Power electronic devices
At 20 A, an IGBT loses less than a MOSFET below about 19 kHz; above that the MOSFET wins, and at 100 kHz the IGBT would dissipate almost twice as much. The switching frequency chooses the device.
01The device as a switch
Power electronics converts electrical energy —from AC to DC, from DC to AC, from one voltage to another— with efficiencies of 90 to 99 %. To do so, the semiconductors do not work in their linear region, as in an amplifier, but as switches: fully closed, with little voltage, or fully open, with little current. The power they dissipate comes from their imperfections: the conduction drop, the leakage when blocking and, above all, the time they take to go from one state to the other.
02The families
| Device | Control | Typical range | Use |
|---|---|---|---|
| Power diode (fast, Schottky, SiC) | Uncontrolled | Up to kV and kA | Rectifiers, freewheeling |
| Thyristor (SCR) | Turned on by the gate; turns off when the current goes to zero | Up to several kV and kA, low frequency | Controlled rectifiers, DC transmission |
| Triac | Bidirectional thyristor | Hundreds of V, tens of A | AC phase control, solid-state relays |
| Power MOSFET | Gate voltage, turn-on and turn-off | Up to a few hundred V; hundreds of kHz | Switching power supplies, low-voltage converters |
| IGBT | MOS gate, bipolar conduction | 600 V to 6.5 kV; up to tens of kHz | Motor inverters, traction, renewable energy |
| SiC MOSFET, GaN HEMT | Gate voltage | SiC 650 V to 3.3 kV; GaN up to 650 V, MHz | Chargers, electric vehicles, compact power supplies |
The thyristor is a four-layer structure that was covered in multijunction devices: once triggered, it stays latched as long as the current exceeds the holding current. The MOSFET conducts like a resistor, \( R_{DS(on)} \), which in silicon grows strongly with the blocking voltage; that is why above about 600 V the IGBT is preferred, which has a nearly fixed drop but turns off more slowly because of the tail current of the minority carriers. The wide-bandgap semiconductors, silicon carbide and gallium nitride, block more voltage with less resistance and switch faster.
03Conduction and switching losses
At each switching event, for a short time, the switch withstands voltage and current at the same time. With a duty cycle \( D \) and a switching frequency \( f \):
A 600 V MOSFET and IGBT switching 400 V at a duty cycle of 0.5. MOSFET: RDS(on) = 0.15 Ω when hot, tr + tf = 50 ns, Coss = 100 pF. IGBT: VCE0 = 0.9 V, rCE = 0.05 Ω, Eon + Eoff = 0.04 mJ per ampere. Thermal resistance to ambient of 1.5 K/W, at 40 °C.
04Safe operating area and gate drive
The safe operating area (SOA) in the datasheet delimits the voltage and current combinations that the device can withstand, depending on the pulse duration; in switching with an inductive load the trajectory must stay inside. The gate of a MOSFET or an IGBT is a capacitor: the gate driver charges and discharges it with peaks of amperes, and the gate resistor sets the switching speed, in a trade-off between losses and overvoltages. In bridges, the high-side switch needs an isolated driver or one with a floating supply, and a dead time that prevents the two switches of a leg from conducting at the same time.
05In the laboratory
With an inductive load and a double-pulse test, measure the voltage and current waveforms at turn-off of a MOSFET and of an IGBT, and integrate the switching energy.
Measure the latching current and the holding current of an SCR with a DC supply and a variable load.
Change the gate resistor and observe the trade-off between switching speed and overvoltage.
06Common mistakes
- Taking RDS(on) at 25 °C. When hot it is between 1.5 and 2 times higher.
- Forgetting the switching losses. At high frequency they can exceed the conduction losses.
- Expecting a thyristor to turn off by removing the gate pulse. It stays latched until the current goes to zero.
- Omitting the dead time in a bridge. The two switches of a leg short-circuit the supply.
07Self-assessment
With the lab data, how much does the MOSFET lose at 20 A and 20 kHz?
30 W of conduction and 4.16 W of switching: 34.2 W.
And the IGBT?
19 W of conduction and 16 W of switching: 35 W. At that frequency they are almost tied.
Why does the IGBT turn off more slowly than the MOSFET?
Because of the tail current: the accumulated minority carriers have to recombine.
What advantage does silicon carbide give?
It blocks more voltage with less conduction resistance and switches fast, at a higher temperature.
08Further reading
- Ned Mohan, Tore M. Undeland and William P. Robbins. Power Electronics: Converters, Applications, and Design. 3rd ed., Wiley, 2003. The classic text of the subject, with the devices and their drivers.
- Muhammad H. Rashid. Power Electronics: Devices, Circuits, and Applications. 4th ed., Pearson. (Spanish edition: “Electrónica de potencia”.) The devices and all the converters, with many worked examples.
- B. Jayant Baliga. Fundamentals of Power Semiconductor Devices. 2nd ed., Springer, 2019. The physics of silicon and wide-bandgap power devices.