FCC Commercial Element 8 pool, section 26

Subelement C

Receiving Systems

Section 26

Power Supplies

826C1

Prior to making "power-on" measurements on a switching power supply, you should be familiar with the supply because of the following:

A.
You need to know where the filter capacitors are so they can be discharged.
Correct Answer
B.
If it does not use a line isolation transformer you may destroy the supply with grounded test equipment.
C.
It is not possible to cause a component failure by using ungrounded test equipment.
D.
So that measurements can be made without referring to the schematic.

826C1

This question does not yet have an explanation! Register to add one

Tags: none

826C2

A constant frequency switching power supply regulator with an input voltage of 165 volts DC, and a switching frequency of 20 kHz, has an "ON" time of 27 microseconds when supplying 1 ampere to its load. What is the output voltage across the load?

A.
It cannot be determined with the information given.
B.
305.55 volts DC.
Correct Answer
C.
89.1 volts DC.
D.
165 volts DC.

826C2

From Alov:

 Vo=V in x D, 

 D= T on x F

To find the output voltage across the load, we can use the formula for the average output voltage of a switching power supply:

Output Voltage = (ON Time / Switching Period) * Input Voltage

First, let's calculate the switching period:

Switching Period = 1 / Switching Frequency Switching Period = 1 / 20,000 Hz Switching Period ≈ 0.00005 seconds (50 microseconds)

Now, we can use the formula to calculate the output voltage:

Output Voltage = (27 microseconds / 50 microseconds) * 165 volts DC Output Voltage ≈ (0.54) * 165 volts DC Output Voltage ≈ 89.1 volts DC

Explanation: The switching power supply operates by rapidly turning ON and OFF its switching element (usually a transistor) at a high frequency (20 kHz in this case). The ON time is the duration for which the switching element is turned ON during each switching cycle.

The average output voltage across the load is calculated by dividing the ON time by the switching period (the time for one complete ON-OFF cycle) and then multiplying it by the input voltage.

Last edited by kacela. Register to edit

Tags: none

826C3

The circuit shown in Fig. 8C10 is the output of a switching power supply. Measuring from the junction of CR6, CR7 and L1 to ground with an oscilloscope, what waveform would you expect to see?

A.
Filtered DC.
B.
Pulsating DC at line frequency.
C.
AC at line frequency.
Correct Answer
D.
Pulsating DC much higher than line frequency.

826C3

This question does not yet have an explanation! Register to add one

Tags: none

826C4

With regard to the comparator shown in Fig. 8C11, the input is a sinusoid. Nominal high level output of the comparator is 4.5 volts. Choose the most correct statement regarding the input and output.

Correct Answer
A.
The leading edge of the output waveform occurs 180 degrees after positive zero crossing of the input waveform.
B.
The rising edge of the output waveform trails the positive zero crossing of the input waveform by 45 degrees.
C.
The rising edge of the output waveform trails the negative zero crossing of the input waveform by 45 degrees.
D.
The rising edge of the output waveform trails the positive peak of the input waveform by 45 degrees.

826C4

A comparator is an electronic device that compares the amplitude of two input signals and produces an output based on their relative levels. In this case, the input to the comparator is a sinusoidal waveform.

When the input waveform is a sinusoid, it goes through positive and negative zero crossings as it oscillates. The "positive zero crossing" refers to the point where the sinusoid changes direction from a negative value to zero and then becomes positive.

For the comparator output, the "leading edge" is the rising edge of the waveform. In this context, it means the point where the output transitions from a low voltage level to a high voltage level.

The statement indicates that the leading edge of the output waveform occurs 180 degrees after the positive zero crossing of the input waveform. This means that the output waveform rises to its high level after the input waveform crosses the positive zero point and continues to rise for half a cycle of the sinusoid.

Mnemonic: "POLE - Positive, Output, Leading, Edge"

Last edited by kacela. Register to edit

Tags: none

826C5

When monitoring the gate voltage of a power MOSFET in the switching power supply of a modern RADAR, you would expect to see the gate voltage change from "low" to "high" by how much?

A.
1 volt to 2 volts.
B.
300 microvolts to 700 microvolts.
Correct Answer
C.
Greater than 2 volts.
D.
1.0 volt to 20.0 volts.

826C5

This question does not yet have an explanation! Register to add one

Tags: none

826C6

The nominal output high of the comparator shown in Fig. 8C11 is 4.5 volts. Choose the most correct statement which describes the trip points.

A.
Upper trip point is 4.5 volts. Lower trip point is approximately 0 volts.
B.
Upper trip point is 2.5 volts. Lower trip point is approximately 2.0 volts.
C.
Upper trip point is 900 microvolts. Lower trip point is approximately 0 volts.
Correct Answer
D.
Upper trip point is +1.285 volts. Lower trip point is -1.285 volts.

826C6

This question does not yet have an explanation! Register to add one

Tags: none

Log in

- or -