25-04-2014, 03:43 PM
Thyristor Devices Silicon Controlled Rectifiers (SCR)
INTRODUCTION
SCR is most popular of thyristor family due to its
Fast switching action , small size and high voltage and current ratings.
It is commonly used in power electronic applications.
SCR has 3 terminals (gate provides control)
SCR is turned on by applying +ve gate signal
when anode is +ve with repect to cathode.
SCR is turned off by interrupting anode
current.
SCR RATINGS
(a) SCR Current Ratings
1- Maximum Repetitive RMS current Rating
Average on-state current is the maximum average current value that can be carried by the SCR in its on state.
RMS value of nonsinusoidal waveform is simplified by approximating it by rectangular waveform.
This approximation give higher RMS value, but leaves slight safety factor.
SCR RATE-OF-CHANGE RATINGS
1- (di/dt rating)
Critical rate of rise of on-state current. It is the rate at which anode current increases and must be less than rate at which conduction area increases.
To prevent damage to SCR by high di/dt value, small inductance is added in series with device. Vaue of required inductance is
L>= Vp
(di/dt)max
2- dv/dt rating
Maximum rise time of a voltage pulse that can be applied to the SCR in the off state without causing it to fire. Unscheduled firing due to high value of dv/dt can be prevented by using RC snubber circuit.
PULSE SIGNALS
Instead of continuous DC signal, single pulse or train of pulses is generated.
It provides precise control of point at which SCR is fired.
It provides electrical isolation between SCR and gate-trigger circuit.