GTO gate drive circuit
Gate drive circuit of a GTO:
A GTO gate drive has to fulfill the following functions:
- Turn the GTO on by means of a high current pulse (IGM)
- Maintain conduction through provision of a continuous gate current (IG, also known as the “back-porch current”).
- Turn the GTO off with a high negative gate current pulse.
- Reinforce the blocking state of the device by a negative gate voltage.
- It is assumed that there is a potential difference of several kVs between the master control and individual gate units.
- The ON and OFF pulses for a GTO is communicated to individual gate units through fiber optic cables. These optical signals are converted to electrical signals by a optical electrical converter. These electrical signals through the control logic then produces the ON and OFF signal for the output stage which in turn sends positive and negative gate current to the GTO.
- Depending on the requirement the control logic may also supervise GTO conduction by monitoring the gate-cathode voltage. Any fault is relayed back via fiber optic cable to the master control. Power supply for the Gate drive units are derived from a common power supply through a high frequency SMPS (Blocks A, B & C) arrangement.
- The top switch T1 sends positive gate pulse to the GTO gate. At the instant of turn on of T1 ,C2 acts almost as a short circuit and the positive gate current is determined by the parallel combination of R1 and R2. However, at steady state only R1 determines the gate current IG.
- The bottom switch T2 is used for biasing the GTO gate negative with respect to the cathode. Since, relatively large negative gate current flows during turn off, no external resistance is used in series with T2.
- The ON state resistance of T2 is utilized for this purpose. In practice, a large number of switches are connected in parallel to obtain the required current rating of T2. A low value resistance R3 is connected between the gate and the cathode terminals of the GTO to ensure minimum forward blocking voltage.