Lecture 1: Digital Electronic Systems (1) PDF

Higher Institute of Engineering, El Shorouk Biomedical and Systems Engineering Department BIS 471 and BIS 461 Digital Electronic Systems (1) Lecture 1 Dr. Mostafa Elhussien Dr. Mostafa Elhussien Digital Electronic Sy...

Higher Institute of Engineering, El Shorouk Biomedical and Systems Engineering Department BIS 471 and BIS 461 Digital Electronic Systems (1) Lecture 1 Dr. Mostafa Elhussien Dr. Mostafa Elhussien Digital Electronic Systems (1) Lec. 1 1  POWER SUPPLIES The Basic DC Power Supply The dc power supply converts the standard 120 V, 60 Hz ac available at wall outlets into a constant dc voltage. The dc voltage produced by a power supply is used to power all types of electronic circuits, such as Television receivers, Stereo systems, VCRs, Computers, Laboratory equipment. Dr. Mostafa Elhussien Digital Electronic Systems (1) Lec. 1 2  POWER SUPPLIES The Basic DC Power Supply Power supplies are an essential part of all electronic systems from the simplest to the most complex. A basic power supply consists of a rectifier, a filter, and a regulator. The rectifier converts the ac input voltage to a pulsating dc voltage, which is half-wave rectified or full-wave rectified. A capacitor filter eliminates the fluctuations in the rectified voltage and produces a relatively smooth dc voltage. The regulator is a circuit that maintains a constant dc voltage for variations in the input line voltage or in the load. Regulators vary from a single device to more complex integrated circuits. Dr. Mostafa Elhussien Digital Electronic Systems (1) Lec. 1 3  POWER SUPPLIES The Basic DC Power Supply Half-wave rectifier Power supplies Rectifier Full-wave rectifier Filter Capacitors Integrated circuit Regulator voltage regulators Dr. Mostafa Elhussien Digital Electronic Systems (1) Lec. 1 4  POWER SUPPLIES The Basic DC Power Supply Dr. Mostafa Elhussien Digital Electronic Systems (1) Lec. 1 5  POWER SUPPLIES Capacitor-Input Filter A half-wave rectifier with a capacitor-input filter is shown in Figure. During the positive first quarter-cycle of the input, the diode is forward-biased, allowing the capacitor to charge to approximately the diode drop of the input peak, as illustrated in Figure (a). When the input begins to decrease below its peak, as shown in part (b), the capacitor retains its charge and the diode becomes reverse-biased. During the remaining part of the cycle, the capacitor can discharge only through the load resistance at a rate determined by 𝑅𝐿 𝐶 the time constant. The larger the time constant, the less the capacitor will discharge. Dr. Mostafa Elhussien Digital Electronic Systems (1) Lec. 1 6  POWER SUPPLIES Capacitor-Input Filter Because the capacitor charges to a peak value equal to 𝑉𝑝 𝑖𝑛 the peak inverse voltage of the diode in this application is 𝑃𝐼𝑉 = 2𝑉𝑝 𝑖𝑛 During the first quarter of the next cycle, as illustrated in Figure (c), the diode again will become forward-biased when the input voltage exceeds the capacitor voltage. Dr. Mostafa Elhussien Digital Electronic Systems (1) Lec. 1 7  POWER SUPPLIES Capacitor-Input Filter (Ripple Voltage) The capacitor quickly charges at the beginning of a cycle and slowly discharges after the positive peak (when the diode is reverse-biased). The variation in the output voltage due to the charging and discharging is called the ripple voltage. The smaller the ripple, the better the filtering action, as illustrated in Figure. Dr. Mostafa Elhussien Digital Electronic Systems (1) Lec. 1 8  POWER SUPPLIES Capacitor-Input Filter (Ripple Voltage) For a given input frequency, the output frequency of a full-wave rectifier is twice that of a half- wave rectifier. As a result, a full-wave rectifier is easier to filter. When filtered, the full-wave rectified voltage has a smaller ripple than does a half-wave signal for the same load resistance and capacitor values. A smaller ripple occurs because the capacitor discharges less during the shorter interval between full-wave pulses, as shown in Figure. A good rule of thumb for effective filtering is to make 𝑅𝐿 𝐶 ≥ 10𝑇 where T is the period of the rectified voltage. Dr. Mostafa Elhussien Digital Electronic Systems (1) Lec. 1 9  POWER SUPPLIES Capacitor-Input Filter (Ripple Voltage) The ripple factor 𝑟 is an indication of the effectiveness of the filter and is defined as the ratio of the ripple voltage 𝑉𝑟 to the dc (average) value of the filter output voltage 𝑉𝐷𝐶. 𝑉𝑟 𝑟= 100% 𝑉𝐷𝐶 These parameters are illustrated in Figure. The lower the ripple factor, the better the filter. The ripple factor can be decreased by increasing the value of the filter capacitor. Dr. Mostafa Elhussien Digital Electronic Systems (1) Lec. 1 10  POWER SUPPLIES IC Regulated Power Supplies While filters can reduce the ripple from power supplies to a low value, the most effective filter is a combination of a capacitor-input filter used with an integrated circuit (IC) voltage regulator. An integrated circuit regulator is a device that is connected to the output of a filtered rectifier and maintains a constant output voltage despite changes in the input, the load current, or the temperature. The capacitor-input filter reduces the input ripple to the regulator to an acceptable level. The combination of a large capacitor and an IC regulator is inexpensive and helps produce an excellent small power supply. Dr. Mostafa Elhussien Digital Electronic Systems (1) Lec. 1 11  POWER SUPPLIES IC Regulated Power Supplies The most popular IC regulators have three terminals: an input terminal, an output terminal, and a reference (or adjust) terminal. The input to the regulator is first filtered with a capacitor to reduce the ripple to less than 10%. The regulator further reduces the ripple to a negligible amount. In addition, most regulators have an internal voltage reference, short-circuit protection, and thermal shutdown circuitry. They are available in a variety of voltages, including positive and negative outputs, and can be designed for variable outputs with a minimum of external components. Typically, IC regulators can furnish a constant output of one or more amps of current with high ripple rejection. IC regulators are available that can supply load currents of over 5 A. Dr. Mostafa Elhussien Digital Electronic Systems (1) Lec. 1 12  POWER SUPPLIES IC Regulated Power Supplies Three-terminal regulators designed for a fixed output voltage require only external capacitors to complete the regulation portion of the power supply, as shown in Figure (a). Filtering is accomplished by a large-value capacitor between the input voltage and ground. Sometimes a second smaller-value input capacitor is connected in parallel, especially if the filter capacitor is not close to the IC, to prevent oscillation. This capacitor needs to be located close to the IC. Finally, an output capacitor (typically 0.1𝜇𝐹 to 1𝜇𝐹) is placed in parallel with the output to improve the transient response. Dr. Mostafa Elhussien Digital Electronic Systems (1) Lec. 1 13  POWER SUPPLIES IC Regulated Power Supplies Examples of three-terminal regulators are the 78XX series of positive regulators (and the corresponding 79XX series of negative regulators.) The last two digits in the number stand for the output voltage; thus, the 7812 is a positive regulator with a +12𝑉 output, and the 7912 is a negative regulator with a −12𝑉 output. Dr. Mostafa Elhussien Digital Electronic Systems (1) Lec. 1 14  POWER SUPPLIES IC Regulated Power Supplies A basic fixed power supply with a 7805 regulator is shown in Figure. The output of the bridge is filtered by a relatively large electrolytic capacitor to reduce ripple and present a dc to the input of the regulator. This voltage should be about 2.5 V larger than the output voltage for the 7805 regulator to be able to hold regulation—or at least 7.5 V out of the bridge. The schematic shows a standard 12.6 V transformer, but for the 7805, a lower voltage can be supplied by the transformer and still work nicely, as long as the peak secondary voltage is at least 9 V (to allow for diode drops and head room for the regulator). Generally, heat sinking is required to prevent the regulator from overheating. Dr. Mostafa Elhussien Digital Electronic Systems (1) Lec. 1 15  POWER SUPPLIES Percent Regulation The regulation expressed as a percentage is a figure of merit used to specify the performance of a voltage regulator. It can be in terms of input (line) regulation or load regulation. Line regulation specifies how much change occurs in the output voltage for a given change in the input voltage. It is typically defined as a ratio of a change in output voltage for a corresponding change in the input voltage expressed as a percentage. ∆𝑉𝑂𝑈𝑇 𝐿𝑖𝑛𝑒 𝑟𝑒𝑔𝑢𝑙𝑎𝑡𝑖𝑜𝑛 = 100% ∆𝑉𝐼𝑁 Load regulation specifies how much change occurs in the output voltage over a certain range of load current values, usually from minimum current (no load, NL) to maximum current (full load, FL). It is normally expressed as a percentage and can be calculated with the following formula: 𝑉𝑁𝐿 − 𝑉𝐹𝐿 𝐿𝑜𝑎𝑑 𝑟𝑒𝑔𝑢𝑙𝑎𝑡𝑖𝑜𝑛 = 100% 𝑉𝐹𝐿 Where 𝑉𝑁𝐿 is the output voltage with no load and 𝑉𝐹𝐿 is the output voltage with full (maximum) load. Dr. Mostafa Elhussien Digital Electronic Systems (1) Lec. 1 16  POWER SUPPLIES Percent Regulation (Example) Assume a certain 7805 regulator has a measured no-load output voltage of 5.18 V and a full-load output of 5.15 V. What is the load regulation expressed as a percentage? Solution 𝑉𝑁𝐿 − 𝑉𝐹𝐿 𝐿𝑜𝑎𝑑 𝑟𝑒𝑔𝑢𝑙𝑎𝑡𝑖𝑜𝑛 = 100% 𝑉𝐹𝐿 5.18 𝑉 − 5.15 𝑉 𝐿𝑜𝑎𝑑 𝑟𝑒𝑔𝑢𝑙𝑎𝑡𝑖𝑜𝑛 = 100% 5.15 𝑉 𝐿𝑜𝑎𝑑 𝑟𝑒𝑔𝑢𝑙𝑎𝑡𝑖𝑜𝑛 = 0.58 % Dr. Mostafa Elhussien Digital Electronic Systems (1) Lec. 1 17  Addressing the limitations of linear power supplies Advantages of linear supplies: Greatest disadvantage is large size and weight Simple, Due primarily to size of AC step-down transformer Low-cost, Size / weight increase as current rating Robust, increases Clean output, Another disadvantage is lack of flexibility Low electromagnetic Ability to operate on different supply voltages / emissions frequencies Ability to produce different output voltages Challenge: how to reduce the size of the transformer? Transformer size decreases as frequency increases But AC mains frequency is normally 50-60 Hz (fixed) Dr. Mostafa Elhussien Digital Electronic Systems (1) Lec. 1 18  Switching Mode Power Supplies (SMPS) Also called "switch" or "switched" mode Mains AC is rectified and filtered to produce high-voltage DC High voltage DC is "chopped" into a high frequency pulsed signal using a switch (usually a MOSFET) A transformer converts this high voltage, high frequency pulsed signal into a lower voltage (but still high frequency) pulsed signal High frequency → smaller transformer Lower amplitude pulsed DC is then rectified and filtered to produce a constant (non-pulsed) DC output Regulation of output voltage is performed by varying the duty cycle of the switch / chopper Dr. Mostafa Elhussien Digital Electronic Systems (1) Lec. 1 19  Basic AC-DC SMPS block diagram Dr. Mostafa Elhussien Digital Electronic Systems (1) Lec. 1 20  AC rectifier and filter Mains AC is rectified using diodes Usually using a bridge rectifier Rectifier output is then filtered / smoothed using capacitors Produces (very) high voltage DC Dr. Mostafa Elhussien Digital Electronic Systems (1) Lec. 1 21  Switcher (Chopper) "Chops" the high voltage DC into pulses Usually, a MOSFET is used as switch Other types of power transistors can also be used High frequency pulsed signal at gate switches the MOSFET between on and off states FET has high efficiency when operated this way Low power dissipation in on and off states Switching signal frequency is typically in the range of tens of kHz to several MHz Generated by a regulator / feedback circuit Duty cycle of switching signal controls the output voltage Dr. Mostafa Elhussien Digital Electronic Systems (1) Lec. 1 22  Transformer Steps the high-voltage, high-frequency, pulsed DC down to a lower voltage High voltage pulses → lower voltage pulses Increasing frequency decreases size of Transformer Filtering capacitors Higher frequency allows SMPS to be smaller / lighter than comparable linear power supplies Transformer also provides electrical isolation between high and low voltages Important for safety Dr. Mostafa Elhussien Digital Electronic Systems (1) Lec. 1 23  Pulsed DC rectifier and filter The stepped down, pulsed DC is rectified to produce non-pulsed DC Done using various combinations of diodes, capacitors, and/or inductors Because of the higher frequency, the filtering capacitors in this stage can be smaller than those used to rectify lower frequency, AC mains voltage Dr. Mostafa Elhussien Digital Electronic Systems (1) Lec. 1 24  Voltage regulator / controller Maintains a constant output voltage as load impedance (and/or source voltage) changes Monitors the DC output voltage Compares it to a stable reference voltage Duty cycle of the pulsed gate signal controls the output voltage If output too high, duty cycle is decreased If output too low, duty cycle is increased Feedback is usually via an opto-isolator For electrical isolation / safety Dr. Mostafa Elhussien Digital Electronic Systems (1) Lec. 1 25  Advantages and disadvantages of SMPS Advantages Disadvantages Small size / weight Complicated High efficiency More expensive Flexible line voltage Higher conducted and and frequency radiated emissions Dr. Mostafa Elhussien Digital Electronic Systems (1) Lec. 1 26  DC-DC conversion SMPS are also for DC-DC conversion Step up or step-down DC voltages Very common application of SMPS Different types of DC-DC converters Buck: reduces (steps down) input voltage Boost: increases (steps up) input voltage Buck-Boost: can step voltage up or down Note however that if no transformer is used, there is no electrical isolation between input and output Dr. Mostafa Elhussien Digital Electronic Systems (1) Lec. 1 27  Summary Switching mode power supplies can be used to convert AC to DC or convert between DC voltages A semiconductor switch (often a MOSFET) is used to "chop" a DC voltage into a high frequency pulsed signal Reduces the required transformer / capacitor sizes Allows control of output voltage by varying the duty cycle of a pulsed signal at the gate Transformers are used to step down voltages and for electrical isolation (safety) Capacitors, inductors, and diodes are used to rectify and filter the voltage at various points in the supply SMPS offer significant advantages over linear supplies Dr. Mostafa Elhussien Digital Electronic Systems (1) Lec. 1 28