Microelectronics Circuit Analysis and Design. Rectifier Circuits. Donald A. Neamen. Chapter 2. Diode Circuits. In this chapter, we will:

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1 icroelectronics Circuit Analysis and Design Donald A. Neamen Chapter 2 Diode Circuits n this chapter, we will: Determine the operation and characteristics of diode rectifier circuits, which is the first stage of the process of converting an ac signal into a dc signal in the electronic power supply. Apply the characteristics of the ener diode to a ener diode voltage regulator circuit. Apply the nonlinear characteristics of diodes to create wave shaping circuits known as clippers and clampers. Examine the techniques used to analyze circuits that contain more than one diode. Rectifier Circuits A basic rectifier converts an ac voltage to a pulsating dc voltage. A filter then eliminates ac components of the waveform to produce a nearly constant dc voltage output. Rectifier circuits are used in virtually all electronic devices to convert the 120-V 60-Hz ac power line source to the dc voltages required for operation of electronic devices. n rectifier circuits, the diode state changes with time and a given piecewise linear model is valid only for a certain time interval. Chap 3-4

2 Half Wave Rectification PV Vs+Vγ f r f is zero, when diode is on, v o v s -v γ Figure 2.6 Full-wave rectifier: (a) circuit with center-tapped transformer, (b) voltage transfer characteristics, and (c) input and output waveforms Full-Wave Rectifiers Full-wave rectifiers cut capacitor discharge time in half and require half the filter capacitance to achieve a given ripple voltage. All specifications are the same as for half-wave rectifiers. Reversing polarity of the diodes gives a fullwave rectifier with negative output voltage.

3 Figure 2.7 A full-wave bridge rectifier: (a) circuit showing the current direction for a positive input cycle, (b) current direction for a negative input cycle, and (c) input and output voltage waveforms Full-Wave Bridge Rectification The requirement for a centertapped transformer in the fullwave rectifier is eliminated through use of 2 extra diodes. All other specifications are the same as for a half-wave rectifier except PV V s (max)- v γ. Rectifier Topology Comparison Filter capacitors are a major factor in determining cost, size and weight in design of rectifiers. For a given ripple voltage, a full-wave rectifier requires half the filter capacitance as that in a halfwave rectifier. Reduced peak current can reduce heat dissipation in diodes. Benefits of full-wave rectification outweigh increased expenses and circuit complexity (an extra diode and center-tapped transformer). The bridge rectifier eliminates the center-tapped transformer, and the PV rating of the diodes is reduced. Cost of extra diodes is negligible. Chap 3-12

4 Power Supply Applications Filters, Ripple Voltage, and Regulators Power Supply Applications Filter Networks Output Voltage of Full-Wave Rectifier with RC Filter ost electronic applications require smooth dc current to operate properly. Filtering pulsating dc circuits accomplishes this. Adding a capacitor to the output of a half-wave rectifier filters the pulsating dc into smooth dc. The ripple on the dc output is V Vr 2 frc where f 1 2T P

5 Power Supply Applications Full-wave Rectifier with Filter A capacitive filter added to the output of a full-wave bridge rectifier is shown at the left. One drawback of a half-wave rectifier is the higher level of ripple voltage after filtering. Full-wave rectification reduces this ripple voltage. Power Supply Circuits Filters and Regulators Smoothening the Output Voltage of a Rectifier Add a Capacitor across Power Supply Circuits Filters and Regulators A capacitor-input filter will charge and discharge such that it fills in the gaps between each peak. This reduces variations of voltage. This voltage variation is called ripple voltage. Power Supply Circuits Filters and Regulators 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. V ripple

6 Power Supply Applications Full-wave rectifier with filter design Half-wave Rectifier with Filter Output voltage of a full-wave rectifier with an RC filter Power Supply Circuits Design of Filter Capacitor Voltage across the capacitor: t' τ t' RC vo( t) Ve Ve time Output voltage of a full-wave rectifier with an RC filter inimum out voltage: T ' RC VL Ve T : discharge time Ripple voltage: V ripple V V V L T ' RC ( 1 e ) V ripple V T' RC T << RC TP T T P Vripple V RC Assume capacitor takes negligible time to charge f 1 2 T P V ripple V 2 f RC

7 Example 2.3 Design a full-wave rectifier to meet particular specification. A full-wave rectifier is to be designed to procedure a peak output voltage of 12 V, deliver 120 ma to the load, and produce an output with a ripple of not more than 5.0 %. An input line voltage of 120 V (rms), 60 Hz is available. Full-wave rectifier design P61 H W # 2 Variation on Problem 1.62 Root ean Square V VRS 0.707V 2 ean value of sinusoidal over one period signal is zero Variation con t Variation con t For -0.7V < V < 0.7V, 0 When V 0.7V, changes linearly with voltage The device under test (DUT) acts like an open and can be modeled as such over this voltage range. 5V 0.7V r f 2.35kΩ and Vγ 0. 7V 2mA

8 Variation con t Since the -V characteristics of the device under test (DUT) are symmetrically about V D 0, a similar model can be used for V - 0.7V as for V 0.7V For V -0.7V: 5V 0.7V r f 2.35kΩ and Vγ 0. 7V 2mA

9 ener Diodes ener diodes are available with voltage breakdowns of 1.8 ~ 200 V. at least >10% Power Supply Applications ener Voltage Regulator Circuit Output voltage remain constant, even when output load resistance varies over a wide range, and when input voltage varies over a specific range. nput resistance Limit the current through the ener diode and drop the excess voltage between V PS and V. Varying voltage source Variable load conditions This curve illustrates the minimum and maximum ranges of current operation that the ener can effectively maintain it s voltage. Voltage Rectifier with nonzero ener resistance ener Diode Characteristics The ener diode begins to conduct when V PS V. When V PS V : V L V L V /R L,, but V constant 1 (V PS V )/R i 1 - L

10 Sizing Series Resistance R i V ps V + L z Or V ps V R i z L Case 1: V z > V zo V ps min, z min, L max Case 2: P z rated diode dissipation V ps max, z max, l min Case 1: Case 2: Sizing Series Resistance R R i i V V ps (min) V (min) + ps (max) + L (max) V L z (max) z (min) VPS V Ri nput Assume the ener resistance is zero for ideal diode Power Supply Applications VPS V R i V R For proper operation, the diode must remain in the breakdown region and power dissipation in diode must not exceed its rated value. n other words, 1. The (min) when (max) and V PS (min). 2. The (max) when (min) and V PS (max). VPS (min) V Ri (min) + (max) VPS (max) V Ri (max) + (min) [ V (min) V ] [ (max) + (min)] [ V (max) V ] [ (min) + (max)] PS ener Voltage Regulator Circuit + nput f minimum requirement is (min) 0.1 (max), PS [ V (max) V ] (min)[ V (min) V ] (max) PS PS (max) i VPS (min) 0.9V 0.1VPS (max) Current-limiting resistor R

11 Voltage Regulation VL(max) VL(min) %regulation x100 V ( nom ) where: V L (nom) the nominal output voltage Voltage regulation is the measure of circuit ability to maintain a constant output even when input voltage or load current varies %regulation is used to measure how well the regulator is performing its function L Example 2.5 P68 Example Demonstration of ener diode as a voltage regulator n the Circuit given in Fig, the resistance R 1 kω, V L 10 V at 1 ma, and r 30 Ω. Given that V in changes from 11 V to 20 V, calculate the ener current change and the output voltage change. Three-Terminal C Voltage Regulators Solution: When V in 11 V V z z r z Regulators use feedback with high-gain amplifiers to reduce ripple voltage at the output. Bypass capacitors provide low-impedance paths for highfrequency signals to ensure proper operation of the regulator. Regulators provide excellent line and load regulation, maintaining constant voltage even if the output current changes by many orders of magnitude.

12 Voltage ultipliers Homework solution (page 108) Voltage triplers and quadruplers utilize three and four diode-capacitor arrangements respectively.

13 V o clipped when f V γ 0.7 V and V B 5 V then clip when V 1 < 4.3 V V 1 < V B -V γ

14 Clamper Circuits Action of a Diode Clamper Circuit Diode OR Logic Circuits v nput V sinω t Assume V γ 0 r f 0 (a) a typical diode clamper circuit (b) the sinusoidal input signal v Capacitor deally, the capacitor cannot discharge, remains constant (c) the capacitor voltage (d) the output voltage Kirchhoff s voltage law v Out v + v V + V sinω t V (sinω t 1) Capacitor nput Diode AND Logic Circuits ultiple-diode Circuits Photodiode Circuit V 1 (V) V 2 (V) V O (V)

15 iscellaneous Diode Applications iscellaneous Diode Applications There are many practical applications for diodes beyond power supplies. Some of these applications include: Clipper circuits that serve to protect circuits from damage as a result of over-voltage conditions. Clippers are common in computer circuits. solation diodes are used to isolate various sections of circuits from another. An example of this is the battery backup for computer memory.