turn-on time
The amount of time required for a device to change from a forward blocking mode to a forward conduction mode when a gate pulse is applied.
turn-on time: how fast a switch actually closes
Turn-on time is the elapsed period between the arrival of a gate signal and the moment a semiconductor switch (typically a thyristor, MOSFAT, or IGBT) reaches full forward conduction. It is measured in microseconds or nanoseconds, depending on the device type and circuit conditions. This interval comprises several distinct phases: delay time (time before any change occurs), rise time (the transition from blocking to conducting), and storage time in some devices. The switch does not snap on instantly; it transitions through intermediate impedance states.
In thyristors and IGBTs, turn-on time depends heavily on junction capacitance, doping profiles, and the magnitude of the gate current applied. A larger gate pulse (higher voltage or current) reduces turn-on time by charging the gate capacitance faster. Temperature also affects it: higher junction temperatures typically increase turn-on time because carrier mobility decreases. MOSFETs generally exhibit shorter turn-on times than thyristors, often in the nanosecond range, because they rely on field effect rather than carrier injection.
Why this matters in practice
During turn-on, the device dissipates energy as i2r losses while voltage and current transition across it simultaneously. A long turn-on time means extended overlap between high current and high voltage, generating unwanted heat and stress. In high-frequency switching supplies and inverters operating at kHz or MHz rates, accumulated turn-on losses can become a major thermal burden. Designers choose faster switches or gate drivers to reduce this dead time, but faster switches introduce noise and electromagnetic interference problems.
Specification sheets list turn-on time under defined test conditions: a fixed supply voltage, gate drive voltage, load current, and ambient temperature. Variations from these baseline conditions will shift the actual turn-on time in the field. Inductive snubbers, gate resistors, and driver circuit design all influence the real turn-on speed observed in equipment. In power electronics, turn-on time feeds directly into commutation losses and efficiency calculations for motors, converters, and welding equipment.