Power Electronics Trivia Questions And Answers

A MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) is used as the main switching element in a switched mode power supply operating in the 20kHz to 100kHz range. This is because MOSFETs have fast switching speeds, low on-resistance, and high efficiency, making them suitable for high-frequency applications. Thyristors, TRIACs, and UJTs are not typically used as the main switching element in this frequency range.

The most suitable device for high frequency inversion in SMPS is the MOSFET. MOSFETs have a fast switching speed, low on-resistance, and high breakdown voltage, making them ideal for high frequency applications. They also have a low gate drive power requirement and high efficiency, which is crucial in SMPS designs. Additionally, MOSFETs have a high input impedance, allowing for easy integration with control circuits. Overall, the characteristics of MOSFETs make them the best choice for high frequency inversion in SMPS.

The turn-on and turn-off times of a transistor depend on the junction capacitance. Junction capacitance is the capacitance between the different regions of the transistor, such as the base-emitter and base-collector junctions. During turn-on and turn-off, charges need to be stored or released in these junctions, and the junction capacitance affects the speed at which this can happen. Therefore, the junction capacitance plays a crucial role in determining the turn-on and turn-off times of a transistor.

A MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) should be used as a switch in a low power switched mode power supply (SMPS). This is because MOSFETs have a high switching speed, low on-resistance, and can handle high currents. They are commonly used in SMPS circuits due to their efficiency and reliability. GTO (Gate Turn-Off Thyristor), TRIAC (Triode for Alternating Current), and THYRISTOR are not suitable for this application as they have different characteristics and are better suited for other types of circuits.

When an UJT (Unijunction Transistor) is used for triggering an SCR (Silicon Controlled Rectifier), the waveshape of the voltage obtained from the UJT circuit is a saw-tooth wave. This is because a UJT generates a saw-tooth waveform at its emitter terminal, which is then used to trigger the SCR. The saw-tooth wave has a gradual rise and a rapid fall, making it suitable for triggering the SCR at the desired point in the waveform.

In d.c. choppers, the duty cycle represents the ratio of the ON time to the total time period. The ripple in the output voltage of a d.c. chopper is directly proportional to the duty cycle. So, when the duty cycle is 0.5 (or 50%), the ripple will be maximum. This is because at this duty cycle, the ON time and OFF time are equal, resulting in a larger fluctuation in the output voltage.

In a step-down cyclo converter, the commutation required is natural. This means that the commutation process occurs automatically without the need for any external control or intervention. The cyclo converter operates by allowing the current to naturally reverse direction when the input voltage crosses zero. This natural commutation process allows for efficient and smooth conversion of the input voltage to a lower output voltage.

Triacs cannot be used in an a.c voltage regulator for an inductive load because inductive loads have a reactive component that causes the current to lag behind the voltage. Triacs are not suitable for controlling inductive loads as they are designed to handle resistive loads. The reactive nature of inductive loads can cause voltage spikes and current surges, which can damage the triac and lead to improper regulation of the voltage. Therefore, alternative devices such as thyristors or solid-state relays are typically used for controlling inductive loads in a.c voltage regulators.

A cyclo converter is a type of AC-AC converter that converts AC power at one frequency to AC power at another frequency. It is considered to have 1 stage because it uses a single set of power electronic switches to perform the conversion. This is in contrast to converters with multiple stages, which use multiple sets of switches to achieve the desired conversion. Therefore, the correct answer is 1 stage.

A 4-quadrant chopper is a device that can control the flow of current in both directions, allowing for bidirectional power flow. It can be operated as a 1-quadrant chopper, inverter, or bi-directional rectifier, as it has the capability to control the current flow in these configurations. However, it cannot be operated as a cyclo converter, which is a device used to convert AC power to DC power or vice versa. The operation and control mechanisms of a 4-quadrant chopper are different from those of a cyclo converter, making it unable to function as one.

IGBT (Insulated Gate Bipolar Transistor) is a semiconductor device that combines the advantages of both MOSFET and BJT. It is commonly used in medium power applications because it can handle higher voltages and currents compared to MOSFETs, but is more efficient and has faster switching speeds compared to BJTs. This makes IGBTs suitable for applications that require a moderate amount of power, such as motor drives, power supplies, and inverters. Therefore, the correct answer is medium power.

Power diodes are designed to handle high power levels, making them suitable for applications that require high voltage and current. However, these diodes have a slower switching speed compared to other types of diodes. This is because the diode needs to handle the high power levels, which results in a longer time for the diode to turn on and off. Therefore, power diodes have the characteristics of high power but low speed.

In a 1-phase full converter, the average thyristor current can be calculated by considering the conduction angle. Since the load current is ripple-free, it means that the thyristor conducts for the entire half-cycle. In a full converter, each thyristor conducts for half of the cycle. Therefore, the average thyristor current is equal to half of the load current, which is 1/2*I.

Inverters are used in both induction heating and UPS systems. In induction heating, inverters are used to convert the direct current (DC) power into alternating current (AC) power, which is required for the induction heating process. In UPS systems, inverters are used to convert the DC power from the battery into AC power, allowing the connected devices to continue operating during a power outage. Therefore, the correct answer is both.

A gate turn-off thyristor (GTO) is a power electronic device that has poor turn-off gain. This means that it is not as efficient at turning off the current flow as other devices. GTOs are commonly used in high-power applications where fast switching is required. They have a gate terminal that allows for control of the device's turn-on and turn-off characteristics. The poor turn-off gain of a GTO can result in higher switching losses and lower efficiency compared to other power electronic devices.

In a 3-phase semi-converter, the free wheeling diode conducts when the firing angle is less than or equal to 60 degrees. This means that the diode allows the current to flow in the opposite direction when the thyristor is not conducting. Therefore, the free wheeling diode conducts for 0 degrees, as it allows the current to flow continuously without any interruption.

A 3-phase semi converter is a converter that can operate in both 3-phase and 6-pulse modes. It is called a semi converter because it can only convert the positive half cycles of the input waveform. In 3-phase mode, it can convert all three phases simultaneously, while in 6-pulse mode, it converts the positive half cycles of each phase one at a time. This flexibility allows the converter to be used in various applications where both 3-phase and 6-pulse operations are required.

The commutation overlap in the phase controlled ac to dc converter is due to the source inductance. The source inductance helps in providing a smooth transition of current from one phase to another during the commutation process. It helps in reducing the rate of change of current and prevents any abrupt changes or spikes in the converter. This is important for the proper functioning of the converter and to ensure a stable and efficient conversion of AC to DC power.

An SCR (Silicon Controlled Rectifier) is a semiconductor device that is used for controlling the flow of electric current. High current, high temperature rise, and high rate of rise of voltage can all cause permanent damage to an SCR. However, a high rate of rise of current does not cause permanent damage to an SCR. This is because SCRs are designed to handle high current levels, and a sudden increase in current does not necessarily harm the device.

Modern ac-dc converters employ self commutated devices like GTOs or IGBTs instead of thyristors in order to achieve low reactive power loading on the source. This is because self commutated devices have the ability to control the switching frequency and timing, allowing for a more efficient and precise conversion of power. By reducing the reactive power loading on the source, the converter can operate more efficiently and effectively, resulting in improved overall performance.