Power Electronics - Power Semiconductor Devices Complete Study Notes

Power Semiconductor Diode

Power diodes belong to the class of uncontrolled power semiconductor devices. They are like p-n junction diodes but having large voltage and current rating. It has 3-layer diode which makes it suitable for high power application as they are constructed with n^- layer between p⁺ and n⁺ layers to support large blocking voltage by controlling the width of depletion region.

Constructional Structure and symbol of Power diode

Transfer Characteristics of a Power Diode:

When anode is positive with respect to cathode, diode is forward biased. When forward voltage across diode is slowly increased from 0 to cut-in voltage, diode current is almost zero. Above cut-in voltage, the diode current rises rapidly and the diode is said to conduct. When anode is negative with respect to cathode, diode is reverse biased.

Reverse recovery characteristics of Diode:

Reverse recovery time (t_rr): [T_OFF ≫ T_ON]

When a diode is changed from forward biased state to reverse biased state, the diode continues to conduct in the reverse direction because of stored charges in two layers. The reverse current flows for reverse recovery time, t_rr. The reverse recovery time is defined as the time between the instant forward current becomes zero and the instant reverse recovery current decays to 25% of reverse peak value, I_RM.

Reverse recovery time, t_rr = t_a + t_b

Classification of Power Diodes:

Power diodes can be classified as below based on their use case. This classification is different from that of classification based on softness factor.

• General Purpose Diodes: These diodes have high reverse recovery time, t_rr. Application of this type of diodes includes battery charging electric traction and uninterruptible power supplies (UPS).
• Fast Recovery Diodes: These diodes have low t_rr. To get low t_rr, platinum or gold doping is done while manufacturing these diodes. Hence these diodes have more forward voltage drop. These diodes are mainly used in choppers, commutation circuits and switched mode power supplies (SMPS).
• Schottky Diodes: These diodes use metal to semi-conductor junction. Hence these diodes have lesser t_rr and lesser forward voltage drop. In these diodes, current flow is by majority carriers only and hence there is no turn off delay due to absence of minority carries combination.

Power bipolar junction transistor: (Power BJT)

Power transistor is a current controlled device and the control current is made to flow through base terminal. Thus, the device can be switched ON or OFF by applying a positive/negative signal at base. The transistor remains in on-state if control signal is present. The need for a large blocking voltage in the off state and a high current carrying capability in the on state means that a power Bipolar junction Transistor (BJT) must have a substantially different structure than its logic level counterpart.

Transistor Switching Characteristics:

Let I_CS be the collector current during saturation, I_CEO be the collector leakage current when transistor is off and all the remaining quantities as shown in figure. Different quantities related to switching characteristics of a power transistor are given below. (Here I_CS is the saturation collector current,)

• Delay time t_d is the time taken for the collector saturation current to start rising
• Rise time t_r is the time taken for the collector current to reach I_CS
• Storage time t_s is the time taken for charges to be removed from depletion region
• Fall time t_f is the time taken for collector current to fall to 0, On time, t_ON = t_d + t_r
• OFF time, t_OFF = t_S + t_f

Power MOSFET

MOSFET is a voltage-controlled device. It has three terminals, called drain, source and gate. As its operation is based on flow of majority carriers only, MOSFET is unipolar device. A metal oxide semiconductor field effect transistor (MOSFET) has three terminals called drain (D), source (S) and gate (G).

As power MOSFET is unipolar device, there is no minority storage effect so that high switching speed is possible. Here switching speed is limited by inherent capacitance only. Also due to large drain area, secondary breakdown and thermal runaway that destroy the device do not occur.

The basic structure and circuit symbol of power MOSFET is,

• Generally, MOSFET are low voltage and high current devices.
• These are very popular in dc to dc conversion (choppers).
• These are very fast devices compared to BJT.
• BJT is a minority carrier device where MOSFET is a majority carrier device.
• MOSFET has a very high input impedance.
• Gate is insulated from the rest of the device.
• No steady current flows through the gate. (Only displacement current like in parallel plate capacitor will flow.)
• MOSFET is in cut-off region when gate to source voltage (V_GS) is less than threshold value.
• When V_GS> Threshold (V_Th). It converts silicon surface below the gate into an N-type channel.
• The threshold value depends upon oxide layer and it can be reduced by reducing the thickness of SiO₂
• A BJT is a current controlled device whereas a power MOSFET is a voltage-controlled device.
• The control signal, or base current in BJT is much larger than the control signal (or gate current) required in a MOSFET. This is because gate circuit impedance in MOSFET gate to be driven directly from microelectronic circuits.

Silicon controlled rectifier (SCR) OR Thyristor

It is a solid-state device like transistor. It is a four-layer three junction p-n-p-n device and has three terminals; anode, cathode and gate. SCR can be turned on by using a gate signal controlling the charge near the p-n junction. Hence SCR is a charge-controlled device. However, SCR can’t be turned off by using gate signal. Thus,

SCR belongs to the class of semi-controlled semi-conductor device.

SCR is made up of silicon, it acts as a rectifier. It has very low resistance in the forward direction and high resistance in the reverse direction. It is unidirectional device.

The schematic diagram and circuit symbol of thyristor are shown in figure below:

 Operating Modes of Thyristor:

Depending on polarity of Anode (A) and Cathode (K) voltage and gate signal, SCR can be operated in following modes:

  Reverse Blocking Mode:

In this mode, terminal K is positive with respect to terminal A and also the gate terminal is open. Hence the junctions, J₁ and J₃ are reverse biased and J₂ is forward biased. If reverse voltage is increased above V_BR, avalanche breakdown occurs at J₁ and J₃ and reverse current increases rapidly. A large current associated with V_BR gives rise to more losses in SCR. This may lead to thyristor damage as the junction temperature may exceed permissible limit. Therefore, it should be ensured that maximum reverse voltage doesn’t exceed V_BR.

When cathode of the thyristor is made positive with respect to anode with switch open thyristor is reverse biased. Junctions 𝐽₁ and 𝐽₂ are reverse biased where junction 𝐽₂ is forward biased. The device behaves as if two diodes are connected in series with reverse voltage applied across them.

A small leakage current of the order of few mA only flows. As the thyristor is reverse biased and in blocking mode. It is called as acting in reverse blocking mode of operation.

Now if the reverse voltage is increased, at a critical breakdown level called reverse breakdown voltage 𝑉_𝐵𝑅,an avalanche occurs at 𝐽₁ and 𝐽₃ and the reverse current increases rapidly. As a large current associated with 𝑉_𝐵𝑅 and hence more losses to the SCR. This results in Thyristor damage as junction temperature may exceed its maximum temperature rise.

 Forward Blocking Mode:

In this mode, terminal A is positive with respect to terminal K and gate terminal is open. Hence junction J₁ and J₃ are forward biased and J₂ and SCR starts conducting. But this method of triggering the SCR is not preferred as it may damage the device. Hence in forward blocking state, thyristor can be treated as an open switch. When anode is positive with respect to cathode, with gate circuit open, thyristor is said to be forward biased.

Thus, junction 𝐽₁ and 𝐽₃ are forward biased and 𝐽₂ is reverse biased. As  the  forward voltage is increases junction 𝐽₂ will have an avalanche breakdown at a voltage called forward breakover voltage 𝑉_𝐵𝑂. When forward voltage is less then 𝑉_𝐵𝑂 thyristor offers high impedance. Thus, a thyristor acts as an open switch in forward blocking mode.

 Forward Conduction Mode:

A thyristor is brought from forward blocking mode to forward conduction mode by increasing V_AK above V_BO or by applying a gate pulse between gate and cathode. In this mode, thyristor is in on-state and behaves like a closed-switch.

Here thyristor conducts current from anode to cathode with a very small voltage drop across it. So, a thyristor can be brought from forward blocking mode to forward conducting mode:

• By exceeding the forward break over voltage.
• By applying a gate pulse between gate and cathode.

During forward conduction mode of operation thyristor is in on state and behave like a close switch. Voltage drop is of the order of 1 to 2mV. This small voltage drop is due to ohmic drop across the four layers of the device.

Static V-I Chacteristics of SCR

An elementary circuit diagram for obtaining static V-I characteristics of SCR is shown in figure

• The Anode and Cathode are connected to main source through the load.
• The Gate and Cathode are fed from another source ‘E_g’
• The static V-I characteristics of SCR are shown below:

Where,

V_a=Anode voltage:

I_a=Anode current

V_BO = Forward break over voltage

V_BR= Reverse breakdown voltage

Significance of latching current:

Latching current is related to turn-on process. Gate signal initiates the turn-on process but once the SCR is in on-state, gate losses control on the device. Therefore, we generally remove the gate signal when SCR becomes on to avoid the continuous Gate power loss.

If gate signal is removed when anode current is lesser than the latching value, then SCR fails to turn-on. Therefore, we must maintain gate pulse-width at least for a period until the anode current reaches just above the minimum value which is latching current.

• Therefore, latching current is specified to estimate the minimum gate pulse width requirements to turn -ON the SCR successfully.

Significance of holding current:

Holding current is related to turn -off process. Gate has no-control to turn-off the SCR. In some of the cases when the supply is DC we required commutation circuit to turn-off the SCR.

The conducting thyristor stops conducting only when the anode-current falls below the holding current.

Holding current is the minimum anode current below which the SCR stops conducting. The commutation circuit force the anode current to reduce below the holding current and applies reverse voltage across the SCR at least for a period until the complete excess charge is removed in device.

Switching characteristics Of SCR

• SCR voltage and current waveforms during turn-on and turn–of process.
• Switching characteristics are also known as dynamic characteristics or transient characteristics.
• The time variations of the voltage across the SCR and the current through it during turn-on and turn-off processes give the dynamic or switching characteristics.

Switching Characteristics During Turn-on:

SCR turn on time, is defined as the time during which SCR changes from forward blocking mode to final on state.

Total turn on-time can be divided into three intervals:

1. Delay time (t_d)
2. Rise time (t_r)

iii. Spread time (t_p)

Delay Time (t_d):

The delay time (t_d) is the time between the instant at which gate current reaches 0.9 I_g to the instant at which anode current reaches 0.1 I_a. Here I_g and I_g are respectively the final values of gate and anode currents or the delay time (t_d) may also be defined as the time during which anode voltage falls from V_a to 0.9 V_a where V_a =initial value of anode voltage. The time during which anode current rises from forward leakage current to 0.1 I_a=final value of anode current.

Rise Time (t_r):

The time taken by the anode current to rise from 0.1I_a to 0.9I_a. The rise time is also defined as the time required for the forward blocking off state voltage to fall from 0.9 to 0.1 of its initial value OA. During rise time, turn-on losses in the thyristor are high due to high anode voltage (V_a) and large anode current (I_a) occurring together in the thyristor.

 Spread Time(t_p):

The time taken by the anode current to rise from 0.9I_a to I_a. It is also defined as the time for the forward blocking voltage to fall from 0.1 of its initial value to the on-state voltage drop.

Switching Characteristics During Turn-off:

SCR turn-off means that it has changed from on to off state and can block the forward voltage. The dynamic process of the SCR from conduction state to forward blocking state is called commutation process or turn-off process.

Note: If forward voltage is applied to the SCR now its anode current falls to zero, the device will not be able to block this forward voltage, as the carriers (holes and electrons) in the four layers are still favourable for conduction. The device will therefore go into conduction immediately even though gate signal is not applied. So to solve this problem it is essential that the thyristor is reverse biased for a finite period after the anode current has reached zero.

Turn-off time (t_q):

It is the time between the instant anode current becomes zero and the instant SCR regains forward blocking capability.

During this time (t_q) all the excess carriers from four layers of SCR must be removed.

The turn-off time is divided into two intervals:

1. Reverse recovery time (t_rr)
2. Gate recovery time (t_gr)

After t₁: anode current builds up in the reverse direction with the same di/dt slope. The reason for the reversal of anode current is due to the presence of charge carriers stored in the four layers.

At instant t₃ when reverse recovery current has fallen to nearly zero value, end junction J₁ and J₃ recover and SCR is able to block the reverse voltage.

At the end of reverse recovery period t₃: the middle junction J₂ still has charges, therefore, the thyristor is not able to block the forward voltage at t₃

The charge carriers at J₂ cannot flow to the external circuit, therefore they must decay only by recombination. This is possible if a reverse voltage is maintained across SCR. The time taken for this (t₄ - t₃) is called gate recovery time (t_gr).

The thyristor turn-off time t_q is depended upon magnitude of forward current, di/dt at the time of commutation and junction temperature.

Circuit Turn-off Time ‘t_c’:

It is defined as the time between the instant anode current becomes zero and the instant reverse voltage due to practical circuit reaches zero.

Note: t_c > t_q for reliable turn-off, otherwise the device may turn-on at an undesired instant, a process called commutation failure.

• Thyristors with slow turn-off time are called converter grade SCR’s.

EX: Phase controlled rectifiers, cyclo-converters and ac voltage controllers.

• SCR with fast turn-off time are called inverter grade SCR’s.

EX: Inverters, choppers and forced commutation converters.

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