Power Transistor – Structure, Biasing, Characteristics, Advantages

A transistor is a three-terminal semiconductor device. These devices act as switching devices and are operated at the saturation area, producing a low on-state voltage drop. When a current signal flows to the base or control terminal, they switch on. As long as the control signal is there, the transistor stays on.

Power Transistor

The functioning of a power transistor is identical to that of a regular transistor. The transistor is constructed to handle high voltage and current due to the advantages of traditional power transistors.

The switching speed of transistors is substantially higher than that of thyristors. Low- to medium-power applications employ transistors. Since transistors are current-controlled devices, a constant base current is necessary to maintain their conducting condition.

Types of Transistors

A power transistor is a three-terminal device that is specially made to regulate high current-voltage ratings and handle a wide range of power levels in a device or circuit.

They are divided into two groups based on their structural characteristics: BJT- bipolar junction transistors (transistors like junction transistors, NPN transistors, and PNP transistors) and FET- field-effect transistors (transistors like junction function transistors and metal oxide transistors, N- channel MOSFET, and P- channel MOSFET).

The other types of associated types of transistors are Static induction transistors (SITs), Insulated-gate bipolar transistors (IGBTs). The functionality of transistors is small-signal transistors, Amplifiers, switching transistors, power transistors, high-frequency transistors, phototransistors, etc.

A three-layer, three-terminal device is a bipolar junction transistor. The transistor has three terminals emitter, base, and collector are the three terminals. Collector-base junction (CB) and emitter-base junction (EB) are its two junctions. NPN and PNP transistors are the two kinds of transistors.

Fig 1 Symbol of Transistor

Power Transistor Structure

The conventional transistor is three terminals, three-layer device. But Power transistor is Four-layer three terminal device. The two-transistor is different in structures. A power transistor is a four-layer device with alternating p- and n-type layers that is vertically orientated. Because it maximizes the cross-sectional area through which the device’s current flows, the vertical construction is recommended.

Additionally, this reduces the on-state resistance and hence the transistor’s power dissipation. Emitter layer and collector layer doping generally amounts to 10¹⁹ cm^-3. It has a light doping level of 10¹⁴ in a specific layer known as the collector drift zone (n^–).

Fig 2 Layer Diagram of NPN Power Transistor

The transistor’s breakdown voltage is dependent on the thickness of the drift area. To have strong amplification capabilities, the base thickness must be as thin as possible, although doing so compromises the transistor’s capacity to withstand a breakdown voltage. Many emitter’s arrangement it lowers the base current path’s parasitic ohmic resistance, which lowers the transistor’s power dissipation.

Biasing

All types of semiconductor devices depend heavily on biassing. This determines the device’s functionality. Most power electronics components function as switches. The same applies to the power transistors also. Using two DC sources, V_BB and V_CC, biassing in transistor circuits is accomplished.

The most popular techniques for biasing of transistors are the Base Resistor approach, Collector to Base bias, Collector feedback resistor bias, and Voltage-Divider bias.

The fundamental idea behind each of these approaches is to use V_CC to derive the needed values of I_B and I_C in situations of zero signal.

For power transistor works in Common Emitter Configuration. This type of biasing is recommended for higher voltage and current ratings. Since the power transistor is act as a switch in high-voltage and current applications.

Power BJT as Switch

To turn on or off current to a load in logic circuits, a BJT can be utilized as a switching component. The transistor typically operates as a switch in either cutoff (load is OFF) or saturation (load is ON).

Fig 3 Biasing Circuit of Power Transistor

Input side Equation

Apply KVL

V_BB = I_B.R_B+V_BE

I_B  = (V_BB – V_BE)/R_B

Output side Equation

Apply KVL

V_CC = I_C.R_C+V_CE

V_CE   =  V_CC – I_C.R_C

We know that I_β = IC/IB

I_C   = I_B/ I_β

BUT,  I_B  =  (V_BB – V_BE)/R_B            ,

Therefore, V_CE

V_CE   = (V_CC – β) . [ {(V_CC – V_BE).R_C }/R_B ]

We know that, VCE =  V_CB+V_BE

(OR)

V_CB   =  V_CE-V_BE

As long V_CE>V_BE the collector-base junction is reverse biased and transistor works in active region.

When V_CB  = 0 and  V_BE =V_CE the collector current is maximum in active region

I_CMAX   =  (V_CC – V_CE)/R_C

If I_B > I_BMAX , Then , V_BE and I_C increase and V_CE falls below V_BE. This continues until the BJT enters the saturation area and the Collector-Base junction becomes forward biased.

ODF is defined  as the ratio between I_B to I_BS

I_CM   = I_B / I_BS

The ratio of I_C to I_CS is called forced β

β_forced  =   I_CS / I_R

The total power loss in the two function is

P_T   = V_BE. I_B + V_CE I_C

Various Notations Used in Power Transistor

S. No	Notation	Expansion
1	VBB	Base Voltage
2	VBE	Emitter to Base Voltage
3	VCE	Emitter to Collector Voltage
4	VCC	Collector Voltage
5	IB	Base Current
6	IC	Collector Current
7	ICMAX	Maximum Collector Current
8	IBMAX	Maximum Base Current
9	ICS	Saturated Collector Current
10	IBS	Saturated Base Current
11	β	Collector to Base Gain
12	PT	Total Power Loss

A high value of ODF cannot considerably lower the C_E voltage. Power loss increases as V_BE rises as a result of the rising base current. The C_E voltage does not decrease in reaction to an increase in base current after the transistor is saturated.

When the power is raised at a high ODF value, the transistor might be harmed by thermal runaway. When a transistor is underdriven from I_B to I_BS, it may function in the active area, which causes V_CE to rise and more power loss.

VI Characteristics

The main application of any transistor is Amplifier and Switch. A typical transistor operated in three regions. Cut-off, Active, and Saturation region.

1. In the Active region, there will be different base current. So, the collector current increase proportionally with gain I_c = β.I_b.
2. In the Cutoff region, the base current is Zero. Therefore, the device behaves as open(No current Flow). It is considered as open Switch
3. In the Saturation region, the Base Current is Maximum. The device behaves as a single conductor(i.e., closed switch)

Input Characteristics

The input characteristics are drawn I_E Versus V_BE by keeping V_CE Constant. It operates like a diode at the base-emitter junction. The input characteristics resemble the character curve of PN Junction Forward bias.

For the different values of V_CE, the breakdown curve of the BJT varies proportionately.

Fig 4 Input Characteristics

Output Characteristics

The output characteristics are drawn V_CE versus I_C by keeping the different base currents. The base current increased from zero to maximum. The Curve has three regions. Cut-off, Active, and Saturation region.

Cut-Off Region

The collector current (I_C) is negligible and the transistor is forced into the cutoff zone when the base current (I_B) is zero. In this condition, the transistor is off. The transistor operates as an open switch when it is in the cutoff region, which is also known as the OFF state.

The collector-base and base-emitter junctions are reverse-biased at this time. In this area, the collector-emitter voltage (V_CE) is equal to the supply voltage (V_CC), and the circuit current is zero.

Active Region

In the active region, different base current is given which is above the threshold i.e base-emitter junction is forward-biased while the collector-base junction is reverse-biased. In this region it the transistor behaves as the amplifier.

Saturation Region

The transistor reaches saturation when the base current is maximum. The transistor behaves like a closed switch when it is saturated because both junctions are forward-biased. Base drive is applied and the transistor is said to be on in quasi- and hard-saturation states. During this condition I_C = V_CC /R_C and V_CE = 0.

Fig 5 Output Characteristics

Switching Characteristics

A depletion layer capacitance (C_CB) and a diffusion capacitance (C_BE) may be seen in parallel at a forward-biased p-n junction. Only depletion capacitance exists in a reverse-biased p-n junction.

Fig 6 Equivalent Circuit of Power Transistor

The presence of internal capacitance prevents a transistor from turning on instantaneously. Base current increases to I_BS when base-emitter voltage V_BE is provided, whereas collector current is equal to collector-emitter leakage current I_CEO or stays at zero.

The collector current starts to increase after a delay, delay td or delay time. This delay occurs because it takes time to charge the base-emitter capacitance to V_BES=0.7 V.

Collector current increases to its steady state value, I_CS, in time tr after delay t_d, which is referred to as the rise time. This indicates that the BJT’s turn-on time is t_on=t_d+t_r. The rise time is influenced by the input capacitances.

The collector-emitter voltage t_r decreases from V_CC to V_CES throughout the rising period. In order to sustain forward base current, the transistor remains turned on in a saturated condition.

Fig 7 Switching  Characteristics of Power Transistor

The collector current does not charge for a period ts, often known as the storage time, after the base-emitter voltage V_BE is eliminated at time t1. The base’s saturating charge is eliminated during ts. Collector current starts to decrease time after ts, I_C drops to I_CEO, and collector-emitter voltage increases to V_CC.

The transistor turn-off time, t_off=t_s+t_f, is the sum of the storage time and fall time. The time t_o is the entire OFF duration, and t_n is the steady state duration in the given figure.

Safe Operating Area

The collector current and voltage must always be in this range for the transistor to operate reliably. Actually, the manufacturers FBSOA and RBSOA specify two different forms of SOA.

Fig 8 Forward Biased Safe Operating Area (FBSOA)

When the base-emitter junction is forward-biased to turn on the transistor, the transistor operates according to the FBSOA. It is a logarithmic scale.

For VCE less than or equal to 80%, boundary AB is the upper limit for dc and continuous current. Collector current must be lowered to boundary V_CE, for V_CE greater than 80%. Current should be further decreased in order to prevent secondary breakdown limit and restrict the junction temperature to safe values for further larger V_CE.

This secondary breakdown limit is determined by boundary DC. The maximum voltage capacity for this specific transistor is provided by boundary DE. It displays the collector voltage and current combinations that must not be exceeded in order to prevent a second breakdown. This limit also shows as a straight limit on the FBSOA’s log-log scale.

The second breakdown limit differs for “DC” and “Pulsed” operations with various pulse lengths, similar to the maximum power dissipation limit. It is also the same to understand the pulse length as it relates to a certain restriction.

Fig 9 Reverse Biased Safe Operating Area (RBSOA)

A transistor is subjected to increased current and voltage during turn-off, with the base-emitter junction being reverse-biased. Reverse blocking safe operating area, or RBSOA is designated as the safe working region for transistors during shut off. When the base current is zero or the base-emitter junction is reverse-biased during transistor turn-off, this condition is known as RBSO. RBSOA shrinks in size when reverse bias increases.

The RBSOA is more rectangular in form and plotted on a linear scale. Since a transistor cannot conduct current for any meaningful amount of time when it is reverse-biased, RBSOA is a switching SOA. It displays the maximum permitted V_CE & I_C configurations when the base-emitter junction is reverse-biased. The highest horizontal limit is identical to that in the FBSOA and corresponds to the maximum permitted collector current (ICM).

Advantages of Power Transistor

• The switching frequency range for the power transistor is between 10 and 15 kHz.
• The power transistor may block extremely high voltage while it is off and transport massive currents when it is on.
• Low voltage drops across the power transistor in the ON state.
• The power transistor is extremely simple to ON and OFF.
• It may be used to regulate the amount of power sent to the load, inverters, and helicopters.

Disadvantages of Power Transistor

• The power transistor has a very low reverse-blocking capability.
• Above a switching frequency of 15 kHz, the power transistor cannot function effectively.
• It could sustain thermal runway damage or a subsequent collapse.

Applications of Power Transistor

• Switch mode power supply
• Relay drivers
• DC motor speed control
• Lighting systems
• PWM applications

Reference

1. “Power Electronics: Circuits, Devices and Applications“, M H Rashid,  Pearson     Education
2. “ Power Electronics”  P. S. Bimbhra, Khanna Publishers, 2012

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