Slow turn-on

08.12.2010 22:05

Continuing on the topic of thyristors, here's an interesting graph I just measured:

SCR anode-cathode voltage versus current

This is path anode-cathode voltage and current make on the I(U) graph during a single commutation cycle. The blue part of the plot is during the switch-on and the red part is during switch-off. Each dot represents a sample (sampling period was 10 μs)

Interesting thing to note here is that this graph shows the slow turn-on. Ideally the blue line should not deviate too much from the red one. The slowly rising line on the right means a considerable current flows through the device while there's still a large voltage drop on it, causing increased heat dissipation. This happens because after the gate pulse not all surface of the junction gets turned on at the same moment.

The second useful thing this graph shows is the magnitude of the voltage drop of the two PN junctions (around 1 V, quite low actually) and the equivalent series resistance (around 80 mΩ). These two components help determine the safe operating area of the device: the dissipated power on the constant voltage part depends on the average current value while the resistive part is sensitive to the RMS value.

Posted by Tomaž | Categories: Analog

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