Silicon controlled rectifier (SCR), also called thyristor, is the earliest semi-controlled switching device used in power electronics.
①The main parameters of SCR
(1) Off-state repetitive peak voltage UDRM.
UDRM refers to the maximum off-state pulse voltage with an open gate, the repetition rate is 50 times per second, and the duration of each time is not greater than 10ms. UDRM=90% UDSM, where UDSM is the off-state non-repetitive peak voltage. UDSM should be smaller than UBO, and the margin left is determined by the manufacturer.
(2) Repetitive peak reverse voltage URRM.
Its definition is similar to UDRM, URRM=90%UDSM, where UDSM is the reverse non-repetitive peak voltage.
(3) Rated voltage.
Choose the smaller value of UDRM and URRM as the rated voltage. When selecting, the rated voltage should be 2-3 times of the normal working peak voltage, and it should be able to withstand frequent overvoltages.
②Precautions for using SCR
Note when selecting and using SCR: Even the same type of SCR, its control electrode and trigger characteristics are also quite different, and the trigger voltage and trigger current of the control electrode of the same SCR change with the change of temperature. . The maximum control electrode trigger voltage and maximum control electrode trigger current given in the SCR user manual do not refer to allowable values, but refer to the minimum control electrode voltage and minimum control electrode current required for all SCRs of this model to ensure triggering.
When the temperature changes, the trigger voltage and trigger current will change significantly. Generally, the trigger voltage and trigger current required at a high temperature of 100°C are 2 to 3 times lower than that at room temperature, and the trigger voltage and current required at a low temperature of -40°C are 2 to 3 times higher than that at room temperature.
In addition, the trigger current value given in the SCR manual is a DC value. In practical applications, pulse triggering is generally used, and the duration of the pulse should be sufficient to make the anode current rise above the holding current. At this time, the SCR can be maintained to conduct even if the trigger pulse disappears. If the pulse duration is very short, the amplitude of the trigger pulse current must be increased. The shorter the duration, the greater the required amplitude. The multiple of the increase in amplitude is much greater than the multiple of the decrease in duration. However, excessive voltage and current cannot be applied to the control electrode of the SCR. Generally speaking, the forward control electrode voltage cannot exceed 10V, and the reverse control electrode voltage cannot exceed 5V. Excessive control electrode voltage and current will cause the control electrode to burn out.
Switching time is also an important parameter when using SCR. The turn-on time ton and turn-off time toff given in the SCR manual are typical values. The switching time of different types of SCRs is measured under different specified states, so you must pay attention to it when quoting. The main factors affecting the conduction time are temperature, forward voltage before conduction and forward current after conduction. The higher the temperature, the greater the voltage, the shorter the on-time, the greater the current required, and the longer the time required. The main factors affecting the turn-off time are temperature, forward current before turn-off, reverse voltage, reverse current rise rate, etc. The higher the temperature, the greater the forward current, and the longer the turn-off time; the greater the reverse voltage, the higher the reverse current rise rate, and the shorter the turn-off time.
It can be seen from this that when selecting SCR, the typical value and specific use conditions must be considered together.