# 8.8: Problems

• • Contributed by James M. Fiore
• Professor (Electrical Engineering Technology) at Mohawk Valley Community College

## 8.8.1: Review Questions

1. What is the function of a voltage regulator?

2. What is the difference between load regulation and line regulation?

3. Why do regulators need a reference voltage?

4. What is the functional difference between a linear regulator and a switching regulator?

5. What are the main advantages of using linear regulators versus switching regulators?

6. What are the main advantages of using switching regulators versus linear regulators?

7. What is the function of a pass transistor?

8. Describe two ways in which to increase the output current of an IC-based regulator.

9. How can fixed “three-pin” regulators be used to regulate at other than their rated voltage?

10. What is the purpose of the output inductor and capacitor in the switching regulator?

11. Explain the correlation between the output current demand and the pulsewidth modulator used in switching regulators.

12. What is the purpose of a heat sink?

13. What is meant by the term thermal resistance?

14. What are the thermal resistance elements that control heat flow in a typical power-device/heat-sink connection?

15. What are the general rules that should be considered when using heat sinks?

## 8.8.2: Problems

### Analysis Problems

1. If the average input voltage to the circuit of Problem 9 is 22 V, determine the maximum device dissipation for a 900 mA output.

2. If the average input voltage is 25 V for the circuit of Problem 11, determine the maximum output current for each output voltage. Use the TO-220 case style ($$P_D$$ = 15W, $$I_{limit}$$ = 1.5A).

3. Draw a block diagram of a complete $$\pm$$12 V regulated power supply using LM78XX and LM79XX series parts.

4. Determine the maximum allowable thermal resistance for a heat sink given the following: Ambient temperature = 50$$^{\circ}$$C, maximum operating temperature = 150$$^{\circ}$$C, TO-3 case style with thermal grease and Thermalfilm isolator, power dissipation is 30 W, and the device's thermal resistance is 1.1 C$$^{\circ}$$/W, junction to case.

5. A pass transistor has the following specifications: maximum junction temperature = 125$$^{\circ}$$C, TO-220 case, junction to case thermal resistance = 1.5 C$$^{\circ}$$/W. Determine the maximum power dissipation allowed if this device is connected to a 20 C$$^{\circ}$$/W heat sink with thermal grease, using a 0.003 mica insulator. The ambient temperature is 35$$^{\circ}$$C.

6. The thermal resistance of the LM723 is 25 C$$^{\circ}$$/W, junction to case. Its maximum operating temperature is 150$$^{\circ}$$C. For a maximum dissipation of 500 mW and an ambient temperature of 30$$^{\circ}$$C, determine the maximum allowable thermal resistance for the heat-sink/insulator-interface combination.

### Design Problems

7. Using Figure 8.3.1, design a 15 V regulator using a 3.3 V Zener. The Zener bias current should be 2 mA, the output should be capable of 500 mA.

8. Using Figure 8.3.1 as a guide, design a variable power supply regulator with a 5 to 15 V output range using a 3.9 V Zener. $$I_{zener}$$ = 3 mA.

9. Design a +12 V regulator using the LM317. The output current capability should be at least 900 mA.

10. Design a +3 to +15 V regulator using the LM317. The output should be continuously variable.

11. Using the LM317, configure a regulator to produce either +5V, +12V, or +15V.

12. Design a +12 V regulator using the LM7805.

13. Design a +9 V regulator using the LM723. Use a current limit of 100 mA.

14. Design a +5 V regulator with 100 mA current limiting using the LM723.

15. Configure a $$\pm$$12 V regulator with 70 mA current limiting. Use the LM326

16. Reconfigure the circuit of Problem 15 for $$\pm$$15 V.

17. Using the LM3578A, design a 5 V, 400 mA, regulator. The input voltage is 15 V. Use a discontinuity factor of 0.2, and an oscillator frequency of 75 kHz. No more than 10 mV of ripple is allowed.

18. Repeat Problem 17 for a 9 V output.

### Challenge Problems

19. Based on the LM723 adjustable regulator example, design a regulator that will produce a continuously variable output from 5 V to 12 V.

20. The LM317 has a maximum operating temperature of 125$$^{\circ}$$C. The TO-220 case version shows a thermal resistance of 4 C$$^{\circ}$$/W, junction to case. It also shows 50 C$$^{\circ}$$/W, junction to ambient (no heat sink used). Assuming an ambient temperature of 50$$^{\circ}$$C. What is the maximum allowable power dissipation for each setup? Assume that the first version uses a 15 C$$^{\circ}$$/W heat sink with a 2 C$$^{\circ}$$/W case to heat sink interconnection.

21. Forced air cooling of a heat-sink/power-device can significantly aid in removing heat energy. As a rule of thumb, forced air cooling at a velocity of 1000 feet per minute will effectively increase the efficiency of a heat sink by a factor of 5. Assuming such a system is applied to the circuit of Problem 17, calculate the new power dissipation.

22. An LM317 (TO-3) is used for a 5 V, 1 A power supply. The average voltage into the regulator is 12 V. Assume a maximum operating temperature of 125$$^{\circ}$$C, and an ambient temperature of 25$$^{\circ}$$C. First, determine whether or not a heat sink is required. If it is, determine the maximum acceptable thermal resistance for the heat-sink/insulator combination. For the LM317, thermal resistance = 2.3 C$$^{\circ}$$/W, junction to case, and 35 C$$^{\circ}$$/W, junction to ambient.

### Computer Simulation Problems

23. Using a simulator, plot the time-domain response of the circuit of Figure 8.3.10, assuming an input of 22 V with 3 V peak ripple. How does the simulation change if the ripple is increased to 8 V peak?

24. Verify the output waveform for the circuit of Figure 8.3.14 using a simulator. Use various loads in order to test the current limit operation. The source is 18 V DC, with 2 V peak ripple.

25. Verify the adjustment range for the regulator designed in Example 8.3.5 in the text using a simulator. Use a load of 200 $$\Omega$$, and a source equal to 10 V, with 1 V peak ripple.

26. Use several different loads with a simulator in order to test the current limit portion of the regulator designed in Example 8.3.5 in the text.