Power MOSFET MCQ Quiz in తెలుగు - Objective Question with Answer for Power MOSFET - ముఫ్త్ [PDF] డౌన్‌లోడ్ కరెన్

Output i-v characteristics of power MOSFET:

Transconductance is the change in the drain current divided by the small change in the gate/source voltage with a constant drain/source voltage.

\({g_m} = \frac{{{\rm{\Delta }}{I_D}}}{{{\rm{\Delta }}{V_{GS}}}}\)

It is an important factor in the steady-state characteristics.

The correct answer is Both I and II.

Key Points

• Power MOSFET - It is a voltage controlled device and requires only a small input current.
  • **MOSFET** stands for **Metal-Oxide-Semiconductor Field-Effect Transistor**.
  • It is a **voltage-controlled device**, meaning its operation is governed by the voltage applied to the gate terminal.
  • **Input current** required is minimal, making it highly efficient for switching applications.
  • **Hence, statement I is correct**.
• IGBT - There is no second breakdown problem as with BJTs.
  • **IGBT** stands for **Insulated Gate Bipolar Transistor**.
  • **Second breakdown** is a failure mode in BJTs (Bipolar Junction Transistors) that occurs due to thermal runaway.
  • IGBTs do not suffer from this issue, making them more robust and reliable in high-power applications.
  • **Hence, statement II is correct**.

 Additional Information

• Power MOSFETs
  • Commonly used in **power electronics** for their high efficiency and fast switching.
  • Applications include **switching power supplies**, **motor controllers**, and **DC-DC converters**.
  • They have a **high input impedance** and are capable of handling high currents.
• IGBTs
  • Combine the **high input impedance** of MOSFETs with the **low on-state voltage drop** of BJTs.
  • Widely used in **inverters**, **electric vehicle controllers**, and **high-voltage applications**.
  • They provide **better efficiency** and **thermal stability** compared to traditional BJTs.
• Second Breakdown
  • A phenomenon in BJTs where excessive current causes localized heating, leading to device failure.
  • **Thermal runaway** is the main cause, which is mitigated in IGBTs by their design.

MOSFETs are majority carrier devices that mean flow of current inside the device is carried out either flow of electrons (N-Channel MOSFET) or flow of holes (P-Channel MOSFET). So, when the device turns off, the reverse recombination process will not happen. It leads to short turn ON/OFF times. As switching time is less, loss associated with it less and hence it gives the highest switching speed.

Therefore, both Statement (I) and Statement (II) are individually true and Statement (II) is the correct explanation of Statement (I).

Important Points:

The turn-off times of different power electronic devices are given below.

• MOSFET has the lowest switching off time in the order of nanoseconds.
• BJT has the turn-off time in the order of nanoseconds to microseconds.
• IGBT has the turn-off time in the order of microseconds (about 1 μs).
• Thyristor has the turn-off time in the order of microseconds (about 5 μs).

 

Therefore, the increasing order of turn-off times is:

MOSFET > BJT > IGBT > Thyristor

MOSFET:

• MOSFET is a voltage-driven/controlled device.
• The current through the two terminals is controlled by a voltage at the third terminal (gate)
• It is a unipolar device (current conduction is only due to one type of majority carrier either electron or hole)
• It has a high input impedance.

BJT:

• BJT's are current-driven devices.
• The current through the two terminals is controlled by a current at the third terminal (base).
• It is a bipolar device (current conduction by both types of carriers, i.e. majority and minority electrons and holes)
• It has a low input impedance.

Explanation:

Three-Phase Full Bridge Voltage Source Inverter

Definition: A three-phase full bridge voltage source inverter (VSI) is an electronic device used to convert DC power into three-phase AC power. It is extensively used in industrial applications, renewable energy systems, and motor drives. The inverter employs power semiconductor switches, typically MOSFETs or IGBTs, to generate a three-phase AC output with adjustable frequency and amplitude.

Working Principle: The operation of a three-phase full bridge VSI is based on switching power semiconductor devices in a specific sequence to produce an AC output. The inverter consists of six switches (MOSFETs in this case), arranged in three pairs. Each pair corresponds to one phase of the three-phase system, and the switches work in complementary pairs to ensure that each phase receives the appropriate voltage waveform. The DC supply is connected to the inverter, and the switching pattern determines the AC output waveform.

Key Components:

• DC Input: A DC voltage source is required to provide input power to the inverter.
• MOSFET Switches: Six MOSFET switches are used to control the flow of current in the inverter circuit. These switches are arranged in three pairs (one pair per phase: A, B, and C).
• Freewheeling Diodes: Each MOSFET is typically accompanied by a freewheeling diode to handle the inductive load and prevent voltage spikes during switching.
• Control Circuit: A control circuit or microcontroller generates the necessary gate signals to drive the MOSFET switches in the required sequence.

Analysis of the Correct Option:

The correct answer is:

Option 3: 6 MOSFETs

In a three-phase full bridge voltage source inverter, six MOSFETs are required. These switches are arranged in three legs (or arms), with each leg containing two MOSFETs. Each leg corresponds to one phase of the output (Phase A, Phase B, and Phase C). The switches in each leg operate in a complementary fashion, meaning when the upper switch is ON, the lower switch is OFF, and vice versa. This complementary operation ensures that the load receives the appropriate voltage and current waveforms for three-phase AC power generation.

The six MOSFETs are labeled as follows:

• Upper switches: S1, S3, S5
• Lower switches: S2, S4, S6

The switching sequence is carefully controlled to produce a balanced three-phase AC output. The control signals are typically generated using pulse width modulation (PWM) techniques, enabling precise control of the output voltage and frequency.

Advantages of Using 6 MOSFETs:

• Allows generation of a balanced three-phase AC output.
• Enables efficient conversion of DC power to AC power.
• Facilitates precise control of output voltage and frequency.
• Ensures smooth operation of three-phase loads, such as motors or industrial equipment.

Applications:

• Industrial motor drives
• Renewable energy systems (e.g., solar inverters)
• Uninterruptible power supplies (UPS)
• Electric vehicle powertrain systems

Important Information

To further understand the analysis, let’s evaluate the other options:

Option 1 (5 MOSFETs):

This option is incorrect because a three-phase full bridge voltage source inverter requires six switches, not five. Using only five switches would leave one phase incomplete, resulting in an unbalanced or incomplete AC output, which is not suitable for three-phase systems.

Option 2 (4 MOSFETs):

This option is incorrect as well. A configuration with four MOSFETs is typically used in single-phase full bridge inverters, not three-phase inverters. A three-phase system requires six MOSFETs to handle the three phases of the AC output.

Option 4 (3 MOSFETs):

Using only three MOSFETs is not feasible for a three-phase full bridge inverter. Each phase of the output requires a pair of switches (upper and lower), and three MOSFETs would only be sufficient for one and a half phases, leading to an incomplete and non-functional inverter design.

Option 5 (Other configurations):

Any configuration with fewer than six switches cannot generate a proper three-phase AC output in a full bridge inverter topology. Six switches are the minimum requirement for a three-phase full bridge VSI.

Conclusion:

The three-phase full bridge voltage source inverter requires six MOSFET switches to function correctly. These switches are arranged in three pairs, with each pair corresponding to one phase of the AC output. The complementary operation of the switches ensures the generation of balanced three-phase AC power, which is essential for powering industrial equipment, motors, and other applications. Understanding the configuration and operation of the inverter is crucial for designing and implementing efficient power conversion systems.

MOS (Metal-Oxide-Semiconductor) power transistors offer significant advantages over bipolar power transistors in pulse power supplies due to their very high input resistance and extremely low input currents (in the order of nanoamperes). This high input resistance ensures minimal power loss and higher efficiency, making MOS power transistors more suitable for applications where power efficiency and fast switching capabilities are critical. The input current required for MOS transistors is extremely low (in the order of nanoamperes (nA), as there is negligible current flow into the gate terminal.

MOSFET is not a current triggered device.

Explanation:

 

The gate to source voltage (V_GS) is used to trigger the MOSFET, hence it is voltage controlled device.

In SCR, gate current makes the SCR to conduct.

In BJT, base current is responsible for conduction whereas in IGBT, the gate current is responsible for conduction.

Concept:

The drain current in MOSFET in saturation is given by:

\({I_D} = \frac{{{\mu _n}{C_{ox}}W}}{{2L}}\left( {{{\left( {{V_{GS}} - {V_T}} \right)}^2}} \right)\)

Calculation:

\(v_i - V_{GS_p} - 5 = 0\)

\(V_{GS_p} = v_i - 5\)

and \(v_i - V_{GS_n} = 0\)

\(V_{GS_n} = v_i\)

\( I_{DS_p}=-\frac{1}{2}k_p\left(\frac{W}{L}\right)_p(V_{GS_p - V_{T_p}})^2\)

\( I_{DS_p}=-\frac{1}{2}k_p\left(\frac{W}{L}\right)_p(v_i - 5 + 1)^2\)

\(I_{DS_p} = -\frac{1}{2}k_p \left(\frac{W}{L}\right)_p (v_i - 4)^2\)

\(I_{DS_n} = -\frac{1}{2}k_p \left(\frac{W}{L}\right)_p (V_{GS_n} - V_{T_n})^2\)

\(I_{DS_n} = \frac{1}{2}k_n \left(\frac{W}{L}\right)_n (v_i - 1)^2\)

\(I_{DS_n} = -I_{DS_p}\)

\(\frac{I_{DS_p}}{I_{DS_n}}=-1\)

\(\frac{-k_p\left(\frac{W}{L}\right)_p (v_i - 4)^2}{k'_n\left(\frac{W}{L}\right)(v_i - 1)^2}= -1\)

\(-\frac{1}{4}\times 16 \times \frac{(v_i - 4)^2}{(v_i - 1)^2}=-1\)

\(\frac{(v_i - 4)^2}{(v_i - 1)^2}= \frac{1}{4}\)

\(\frac{v_i - 4}{v_i - 1}= ±\frac{1}{2}\)

⇒ v_i = 7V, 3V

From the transfer characteristic of CMOS, we can see that v_i = 3V.

• Power MOSFET has lower switching losses but its on-resistance and conduction losses are more. A BJT has higher switching losses but lower conduction losses.
• MOSFET is a voltage-controlled device whereas BJT is a current controlled device.
• MOSFET has positive temperature coefficient for resistance. This makes parallel operation of MOSFETs easy. If a MOSFET shares increased current initially, it heats up faster, its resistance rises, and this increased resistance causes this current to shift to other devices in parallel.
• A BJT has negative temperature coefficient, so current-sharing resistors are necessary during parallel operation of BJTs.
• In MOSFET, secondary breakdown does not occur, because it has positive temperature coefficient.
• As BJT has negative temperature coefficient, secondary breakdown occurs. With decrease in resistance, the current increases. This increased current over the same area results in hot spots and breakdown of the BJT.

R = 0.2 and I = 20 A

So average power loss

\(\begin{array}{l} = \frac{1}{{\left( {\frac{{2\pi }}{\omega }} \right)}}\mathop \smallint \limits_0^{\frac{\pi }{\omega }} {I^2}Rdt\\ = \frac{\omega }{{2\pi }} \times {10^2} \times 0.2 \times \frac{\pi }{\omega } = 10\;W\; \end{array}\)