1. CHAPTER 2 DIODE APPLICATIONS
DMT 121 – ELECTRONIC 1
CHAPTER 2
DIODE APPLICATIONS

2. Load Line Analysis VD= 0.7 V
The analysis of electronic circuits can follow one of the two paths :
Actual characteristic or approximate model of the device.
Approximate model will be always used in the analysis
VD= 0.7 V

3. Load Line Analysis
The load line plots all possible current (ID) conditions for all voltages applied to the diode (VD) in a given circuit. E / R is the maximum ID and E is the maximum VD.
Where the load line and the characteristic curve intersect is the Q-point, which specifies a particular ID and VD for a given circuit.
Fig. 2.1 Drawing the load line and finding the point of operation

4. Load Line Analysis The intersection of load line in Fig. 2.2 can
be determined by applying Kirchhoff’s
voltage in the clockwise direction, which
results in:
Fig. 2.2 Series diode configuration
ID and VD are the same for Eq. (2.1) and plotted load line in Fig. 2.2 (previous slide).
Set VD = 0 then we can get ID, where
Set ID = 0 then we get VD, where

5. Example
For the series diode configuration of Fig. 2.3a, employing the diode characteristics of Fig. 2.3b, determine VDQ, IDQ and VR
Fig (a) Circuit; (b) characteristics.

6. Solution From the result, plot the straight line across ID and VD.
The resulting load line appears in Fig The Q points occurred at
VDQ  0.78 V IDQ  18.5mA
VR=IRR=IDQR=(18.5 mA)(1k) =18.5 V

7. Figure 2.4

8. Example
For the series diode configuration of Fig. 2.13, determine VD, VR and ID.
Solution:

9. Example Repeat example 2.4 with the diode reversed Solution:
Open Circuit

10. Diode as Rectifier
Rectifier: An electronic circuit that converts AC to pulsating DC.
Basic function of a DC power supply is to convert an AC voltage to a smooth DC voltage.

11. Half-Wave Rectifier The diode conducts during the positive half cycle.
The diode does not conducts during the negative half cycle.

12. Sinusoidal Input: Half Wave Rectification
Fig Half-wave rectifier.
Fig Nonconduction region (T/2  T).
Fig Conduction region (0  T/2).

13. Average Value of Half Wave Output Voltage
The average value of the half-wave rectified output voltage (also known as DC voltage) is
The process of removing one-half the input signal to establish a dc level is called half-wave rectification

14. Example
What is the average value of the half-wave rectified voltage? Solution: Vm/π = 15.9 V

15. Effect of Barrier Potential (Silicon diode)
Applied signal at least 0.7 for diode to turn on (Vk = 0.7V)
i ≤ 0.7 V  diode in open circuit and o = 0V
When conducting, Vk=0.7V ,then o= i – Vk  this cause reduction in o, thus reduce the resulting dc voltage level.
Now Vdc  (Vm – Vk)

16. Example
Draw the output voltages of each rectifier for the indicated input voltages.

17. Peak Inverse Voltage (PIV)
PIV=peak input voltage and is the maximum voltage across the diode when it is not conducting/reverse bias.
Can be found by applying Kirchhoff’s voltage law. The load voltage is 0V so the input voltage is across the diode at tp.

18. Peak Inverse Voltage (PIV)
Because the diode is only forward biased for one-half of the AC cycle, it is also reverse biased for one-half cycle.
It is important that the reverse breakdown voltage rating of the diode be high enough to withstand the peak, reverse-biasing AC voltage.
PIV=Vm OR accurately
PIV (or PRV)  Vm
PIV = Peak inverse voltage
PRV = Peak reverse voltage
Vm = Peak AC voltage
Diode must capable to withstand certain amount of repetitive reverse voltage

19. Full-Wave Rectifier
A full-wave rectifier allows current to flow during both the positive and negative half cycles or the full 360°.
Output frequency is twice the input frequency.
VDC or VAVG = 2Vm/π

20. Full-Wave Rectification
Half Wave Rectifier
Full Wave Rectifier
The rectification process can be improved by using more diodes in a full-wave rectifier circuit.
Full-wave rectification produces a greater DC output:
Half-wave: Vdc =0.318Vm =Vm/π
Full-wave: Vdc =0.636Vm =2Vm/π

21. Example
Find average value of the full-wave rectified voltage?

22. Full-Wave Rectification
Center-Tapped Transformer Rectifier
Requires
Two diodes
Center-tapped transformer
VDC=0.636(Vm)

23. Full-Wave Center Tapped
Current flow direction during both alternations. The peak output is about half of the secondary windings total voltage.
Each diode is subjected to a PIV of the full secondary winding output minus one diode voltage drop
PIV=2Vm(out)+0.7V
During positive half-cycles, D1 is forward-biased while D2 is reverse-biased.
During negative half-cycles, D2 is forward-biased while D1 is reverse-biased.

24. Full-Wave Rectification
Bridge Rectifier
Four diodes are required
VDC = Vm

25. Full-Wave Bridge Rectifier
The full-wave bridge rectifier takes advantage of the full output of the secondary winding.
It employs four diode arranged such that current flows in the direction through the load during each half of the cycle.
During positive half-cycle of the input, D1 and D2 are forward-biased and conduct current. D3 and D4 are reverse-biased.
During negative half-cycle of the input, D3 and D4 are forward-biased and conduct current. D1 and D2 are reverse-biased.

26. Full-Wave Bridge Rectifier
The PIV for a bridge rectifier is approximately half the PIV for a center-tapped rectifier.
PIV=Vm(out)+0.7V

27. Summary of Rectifier Circuits
Ideal VDC
Practical (approximate) VDC
PIV
Half-Wave Rectifier
VDC = 0.318(Vm)
= Vm/π
VDC = 0.318(Vm)-0.7
PIV=Vm
Full-Wave Bridge Rectifier
VDC = 0.636(Vm)
=2 Vm/π
VDC = 0.636(Vm)-2(0.7)
PIV=2Vm-0.7V
Center-Tapped Transformer Rectifier
VDC = 0.636(Vm)-0.7
PIV=Vm-0.7V
Vm = peak of the AC voltage = Vp
In the center tapped transformer rectifier circuit, the peak AC voltage is the transformer secondary voltage to the tap.

28. Power Supply Filters and Regulators
In most power supply – 60 Hz ac power line voltage  constant dc voltage
Pulsating dc output must be filtered to reduce the large voltage variation
Small amount of fluctuation in the filter o/p voltage - ripple

29. Power Supply Filters and Regulators – Capacitor-Input Filter

30. Power Supply Filters and Regulators – Capacitor-Input Filter
Surge Current in the Capacitor-Input Filter:
Being that the capacitor appears as a short during the initial charging, the current through the diodes can momentarily be quite high. To reduce risk of damaging the diodes, a surge current limiting resistor is placed in series with the filter and load.
The min. surge
Resistor values:
IFSM = forward surge current rating
specified on diode data
sheet.

32. Power Supply Filters and Regulators
Regulation is the last step in eliminating the remaining ripple and maintaining the output voltage to a specific value. Typically this regulation is performed by an integrated circuit regulator. There are many different types used based on the voltage and current requirements.

33. Power Supply Filters and Regulators
How well the regulation is performed by a regulator is measured by it’s regulation percentage. There are two types of regulation, line and load.
Line and load regulation percentage is simply a ratio of change in voltage (line) or current (load) as a percentage.

34. Capacitor Input Filter – Ripple Voltage
Ripple Voltage: the variation in the capacitor voltage due to charging and discharging is called ripple voltage
Ripple voltage is undesirable: thus, the smaller the ripple, the better the filtering action
The advantage of a full-wave rectifier over a half-wave is quite clear. The capacitor can more effectively reduce the ripple when the time between peaks is shorter. Figure (a) and (b)
Easier to filter
-shorted time between
peaks.
-smaller ripple.

35. Capacitor Input Filter – Ripple Voltage
Ripple factor: indication of the effectiveness of the filter
[half-wave rectifier]
Vr(pp) = peak to peak ripple voltage; VDC = VAVG = average value of filter’s
output voltage.
Lower ripple factor  better filter
[can be lowered by increasing the value of filter capacitor
or increasing the load resistance]
For the full-wave rectifier:
Vp(rect) = unfiltered
peak.

36. Diode Limiters (Clipper)
Clippers are networks that employ diodes to “clip” away a of an input signal without distorting the remaining part of the applied waveform. Clippers used to clip-off portions of signal voltages above or below certain levels.

37. Diode Limiters (Clipper)

38. Example
What would you expect to see displayed on an oscilloscope connected across RL in the limiter shown in above figure.

39. Solution

40. Biased Limiters (Clippers)
A positive limiter
The level to which an ac voltage is limited can be adjusted by adding a bias voltage, VBIAS in series with the diode
The voltage at point A must equal VBIAS V before the diode become forward-biased and conduct.
Once the diode begins to conduct, the voltage at point A is limited to VBIAS V, so that all input voltage above this level is clipped off.

41. Biased Limiters (Clippers)
A negative limiter
In this case, the voltage at point A must go below –VBIAS – 0.7V to forward-bias the diode and initiate limiting action as shown in the above figure.

42. Modified Biased Limiters (Clippers)

43. Example
Figure above shows combining a positive limiter with a negative limiter. Determine the output voltage waveform?

44. Solution

45. Summary Limiters (Clippers)
In this examples VD = 0
In analysis, VD = 0 or VD = 0.7 V can be used. Both are right assumption.

46. Summary Limiters (Clippers)
In this examples VD = 0
In analysis, VD = 0 or VD = 0.7 V can be used. Both are right assumption.

47. Diode Clampers
A clamper is a network constructed of a diode, a resistor, and a capacitor that shifts a waveform to a different dc level without changing the appearance of the applied signal.
Sometimes known as dc restorers
Clamping networks have a capacitor connected directly from input to output with a resistive element in parallel with the output signal. The diode is also parallel with the output signal but may or may not have a series dc supply as an added elements.

48. Diode Clampers
Positive clamper operation. (Diode pointing up – away from ground)

49. Diode Clampers
Negative clamper operation (Diode pointing down – toward ground)

50. Diode Clamper
If diode is pointing up (away from ground), the circuit is a positive clamper.
If the diode is pointing down (toward ground), the circuit is a negative clamper

51. Diode Clamper (Square Wave)
Diode ‘OF’ state
Output
Diode ‘ON’ state
V – Vc = 0 ; Vc = V; Vo = 0.7 V but ideal Vo = 0V
-V - Vc - Vo = 0; Vc = V
Vo = -2 V

52. Summary of Clamper Circuits

53. Voltage Multipliers
Voltage multiplier circuits use a combination of diodes and capacitors to step up the output voltage of rectifier circuits.
Voltage Doubler
Voltage Tripler
Voltage Quadrupler

54. Voltage Doubler Vout = VC2 = 2Vm
This half-wave voltage doubler’s output can be calculated by:
Vout = VC2 = 2Vm
where Vm = peak secondary voltage of the transformer

55. Half-Wave Voltage Doubler
Positive Half-Cycle
D1 conducts
D2 is switched off
Capacitor C1 charges to Vm
Negative Half-Cycle
D1 is switched off
D2 conducts
Capacitor C2 charges to Vm
Vout = VC2 = 2Vm

56. Full-Wave Voltage Doubler
Positif Half-Cycle
D1 forward-biased → C1 charges to Vp
D2 reverse-biased
Negative Half-Cycle
D1 reverse-biased
D2 forward-biased → C2 charges to Vp
Output voltage=2Vp (across 2 capacitors in series

57. Voltage Tripler and Quadrupler

58. Voltage Tripler Positive half-cycle: C1 charges to Vp through D1
Negative half-cycle: C2 charges to 2Vp through D2
Positive half-cycle: C3 charges to 2Vp through D3
Output: 3Vp across C1 and C3

59. Output: 4Vp across C2 and C4
Voltage Quadrupler
Output: 4Vp across C2 and C4

60. The Diode Data Sheet
The data sheet for diodes and other devices gives detailed information about specific characteristics such as the various maximum current and voltage ratings, temperature range, and voltage versus current curves (V-I characteristic).
It is sometimes a very valuable piece of information, even for a technician. There are cases when you might have to select a replacement diode when the type of diode needed may no longer be available.
These are the absolute max. values under which the diode can be operated without damage to the device.

61. The Diode Data Sheet (Maximum Rating)
Symbol
1N4001
1N4002
1N4003
UNIT
Peak repetitive reverse voltage
Working peak reverse voltage
DC blocking voltage
VRRM
VRWM VR 50
100
200 V
Nonrepetitive peak reverse voltage
VRSM 60
120
240
rms reverse voltage
VR(rms) 35 70
140
Average rectified forward current (single-phase, resistive load, 60Hz, TA = 75oC Io 1 A
Nonrepetitive peak surge current (surge applied at rated load conditions)
IFSM
30 (for 1 cycle)
Operating and storage junction temperature range
Tj, Tstg
-65 to +175 oC

62. Zener Diodes
The zener diode – silicon pn-junction device-designed for operate in the reverse-biased region
Zener diode symbol
Schematic diagram shown that this particular zener circuit will work to maintain 10 V across the load

63. Zener Diodes
Breakdown voltage – set by controlling the doping level during manufacture
When diode reached reverse breakdown – voltage remains constant- current change drastically
If zener diode is FB – operates the same as a rectifier diode
A zener diode is much like a normal diode – but if it is placed in the circuit in reverse bias and operates in reverse breakdown.
Note that it’s forward characteristics are just like a normal diode.
1.8V – 200V

64. Zener Diodes
The reverse voltage (VR) is increased – the reverse current (IR) remains extremely small up to the “knee”of the curve
Reverse current – called the zener current, IZ
At the bottom of the knee- the zener breakdown voltage (VZ) remains constant although it increase slightly as the zener current, IZ increase.
IZK – min. current required to maintain voltage regulation
IZM – max. amount of current the diode can handle without being damage/destroyed
IZT – the current level at which the VZ rating of diode is measured (specified on a data sheet)
The zener diode maintains a constant voltage for value of reverse current rating from IZK to IZM

65. Zener Diodes (Zener Equivalent Circuit)
Since the actual voltage is not ideally vertical, the change in zener current produces a small change in zener voltage
By ohm’s law:
Normaly -Zz is specified at IZT

66. Zener impedance

67. Zener Diodes (Temp Coeff & Zener Power Dissipation and Derating)
As with most devices, zener diodes have given characteristics such as temperature coefficients and power ratings that have to be considered. The data sheet provides this information

68. Zener Diodes Applications
Zener diode can be used as
Voltage regulator for providing stable reference voltages
Simple limiters or clippers

69. Zener Regulation with Varying Input Voltage
As i/p voltage varies (within limits) – zener diode maintains a constant o/p voltage
But as VIN changes, IZ will change, so i/p voltage variations are set by the min. & max. current value (IZK & IZM) which the zener can operate
Resistor, R –current limiting resistor

70. Zener Regulation with a Variable Load
The zener diode maintains a nearly constant voltage across RL as long as the zener current is greater than IZK and less than IZM
When the o/p terminal of the zener diode is open (RL=∞)-load current is zero and all of the current is through the zener
When a load resistor (R) is connected, current flow through zener & load RL, IL, IZ
The zener diode continues to regulate the voltage until IZ reaches its min value , IZK
At this point, the load current is max. , the total current through R remains essentially constant.

71. Zener Limiting
Zener diode also can be used in ac applications to limit voltage swings to desired level
(a) To limit the +ve peak of a signal voltage to the selected zener voltage
- During –ve alternation, zener arts as FB diode & limits the –ve voltage to -0.7V
(b) Zener diode is turn around
-The –ve peak is by zener action & +ve voltage is limited to +0.7V
(c) Two back-to-back zeners limit both peaks to the zener voltage ±7V
-During the +ve alternation, D2 is functioning as the zener limiter – D1 is functioning as a FB diode.
-During the –ve alternation-the roles are reversed