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The gate CHARGE is DEFINED as the amount of charge required for the DEVICE during turn-on and turn-off time. It is the most important parameter we have to CONSIDER while selecting the MOSFET. The switching speed of the MOSFET depends on the speed at which the gate driver can charge or discharge the input gate charge.

The gate charge is defined as the amount of charge required for the device during turn-on and turn-off time. It is the most important parameter we have to consider while selecting the MOSFET. The switching speed of the MOSFET depends on the speed at which the gate driver can charge or discharge the input gate charge.

To MEET the high current demand we will CONNECT the SCRs in a parallel MANNER.

Similarly to meet the high voltage demand we have to connect the SCRs in SERIES.

To meet the high current demand we will connect the SCRs in a parallel manner.

Similarly to meet the high voltage demand we have to connect the SCRs in series.

In GENERAL, AC voltages and currents are mentioned in RMS values only. It is sensible to compare with steady DC voltages and currents.

For example 230V AC supply MEANS, 230Vrms AC Supply.

In general, AC voltages and currents are mentioned in RMS values only. It is sensible to compare with steady DC voltages and currents.

For example 230V AC supply means, 230Vrms AC Supply.

• ASYMMETRICAL GTO
• SYMMETRICAL GTO

Some of the level sensitive devices are

• MOSFET
• IGBT
• MCT
• IGCT

In order to KEEP these devices in the ON state condition, we have to APPLY gate current/voltage continuously to these devices.

Some of the level sensitive devices are

In order to keep these devices in the ON state condition, we have to apply gate current/voltage continuously to these devices.

A DEVICE which converts dc power into ac power at DESIRED output voltage and FREQUENCY is called as Inverter.

Inverters are BROADLY classified into

• Voltage Source Inverter (VSI)
• Current Source Inverter (CSI)

A device which converts dc power into ac power at desired output voltage and frequency is called as Inverter.

Inverters are broadly classified into

• di/dt is the rate of change of current in a device.
• When SCR is forward biased and is turned ON by the gate signal, the anode current flows.
• The anode current requires some time to spread INSIDE the device. (Spreading of charge carriers)
• But if the rate of RISE of anode current (di/dt) is greater than the spread velocity of charge carriers then local hot spots is created near the gate DUE to INCREASED current density. This localized heating may damage the device.
• Local spot heating is avoided by ensuring that the conduction spreads to the whole area very rapidly. (OR) The di/dt value must be maintained below a threshold (limiting) value.
• This is done by means of connecting an inductor in series with the thyristor.
• The inductance L opposes the high di/dt variations.
• When the current variation is high, the inductor smooths it and protects the SCR from damage. (Though di/dt variation is high, the inductor ‘L’ smooths it because it takes some time to charge). L ≥ [VS / (di/dt)]

dv/dt Protection:-

• dv/dt is the rate of charge of voltage in SCR.
• We know that iC=C.dv/dt. ie, when dv/dt is high, iC is high.
• This high current(iC) may turn ON SCR even when gate current is zero. This is called as dv/dt turn ON or false turn ON of SCR.
• To protect the thyristor against false turn ON or against high dv/dt a “Snubber Circuit” is used.

dv/dt Protection:-

• For reliable OPERATION of SCR, it should be operated within the specific RATINGS.
• SCRs are very delicate devices and so they MUST be protected against abnormal operating conditions.

Various PROTECTION of SCR is

1. di/dt Protection
2. dv/dt Protection
3. Over voltage Protection
4. Over Current Protection

Various protection of SCR is