Class C, C or L-C switched by another load–carrying SCR & Class D, L-C or C switched by an auxiliary SCR
Class C, C or L-C switched by another load–carrying SCR:
Fig: Class C, turn off, SCR switched off by another load-carrying SCR
- This configuration has two SCRs. One of them may be the main SCR and the other auxiliary. Both may be load current carrying main SCRs.
- The configuration may have four SCRs with the load across the capacitor, with the integral converter supplied from a current source.
- Assume SCR2 is conducting. C then charges up in the polarity shown. When SCR1 is triggered, C is switched across SCR2 via SCR1 and the discharge current of C opposes the flow of load current in SCR2.
Class D, L-C or C switched by an auxiliary SCR:
Fig: Class D turn OFF. Class D commutation by a C ( or LC) switched by an auxiliary SCR
- The circuit of class B self-commuted by LC circuit can be converted to Class D if the load current is carried by only one of the SCR’s, the other acting as an auxiliary turn-off SCR. The auxiliary SCR would have a resistor in its anode lead of say ten times the load resistance.
- SCRA must be triggered first in order to charge the upper terminal of the capacitor as positive. As soon as C is charged to the supply voltage, SCRA will turn off.
- If there is substantial inductance in the input lines, the capacitor may charge to voltages in excess of the supply voltage.
- This extra voltage would discharge through the diode-inductor-load circuit.
- When SCRM is triggered the current flows in two paths: Load current flows through the load and the commutating current flows through C- SCRM -L-D network. The charge on C is reversed and held at that level by the diode D.
- When SCRA is re-triggered, the voltage across C appears across SCRM via SCRA and SCRM is turned off. If the load carries a constant current, the capacitor again charges linearly to the dot as positive.
Questions of this topic
A Class D turn-off circuit has a commutating capacitor of 10 micro F. The load consists of a clamped inductive load such that the load current is reasonably constant at 25 amperes. The Inverter grade SCR has a turn-off time of 12 microsecs. Determine whether the SCR will be satisfactorily commutated. Also dimension the commutating inductor. The supply voltage is 220 VDC.