Subelement C: Receiving Systems – 10 Key Topics – 10 Exam Questions – 4 Drawings— Topic 26: Power Supplies
Question 8-26C2
Element 8 (RADAR)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?
Explanation
A switching power supply (SMPS) regulates its output voltage by rapidly turning an input voltage ON and OFF, controlling the ratio of the "ON" time to the total switching period. This ratio is known as the **duty cycle (D)**. For a step-down (buck) converter, which is typical when the output voltage is less than the input, the output voltage (Vout) is the input voltage (Vin) multiplied by the duty cycle.
1. **Calculate the total switching period (T):**
T = 1 / frequency = 1 / 20 kHz = 1 / 20,000 Hz = 0.00005 seconds = 50 microseconds (µs).
2. **Determine the duty cycle (D):**
D = ON time / Period = 27 µs / 50 µs = 0.54.
3. **Calculate the output voltage (Vout):**
Vout = Vin * D = 165 V * 0.54 = 89.1 V DC.
This efficient voltage regulation method, minimizing resistive losses, is fundamental in modern radio equipment. The other options are incorrect as they either assume a different type of converter (e.g., boost) or an incorrect understanding of the duty cycle's effect on output voltage.
Related Questions
8-25C6 The sensitivity time control (STC) circuit:8-26C1 Prior to making “power-on” measurements on a switching power supply, you should be familiar with the supply because of the following: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?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.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?