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.

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.

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 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.

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.

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.

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.

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.

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.

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.

The above figure shows a linear voltage regulator. It produces a constant output voltage U_out that is smaller than unregulated input voltage U_in.

In this circuit, the power transistor operates as a common emitter amplifier and its collector-emitter voltage U_CE compensates for the difference between input and output voltages. To this aim, the control circuit follows the output voltage and generates an appropriate base current that regulates the internal resistance of the transistor. Because the entire load current passes continuously through the transistor, its power dissipation has high values as is shown.

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.

The above figure illustrates the relationship of resistance R_ON to gate-source voltage U_GS and drain current I_D.

Usually, the threshold voltage of n-MOS power transistors is about 4-5 V. The higher the applied gate-source voltage, the wider the channel, and therefore the value of resistance R_ON decreases.

When the drain current of a turned-on MOS transistor with fixed value of U_GS increases, the transistor channel narrows and resistance R_ON increases.

The above figure shows the values of the normalized drain-source on-resistance R_ON versus drain current I_D with parameter junction temperatures T_j. The reference values for R_ON(ref) are: T_j = 25 ^oC and I_D =2A.

The on-resistance R_ON increases with increases in temperature, thus avoiding thermal runaway. If unwanted heating occurs, R_ON increases, causing I_D to decrease. This will diminish the power dissipation and thus the device temperature.

MOS transistors can easily be connected in parallel. Any imbalance between transistors will lead to an increase in the resistance R_ON in the transistor with a higher current. This will cause the higher current to be partially diverted to the other parallel device, thus avoiding thermal runaway.

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).

In power circuits, the IGBT is used as a switch. When it is set to turn-off, the switch is open. When it is set to turn-on, the switch is closed. IGBTs combine the advantages of bipolar and MOS transistors. They have higher collector to emitter voltages than BJT devices.

The above figure shows the typical relationship between U_CE(ON) and junction temperature. At lower levels of collector current, U_CE(ON) decreases with increasing temperature. At higher collector current U_CE(ON) increases with increasing temperature.

The transfer characteristic is the relationship between collector current and gate voltage, at specified collector voltage. It is recomended the value of U_GE to be up to 15 V.

The above figure illustrates linear and switching operating modes of transistors. In linear mode, the transistor operates around point C; thus half of the supply voltage and half of the maximum collector current I_C = U_CC/R_C will be dissipated, even if there is no ac input voltage. In switching mode, if the base current is high, the transistor turns on (point A). In this state, I_C reaches max, and the voltage drop U_CE across the switch is very low. There is close to zero power dissipation (also referred to as conduction losses). If the base current is zero, the transistor turns off (point B) and its losses are negligible. When in switching mode, if the circuit is rapidly switched between operating at A and operating at B, the average point of operation will still be at C, but now with nearly zero losses.

The above figure shows the ideal behavior of a transistor switch. When the input voltage U_in is zero, the transistor is turned off. No output current flows I_out through the transistor. The output voltage U_out equals the supply voltage U_CC. When a positive input signal is applied, the device turns on. The output current jumps to its maximal value. The output voltage falls to zero. In this case, the global losses are zero.

In reality, switching characteristics are not so clear cut. They depend on the load type, the device characteristics, the type of driving circuit, etc.

The above figure shows the real switching characteristics of a BJT with a resistive load. When the input signal U_in is zero, the transistor is cut off and the collector current I_C is near zero. If the positive input signal U_in is applied, I_B increases to I_B1 and after delay time t_d the collector current reaches 10% of its on-state value. The current I_C continues to rise and after rise time t_r it reaches 90% of its on-state value. The sum of both times is called turn-on time t_on.

When the input signal U_in falls to zero, I_C continues to flow for storage time t_s. After that, I_C falls from 90% to 10% of its maximal value for fall time t_f. The sum of both times is called turn-off time t_off. Usually the turn-off time t_off is longer than the turn-on time t_on.

If the input signal U_in is zero, the transistor is cut off, the collector current I_C is zero and the collector-emitter voltage U_CE equals the supply voltage U_CC. Due to inductive load, the current I_L is unchangeable and flows through the diode.

When the input voltage U_in is applied, the transistor starts conducting. The collector current increases slowly to its on-state level I_Csat within time interval t₁ - t₂. During this period the free-wheeling diode is still on. As a result, the collector-emitter voltage U_CE stays unchanged. After t₂, the transistor goes to its saturation point and thus U_CE decreases to U_CEsat.

If the input signal U_in is positive, the transistor is on, and the collector current I_C is in its on-state level. Collector-emitter voltage U_CE equals the saturation value U_CEsat.

When the input voltage U_in falls to zero (at t₁), the collector current remains constant within time interval t₁ - t₂. During the time interval t₂ - t₃, the collector current I_C decreases to zero. At the time t₂ U_CE jumps to the supply voltage U_CC.

The lag between the decrease in I_C and the increase in U_CE can provoke the appearance of very high values for the power dissipation within time interval t₂ - t₃.

Use the appropriate blocks to construct a switched-mode transistor circuit for dc motor speed control.

The above figure illustrates the switching behavior of a MOS transistor with resistive load.

When the input signal rises up to a positive level, the transistor turns on and its drain current I_D starts increasing. After the rise time t_on, the current I_D has reached its on-state level. This level depends on supply voltage U_DD and resistor R. The U_DS voltage is determined by the transistor's R_ON value.

When the input voltage is removed, the transistor turns off after fall time t_off. The drain current drops to zero and U_DS voltage increases to U_DD.

The above figure demonstrates the switching behavior of a MOS transistor with a clamped inductive load.When the input level is smaller than the threshold voltage, the transistor is turned off, drain current I_D is zero, and U_DS voltage is equal to the supply voltage U_DD.

When the input jumps to positive, the transistor turns on and U_DS voltage falls to near zero. The current I_D flows through the inductor and its value increases. When the input falls to zero, the transistor turns off; at this point the magnetic field around the coil collapses, inducing a large voltage of opposite polarity across the coil. This raises the potential of a drain and so the diode turns on, increasing the U_DS voltage to the battery voltage U_CL.

The above figure shows a step-down switching regulator that directly converts a fixed voltage dc supply to a variable output dc voltage.

The rectangular pulses on the base of the power transistor alternately saturate and cut off the pass transistor during each cycle. When the transistor is off, the voltage drop U_CE is high, but the current I_T is near zero. When the transistor is on, the current is high with negligible voltage drop. This way, the transistor power dissipation is always near to zero. L_C circuit blocks the ac component of the rectangular voltage at the diode and forms dc at the output. Changing the t_on/T ratio of the control pulses can regulate the output voltage value.

The above figure demonstrates the switching behavior of a MOS transistor with a capacitive load. In the initial state, the input level is near to zero, the transistor is turned off, drain current I_D is zero, U_DS voltage is equal to the supply voltage U_DD, and the capacitor C is charged.

When the input jumps to positive, the transistor turns on and the capacitor starts discharging. The current I_D through the transistor is at this point the sum of the current through resistor I_R and the discharge current of the capacitor C. After time interval t_on, the capacitor discharges; subsequently, the voltage U_DS drops to near zero, and the current I_D becomes determined only by the current running through resistor R. When the input changes to zero, the transistor turns off, the capacitor starts charging, and the switch resets to the initial state.

The above figure illustrates the switching waveforms of an IGBT. The switching times are:

Turn-on delay time t_d(on): the time it takes for the collector current to reach 10% of I_C on-state value from the time a pulse is injected into the gate.

Rise time t_r: the time it takes for the collector current to reach 90% of I_C from 10%.

Turn-off delay time t_d(off): the time it takes for the collector current to reach 90% of I_C on-state value from the time a pulse is removed from the gate.

Fall time t_f: the time it takes for the collector current to reach from 90% of I_C to 10%.

These times generally rise when the current and the temperature of the system increase.

Drive circuits ensure the safe switching of power transistors. The type of drive circuit necessary depends on various requirements that different devices impose on the input control. Drive circuits are divided into two basic groups - circuits for current control and circuits for voltage control.

Current control circuits are used to drive bipolar power transistors. They have to ensure a high-value base current in saturation mode (ON switch position) and reliable setting of the transistor in cut-off mode (OFF switch position). Voltage control circuits are used to drive MOS transistors and IGBTs. They have to guarantee input voltage values higher than the threshold when the transistor is turned on, and values lower than the threshold when the transistor is turned off.

The above figure shows a typical drive circuit for a power BJT.

A positive voltage level is applied to the drive circuit for setting the transistor to its saturation point. The voltage is transformed via current I_B = I_B1 through the resistor R. This current should be much higher than base saturation current I_Bsat to ensure fast switching. The higher the base current in saturation, the shorter the turn-on switching time, but the larger the turn-off time.

When a negative input voltage is applied, the transistor cuts off after t_off time. To minimize this time, the base current I_B2 should be high.

To reduce the turn-on time of a BJT without increasing the turn-off time, a capacitor C is added in parallel to the resistor.

When a high control voltage is applied, the base current I⁺_max is much higher than I_Bsat.; which ensures fast transistor switching. The capacitor charges and the input current goes to a value a little bit bigger than I_Bsat.

At zero input voltage, the base appears inversely biased due to the charged capacitor. During capacitor discharging, the base current changes its direction thus ensuring a reliable and fast turn-off of the transistor.

The above figure shows a simple drive circuit for MOS and IGBTs implemented with a CMOS inverter. The gate resistor R is used to avoid undesired parasitic oscillations in the power transistor input.

If the drive voltage U_DR increases, the input parasitic capacitor C_GS charges through resistor R. The transistor turns on when the capacitor voltage reaches the threshold voltage U_TH. After that the output current increases to its maximum value. If the drive voltage U_DR falls, the input capacitor discharges. When the capacitor voltage decreases under threshold voltage U_TH, the transistor turns off.

The above figure shows a fast drive circuit for a MOS and an IGBT.

For fast charging and discharging of the input capacitance, the value of R should be as small as possible. A small gate resistance will overload the inverter output. To prevent this, parallel connected inverters are used. This increases the maximum allowed output current, thus allowing resistance R to be decreased.

When the drive voltage U_DR increases, the input capacitor rapidly charges through resistor R.

When the drive voltage U_DR decreases, the input capacitor discharges through the diode. This accelerates the switching process.

Switching transients cause over-current, over-voltage, or rates of current (di/dt) and voltages (du/dt) in excess of critical rates to appear in power devices. Switching transients expand power losses, thus degrading device performances and sometimes even destroying the transistor.

The above figure illustrates the behavior of a typical inductive load switch. The power dissipation P_dis increases in both switching periods (t₁ - t₂) and (t₃ - t₄). The power losses increase when the switching times increase or switching frequency increases.

The trade-off between switching speed and power losses is a consideration in the normal operation of the circuits.

Snubber circuits are used to avoid the simultaneous occurrence of peak voltage and peak current, or to diminish the rates of change of currents and voltages.

The above figure shows the voltage-current switching locus of a typical power BJT switch. When the snubber is missing, the transition from on to off state and vice versa passes through the region with high voltage and current; this is called “hard switching”.

Snubber circuits ensure an alternate path for current and voltage, which significantly reduces power losses; this is called “soft switching”. Turn on snubber limits the rate of the rise of the current while the turn off snubber limits the rate of the rise of the voltage.

The above figure shows the turn-off switching of a power BJT with an inductive load. Without a snubber, peak voltage and peak current occur simultaneously and there is a very high amount of power dissipation.

With a RCD snubber, during turn off I_C decreases, the capacitor charges and the collector-emitter voltage U_CE increases. This avoids a simultaneous occurrence of high voltage and current in the output; it also avoids exceeding the critical rate of change of U_CE voltage. The diode prevents a parasitic oscillation of the circuit.

The capacitor discharges through the snubber resistor when the device turns on.

The above figure shows some of the more widespread applications of power transistors in linear and switching modes.

In linear applications, continuous currents and voltages are applied between the terminals of power devices. In this operating mode, a large mount of power is dissipated.

When the power transistors operate in switching mode, the pulse currents and voltages are applied to its terminals. In this mode, a low average amount of power is dissipated. Power dissipation is high only during the small time interval when the transistor switches.

When the load of an amplifier has a high resistance (such as in a set of headphones) the necessary output power is very low (just a fraction of a watt). In this case, amplification can be accomplished with just a simple integrated amplifier.

If the load is larger (such as for a loudspeaker) more power is needed. This can be ensured with serially connected power bipolar transistors.

When high power sound systems must be supplied, parallel connected MOS power transistors are used to guarantee high amplification without thermal runaway problems.

The above figure shows a circuit for dc motor control using a half bridge chopper. It is composed from two power MOSFETs controlled by pulse signals S₁ and S₂. The same structure can be implemented with BJTs and IGBTs.

The motor mode of the dc machine is controlled by the upper transistor and the diode D₂. The motor speed can be regulated by changing the turn on transistor switching time.

If the lower transistor and the diode D₁ are operated, the dc machine works in regenerative braking mode. Then S₂ switches at regular intervals and this way the stopping process is controlled.

The full bridge configuration of the single-phase voltage source inverter is shown.

Alternatively fired IGBT switches S₁ and S₄ determine the voltage U_A (at point A). Switches S₂ and S₃ determine U_B (at point B). The firings of the right leg (S₂, S₃) is shifted by an angle φ° with respect to the left leg (S₁, S₄) of the inverter. This produces resulting load voltage U_AB with pulse width of φ°.

When an inductive load is connected, sinusoidal current appears through it. The amplitude of this current can be regulated by changing the angle φ°.

The figure shows the recommended operating ranges of power transistors depending on switched voltages and currents for different frequencies. Some additional factors as on-state losses, current density, reliability, prices, etc. must also be taken into account.

A BJT (including a Darlington connection) has the widest range of application utilizing switched power, but its frequency of operation is the lowest. It can be used in linear and switched mode voltage regulators, TV and monitors, motor control, etc. Typical MOSFET applications include switched mode power supplies, battery charging devices, power amplifiers. An IGBT is used in motor control, uninterruptible power supply (UPS), and low frequency lighting.