The following presentation is a part of the level 5 module -- Electronic Engineering. This resources is a part of the 2009/2010 Engineering (foundation degree, BEng and HN) courses from University of Wales Newport (course codes H101, H691, H620, HH37 and 001H). This resource is a part of the core modules for the full time 1st year undergraduate programme. The BEng & Foundation Degrees and HNC/D in Engineering are designed to meet the needs of employers by placing the emphasis on the theoretical, practical and vocational aspects of engineering within the workplace and beyond. Engineering is becoming more high profile, and therefore more in demand as a skill set, in today’s high-tech world. This course has been designed to provide you with knowledge, skills and practical experience encountered in everyday engineering environments.

1. Stabilised Power Supplies Electronic Engineering © University of Wales Newport 2009 This work is licensed under a Creative Commons Attribution 2.0 License .

3. Simple Power Supply Block Diagram Stabilised Power Supplies

5. Zener Diode This has the same forward characteristics of a normal diode, but in the reverse direction the diode breaks down at a well-defined level. This breakdown, unlike normal diodes is recoverable.

6. It is used in the following way. If Vin < Vzd the Zener Diode does not conduct, no voltage is dropped across the resistor Rs and Vout = Vin. If V in > Vzd then the zener diode conducts, so as to drop the excess voltage (Vin-Vzd) across the series resistor. Vout = Vzd. Stabilised Power Supplies Vin Vzd Vout Rs I L

7. When working normally : If Vin  Vout  Vzd  Izd  IRs  VRs  Vout  . If IL  IRs  VRs  Vout  Vzd  Izd  IRs  VRs  Vout  . Stabilised Power Supplies

8. Zener Diodes Stabilised Power Supplies V RBV 400 mW 500 mW 1.3 W V RBV 400 mW 500 mW 1.3 W 2.4 BZX79C2V4 BZX55C2V4 9.1 BZX79C9V1 BZX55C9V1 1N5346B 2.7 BZX79C2V7 BZX55C2V7 10 BZX79C10 BZX55C10 1N5347B 3.0 BZX79C3V0 BZX55C3V0 11 BZX79C11 BZX55C11 1N5348B 3.3 BZX79C3V3 BZX55C3V3 1N5333B 12 BZX79C12 BZX55C12 1N5349B 3.6 BZX79C3V6 BZX55C3V6 13 BZX79C13 BZX55C13 1N5350B 3.9 BZX79C3V9 BZX55C3V9 1N5335B 15 BZX79C15 BZX55C15 1N5352B 4.3 BZX79C4V3 BZX55C4V3 1N5336B 16 BZX79C16 BZX55C16 1N5353B 4.7 BZX79C4V7 BZX55C4V7 1N5337B 18 BZX79C18 BZX55C18 1N5355B 5.1 BZX79C5V1 BZX55C5V1 1N5338B 20 BZX79C20 BZX55C20 1N5357B 5.6 BZX79C5V6 BZX55C5V6 1N5341B 22 BZX79C22 BZX55C22 1N5358B 6.2 BZX79C6V2 BZX55C6V2 1N5342B 24 BZX79C24 BZX55C24 1N5359B 6.8 BZX79C6V8 BZX55C6V8 1N5343B 27 BZX79C27 BZX55C27 1N5361B 7.5 BZX79C7V5 BZX55C7V5 1N5344B 30 BZX79C30 BZX55C30 1N5363B 8.2 BZX79C8V2 BZX55C8V2 1N5345B

9. EXAMPLES. 1. A cars battery voltage is monitored are changes between 10.8v and 14.4v. We need to build a 9v supply to run an electronic device. The maximum current drawn by the device is 100 mA. Design a circuit and suggest component values. 2. An unstabilised power supply produces 9v  10%. We are using a 1N5338B to produce a fixed output voltage. What current can be certain of driving to the output circuit? In both cases the zener requires 5 mA flowing through it to maintain its voltage. Stabilised Power Supplies

10. Practically it is observed that Vout still changes by unacceptable levels if Vin and/or IL fluctuate excessively. By modifying the design we can maintain Vout even if changes are large. Vin charges excessive. E.g. Vin = 12 volts, Vzd 1 = 9 volts, Vzd 2 = 5 volts. As Vin changes, Vzd 1 will vary by a small amount and Vzd 2 by an even smaller amount. Vin Rs 1 Vzd 2 Rs 2 Vzd 1

11. If I LOAD changes excessive. I b (and hence changes in Izd) will be less than I LOAD (typically ) Therefore Vout will be much more stable. The disadvantage of the circuits that have been looked at so far is that there is no ability to set the output and the desired level. We can achieve this control by using a series control feedback regulator. LOAD I LOAD Ib

12. The Block Diagram for such a circuit is shown below: This can be realised in the following way. Stabilised Power Supplies REGULATOR COMPARATOR R E F E R E N C E F E E D B A C K Vin Vout

13. Regulator Current flows through R into the base of the transistor turning it on – current flows through Q1. If current is drawn from the base of Q1 then this will reduce the drive to Q1, and it will conduct less. Stabilised Power Supplies REGULATOR Q1 R

14. Comparator Vref and Vfb are compared in the following way: If Vfb > Vref + 0.6v then Q2 conducts more If Vfb < Vref + 0.6v then Q2 conducts less Stabilised Power Supplies COMPARATOR Q2 Vref Vfb

15. Reference The reference voltage generated will be equal to the zener voltage of the diode. Stabilised Power Supplies R E F E R E N C E Rs Zd Vref

16. Feedback The value of the feedback voltage Vfb is given by: Stabilised Power Supplies F E E D B A C K R1 R2 Vfb Vout

17. This can now be pieced together to give use a circuit for the Series Control Feedback Regulator. Q1 R Q2 Vfb Rs Zd Vref R1 R2 Vin Vout

18. The output voltage Vout is given by: but Rearranging gives us: Variable Output How can we change the value of Vout? 1. Change the value of Vref – change the zener (limited) 2. Change the values of the feedback resistors. If the feedback resistors are replaced by the resistor chain incorporating a variable resistor we can make the voltage variable over a fixed range of values.

19. EXAMPLES 1. We are using a 5.1v zener, Rv is a 1k  variable, Ra is 2.2k  and Rb is 4.7k  . Over what range can the output be varied? Are there any conditions which might effect this? 2. We require supply with a variable output of 10v to 15v suggest components to achieve this. Stabilised Power Supplies Ra Rb Rv

20. Current Limit This can be achieved by replacing Q1 with the following circuit: For D1 and D2 to conduct they require 0.6 + 0.6 = 1.2 volts across them. The voltage which is actually across them is Iload x Rlimit + Vbe of Q1 = Iload x Rlimit + 0.6 Q1 Vout D1 D2 R limit I load

21. So we can see that if Iload x Rlimit  0.6 the diodes conduct. When the diodes conduct the current which is normally flowing into the base of the transistor to turn it on flows through the diodes instead -- this has the effect of limiting the current flow through the transistor. The limiting point is when Iload x Rlimit = 0.6 Therefore Iload max = 0.6/ Rlimit or Rlimit = 0.6/Iload max Stabilised Power Supplies

22. Series Regulator Operation We can think of a Series Regulator in the following way. We can also control the output voltage by using a parallel regulator in the following way. Stabilised Power Supplies L O A D Regulator L O A D Regulator Resistor

23. The Parallel Regulator has the following advantages. 1 Built in current limit due to the resistor. 2 Constant current consumption? 3 Load current does not flow through the parallel regulator. Stabilised Power Supplies

24. Switching Regulators With both the series and parallel regulators there is a waste of power in the regulator. Switch-Mode Regulators overcome this by using a switch to control the power. The switch opens and closes and this charges and allows the capacitor to discharge accordingly. Stabilised Power Supplies L O A D

25. The duty cycle of the switch will determine the output voltage and will be controlled according to the load current. Large Load Resistance (small load current). Switch state Stabilised Power Supplies On Off Desired output Rapid charge up Slow discharge

26. Small Load Resistance (large load current). Stabilised Power Supplies Slow charge up Rapid discharge

27. The Block Diagram for this type of regulator is similar to that of a series regulator. The P.W.M. is a pulse width modulator. Stabilised Power Supplies SWITCH COMPARATOR R E F E R E N C E F E E D B A C K Vin Vout P.W.M.

28. The P.W.M. takes in the signal from the comparator and generates a fixed frequency rectangular waveform whose duty cycle is dependent upon the input. One design uses a simple triangular wave generator and a comparator. A comparator compares its two inputs and when the one on the + input is greater than the one on the – input the output is high and when the opposite way the output is low. Stabilised Power Supplies

29. To switch Triangular wave generator Signal from comparator - + Signal to the switch Triangular wave Signal to P.W.M. The operation is shown below Stabilised Power Supplies

30. In addition to the advantages of lower wasted power in the regulator, the switching regulator can also be configured to provide outputs, which the series and parallel regulator could not produce. Stabilised Power Supplies

31. Configurations available with SWITCHING REGULATORS Step-down Regulator S is controlled by the regulation circuitry. When the switch S is closed it conducts - current flows through L to charge C and to supply the output. The inductor has a magnetic field and the potential across the inductor is positive + on the left, and negative - on the right. The diode will not conduct. + - + - + - + - L C D S Vout Vin

32. When the switch is opened, the magnetic field in the inductor collapses generating a reversed voltage. This maintains a charging current to the capacitor via the diode. This ensures that the ripple due to the switching on and off is minimised as the output is always being supplied. - + + - + - Stabilised Power Supplies L C D S Vout Vin

33. Step Up Regulator. With the switch S closed it conducts and a magnetic field is built up within the coil. Polarity is positive + on the left, negative - on the right. The diode does not conduct as its anode is at ground potential. + - + - Stabilised Power Supplies L C D S Vout Vin

34. With S open the field in the inductor collapses - the polarity reverses and adds to the input. This now charges the capacitor C via the diode D. As the inductor voltage and the input voltage are summed, Vout > Vin. - + + - + - Stabilised Power Supplies L C D S Vout Vin

35. Inverting With S closed, current flows through L building up a magnetic field, positive + at the top. The diode does not conduct as the cathode is positive. + - + - Stabilised Power Supplies L C D S Vout Vin

36. + - - + - + - + With S open the magnetic field collapses and the voltage across it reverses and C charges negatively to give a negative value for Vout. Note. With the second two methods of connecting to the output, only limited current can be drawn from the output. L C D S Vout Vin

37. This is useful where we have a single supply and we need reference voltage of other levels or polarities around a circuit: Stabilised Power Supplies 5v supply Step up Invert 9v 5v -5v GND

38. I. C. Regulators There are available a range of integrated circuit regulators, examples of these are:- Positive Output LM 340 series and the 78XX series e.g. LM 340-12 and 7812 + 12 volt regulators LM 340 -5 and 7805 + 5 volt regulators Negative Output LM 320 series and the 79XX series e.g. LM 320-9 and 7909 - -9 volt regulator LM 320-18 and 7918 - -18 volt regulator Stabilised Power Supplies

39. Maximum current 5A Stabilised Power Supplies

40. Maximum current 1A/1.5A 7805T – 1A 7805K – 1.5A Maximum current 100mA Stabilised Power Supplies

41. It is possible to use a fixed regulator to produce a different (larger) output. This is done in the following way. As this is a 5 volt regulator. The voltage across R1 = 5 V As long as I is small compared to IR 1 but 7805 Vin Vout R 1 R 2 I

42. Varying Output There is available an I.C. specifically designed for varying output. Stabilised Power Supplies LM317 Vin Vout R 1 R 2

43. This resource was created by the University of Wales Newport and released as an open educational resource through the Open Engineering Resources project of the HE Academy Engineering Subject Centre. The Open Engineering Resources project was funded by HEFCE and part of the JISC/HE Academy UKOER programme. © 2009 University of Wales Newport This work is licensed under a Creative Commons Attribution 2.0 License . The JISC logo is licensed under the terms of the Creative Commons Attribution-Non-Commercial-No Derivative Works 2.0 UK: England & Wales Licence. All reproductions must comply with the terms of that licence. The HEA logo is owned by the Higher Education Academy Limited may be freely distributed and copied for educational purposes only, provided that appropriate acknowledgement is given to the Higher Education Academy as the copyright holder and original publisher. The name and logo of University of Wales Newport is a trade mark and all rights in it are reserved. The name and logo should not be reproduced without the express authorisation of the University. Stabilised Power Supplies