Diode Applications. Half-Wave Rectifier. Full-Wave Rectifier. Bridge Rectifier

Transcription

1 Rectification

2 Diode Applications rec ti fi er Pronunciation: 'rek-t&-"fi(-&)r Function: noun : one that rectifies; specifically : a device for converting alternating current into direct current Half-Wave Rectifier Full-Wave Rectifier Bridge Rectifier

3 Rectification Conversion of ac to dc. Many devices (transistors) are unidirectional current devices DC required for proper operation.

4 Half Wave Rectifier

5 Full Wave Rectifier

6 Bridge Rectifier

7 Bridge Rectifier

8 Full Wave Rectifier

9 SINUSODIAL INPUT HALF-WAVE RECTIFICATION

10 Half-Wave Rectifier Vdc 0.318Vm V dc 0.318( V m V T )

11 Effect of VT on half-wave rectified signal

12 Determining the required PIV ratting for the half wave rectifier

13 The peak Inverse voltage PIV for half wave rectifier PIVrating V m

14 Bridge Rectifier

15

16 Input and output waveforms for a full wave rectifier full wave rectifier

17 Determining the required PIV ratting for the bridge configuration PIV V m

18 Full wave rectification

19 Full wave rectification

20 Full wave rectification

21 Full wave rectification

22 Full wave rectification

23 Power supply filtering

24 Comparison of ripple voltages for half- wave and full- wave for the same filter capacitor

25 The voltage when the filter capacitor discharge through Rl

26 Concept Question Sketch a circuit for a half-wave rectifier. Also, make a plot of voltage versus time for the voltage output across the load resistor.

27 Concept Question Sketch a circuit for a full-wave rectifier. Also, make a plot of voltage versus time for the voltage output across the load resistor.

28 Filters We have now used diodes to produced a pulsed dc signal. Most equipment requires regulated dc We must remove the ripple Ripple is departure of waveform from pure dc (flat, constant voltage level) Frequency so far we have seen pulsed dc at the same frequency as the input (½ wave) or twice the line frequency (full wave rectifier) Amplitude a measure of the effectiveness of the filter

29 Alternate Definition Defined also for current I ac = effective value of ac harmonic component I dc = average of dc component I rms I 2 dc I 2 ac so, I ac I 2 rms I 2 dc For ½-wave rectifier r = 1.21 For full-wave rectifier r = 0.48 r I I ac dc r I rms I dc 2 1

30 Diode Applications Half wave rectifier and equivalent circuit with piecewise linear model Ideal V c R f v i v i v i = V M sin ( t)

31 Half Wave Rectifier We initially consider the diode to be ideal, such that V C =0 and R f =0

32 Half Wave Rectifier The (ideal) diode conducts for v i >0 and since R f =0 v 0 v i For v i < 0 the (ideal) diode is an open circuit (it doesn t conduct) and v 0 0.

33 Half Wave Rectifier In this simplified (ideal diode) case the input and output waveforms are as shown The diode must withstand a peak inverse voltage of V M

34 Half Wave Rectifier The average d.c. value of this half-wave-rectified sine wave is V AV 2 VM sin d VM VM cos cos 2 0

35 Half Wave Rectifier So far this rectifier is not very useful. Even though the output does not change polarity it has a lot of ripple, i.e. variations in output voltage about a steady value. To generate an output voltage that more closely resembles a true d.c. voltage we can use a reservoir or smoothing capacitor in parallel with the output (load) resistance.

36 Smoothed Half Wave Rectifier Circuit with reservoir capacitor Output voltage The capacitor charges over the period t 1 to t 2 when the diode is on and discharges from t 2 to t 3 when the diode is off.

37 Smoothed Half Wave Rectifier When the supply voltage exceeds the output voltage the (ideal) diode conducts. During the charging period (t 1 < t< t 2 ) v o = V M sin ( t) (The resistance in the charging circuit is strictly R f which we have assumed to be zero. Even for a practical diode R f C will be very small)

38 Smoothed Half Wave Rectifier When the supply voltage falls below the output voltage the diode switches off and the capacitor discharges through the load. During the discharge period (t 2 < t< t 3 ) and v o = V M exp {- t /RC} where t = t- t 2 At time t 3 the supply voltage once again exceeds the load voltage and the cycle repeats

39 Smoothed Half Wave Rectifier The resistance in the discharge phase is the load resistance R. RC can be made large compared to the wave period. The change in output voltage (or ripple) can then be estimated using a linear approximation to the exponential discharge.

40 Ripple Factor r V rms ( ripple voltage out) V(average out) Low r indicates better filtering

41 Smoothed Half Wave Rectifier v o = V M exp {- t /RC} V M [ 1- (t /RC)] The change in voltage V is therefore approximately given by V M t /RC For a the half wave rectifier this discharge occurs for a time (t 3 - t 2 ) close to the period T = 1/f, with f= frequency. Giving the required result: ΔV V M T RC

42 Smoothed Half Wave Rectifier We can define a ripple factor as Ripple factor where V d.c. = (V M - V/2) ΔV Vd.c The lower the ripple factor the better

43 Half Wave Rectifier If we don t consider the diode to be ideal then from the equivalent circuit we obtain, for v i >V c: v i V c i R f - ir =0 i.e. Giving i vi Vc ( Rf R) v o ir ( vi Vc) f ( R R R) v i V c

44 Non-Ideal Half Wave Rectifier V M

45 Non-Ideal Half Wave Rectifier A plot of v 0 against v i is known as the transfer characteristic R/(R + R f ) V C v i

46 Non-Ideal Half Wave Rectifier We usually have R>> R f so that R f can be neglected in comparison to R. Often V M >> V c so V c can also be neglected. The transfer characteristic then reduces to v 0 v i

47 Full-Wave (Bridge) Rectifier vi We initially consider the diodes to be ideal, such that V C =0 and R f =0 The four-diode bridge can be bought as a package

48 Full-Wave (Bridge) Rectifier vi During positive half cycles v i is positive. Current is conducted through diodes D1, resistor R and diode D2 Meanwhile diodes D3 and D4 are reverse biased.

49 Full-Wave (Bridge) Rectifier vi During negative half cycles v i is negative. Current is conducted through diodes D3, resistor R and diode D4 Meanwhile diodes D1 and D2 are reverse biased.

50 Full-Wave (Bridge) Rectifier Current always flows the same way through the load R. Show for yourself that the average d.c. value of this full-wave-rectified sine wave is V AV = 2V M / (i.e. twice the half-wave value)

51 Full-Wave (Bridge) Rectifier Two diodes are in the conduction path. Thus in the case of non-ideal diodes v o will be lower than v i by 2V C. As for the half-wave rectifier a reservoir capacitor can be used. In the full wave case the discharge time is T/2 and VMT ΔV 2RC

52 Half Wave Capacitive Filter Improving the ripple factor During forward bias half-cycle, capacitor is charging During the reverse bias half-cycle, the capacitor discharges through the output resistor

53 Full Wave Capacitive Filter Even better ripple factor.

54 Zener Regulation Circuit Since the load is in parallel with the diode, the voltage drop across RL is always the same as across VR1 and is VZ = constant Zener voltage The input voltage V must be greater than VZ. Zener MUST be operated under load. If not, the zener is still delivering power (more than usual) and may melt. Recall that the zener can draw large currents all at the same voltage.