Diode Circuits and Applications Questions and Answers

Q: A diode dissipates \( 1 \ \mathrm{W} \) with a thermal resistance of \( 60 \ \mathrm{^\circ C/W} \) in a \( 40 \ \mathrm{^\circ C} \) ambient. Its junction reaches

Adding the rise to the ambient:\[ T_{j} = 40 + (1 \times 60) = 100 \ \mathrm{^\circ C} \]giving 100 degrees, safely below a silicon limit of 150 but leaving little margin for a hotter ambient. Sixty degrees is the rise alone and forty the ambient alone. Whether a device survives is decided by the sum rather than either term, which is why an ambient specification is as much part of a rating as the power figure.

Q: With a heatsink halving that thermal resistance, the same dissipation gives a junction temperature of

Halving the resistance halves the rise:\[ T_{j} = 40 + (1 \times 30) = 70 \ \mathrm{^\circ C} \]so 70 degrees, a thirty degree improvement bought entirely with metal. Fifty degrees would need a quarter of the original resistance. Because the rise is proportional to the resistance, halving one halves the other exactly, which makes heatsink sizing one of the more straightforward calculations in a power design.

Q: Why does leakage current matter more in a hot rectifier?

Reverse saturation current roughly doubles for every ten degrees, so a device that leaks microamps at room temperature can leak milliamps when hot - and since that leakage dissipates power under reverse voltage, it can feed a thermal runaway. It does not reverse direction, remains largely independent of reverse voltage, and stays far below the forward current. Any mechanism whose effect grows with the temperature it produces has to be checked for runaway rather than merely tabulated.

Q: What is the practical consequence of a diode's exponential characteristic for measurement?

Because sixty millivolts changes the current tenfold, setting a diode's current by applying a voltage is hopelessly imprecise, and the current must be set by a series element. The forward drop is easy to measure at a known current, leakage is measurable with care, and frequency is a separate matter. Every device with an exponential characteristic is driven from a current rather than a voltage for exactly this reason, which is why the same arrangement appears with light emitting diodes and transistor base junctions.

151.

A diode dissipates \( 1 \ \mathrm{W} \) with a thermal resistance of \( 60 \ \mathrm{^\circ C/W} \) in a \( 40 \ \mathrm{^\circ C} \) ambient. Its junction reaches

A.40 degrees
B.100 degrees
C.60 degrees
D.140 degrees

Answer: B

Adding the rise to the ambient:

\[ T_{j} = 40 + (1 \times 60) = 100 \ \mathrm{^\circ C} \]

giving 100 degrees, safely below a silicon limit of 150 but leaving little margin for a hotter ambient. Sixty degrees is the rise alone and forty the ambient alone. Whether a device survives is decided by the sum rather than either term, which is why an ambient specification is as much part of a rating as the power figure.

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152.

With a heatsink halving that thermal resistance, the same dissipation gives a junction temperature of

A.50 degrees
B.130 degrees
C.70 degrees
D.100 degrees

Answer: C

Halving the resistance halves the rise:

\[ T_{j} = 40 + (1 \times 30) = 70 \ \mathrm{^\circ C} \]

so 70 degrees, a thirty degree improvement bought entirely with metal. Fifty degrees would need a quarter of the original resistance. Because the rise is proportional to the resistance, halving one halves the other exactly, which makes heatsink sizing one of the more straightforward calculations in a power design.

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153.

Why does leakage current matter more in a hot rectifier?

A.It reverses direction above a critical temperature
B.It becomes proportional to the reverse voltage applied
C.It exceeds the forward current at high temperatures
D.It roughly doubles every ten degrees of rise

Answer: D

Reverse saturation current roughly doubles for every ten degrees, so a device that leaks microamps at room temperature can leak milliamps when hot - and since that leakage dissipates power under reverse voltage, it can feed a thermal runaway. It does not reverse direction, remains largely independent of reverse voltage, and stays far below the forward current. Any mechanism whose effect grows with the temperature it produces has to be checked for runaway rather than merely tabulated.

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154.

What is the practical consequence of a diode's exponential characteristic for measurement?

A.A small voltage error gives a large current error
B.The forward drop cannot be measured accurately at all
C.The reverse leakage becomes impossible to determine
D.The device cannot be used above a few kilohertz

Answer: A

Because sixty millivolts changes the current tenfold, setting a diode's current by applying a voltage is hopelessly imprecise, and the current must be set by a series element. The forward drop is easy to measure at a known current, leakage is measurable with care, and frequency is a separate matter. Every device with an exponential characteristic is driven from a current rather than a voltage for exactly this reason, which is why the same arrangement appears with light emitting diodes and transistor base junctions.

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155.

Why is a small-signal diode's dynamic resistance inversely proportional to its current?

A.Its stored charge grows in proportion to current
B.Differentiating the exponential law gives that result
C.Its bulk resistance falls as the device warms up
D.Its depletion capacitance falls as bias increases

Answer: B

Differentiating the diode equation gives a slope resistance of the thermal voltage divided by the current, so at one milliamp it is about 26 ohms and at ten it is about 2.6. Bulk resistance, capacitance and stored charge are all real effects but none produces this relation. A current-controlled resistance falling as one over the current is what makes diodes and transistor junctions useful as variable elements, and the same expression reappears throughout small-signal analysis.

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