# 16 Power Electronics Interview Questions (With Answers)

Indeed Editorial Team

Updated 18 March 2023

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## 8 Common Power Electronics Interview Questions With Sample Answers

A hiring manager may ask some power electronics interview questions to test your theoretical and practical knowledge of this field. Here are eight example questions and answers:

### 1. What is the difference between holding and latching current in SCR?

The question aims to test your basic knowledge of power electronics. Try to highlight the main differences between holding and latching current in a silicone-controlled rectifier (SCR). You can use examples from your work experience.

Example: 'The latching current relates to the SCR turn-on process and is the minimum current an anode requires to remain in the forward conduction state after removing the gate current. The holding current relates to the SCR turn-off process and is the minimum current value for an anode to reach so that it can attain an off status. For example, if the value of the latching current is 5mA, then the holding current can stop conduction if it falls below 5mA.'

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### 2. Explain the different ways to turn on an SCR.

Employers may ask this question to test your basic knowledge of power electronics. Start your answer by explaining what turn-on methods are. List different methods and explain their significance.

Example: 'SCR turn-on methods are techniques that bring an SCR into forward conduction mode from blocking mode. There are five main SCR turn-on methods. Choosing one depends on the temperature and voltage of the circuit. For example, forward voltage triggering involves a forward-biased SCR and the voltage significantly increases until SCR turns on. There is also gate triggering, the most efficient method to turn on the SCR. You apply the positive gate voltage between the gate and cathode terminals to turn on the SCR. Other methods include dv/dt triggering, temperature triggering and light triggering.'

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### 3. Can you compare CSI and VSI?

Interviewers may ask you this question to check whether you know how inductive and capacitive energy storage work. You can demonstrate your awareness of developing effective high-voltage direct current (HVDC) transmission lines, induction heating, plasma generators and different switching devices. Try giving examples of your practical experience using these methods during past projects. You can also mention the advantages and disadvantages of the current source inverter (CSI) and voltage source inverter (VSI) to show you can make correct decisions at work.

Example: 'CSI is more reliable since it effectively converts the direct current from the current reactor (SCR) into a one- or three-phase alternating current. CSI maintains a constant but customisable input current while its output current does not depend on the load. Its circuit is easy to develop, containing only capacitors and no feedback diodes. The load impedance affects the magnitude of the output voltage and its waveform.

The VSI has a constant voltage. Besides that, the commutation circuit is complex since it uses both capacitors and inductors. It also utilises feedback diodes. Lastly, the output voltage does not depend on the load, but the magnitude of the output current fluctuates based on the load impedance.'

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### 4. What are the different operation regions of the thyristor or SCR?

Your understanding of thyristors can convince the employer that you can create and utilise architectures of switches to control power output. As thyristors allow extended usability of electrical circuits and easier automation methods, answer with a description of the three operating regions. Offer an example application of thyristors in each operation region to explain your proficiency in developing and designing switching and inverting circuits, direct current motor drives and alternating current/direct current (AC/DC) static switches.

Example: 'Thyristors have three operating regions, each of which depends on the connection mode between the device and circuit. The three regions are the reverse blocking, forward blocking and forward conduction region. The forward blocking region uses the anode and cathode as positive and negative charges, respectively, without applying any gate signal to the thyristor. There is also a small leakage current flow in the forward blocking region mode. The reverse blocking region uses opposite charges for each electrode, which makes the thyristor exhibit reverse blocking behaviour similar to the diode.

The third operating mode is the forward conduction region, where the applied forward voltage between the cathode and anode rises at certain breakover avalanche breakdown points. This makes the SCR begin conducting current in the forward direction. Since this functionality within a device can damage the SCR, it is necessary to apply a gate signal before the forward breakover voltage.'

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### 5. Can you differentiate between an isolator and a circuit breaker?

Recruiters may ask this question to test your knowledge of safety procedures and methodologies. These help you during electrical equipment maintenance or safety design. You can highlight the built-in arc suppression parameter or the power interruption functions of the isolator and circuit breaker.

Example: 'Though isolators and circuit breakers are switches that kill connections between circuit components and a power system, you cannot use isolators with a load, while circuit breakers automatically operate in case of a system error. Isolators are manually run off-loading devices that help separate downstream and upstream circuits when repairing downstream circuits. Circuit breakers are onload devices helpful in breaking circuits in case of overload faults, short circuits and other malfunctions. Also, the breaking capacity of isolators is lower than that of circuit breakers.'

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### 6. Would you choose MOSFET or IGBT for high-frequency applications?

The Metal Oxide Semiconductor Field Effect Transistor (MOSFET) and Insulated Gate Bipolar Transistor (IGBT) are semiconductor voltage-controlled devices. Explaining which one might be preferable in switch mode power supply applications can help narrate an accurate answer. You could talk about the basics of these devices or highlight differences in their input impedance, damage resistance and applications.

Example: 'Choosing a MOSFET for high-frequency applications might be beneficial since this semiconductor device contributes to lesser switching losses in comparison to IGBT. The best practice is to have a maximum frequency range of 200kHz for high-frequency applications and a maximum of 20kHz for low-frequency applications. Another benefit of MOSFET is that its cost is relatively lower than that of IGBT.'

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### 7. In your opinion, what is the primary use of power electronics?

The answer to this question may give a recruiter a detailed insight into your approach to problems and projects relating to power electronics. The primary use of power electronics is to convert and control electrical power between alternative forms. Here is an example of a comprehensive explanation you can provide:

Example: 'Power electronics supports engineering experts in creating high-efficiency electrical systems for motor controls, lighting, automation gear and advanced microprocessor utilities. We can use power electronics principles for AC to DC, DC to AC, AC to AC and DC to DC conversion, rectifier and inverter applications, chopper applications and cycloconverter applications. Knowledge about rectifier and inverter circuits helps convert AC to DC and DC to AC circuits, whereas rectifier and inverter applications help in electroplating, providing better charges, variable DC drives, emergency lighting systems, uninterrupted power supplies and frequency converters.'

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### 8. Why do companies specialising in power electronics avoid using the unidirectional or half-wave AC voltage controller?

Employers may ask this question to assess your understanding of how power electronics companies function. Outline the disadvantages of using a unidirectional or half-wave AC voltage controller. Consider also explaining why companies avoid it.

Example: 'A unidirectional or half-wave AC voltage controller has limited control range due to the presence of a diode in the circuit. You can only use it for low-power resistive load circuits. Since companies specialising in power electronics typically work with complex voltage circuits, a unidirectional voltage controller is not useful for all the operations and has limited application.'