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Posted on 23 April 2019

Driver Signal Pulse Length

 

 

 

 

 

 

 

For the majority of thyristors, the minimum pulse length is 10 μs. Rectifiers with negative voltage and AC power controllers with inductive load, however, cannot trigger reliably until the voltage across the thyristors is positive. Long driver pulses are needed to ensure that this happens.

For the driver transformer, long pulse duration means a larger voltage/time area, i.e. a larger (and hence more expensive) driver unit. The pulse length of the usable driver signal is determined by the main inductance of the transformer. The main inductance of the transformer is determined by the permeability of the core material and is temperature dependent. Very often the voltage/time area Vdt (µVs) is also given; this can then be used to calculate the maximum pulse duration.

In practice, drive pulses > 1 ms are virtually impossible to achieve. For this reason, in such cases a chain of short pulses is often used instead of a single long pulse (frequency 5…10 kHz). If the gaps between pulses cause interference, a second pulse chain can be superposed to achieve a long gap-free pulse. In any case, the transformer only has to be dimensioned for the brief duration of one single pulse in the chain (for 7 kHz e.g. around 70 μs).The diagram above shows several end-stage circuits of driver devices with corresponding pulse shapes.

A diode is always connected between the transformer and thyristor. This is intended to suppress negative drive pulses generated by backswing that the thyristor cannot cope with.

For more information, please read:

Drive Pulse Generation

Driver Units for Thyristors

Pulse Transformer Requirements

 

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