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Thyristors
25 questions By Tony R. Kuphaldt
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Question 16 of 25
One way that SCRs may be triggered into their “on” state is by a transient voltage applied between the anode and cathode terminals. Normally, this method of triggering is considered a flaw of the device, as it opens the possibility of unwanted triggering resulting from disturbances in the power supply voltage.
Explain why a high \(\frac{dv}{dt}\) present on the power supply rail is able to trigger an SCR, with reference to the SCR’s equivalent circuit. Also suggest what means might be employed to prevent false triggering from power supply transients.
Reveal answerParasitic (Miller-effect) capacitances inside the SCR’s bipolar structure make the device vulnerable to voltage transients, large \(\frac{dv}{dt}\) rates creating base currents large enough to initiate conduction. Snubber circuits are typically provided to mitigate these effects:
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Notes:The expression \(\frac{dv}{dt}\) is, of course, a calculus term meaning rate-of-change of voltage over time. An important review concept for this question is the “Ohm’s Law” formula for a capacitance:
i = C dv dtOnly by understanding the effects of a rapidly changing voltage on a capacitance are students able to comprehend why large rates of [dv/dt] could cause trouble for an SCR.
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Question 17 of 25
Identify three different ways that an SCR or a TRIAC may be triggered into its “on” (conducting) state:
- 1.
- 2.
- 3.
Reveal answer- 1. Applying a voltage pulse at the gate terminal
- 2. Exceeding the anode-to-cathode “breakover” voltage
- 3. Exceeding the “critical rate of rise” for anode-cathode voltage \(\frac{dv}{dt}\)
Notes:Although gate triggering is by far the most common method of initiating conduction through SCRs and TRIACs, it is important that students realize it is not the only way. The other two methods, both involving voltage applied between the anode and cathode terminals (or MT1-MT2 terminals) of the device, are often accidental means of triggering.
Be sure to discuss with your students the reason why excessive [dv/dt] can trigger a thyristor, based on an examination of inter-electrode capacitance within the transistors of a thyristor model.
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Question 18 of 25
Identify two different ways that an SCR or a TRIAC may be forced into its “off” (non-conducting) state:
- 1.
- 2.
Reveal answer- 1. Low current dropout (interrupting current with some other switching device)
- 2. “Reverse-firing” the gate with a voltage pulse of the “wrong” polarity
Notes:Although low-current dropout is the most common method of terminating conduction through SCRs and TRIACs, it is important that students realize it is not the only way. The other method, though, is often very difficult to achieve with ordinary SCRs or TRIACs.
Can someone please explain why do we need C1 capacitor in question 22 ? Circuit behaves exactly the same without it and I am confused about it’s purpose.