited secondary winding. If the rotor had been purely inductive, the magnetic fields in the stator and rotor would have been phase-shifted by 180°. However, due to the fact that the rotor impedance also contains a resistive component, the north pole of the rotor field is always 15° behind the south pole of the stator field. This means that a torque cannot be generated when the motor is switched on.

Types of induction motors with squirrel-cage rotors

 

The starting torque can be produced with the aid of a rotating magnetic field. The prerequisite for this is a phase-shift between the currents flowing in the working and auxiliary windings which are situated at 90° to one another. The phase-shifted voltage feed of the auxiliary winding can be achieved either through an equivalent resistance, a capacitor or a choke. Depending on the circuitry used for the phase shift, a distinction can be drawn between five different types of two-phased asynchronous motors.

How the motor works

The phase shift between the currents in the two stator windings is achieved in this motor by implementing a considerable amount of active resistance in the auxiliary winding. The auxiliary winding comprises either a larger number of winding turns, lower wire cross-section or is made of resistive material. If the current in the auxiliary winding leads the current in the power winding, the rotation direction is from the pole of the auxiliary winding to the next pole of the power winding. After start-up the auxiliary winding is normally disconnected using a centrifugal switch or a current relay.

Operating characteristics

 

It is the asymmetrical rotating field which is responsible for the motor's relatively low torque during starting. The starting current amounts to approximately six times the rated current, while the starting torque is about equal to the rated torque. After the auxiliary winding is disconnected, the motor demonstrates the characteristics of single-winding single-phase motors. For that reason such motors are nowadays manufactured exclusively with power levels of under 1 kW and used where they do not need frequent starting.

Capacitro motor with starting capacitor

A capacitor is connected in series with the auxiliary winding. It is selected so that, in phase terms, the current in this winding is ahead of the applied voltage by 40°. The inductance of the power winding is the reason for the fact that current in this winding lags the voltage by 50°. The result of this is a 90° phase shift between the winding currents. When 75% of the nominal speed is reached the auxiliary winding is disconnected from the capacitor. Because the small electrolytic capacitor is only in operation for a few seconds, its value can be chosen to be adequately large – around 200 µF.

Operating response

 

Capacitor motors with starting capacitors are the most popular single-phase AC motors in use. Their starting characteristics are better than those for motors with resistive auxiliary phase windings. The starting torque is higher, the starting current consumed is lower and the power factor is better. After the auxiliary winding is disconnected, the motor demonstrates the operational characteristics of single-winding single-phase motors.

Asynchronous motor

Induction motors with squirrel-cage rotors

 

In the repulsion motor the collector and brushes are only used in the starting phase. After a certain speed has been attained the rotor is short-circuited. The next logical step in the simplification of its construction is the short-circuit rotor, usually referred to as a squirrel-cage. The single-phase asynchronous machines have the same simple mechanical design as three-phase motors. A distinction is drawn between single-phase and two-phase motors depending on the number of phase windings.

Design

This motor has a particularly simple and straight-forward design. The laminated stator is equipped with a power winding taking up 2/3 of the grooves. The squirrel-cage consists of two short-circuit rings, which are interconnected by aluminium or copper bars running close beneath the surface of the rotor core. In order to keep power losses low, a deep-bar rotor is not used. The grooves can take a variety of forms.

Direction of the magnetic fields

 

 If the power winding is connected to an alternating voltage, the motor responds like a transformer with short-circuited secondary winding. There is a 180° phase shift between the magnetic fields in the stator and rotor – the fields oppose each other and there is no starting torque generated when the motor is switched on.

How it works

 

The pulsating alternating field in the stator can be perceived as two magnetic fields rotating in opposite directions, which at standsill build up two torques of opposite and equal magnitude. The resulting torque is equal to zero. The hand-operated pony motor does not start by itself – it must be cranked in one direction or the other. After start-up, one of the partial moments will predominate so that the resulting torque becomes greater than zero – the motor accelerates until it reaches its rated speed. Here the same regularities and functional principles apply as elaborated on in the section on the repulsion motor.

 

Operating attributes

 

Compared to other motor types the hand-operated single-phase motor (pony) demonstrates a number of negative characteristics.
Pony motors are only used for small concrete mixers or grinding machines.