3 Figure 4.20: Block diagram of a dc power supply 4.5 Rectifier CircuitsFigure 4.20: Block diagram of a dc power supplyThe primary application of diode is the rectifier –Electrical device which converts alternating current (AC) to direct current (DC)One important application of rectifier is dc power supply.
4 step #1: increase / decrease rms magnitude of AC wave via power transformer step #2: convert full-wave AC to half-wave DC (still time-varying and periodic)step #3: employ low-pass filter to reduce wave amplitude by > 90%step #4: employ voltage regulator to eliminate ripplestep #5: supply dc load
5 4.5.1 The Half-Wave Rectifier Half-wave rectifier – utilizes only alternate half-cycles of the input sinusoidConstant voltage drop model is employed.
6 4.5.1 The Half-Wave Rectifier Small inputs? Regardless of the model employed, one should note that the rectifier will not operate properly when input voltage is small (< 1V)Those cases require a precision rectifier (diode with op amps).
7 4.5.2 Full-Wave RectifierCenter-tapping of the transformer, allowing “reversal” of certain currents…
8 Full-Wave RectifierWhen instantaneous source voltage is positive, D1 conducts while D2 blocks…
9 4.5.2 Full-Wave Rectifierwhen instantaneous source voltage is negative, D2 conducts while D1 blocks
11 4.5.3 Bridge RectifierAn alternative implementation of the full-wave rectifier is bridge rectifierDoes not require center-tapped transformerFour diodes instead of two
12 4.5.3 Bridge Rectifierwhen instantaneous source voltage is positive, D1 and D2 conduct while D3 and D4 block
13 4.5.3 Bridge Rectifierwhen instantaneous source voltage is negative, D3 and D4 conduct while D1 and D2 block
14 4.5.4. The Rectifier with a Filter Capacitor Why is this example unrealistic?Because for any practical application,the converter would supply a load(which in turn provides a path for capacitor discharging)
17 4.5.4. The Rectifier with a Filter Capacitor Precision rectifier – is a device which facilitates rectification of low-voltage input waveformsHow?
18 4.6: Limiting and Clamping Circuits Q: What is a limiter or clamping circuit?A: One which limits voltage output.
19 single limiters employ one diode double limiters employ two diodes of opposite polaritylinear range may be controlled via string of diodes and dc sourceszener diodes may be used to implement soft limiting
20 soft vs. hard limiter 4.6: Soft vs Hard limiter Q: How are limiter circuits applied?A: Signal processing, used to prevent breakdown of transistors within various devices.Why use soft?
21 4.6.2 The Clamped Capacitor or DC Restorer Q: What is a DC restorer?A: Circuit which provides the dc component of an AC without DC value.Q: Why is this ability important?A:1) Average value of the output is effective way to measure duty cycle2) Duty cycle is modulated to carry digital data (PWM): use DC restorer followed by RC low pass filter
22 Q: What is a voltage doubler? 4.6.3 The Voltage Doublerdc restorerpeak rectifierQ: What is a voltage doubler?A: One which multiplies the amplitude of a wave or signal by two.How?
23 4.7 Special Diodes Schottky-Barrier Diode or Schottky Diode metal and moderately doped semiconductor junctioncurrent flows from metal to semiconductorcurrent is conducted by majority carriers: can switch it on and off faster than p-n junctionforward voltage drop is lower than p-n junction( V)
24 reverse-biased p-n junction 4.7 Special DiodesVaractorsreverse-biased p-n junctionjunction capacitance is a function of reverse bias voltagehow?voltage variable capacitortuning of receivers, Phase locked loopsAnodeCathode
25 4.7 Special Diodes Photodiodes reverse-biased p-n junction expose to light: covalent bonds break, electron-hole pairs are createdfree electrons sweep to n side and holes to p sidereverse current is createdphotocurrent is proportational to intensity of incident lightconvert light into electric signalapplications: CD-ROM, fiber-opticwhat happens when you don't reverse bias the photodiode and expose it to light?AnodeCathodePo...P2= light levels
26 4.7 Special Diodes LEDs convert forward current into light forward bias region: when minority carriers diffuse into p and n sides, they recombine with majority carriers, e.g. electrons with holes.recombination: emission of lightspecial semiconductor material: direct band-gapknown spectra of light when electrons leave orbitemitted light is proportional to number of recombinations which is proportional to the forward currentAnodeCathode