A. It is defined as the time during which a reverse voltage is applied across the thyristor during its commutation process.
A. If the circuit turn off time is less than the thyristor turn off time the device may turn on at an undesired instant resulting in commutation failure.
A. A dc Chopper is equivalent to the transformer in ac circuit. It is a static switch used to get the variable dc voltage from a constant dc voltage.
A. The process of changing the direction of current flow in a particular path of the circuit. It is used to turn off the SCR.
A. Natural commutation
Forced commutation
A. The process of the current flowing through the thyristor goes through a natural zero and enable the thyristor to turn off is called as natural commutation.
A. The process of the current flowing through the thyristor is forced to become zero by external circuitry is called as forced commutation.
A. In step down chopper, the average output voltage is less than the input supply voltage. It is also known as Buck converter.
A. In step up chopper, the average output voltage is more than the input supply voltage. It is also known as Boost converter.
A. The process of a charged capacitor momentarily reverse biases the conducting SCR and turns it off is called as voltage commutation.
A. The process of a current pulse is made to flow in the reverse direction through the conducting SCR and thus made the net SCR current becomes zero, consequently turn off the SCR is called as current commutation.
A. The advantages of current commutated chopper is;
1. Commutation is reliable as load current is less than the peak commutation current
2. The auxiliary SCR is naturally commutated as its current passes through zero value.
3. The capacitor always remains charged with the correct polarity.
A. In load commutation, the load current flowing through the thyristor either becomes zero or is transferred to another device from the conducting SCR.
A. SCR, GTO, GTR
A. IGBT, MCT, IGCT, SIT
A. To turn on these kind of devices single pulse of short duration is sufficient. Continuous gate voltage of entire on time is not required. It will avoid the hard triggering. Ex. Thyristor, GTO
A. In order to maintain these kind of devices in on-state, we need to apply continuous gate current/voltage. Some of the level sensitive devices are MOSFET, IGBT, MCT, IGCT.
A. IGBT has
Lower turn on and turn off times than BJT
Lower on state conduction losses than MOSFET
Excellent safe operating area
A. A snubber circuit consists of a series combination of a resistor and a capacitor in parallel with the
thyristors. It is mainly used for dv / dt protection.
A. Circuit turn off time should be greater than the thyristor turn-off time for reliable turn-off, otherwise the
It converts fixed ac voltage into variable dc voltage.
The delay angle is defined as the angle between the zero crossing of the input voltage and the instant the thyristor is fired..
The commutation period when outgoing and incoming thyristors are conducting is known as overlap period. The angular period, when both devices share conduction is known as the commutation angle or overlap angle
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