Power Electronics Devices: A Comparison
BJT vs. MOSFET
Question 20: Differences in characteristics of gate BJTs and MOSFETs?
High-power bipolar junction transistors (BJTs) are common in power converters operating below 10 kHz, handling up to 1200V and 400A. A BJT has three terminals: base, emitter, and collector. In a common emitter configuration, it typically functions as a switch.
An NPN transistor turns on when the base voltage exceeds the emitter voltage, and sufficient base current drives the transistor into saturation. The transistor remains on as long as the collector-emitter junction is correctly biased.
The forward voltage drop of a conducting BJT ranges from 0.5V to 1.5V. Removing the base excitation voltage turns the transistor off.
Power MOSFETs are used in high-speed power converters, typically handling lower power levels (e.g., 1000V, 50A) at frequencies of tens of kHz.
IGBT, MCT, and SIT Characteristics
Question 21: Characteristic of an IGBT gate?
Insulated gate bipolar transistors (IGBTs) are voltage-controlled devices, faster than BJTs but not as fast as MOSFETs. They offer higher output current and frequency capabilities (up to 20 kHz) and are available up to 1200V and 400A.
Question 22: Characteristic of an MCT gate?
MOS-controlled thyristors (MCTs) are turned on by a small negative voltage pulse at the MOS gate (relative to the anode) and turned off by a small positive voltage pulse.
Question 23: Characteristic of a SIT gate?
Static induction transistors (SITs) are high-power, high-frequency devices similar to triode vacuum tubes and JFETs. They offer low noise, low distortion, and high-frequency audio capabilities with fast switching times (around 0.25 μs). However, their normally-on characteristic and high voltage drop limit their use in power conversion. SITs are available up to 1200V and 300A, with switching speeds up to 100 kHz, making them suitable for high-power, high-frequency applications like audio, VHF/UHF, and microwave amplifiers.
BJT vs. IGBT and MCT vs. GTO
Question 24: Differences between a BJT and IGBT?
- BJTs are current-controlled, while IGBTs are voltage-controlled.
- IGBTs are faster than BJTs.
- IGBTs are better suited for high voltages and currents.
- IGBTs handle frequencies up to 20 kHz, while BJTs are typically used below 10 kHz.
- Both are available up to 1200V and 400A.
Question 25: Differences between MCT and GTO?
- GTOs are turned on by a short positive gate pulse and turned off by a short negative gate pulse.
- MCTs are turned on by a small negative voltage pulse at the gate (relative to the anode) and turned off by a small positive voltage pulse.
- GTOs do not require a commutation circuit and are suitable for forced commutation converters, available up to 4000V and 3000A.
- MCTs have high turn-on gain and are available up to 1000V and 100A.
SIT vs. GTO and Power Diodes
Question 26: Differences between SIT and GTO?
- Both SITs and GTOs are self-commutated thyristors.
- Both are turned on by a short positive gate pulse.
- SITs are available up to 1200V and 300A, while GTOs are available up to 4000V and 3000A.
- SITs are suitable for medium-power converters and handle frequencies up to several hundred kHz, exceeding the GTO’s frequency range.
Power Diodes
Types of Power Diodes
Standard general-purpose diodes, fast recovery diodes, and Schottky diodes.
Diode Leakage Current
When forward-biased (anode positive relative to cathode), a diode conducts. When reverse-biased, a small leakage current (microamps to milliamps) flows, increasing with reverse voltage until avalanche or Zener breakdown.
Reverse Recovery Time
The time it takes for the reverse current to drop to 20% of its peak value (IRR) after switching from forward conduction to reverse blocking. It depends on junction temperature, forward current decay rate, and forward current before switching.
Reverse Recovery Current
The current flowing due to minority carriers when the diode is reverse-biased.
Softness Factor
The ratio of TB to TA. TA is caused by charge storage in the reverse-biased depletion region, contributing to the peak reverse recovery current (IRR). TB is due to charge storage in the semiconductor material.
Types of Recovery Diodes
Fast recovery diodes have recovery times under 5 μs and are used in circuits where recovery speed is critical. Epitaxial diodes offer faster switching than diffused diodes. For voltages above 400V, fast recovery diodes are typically fabricated by diffusion, with recovery time controlled by gold or platinum diffusion.
Cause of Reverse Recovery Time
Forward current in a diode is the net effect of majority and minority carriers. When a conducting diode’s current is reduced to zero, minority carriers stored in the PN junction cause continued conduction.
Effect of Reverse Recovery Time
Limits the rate of rise of forward current and switching speed.
Need for Fast Recovery Diodes in High-Speed Conversion
Faster recovery speeds are needed as switching frequency increases.
Forward Recovery Time
The time it takes for charges in the PN junction to recombine.
PN Junction vs. Schottky Diodes
Schottky diodes minimize charge storage issues, exhibiting much lower recovered charge than PN junction diodes. However, their leakage current is higher. Schottky diodes with lower forward voltage drops have higher leakage currents, and vice versa.
Schottky Diode Limitations
Limited to a maximum voltage (Vmax) of around 100V and current ratings from 1A to 300A. Ideal for high-current, low-voltage DC power supplies, offering greater efficiency in low-current power supplies.
Typical Reverse Recovery Times
General-purpose diodes have relatively high reverse recovery times (typically 25 μs). Fast recovery diodes have much lower times (typically less than 5 μs) and are crucial for high-speed switching applications.
Series-Connected Diodes and Power Electronics Design
Problems with Series-Connected Diodes and Solutions
When high-voltage diodes are unavailable, diodes are connected in series to enhance reverse blocking capability.
Steps in Power Electronics Equipment Design
Four main parts: 1. Power circuit design, 2. Power device protection, 3. Control strategy determination, 4. Control and logic circuit design.
Peripheral Effects of Power Electronics Equipment
Switching power converters introduce harmonic currents and voltages into the power supply and converter output, often requiring input and output filters to mitigate these harmonics.
Thyristors and Power Electronics Fundamentals
GTO vs. Thyristor Gate Characteristics
Thyristors cannot be turned off by the gate signal, while GTOs can be turned off by a negative gate pulse.
Thyristor vs. Transistor Gate Characteristics
Transistors are current-controlled, while thyristors are voltage-controlled.
Power Electronics Definition
The application of solid-state electronics for controlling and converting electrical power.
Types of Thyristors
Phase-controlled thyristors (SCRs), fast-switching SCRs, gate turn-off thyristors (GTOs), bidirectional triode thyristors (TRIACs), reverse-conducting thyristors (RCTs), static induction thyristors (SITs), light-activated SCRs (LASCRs), FET-controlled thyristors (FET-CTHs), and MOS-controlled thyristors (MCTs).
Switching Circuit
A circuit that biases the gate of a thyristor (DIAC, TRIAC, SCR, FET) to enable current flow from anode to cathode.
Conditions for Thyristor Conduction
A thyristor has three terminals: anode, cathode, and gate. When a small current flows from the gate to the cathode, the thyristor conducts as long as the anode potential is higher than the cathode potential.
Turning Off a Conducting Thyristor
Reduce the anode potential to or below the cathode potential. Line-commutated thyristors turn off naturally due to the sinusoidal input voltage, while forced-commutated thyristors require specific circuitry for turn-off.