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Triac vs diac; BJT vs MOSFET selection

Short answer

A diac is a two-terminal bidirectional trigger diode without a gate; a triac is a three-terminal bidirectional thyristor with a gate. For BJT vs MOSFET, choose BJT for low-frequency high-voltage conduction loss, MOSFET for high-frequency low switching loss and easy paralleling; judge by total conduction plus switching loss at operating conditions.

What are the differences between triac and diac? What are the key factors that engineers consider when selecting between power BJTs and power MOSFETs based on their conduction and switching loss characteristics?

Triac versus diac

A diac is a two-terminal, bidirectional trigger diode that conducts in either direction once its breakover voltage (about 30 V) is reached. A triac is a three-terminal bidirectional thyristor (two SCRs in anti-parallel with one gate) used to control a.c. power.

PointDiacTriac
Terminals2 (MT1, MT2)3 (MT1, MT2, gate)
GateNo gateHas a gate
Turn-onOnly by breakover voltageBy gate pulse in either half cycle
LayersTypically 3/5-layer, no gate5-layer bidirectional thyristor
Current ratingSmall (mA)Large (A to tens of A)
UseTrigger device for triac (light dimmer)Main power switch in a.c. controllers

Example: in a fan regulator, the diac fires the triac when the RC network voltage reaches about 30 V.

Choosing between power BJT and power MOSFET

FactorPower BJTPower MOSFET
Conduction lossLow: VCE(sat)≈0.5V_{CE(sat)} \approx 0.5-1 V nearly independent of ratingI2RDS(on)I^2 R_{DS(on)}; RDS(on)R_{DS(on)} rises sharply with voltage rating
Switching lossHigh: slow due to minority-carrier storage timeVery low: majority carrier, switches in tens of ns
Suitable frequencyUp to a few kHzTens to hundreds of kHz
DriveCurrent-driven; large continuous base currentVoltage-driven; almost no steady gate current
TemperatureNegative coefficient; second breakdown and thermal runaway riskPositive coefficient of RDS(on)R_{DS(on)}; easy paralleling, no second breakdown

Key considerations for engineers:

  1. Voltage level: at high voltage (above about 500 V), MOSFET RDS(on)R_{DS(on)} is high, so conduction loss favours the BJT (or IGBT).
  2. Switching frequency: at high frequency, switching loss dominates, so the MOSFET is chosen (SMPS, high-frequency converters).
  3. Total loss: P=Pcond+PswP = P_{cond} + P_{sw}, where Psw∝fP_{sw} \propto f; the device with lower total loss at the working current and frequency is chosen.
  4. Drive circuit cost, ruggedness (safe operating area) and ease of paralleling.

Example: a 100 kHz, 48 V DC-DC converter uses a MOSFET; a low-frequency, high-voltage motor drive traditionally used BJTs (now usually IGBTs).

✓ Answer checked by Claude Opus 5.5 (high effort)

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