Thyristor Components
The term thyristor identifies a general class of solid-state silicon controlled rectifiers (SCRs). These devices are similar to normal rectifiers, but are designed to remain in a blocking state (in the forward direction) until a small signal is applied to a control electrode (the gate). After application of the control pulse, the device conducts in the forward direction and exhibits characteristics similar to those of a common silicon rectifier. Conduction continues after the control signal has been removed and until the current through the device drops below a predetermined threshold, or until the applied voltage reverses polarity.
The voltage and current ratings for thyristors are similar to the parameters used to classify standard silicon rectifiers. Some of the primary device parameters include:
• Peak forward blocking voltage — the maximum safe value that can be applied to the thyristor while it is in a blocking state.
• Holding current — the minimum anode-to-cathode current that will keep the thyristor conduct- ing after it has been switched on by the application of a gate pulse.
• Forward voltage drop — the voltage loss across the anode-to-cathode current path for a specified load current. Because the ratio of rms-to-average forward cur- rent varies with the angle of conduction, power dissipation for any average cur- rent also varies with the device angle of conduction. The interaction of forward voltage drop, phase angle, and device case temperature generally are specified in the form of one or more graphs or charts.
• Gate trigger sensitivity — the minimum voltage or current that must be applied to the gate to trigger a specific type of thyristor into conduction. This value must take into consideration variations in production runs and operating tem- perature. The minimum trigger voltage is not normally temperature sensitive, but the minimum trigger current can vary considerably with thyristor case temperature.
• Turn-on time — the length of time required for a thyristor to change from a nonconducting state to a conducting
state. When a gate signal is applied to the thyristor, anode-to-cathode current begins to flow after a finite delay. A sec- ond switching interval occurs between the point at which current begins to flow and the point at which full anode current (determined by the instantaneous applied voltage and the load) is reached. The sum of these two times is the turn-on time. The turn-on interval is illustrated in Figure 6.14.
• Turn-off time — the length of time required for a thyristor to change from a conducting state to a nonconducting state. The turn-off time is composed of two individual periods: the storage time (similar to the storage interval of a saturated transistor) and the recovery time. If forward voltage is reapplied before the entire turn-off time has elapsed, the thyristor will conduct again.
Failure Modes
Thyristors, like diodes, are subject to damage from transient overvoltages because the peak inverse volt- age or instantaneous forward voltage (or current) rating of the device may be exceeded. Thyristors face an added problem because of the possibility of device misfiring. A thyristor can break over into a conduc- tion state, regardless of gate drive, if either of these conditions occur:
• Too high a positive voltage is applied between the anode and cathode.
• A positive anode-to-cathode voltage is applied too quickly, exceeding the dv/dt (delta voltage/delta time) rating.
If the leading edge is sufficiently steep, even a small voltage pulse can turn on a thyristor. This represents a threat not only to the device, but also to the load that it controls.
Application Considerations
Any application of a thyristor must take into account the device dv/dt rating and the electri- cal environment in which it will operate. A thy- ristor controlling an appreciable amount of energy should be protected against fast-rise- time transients that may cause the device to
break over into a conduction state. The most basic method of softening the applied anode- to-cathode waveform is the resistor/capacitor snubber network shown in Figure 6.15. This standard technique of limiting the applied dv/ dt relies on the integrating ability of the capacitor. In the figure, C1 snubs the excess transient energy, while R1 defines the applied dv/dt with Lt, the external system inductance.
An applied transient waveform (assuming an infinitely sharp wavefront) will be impressed across the entire protection network of C1, R1, and Lt. The total distributed and lumped system inductance Lt plays a significant role in determining the ability of C1 and R1 to effectively snub a transient waveform. Power sources that are stiff (having little series inductance or resistance) will present special problems to engi- neers seeking to protect a thyristor from steep transient waveforms.
Exposure of semiconductors to a high-transient environment can cause a degrading of the device, which eventually may result in total failure. Figure 6.16 shows the energy-vs.-survival scale for several types of semiconductors.