(C)Element 8-26 Power supplies

Question 1

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

Answer 1

if it does not use a line isolation transformer you may destroy the supply with ground test equipment

Question 2

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?

Answer 2

89.1 volts

Question 3

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?

Answer 3

pulsating DC much higher than line frequency

Question 4

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.

Answer 4

The leading edge of the output waveform occurs 180 degrees after positive zero crossing of the input waveform.

Question 5

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?

Answer 5

greater than 2 volts

Question 6

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.

Answer 6

Upper trip point is +1.285 volts. Lower trip point is -1.285 volts.