Power Electronic Switches Compared: Diode, SCR, TRIAC, MOSFET and IGBT
Power electronics converts electrical energy from one form to another, AC to DC, DC to AC, or one voltage to another, using semiconductor switches instead of resistors or rheostats. The switches are either fully on, with a small voltage drop, or fully off, with negligible current, so very little power is lost. Choosing the right switch for a fan regulator, a battery charger, a solar inverter or a motor drive depends on its voltage and current rating, how it is turned on and off, and how fast it can switch.
This article compares the main device families and shows how to estimate conduction loss and junction temperature, the calculation that decides whether a device survives.
The Main Device Families
Power diode
An uncontrolled switch that conducts when its anode is positive with respect to its cathode. Forward drop is typically around 0.7 V to over 1 V depending on current and type. Fast-recovery and Schottky diodes are used where switching speed matters.
SCR (thyristor)
A four-layer PNPN device with anode, cathode and gate. A short gate current pulse turns it on when it is forward biased, and it then stays on without gate current as long as the anode current remains above the holding current. The anode current must first rise above the latching current before the gate pulse is removed. The gate cannot turn it off; on AC it turns off naturally each time current falls to zero, which is called natural or line commutation. Common small SCRs include the BT151 and TYN612 families in TO-220 packages.
TRIAC
Equivalent to two SCRs in inverse parallel with a common gate, so it conducts in both directions. It is used in fan regulators, light dimmers and solid-state relays for AC loads. The BT136 (4 A) and BT139 (16 A) are widely available examples in 600 V versions.
Power MOSFET
A voltage-controlled device. Applying about 10 V between gate and source (or 4.5 to 5 V for logic-level types) turns it fully on, and it behaves like a small resistance RDS(on). It switches very fast, at tens to hundreds of kHz, and is the default choice below a few hundred volts. The IRF540N (100 V) and IRFZ44N (55 V) are common examples; their logic-level counterparts are IRL540N and IRLZ44N.
IGBT
The insulated gate bipolar transistor combines a MOSFET input with a bipolar output. It is voltage driven like a MOSFET but has a nearly constant on-state voltage VCE(sat), typically in the range of about 1.5 to 3 V. It handles high voltages, commonly 600 V, 1200 V and 1700 V classes, at moderate switching frequencies, and dominates motor drives, UPS units, solar inverters and induction cookers.
Side-by-Side Comparison
| Device | Turn-on | Turn-off | Typical switching speed | Typical applications |
|---|---|---|---|---|
| Diode | Forward voltage | Reverse voltage | Depends on type | Rectifiers, freewheeling |
| SCR | Gate current pulse | Current falls below holding current | Line frequency | Controlled rectifiers, soft starters, HVDC valves |
| TRIAC | Gate pulse, either polarity | Current zero | Line frequency | Dimmers, fan regulators |
| MOSFET | Gate voltage | Remove gate voltage | Fastest, up to hundreds of kHz | SMPS, DC-DC converters, low-voltage motor drives |
| IGBT | Gate voltage | Remove gate voltage | Moderate, typically up to tens of kHz | VFDs, inverters, UPS, traction |
Worked Example: Conduction Loss and Junction Temperature
Datasheet ratings such as 33 A are measured with the case held at 25 °C, which is never true in practice. The real limit is junction temperature, set by the power dissipated and the thermal resistance from junction to ambient.
MOSFET conduction loss: P = I² × RDS(on) IGBT conduction loss: P = VCE(sat) × I (approx.) Diode / SCR loss: P = VF × Iavg (approx.) Junction temperature: Tj = Ta + P × (RθJC + RθCS + RθSA) RθJC = junction to case, RθCS = case to sink (pad/grease), RθSA = heatsink to ambient, all in °C/W
Take a load of 10 A DC switched continuously. Assume a MOSFET with RDS(on) of 0.044 Ω at 25 °C, an IGBT with VCE(sat) of 2.0 V, and use a hot-junction multiplier of 1.7 on RDS(on), a typical figure read from MOSFET datasheet curves at around 125 °C.
- MOSFET loss = 10² × 0.044 × 1.7 = 7.5 W.
- IGBT loss = 2.0 × 10 = 20 W.
- At 50 A, MOSFET loss = 2500 × 0.0748 = 187 W, while IGBT loss ≈ 2.0 × 50 = 100 W (VCE(sat) rises somewhat with current).
The MOSFET loss grows with current squared while the IGBT loss grows roughly linearly. That is why MOSFETs win at low voltage and moderate current, and IGBTs win at high voltage and high current, where a MOSFET of equal voltage rating would have a much higher RDS(on).
Heatsink check for the 7.5 W MOSFET case: Ta = 40 °C (Indian summer panel), RθJC = 1.2 °C/W (assumed), RθCS = 0.5 °C/W (thermal pad), RθSA = 8 °C/W (small heatsink) Tj = 40 + 7.5 × (1.2 + 0.5 + 8) = 40 + 72.8 = 112.8 °C Below a 175 °C maximum, with reasonable margin
Switching losses add to conduction losses and grow with frequency. Each turn-on and turn-off transition dissipates energy; datasheets give Eon and Eoff for IGBTs, and switching loss is approximately (Eon + Eoff) × fsw.
Gate Drive and Common Mistakes
- Driving a standard MOSFET from a 3.3 V microcontroller pin. It will be only partly on and overheat. Use a logic-level part or a gate driver IC.
- Leaving the gate floating. Always fit a gate-source pull-down resistor, typically around 10 kΩ.
- Ignoring the dv/dt and snubber needs of SCRs and TRIACs with inductive loads, which can cause false triggering.
- Using a high-side N-channel MOSFET without a bootstrap or isolated driver to lift the gate above the source.
- Forgetting the freewheeling diode across inductive loads such as relays, solenoids and motors.
Summary
Diodes and SCRs suit line-frequency rectification; TRIACs handle simple AC control; MOSFETs dominate fast, low-voltage switching; IGBTs dominate high-voltage, high-power converters. Always check conduction and switching losses against the thermal path, because junction temperature, not the headline current rating, sets the real limit.