AE UNIT II.ppt
- 1. ANALOG ELECTRONICS RECTIFIERS S.ARUN, Assistant Professor/EEE, Kongunadu College of Engineering and Technology.
- 2. RECTIFIER Rectifier is defined as an electronic devices used for converting a.c voltage into unidirectional voltage. A rectifier utilizes unidirectional conduction device like a PN junction diode. Rectifiers are classified depending upon the period of conduction as Half –wave rectifier Full –wave rectifier
- 3. Half wave Rectifier The circuit diagram and its waveform as shown. It conduct only during positive half cycle
- 4. Operation The AC voltage across secondary winding AB changes polarities after every half cycle. During +ve half cycle of input AC voltage end A becomes positive w.r.to B. This makes the diode forward biased and hence it conducts current. During –ve half cycle end A is negative w.r.to end B. Under this condition, the diode is reverse biased and hence it not conduct. Therefore current flows through the diode during +ve half cycle of input AC voltage only.
- 6. Advantages It is simple Low cost Disadvantages Low rectification efficiency High ripple factor Low TUF Since current flows for only one half cycle core saturation result.
- 7. Full Wave Rectifier FWR is a circuit which allows a unidirectional current to flow through the load during the entire input cycle. There are two types of full wave rectifiers as Center tapped full wave rectifier Bridge rectifier
- 8. Center tapped FWR In full wave rectification, current flows through the load in the same direction for both half cycles of the input AC voltage. It uses the center tapped transformer which provides equal voltages above and below the center tapped for both half cycles. The voltage between the center tap and either end of the secondary winding is half of the secondary voltage. The center tap of the secondary winding of a transformer is taken as the ground or zero voltage reference point.
- 9. The circuit uses two diodes, which are connected to the center tapped secondary winding of the transformer as shown Diode D1 utilizes the AC voltage appearing across the upper half (OA) of secondary winding for rectification while diode D2 uses the lower half winding (OB).
- 10. During positive half cycle
- 11. During negative half cycle
- 12. Input and output waveforms
- 13. Advantages The output voltage and transformer efficiency are higher The dc saturation of the core is avoided as current flows through the two halves of the center tapped secondary of the transformer Lower ripple factor Higher TUF Disadvantages Usage of additional diode and bulky transformer is needed, and hence increase in cost PIV of diode is high The output voltage is half of the secondary voltage.
- 14. Full wave Bridge Rectifier The need for a center tapped power transformer is eliminated in the bridge rectifier, as shown
- 15. It contains 4 diodes D1, D2, D3, D4 connected to form bridge. The ac supply to be rectified is applied to the diagonally opposite ends of the bridge through the transformer. Between other two ends of the bridge the load resistance RL is connected. Input Output Waveform as shown:
- 16. During positive half cycle:
- 17. During negative half cycle:
- 18. Advantages The need for center tapped transformer is eliminated The output is twice that of the center tap circuit for the same secondary voltage The PIV is one half that of the center tap circuit Disadvantages It requires four diode As during each half cycle of a.c input two diodes that conduct are in series, therefore, voltage drop in the internal resistance of the rectifying unit will be twice as great as in the center tap circuit.
- 19. FILTER The output of rectifier circuit is not pure d.c, but contains fluctuations or ripple, which are undesired. To minimize the ripple content in the output, filter circuits are used.
- 20. An a.c input is applied to the rectifier. At the output of the rectifier, there will be d.c and ripple voltage present, which is the input to the filter. Ideally the output of the filter should be pure d.c. Practically, the filter circuit will try to minimize the ripple at the output, as far as possible. Basically the ripple is a.c, i.e. varying with time, while d.c. is a constant with respect to time. Ideally, the inductance acts as a short circuit for d.c, but it has a large impedance for a.c. Similarly, the capacitor acts as open for d.c. and almost short for a.c if the large value of capacitance is sufficiently large enough.
- 21. The inductance used in filter circuits is also called “choke”. Similarly, since the capacitance is open for d.c, i.e it blocks d.c, hence it cannot be connected in series with the load. There are basically two types of filters circuits as, Capacitor input filter Choke input filter Looking from the rectifier side, if the first element, in the filter circuit is capacitor then the filter circuit is called capacitor input filter. While if the first element is an inductor, it is called choke input filter. The choke input filter is not in use nowadays as inductors are bulky, expensive and consume more power.
- 22. Capacitor filter The block schematic of capacitor input filter is shown in the figure. Looking from the rectifier side the first element in filter is a capacitor.
- 23. Operation of the filter with Half Wave Rectifier The figure shows that a half wave rectifier with a capacitor input filter. The filter uses a single capacitor in parallel with the load, represents by the resistance RL. In order to minimize the ripple in the output, the capacitor C used in the filter circuit is quite large, of the order of tens of microfarads.
- 24. During the positive half cycle of the input signal es, the diode is forward biased. This charges the capacitor C to peak value of input i.e. Esm. Practically the capacitor C charges to (Esm – 0.7), due to diode forward voltage drop. This initial charging happens only once, immediately when the power is turned on. When the input starts decreasing below its peak value, the capacitor remains charged at Esm and the ideal diode gets reverse biased. This is because the capacitor voltage which is cathode voltage of diode becomes more positive then anode. So during the entire negative half cycle and some part of the next positive half cycle, capacitor discharges through
- 25. Capacitor discharges through load resistance as shown
- 26. The discharging of the capacitor is detected by RLC time constant which is very large and hence capacitor discharges very little from Esm. In the next positive half cycle, when es becomes more then capacitor voltage, the diode becomes forward biased and charges the capacitor C back to Esm. This is shown in figure as capacitor starts charge again.
- 27. The capacitor voltage is same as the output voltage as it is in parallel with RL. It can be seen that the diode conducts only from point B till capacitor gets charged back to Esm. Thus diode conducts only for part of the positive half cycle. From point A to B, the diode remains nonconducting and conducts only for the period from B to C. this is shown in figure
- 28. Operation of the filter with Full Wave Rectifier Capacitor input filter with two diode full wave rectifier as shown
- 29. Immediately when power is turned on, the capacitor C gets charged through forward biased diode D1 to Esm, during first quarter cycle of the rectified output voltage. In the next quarter cycle from pi/2 to pi, the capacitor starts discharging through RL. Once capacitor gets charged to Esm, the diode D1 becomes reverse biased and stops conducting. So during the period from pi/2 to pi, the capacitor C supplies the load current. It discharges to point B shown in figure
- 30. Expression for Ripple Factor The ripple voltage in the output of capacitor filter with full wave rectifier is practically assumed to be triangular as shown in figure The peak to peak ripple voltage Vr is given by, For full wave peak to peak ripple As triangular
- 31. While For half wave peak to peak ripple Ripple Factor = = For half wave
- 32. Approximate Analysis of Capacitor Input Filter Consider an output waveform for a full wave rectifier circuit using a capacitor input filter, as shown in figure During time T1, capacitor gets charged and this process is quick. During time T2, capacitor gets discharged through RL. As time constant RLC is very large, discharging process is very slow and hence T >>T
- 33. Let Vr be the peak to peak value of ripple voltage, which is assumed to be triangular as shown in figure It is known mathematically that the r.m.s value of such a triangular waveform is During the time interval T2, the capacitor C is discharging through the load resistance RL. The charge lost is Q = CVr
- 36. How to Decrease Ripple Factor? It can be seen from the expression of ripple factor that to decreases its value, Increase the value of filter capacitor. Increase the value of load resistance But higher C means larger initial surge current for which higher rating diodes must be used. As RL decreases, the load current increases but ripple increases. Hence filter is not suitable for variable loads or higher loads. The capacitor filter is suitable for lighter loads i.e. small load currents.
- 37. D.C Output Voltage with Capacitor Filter The d,c output voltage from a capacitor filter fed from a full wave rectifier is given by, ………Full wave While the d.c output voltage from a capacitor filter fed from a half wave rectifier is given by, ………Half wave • From the above expressions, it van be seen that as the current drawn by the load increases, the d.c output voltage decreases. • Hence this filter circuit is having poor regulation.
- 38. The load regulation graph i.e. regulation characteristics for the capacitor input filter is shown in figure
- 39. Expression for Ripple Voltage
- 40. Surge Current in a Capacitor Input Filter The capacitor has a feature that initially it acts as a short circuit momentarily. It does not offer any resistance. The forward resistance of the diode is very small. Hence at start, large current flows through the forward biased diode. This is called surge current. Such an initial inrush of surge current can destroy the diodes. The diodes current rating must be so as to withstand high surge current.
- 41. Diode Surge Current as shown Limiting surge current as shown
- 42. The advantages of capacitor input filter are Less number of component Low ripple factor hence low ripple voltage Suitable for high voltage at small load currents The disadvantages of capacitor input filter are Ripple factor depends on load resistance Not suitable for variable loads as ripple content increases as RL decreases Regulation is poor Diodes are subjected to high surge currents hence must be be selected accordingly
- 43. Resistance Capacitance Filter It is possible to further reduce the amount of ripple across a filter capacitor by using an additional RC filter section as shown in figure The purpose of the added RC section is to pass most of the dc component while attenuating (reducing) as much of the ac component as possible.
- 44. Figure shows a full-wave rectifier with capacitor filter followed by an RC filter section. The operation of the filter circuit can be analyzed using superposition for the dc and ac components of signal.
- 45. DC operation of R-C Filter section: Figure shows the dc equivalent circuit to use in analyzing the RC filter circuit Since both capacitors are open-circuit for dc operation, the resulting output dc voltage is given by equation as per voltage division rule.
- 46. AC operation of R-C Filter section: Figure shows the ac equivalent circuit of the RC filter section. Due to the voltage-divider action of the capacitor ac impedance and the load resistor, the ac component of voltage resulting across the load is given by the equation
- 47. For a full-wave rectifier with ac ripple at 100 Hz, the impedance of a capacitor can be calculated using the relation = Ω Where C is in Farads kΩ if C is in μF The main drawback of R-C filter is the large voltage drop in the series resistor R i.e. poor voltage regulation. It is also needs adequate ventilation to dissipate the heat developed in the resistor R. Thus R-C filter is suitable only for light loads (small load current or large load resistance).
- 48. Inductance Capacitance Filter A simple series inductor reduces both the peak and effective values of the output current and output voltage. On the other hand simple shunt capacitor filter reduces the ripple voltage but increases the diode current. The diode may get damaged due to large current and at the same time it causes greater heating of supply transformer resulting in reduced efficiency. In an inductor filter, ripple factor increases with the increase in load resistance RL while in a capacitor filter it varies inversely with load resistance RL. From economical point of view also, neither series
- 49. Practical filter circuits are derived by combining the voltage stabilizing action of shunt capacitor with the current smoothening action of series choke coil. By using combination of inductor and capacitor ripple factor can be lowered, diode current can be restricted and simultaneously ripple factor can be made almost independent of load resistance (or load current). Two types of most commonly used combinations are choke input or L-section filter and capacitor input or pi -filter.
- 50. Choke input filter consists of a choke L connected in series with the rectifier and a capacitor C across the load, as shown in figure This is also sometimes called the L-section filter because in this arrangement inductor and capacitor are connected as an inverted L. Only one filter section is shown, but several identical sections are often employed to improve the smoothening action. The choke L on the input side of the filter readily allows dc to pass but opposes the flow of ac components
- 51. Any fluctuation that remains in the current even after passing through the choke are largely bypassed around the load by the shunt capacitor because XC is much smaller than RL. Ripples can be reduced effectively by making XL greater than XC at ripple frequency. However, a small ripple still remains in the filtered output and this is considered negligible if it less than 1%. The rectified and filtered output voltage waveform from a full wave rectifier with choke input filter are shown in figure
- 52. Regulation:
- 53. Ripple Factor: The main object of the filter is to suppress the harmonics components in the system and for this it is necessary that reactance of the choke coil XL is made much greater than combined parallel impedance of the capacitor C and load resistor RL. The parallel impedance of the capacitor C and load resistor RL can be made small by making the reactance of the capacitor, XC much smaller than load resistance RL. Very little error is caused if it is assumed that the entire alternating current is flowing through the capacitor and none through the load resistor RL.
- 54. Under these condition the net impedance across the input terminals of the filter circuit is approximately XL =2 ꞶL, the reactance of the inductor at the second harmonics frequency. AC current through the circuit is given as Where , the reactance of the capacitor at the second harmonic frequency. Ripple factor =
- 55. Critical Inductance: During the above discussion it has been assumed that a current flows through the circuit at all times. In the absence of inductor, current flows through the diode circuit for a small portion of the cycle, and the capacitor is charged to the peak transformer secondary voltage in each cycle (neglecting diode forward and transformer resistance). When a small inductance is inserted in the circuit, the diode current will exist for a longer duration but cutout may still occur. With the continues increase in inductance, a value is reached for which diode current exist for the whole cycle.
- 56. Current flowing through the load is made up of two components IC given as and ac component of peak value For continuous flow of current through diode it is necessary that Idc should always exceed the negative peak value of ac component so or or • The L-C filter was quite popular at one time. Now, it is becoming obsolete in typical power supplies because of the size and cost of inductors. • For low voltage power supplies, the L-C filter has been replaced by IC voltage regulators, active filters that reduce ripple and hold the output dc voltage constant.