GATE EE 2021 Set 1 — Question 35
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Analog & Digital Electronics → Diode Circuits → Zener Regulator
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Question
In the circuit shown, a Zener diode is used to regulate the voltage across load . The input is an unregulated DC voltage with a minimum value of and a maximum value of . The value of is . The Zener diode has a maximum rated power dissipation of . Assuming the Zener diode to be ideal, the minimum value of is __________ .
Correct answer
29 to 31
Solution
Given:
To maintain regulation, the Zener diode must be in the breakdown region, meaning . Also, to avoid damage, .
The source current is .
By KCL, , where is the load current.To find the minimum value of , we need to maximize while ensuring regulation () for all input voltages.
For a fixed , the worst-case for maintaining regulation is when is at its minimum (), as this results in the minimum source current. At , we must have , so:
Now check the maximum power dissipation constraint at with :
Since , the Zener diode is safe.Thus, the minimum value of is .
To maintain regulation, the Zener diode must be in the breakdown region, meaning . Also, to avoid damage, .
The source current is .
By KCL, , where is the load current.To find the minimum value of , we need to maximize while ensuring regulation () for all input voltages.
For a fixed , the worst-case for maintaining regulation is when is at its minimum (), as this results in the minimum source current. At , we must have , so:
Now check the maximum power dissipation constraint at with :
Since , the Zener diode is safe.Thus, the minimum value of is .
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