ELEC4410 Chap.3 Power Switch Selection and Gate Drives
Power Switch Selection and Gate Drives
A practical power switch is judged against electrical duties rather than by a memorised ranking. Start with blocking voltage, conducted current and current direction. Add the switching frequency, whether commanded turn-on and turn-off are required, the available gate-drive interface and the allowable loss. Finally check junction temperature.
These criteria convert a device comparison into an application-specific selection.
A diode is uncontrolled and follows circuit conditions, making it useful for rectification or a freewheel path but not as the commanded element in a PWM leg. An SCR accepts a trigger for turn-on and remains important in controlled high-power conversion, with commutation governing current extinction.
MOSFETs offer voltage-controlled fast switching, while IGBTs offer insulated-gate control with a different power and switching-loss compromise. None is a universal winner.
The driver belongs inside the design boundary. It supplies and removes gate charge, establishes a safe reference and may provide isolation or high-side level shifting. Dead time prevents simultaneous conduction in complementary devices.
Slower edges increase overlap loss; unnecessarily fast edges can aggravate overshoot and EMI. Selection must therefore connect the semiconductor, driver, layout and cooling path.
What this chapter covers
- 01
Diodes
- 02
SCR commutation and controllability
- 03
MOSFET and IGBT comparison
- 04
Gate charge, reference and dead time
- 05
Switching-loss and EMI verification
Worked example · free
Shortlist a switch for a motor converter
- +1Reject the diode as the main commanded switch and explain why ordinary SCR turn-off behaviour does not suit this high-frequency command.
- +2Keep MOSFET and IGBT candidates, comparing voltage and current rating, conduction behaviour, transition loss and gate-drive requirements.
- +1Verify gate waveform, switching-node overlap, dead time, device temperature and transient voltage before making the final recommendation.
Key terms
- Power Switch Selection
- A core ELEC4410 topic used to organise diodes, SCRs, MOSFETs, IGBTs, controllability, gate drive and switching limits.
Power Switch Selection and Gate Drives FAQ
Why is a diode not the controlled switch in a PWM leg?
A diode has no command terminal that imposes the PWM sequence. It may carry freewheel current when circuit polarity permits, but controller timing cannot independently force its ordinary turn-on and turn-off events.
What does the gate driver change besides logic level?
It supplies gate charge, establishes the correct device reference and may isolate or level-shift a high-side command. Its impedance and timing influence transition speed, overlap loss, overshoot, EMI and shoot-through risk.
How should MOSFET and IGBT candidates be compared?
Use one requirement matrix covering blocking voltage, current, switching frequency, conduction behaviour, switching energy, driver complexity and thermal margin. The application selects the compromise; a device-family slogan cannot replace rated data.
Exam move
Draw a comparison matrix with diode, SCR, MOSFET and IGBT as rows. Fill columns for commanded turn-on, commanded turn-off, current direction, switching emphasis, conduction behaviour and driver needs. Do not add a generic best-device column. Instead, write three short briefs with different frequency, voltage and current priorities and justify a shortlist for each.
For gate-drive revision, sketch a complementary leg and mark the reference of each driver, the dead-time interval and the current path during freewheel operation. Explain aloud how a slower edge changes voltage-current overlap and how a faster edge changes dv/dt, di/dt and EMI.
End each practice answer with the measurements that decide the selection: gate waveform, switching node, current, overshoot and calculated or measured temperature. Repeat the matrix at two switching frequencies so the preferred compromise is allowed to change. Add transient voltage and current margin rather than comparing nominal ratings only.
For a high-side device, draw the moving reference explicitly and explain why a ground-referenced trace may misrepresent gate command.
Practise identifying too little and too much dead time from paired PWM and switching-node sketches, then connect each timing error to its likely current path and loss consequence For each candidate, trace the freewheel current when its commanded device is off and verify that the chosen family supports the required direction and commutation.
Link the final thermal estimate back to the exact transition waveform rather than quoting a temperature detached from driver behaviour Finish by defending one device shortlist without claiming a universal winner or ignoring the gate-drive reference Keep all comparisons at one operating point.
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