8-C-26: Power Supplies
8-26C1:
Prior to making “power-on” measurements on a switching power supply, you should be familiar with the supply because of the following:
If it does not use a line isolation transformer you may destroy the supply with grounded test equipment.
You need to know where the filter capacitors are so they can be discharged.
It is not possible to cause a component failure by using ungrounded test equipment.
So that measurements can be made without referring to the schematic.
8-26C2:
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?
89.1 volts DC.
It cannot be determined with the information given.
305.55 volts DC.
165 volts DC.
8-26C3:
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?
Pulsating DC much higher than line frequency.
Filtered DC.
Pulsating DC at line frequency.
AC at line frequency.
8-26C4:
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.
The leading edge of the output waveform occurs 180 degrees after positive zero crossing of the input waveform.
The rising edge of the output waveform trails the positive zero crossing of the input waveform by 45 degrees.
The rising edge of the output waveform trails the negative zero crossing of the input waveform by 45 degrees.
The rising edge of the output waveform trails the positive peak of the input waveform by 45 degrees.
8-26C5:
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?
Greater than 2 volts.
1 volt to 2 volts.
300 microvolts to 700 microvolts.
1.0 volt to 20.0 volts.
8-26C6:
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.
Upper trip point is +1.285 volts. Lower trip point is -1.285 volts.
Upper trip point is 4.5 volts. Lower trip point is approximately 0 volts.
Upper trip point is 2.5 volts. Lower trip point is approximately 2.0 volts.
Upper trip point is 900 microvolts. Lower trip point is approximately 0 volts.
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