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Posted on 12 June 2019

Pulse Transformer Requirements

 

 

 

 

 

 

 

In addition to pulse duration and amplitude, a driver pulse transformer has to meet other requirements as well.

In bridge circuits, in particular, the cathode potentials of the individual thyristors are not only different but also change rapidly in step with the operating frequency. Owing to the winding capacitances CP of the transformers, charge/discharge currents occur which flow through the control circuits of the thyristors and might trigger the thyristors at the wrong moment. This is particularly dangerous in inverter applications. To prevent this, pulse transformers have to be very low in capacitance and use dual shielding. One shield is connected to the earth potential (ground) of the driver device, while the other is connected to the cathode terminal of the thyristor.

In addition, pulse transformers have to have an isolation voltage that corresponds to the nominal operating voltage. For 400 V networks, 2.5 kV is common, while for 690 V networks, 4 kV is typical.

For transformers, the isolation voltage results in minimum requirements for the clearance and creepage distances (IEC 60664) between the terminals. Level adjustments between the primary and secondary side can be made via the winding ratio. Typical ratios here are 2:1 or 3:1.

Drive pulse transformers not only have to be designed for high isolation voltage, but also have to be immune to fast voltage changes.

 

For more information, please read:

Driver Signal Pulse Length

Drive Pulse Generation

Driver Units for Thyristors

 

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