8-26C: Power Supplies Flashcards
Receiving Systems (4 cards)
8-26C1
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.
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.
ANSWER B
When working on a switching power supply that is turned on, keep in mind that it may not use an isolation transformer and the internal resistance of your test equipment could destory the transistors, unless your test porobe is specifically designed for use with switching power supplies.
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?
A. It cannot be determined with the information given.
B. 305.55 volts DC.
C. 89.1 volts DC.
D. 165 volts DC.
ANSWER C
The formula for the output voltage of a switching power supply is:
Vo=Vin X D
Where Vo = Output voltage in volts
Vin= Input voltage in volts
D= duty cycle = Ton x f
Since D = 27 x 10E-6 X 20 X 10E3 = 0.540
Vo= 165 x .540 = 89.1 V
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?
A. 1 volt to 2 volts.
B. 300 microvolts to 700 microvolts.
C. Greater than 2 volts.
D. 1.0 volt to 20.0 volts.
ANSWER C
The MOSFET is the preferred transistor in switching power supplies, and the MOSFET gate voltage change is always greater than 2.0 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.
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.
D. Upper trip point is +1.285 volts.
Lower trip point is -1.285 volts.
ANSWER D
In a comparator shown in Fig. 8C11we have assigned R1 to the upper resistor and R2 to the lower resistor. The comparator operates from a 15V and 25V supply. The output swings from 14.5V to 24.5V. You calculate the trip points as follows:
Vut=VomaxR2/(R1+R2)
4.5 x 2,000/(5000 + 2000) = 1.285V
Vlt=-Vomax x R2/(R1 + R2)
4.5 x 2,000/(5000 + 2000) = -1.285V
