Power Electronics: SCR, Rectifiers, Choppers and Inverters for SSC JE Electrical
Rahul Kumar
SSC Exam Expert & Content Editor
In this note
- 1.Power Semiconductor Devices
- 2.SCR Basics
- 3.Uncontrolled Rectifiers (Vm = peak voltage)
- 4.Controlled Rectifiers (firing angle α)
- 5.Choppers (DC to DC Converters)
- 6.Inverters and Other Converters
- 7.Solved Examples
- 8.Quick Tips
- 9.Quick Revision Points
- 10.Practice Questions With Answers
- 11.How to Practise This Chapter
- 12.Frequently Asked Questions
PDF Notes: Power Electronics: SCR, Rectifiers, Choppers and Inverters for SSC JE Electrical
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Power electronics is the use of solid-state devices to control and convert electric power. It covers rectifiers (AC to DC), inverters (DC to AC), choppers (DC to DC) and cycloconverters (AC to AC). In the SSC JE electrical paper the questions are mostly on the SCR, rectifier formulas and the conversions. This note gives the devices, formulas, solved examples, practice questions and FAQs.
Power Semiconductor Devices
| Device | Features |
|---|---|
| Power diode | Conducts in one direction only; uncontrolled |
| SCR (thyristor) | Four-layer PNPN device with 3 terminals (anode, cathode, gate); turned on by a gate pulse; turned off by reducing the current below the holding value |
| TRIAC | Bidirectional thyristor; controls both half cycles of AC |
| DIAC | Two-terminal bidirectional trigger device used with a TRIAC |
| GTO | Gate Turn-Off thyristor; can be turned off by a negative gate pulse |
| Power BJT | Current-controlled switch |
| Power MOSFET | Voltage-controlled; fast switching; for low-voltage, high-frequency use |
| IGBT | Combines the MOSFET input and BJT output; widely used in drives and inverters |
SCR Basics
- Latching current is the minimum anode current needed to keep the SCR on immediately after the gate pulse is removed; the holding current is the minimum current to keep it on afterwards. Latching current is greater than holding current.
- The SCR can be turned off by commutation: natural (line) commutation in AC circuits or forced commutation in DC circuits.
- Protection: a snubber (RC) circuit protects against high dv/dt; a series inductor limits di/dt; fuses give overcurrent protection; heat sinks remove heat.
- Triggering methods: gate triggering (the usual), forward-voltage triggering, dv/dt triggering, temperature and light (LASCR) triggering.
- Forward break-over voltage is the voltage at which an SCR turns on with no gate signal.
Uncontrolled Rectifiers (Vm = peak voltage)
| Quantity | Half-wave | Full-wave |
|---|---|---|
| Average (DC) voltage | Vm / π = 0.318 Vm | 2 Vm / π = 0.637 Vm |
| RMS voltage | Vm / 2 | Vm / √2 |
| Form factor | 1.57 | 1.11 |
| Ripple factor | 1.21 | 0.48 |
| Maximum efficiency | 40.6% | 81.2% |
| PIV (diode) | Vm | 2 Vm (centre-tap); Vm (bridge) |
| Ripple frequency | Same as supply (f) | Twice the supply (2f) |
Controlled Rectifiers (firing angle α)
- Single-phase half-wave controlled: Vdc = (Vm / 2π)(1 + cos α).
- Single-phase full converter (fully controlled bridge): Vdc = (2 Vm / π) cos α. It can work as an inverter when α is greater than 90°.
- Single-phase semi-converter (half-controlled): Vdc = (Vm / π)(1 + cos α).
- Increasing the firing angle reduces the average output voltage.
Choppers (DC to DC Converters)
| Type | Output voltage | Note |
|---|---|---|
| Step-down (buck) chopper | Vo = D × Vs | D is the duty cycle = Ton/(Ton + Toff) |
| Step-up (boost) chopper | Vo = Vs / (1 − D) | Output is higher than the input |
| Buck-boost | Vo = D Vs / (1 − D) | Output polarity is reversed |
Inverters and Other Converters
| Converter | Function |
|---|---|
| Rectifier | AC to DC |
| Inverter | DC to AC (used in UPS and solar systems) |
| Chopper | DC to DC with a different voltage |
| Cycloconverter | AC to AC with a different frequency (used in large AC drives) |
| AC voltage controller | Varies the RMS AC voltage (light dimmer, fan regulator) |
| SMPS | Switched-mode power supply: a compact, efficient DC supply using high-frequency switching |
| UPS | Uninterruptible power supply with a battery and an inverter |
- A TRIAC is used in fan regulators and light dimmers.
- Inverter output may be a square wave, modified sine wave or pure sine wave (using PWM).
- Pulse Width Modulation (PWM) is used to control the output voltage and reduce harmonics.
Solved Examples
| Question | Solution |
|---|---|
| Half-wave rectifier with Vm = 100 V. Find Vdc. | Vm / π = 100 / 3.14 = 31.8 V |
| Full-wave rectifier with Vm = 100 V. Find Vdc. | 2 × 100 / 3.14 = 63.7 V |
| Buck chopper with Vs = 100 V and D = 0.4 | Vo = 0.4 × 100 = 40 V |
| Boost chopper with Vs = 50 V and D = 0.5 | Vo = 50 / (1 − 0.5) = 100 V |
| Full converter, Vm = 325 V, α = 60° | Vdc = (2 × 325 / π) × cos 60° = 206.9 × 0.5 ≈ 103.4 V |
Quick Tips
- Remember the table of ripple factors: 1.21 for half-wave and 0.48 for full-wave.
- The ripple frequency of a full-wave rectifier is twice the supply frequency.
- Latching current > holding current is a favourite question.
- A duty cycle D between 0 and 1 lowers the voltage in a buck chopper and raises it in a boost chopper.
Quick Revision Points
- SCR has 3 terminals: anode, cathode, gate.
- Half-wave Vdc = Vm/π; full-wave = 2Vm/π.
- Buck Vo = D Vs; boost Vo = Vs/(1−D).
- Cycloconverter converts AC to AC at a different frequency.
- Snubber circuit protects against dv/dt.
Practice Questions With Answers
- Number of terminals in an SCR?
Answer: 3 (anode, cathode, gate) - What does a snubber circuit protect against?
Answer: High dv/dt - Average output voltage of a half-wave rectifier?
Answer: Vm/π - What is the ripple factor of a full-wave rectifier?
Answer: 0.48 - A chopper converts?
Answer: DC to DC - Which device is a bidirectional thyristor?
Answer: TRIAC - Which converter changes the frequency of AC?
Answer: Cycloconverter - Which is greater, latching or holding current?
Answer: Latching current - PIV of a bridge rectifier diode?
Answer: Vm - Boost chopper output with D = 0.5 and Vs = 20 V?
Answer: 40 V
How to Practise This Chapter
Practise 10 questions every day and write the steps on paper. Then attempt timed sets from our practice papers and the previous year papers page. See all topics on the Power Electronics notes page.
Frequently Asked Questions
What is power electronics?
The use of power semiconductor devices to convert and control electrical power.
What is an SCR?
A silicon controlled rectifier, a four-layer three-terminal thyristor.
How is an SCR turned on?
By applying a suitable gate pulse when it is forward biased.
How is an SCR turned off?
By reducing the anode current below the holding current (commutation).
What is a TRIAC?
A bidirectional thyristor that controls both half cycles of AC.
What is the difference between a MOSFET and an IGBT?
A MOSFET is better for low-voltage, high-frequency use; an IGBT suits higher voltage and current.
What is the ripple factor?
The ratio of the AC component to the DC component of the output.
What is a chopper?
A DC-to-DC converter that changes the DC voltage level.
What is an inverter?
A circuit that converts DC into AC.
What is a cycloconverter?
A converter that changes AC of one frequency into AC of another frequency.
What is PWM?
Pulse Width Modulation, a method of controlling the output by changing the pulse width.
Is this topic asked in SSC JE?
Yes, a few direct questions come from SCR, rectifiers and converters.
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Rahul Kumar
SSC Exam Expert & Content Editor
Rahul is a senior SSC exam strategist with 8+ years of experience helping aspirants crack CGL, CHSL and MTS exams. He writes in-depth notification breakdowns, exam pattern guides and preparation strategies.