8-18B: Power Supplies

Question 1

8-18B2

The characteristics of a field-effect transistor (FET) used in a modern RADAR switching power supply can be compared as follows:

A. “On” state compares to a bipolar transistor.
“Off” state compares to a 1-Megohm resistor.
B. “On” state compares to a pure resistor.
“Off” state compares to a mechanical relay.
C. “On” state compares to an low resistance inductor.
“Off” state compares to a 10-Megohm resistor.
D. “On” state compares to a resistor.
“Off” state compares to a capacitor.

Answer 1

ANSWER B
The field-effect transistor (FET) is the popular choice in switching power supplies where it is desireable for the ON state to be low resistance and the OFF state to offer infinite resistance similar to an open mechanical relay.

Question 2

8-18B3

A pulse-width modulator in a switching power supply is used to:

A. Provide the reference voltage for the regulator.
B. Vary the frequency of the switching regulator to control the output voltage.
C. Vary the duty cycle of the regulator switch to control the output voltage.
D. Compare the reference voltage with the output voltage sample and produce an error voltage.

Answer 2

ANSWER C
As the duty cycle (on time / period of cycle) of the power supply increases the votage increases. Therefore, you can use the duty cycle to regulate the output voltage.

Question 3

8-18B4

ln a fixed-frequency switching power supply, the pulse width of the switching circuit will increase when:

A. The load impedance decreases.
B. The load current decreases.
C. The output voltage increases.
D. The input voltage increases.

Answer 3

Answer A
A switching-mode power supply drops and regulates the output voltage by chopping the dc input into a square wave, and the recovering the voltage with a low-pass filter. As the input voltage or the load impedance changes, the duty cycle of the on/off cycle changes and keeps the voltage constant.