AC Direct Off-Line Power Supplies

Transcription

1 AC Direct Off-Line Power Supplies r Introduction Many DC power supplies found in electronic systems, including those in this Tech School, rectify the 120 volts available at an electric outlet. The initial input circuitry of the power supply rectifies and filters the AC voltage to a filtered DC voltage. The initial rectifier/filter circuits may be referred to as an AC line-derived DC power supply or simply a raw DC power supply. The output DC voltage from the AC line-derived DC power supply feeds the regulator circuits or remaining circuits of a switching power supply. This section covers the theory, precautions, and measurements related to the AC line-derived DC power supply section. AC Line-Derived Half-wave Rectifier A simple half-wave rectifier and filter circuit covered earlier in this course can be supplied with AC voltage from an electric outlet. On the positive AC cycle the diode conducts, charging the filter capacitor to the AC line voltage peak. On the negative cycle of the AC voltage the diode remains open and the capacitor remains charged. With no load resistor or discharge path for the capacitor, the output is a steady voltage of approximately 169 volts. This assumes the electric outlet voltage is near the nominal value of 120 V RMS with a normal peak voltage. By changing the polarity of the diode and filter capacitor in the half-wave supply shown in figure 15, a negative output voltage may be produced. In a half-wave rectifier, the filter capacitor's charge is only replenished on one alternation of the AC cycle. Therefore, a large and expensive filter capacitor is required for high load currents. For this reason, a half-wave rectifier/filter is commonly used in low power circuit applications. Fig. 15: An AC line-derived DC power supply using a half-wave rectifier with a filler capacitor. AC Direct Off-Line Power Supplies27

2 AC Line-Derived Full Wave Rectifier Both alternations of an input AC voltage can be rectified by using a circuit arrangement shown in figure 16. When both the positive and negative AC alternations are rectified it is called full-wave rectification. A full-wave rectifier requires 2 or more diodes. The full-wave rectifier in figure I6 uses two diodes and requires a center-tapped transformer secondary. The center-tap is the common reference point or circuit ground for the load. The voltages at the opposite ends of the secondary are 180 degrees out-of-phase with each other. When the voltage at the top of the secondary swings positive with respect to the center-tap, the voltage at the bottom of the transformer secondary is negative with respect to the center-tap. On the next AC alternation the voltage at the top is negative and the voltage on the bottom positive with respect to the center-tap. D1 Load D1 Load Fig. 16: Full wave rectifier with center - tapped transformer. 28AC Direct Off-Line Power Supplies

3 Consider the circuit operation of the full-wave rectifier in figure 16. When the voltage at the top of the secondary goes positive, D1 is forward biased. Current flows from the transformer center-tap charging Cl and through D1 to the top of the secondary. Current also flows through the load resistor. During the next alternation the voltage at the bottom of the transformer secondary becomes positive forward biasing D2. Current flows from the center-tap recharging CI and through D2 to the bottom of the secondary. Current also flows through the load resistor. The filter capacitor in the full-wave rectifier smoothes the pulsating output voltage. Without the filter capacitor the voltage output of a full-wave rectifier would be DC pulses during the negative and positive alternation as shown in figure 17A. The filter capacitor holds a charge and opposes a voltage change to smooth the output voltage as shown in figure 17B. The output voltage of the supply shown in figure 16 depends on the turn ratio of the transformer. For example a 1:1 turns ratio or full secondary voltage peak of approximately 169 volts results in a voltage output of approximately 85 volts (1/2 of the 169 V peak). A. Full-wave output ( No Capacitor) B. Full-wave output ( With Capacitor) Fig. 17: Output voltage waveforms of a full-wave rectifier without a filter capacitor and with a filter capacitor (B). (A) The full-wave rectifier output voltage requires less capacitance because it is much easier to filter than the half-wave rectifier. However, the full-wave rectifier shown in figure 16 requires a center tapped transformer. This makes it undesirable for most modern day cost-sensitive and efficient operating electronic systems. Full-wave rectification without a transformer is possible with a bridge rectifier circuit arrangement shown in figure 18. A full-wave bridge consists of 4 diodes arranged so current flows in the load in only one direction during both alternations of the AC input voltage. A ground or common circuit reference is designated at the junction of diodes D3 and D4. AC Direct Off-Line Power Supplies 29

4 A. ( Load B. Load Fig. 18: Direct AC line-derived full wave bridge rectifier and filter. Figure ISA shows the current flow during the positive half cycle of the input AC sine wave. The voltage potential forward biased diodes DI and D2. This places the full voltage of the AC line across capacitor Cl and the load. Current flows from the bottom side of the AC plug through D I to charge capacitor Cl. Current flows from the top plate of Cl through diode D2 to the positive side of the AC plug. At the same time current flows through the load. During the negative half cycle of the input AC sine wave, diodes D3 and D4 are forward biased. Current flows from the top side of the AC plug through D3, charging C1 in the same polarity as the previous half cycle. Current flows from the top plate of Cl through D4 to the bottom side of the AC plug. Current also flows through the load as shown. The voltage output of an AC line-derived bridge rectifier is approximately 169 volts with no resistor load or C1 discharge path. The bridge rectifier provides direct AC line-derived full-wave rectification of the AC line voltage without the added cost, weight, and energy losses associated with a power transformer. The bridge rectifier is ideal for most applications and is by far the most popular raw DC power supply found in electronic systems. 30AC Direct Off-Line Power Supplies

5 HOT Grounds A direct AC line-derived supply commonly results in a "HOT" ground or circuit common point in the electronic system. The electronic system may be said to have a "HOT" chassis. A "HOT" chassis or "HOT" circuit ground or common reference results from a direct path to one or both sides of the electrical outlet. This creates a shock hazard for service technicians when servicing and poses a test equipment hazard. To understand what is meant by a "hot" ground in a electronic system, consider an AC electric outlet and the building's electric box shown in figure 19. The 120 volts AC power line voltage potential exists between neutral or earth ground and a hot wire input to the electric box. A white wire is connected from neutral or earth ground to the wide plug of the electric outlet. The hot wire is connected through a circuit breaker in the electric box with a black wire to the narrow or "hot" side of the electric outlet. Electric Box HOT Electronic System DC Power Supply L System Ground Fig. 19: Wiring of an electric outlet and electric box to an electronic systems power supply. Figure 19 shows that when the enclosures of an AC powered electronic system are removed, circuit points in thesystem are connected directly to the hot side of the AC outlet. Touching hot circuit points in the electronic system would be essentially that same as removing the AC outlet cover AC Direct Off-Line Power Supplies 3

6 and touching the black wire connection. An alternating voltage potential exists between the hot side (narrow plug) and earth ground. Current flows through any resistance path between these points of potential. An electronic system with a half-wave rectifier uses a polarized AC plug. The polarized plug insures connection from the wide side of the outlet, neutral or earth ground, to common or ground points of the system. (See figure 20.) This arrangement presents no voltage potential in respect to earth ground if proper electric wiring and earth ground connections have been made. You can safely touch the circuit ground in the system or connect test equipment grounds. But what happens if the AC line cord is plugged into the receptacle backwards with a "cheater" adapter? Now the circuit common or ground is "hot" or at a 120 volt AC potential to earth ground. This creates a potentially lethal shock hazard for a service technician if they touch this point and complete a conduction path to earth ground. A greater servicing problem exists with a full-wave AC line-derived bridge power supply. A direct AC line-derived bridge rectifier uses a common ground reference at the anode junction of two bridge rectifier diodes. During the operation of the bridge rectifier, these diodes are alternately turned on and off, each conducting during the opposite half of each AC cycle. During each negative AC alternation, the common ground in the electronic system is electrically connected to the hot side of the AC plug. A voltage difference of half the AC line potential exists to earth ground no matter how the AC plug is connected. A voltmeter connected from the bridge common ground to earth ground reads approximately 60 volts AC. The voltage created on the "HOT" ground in a line-derived bridge supply is capable of delivering current through any conductive path to earth ground. This includes current through you are unfortunate enough if you to touch the ground of the chassis while your shoes conduct through a damp cement floor. A "HOT" ground can pose a hazard for the power supply and other test equipment when connected. Most test instrument grounds or shields are connected to earth ground for shielding and user safety. Connecting a ground from a test instrument to the "Hot" ground of the power supply causes high current to flow through the bridge rectifier diode as shown in figure 3. The high current may cause damage to the power supply components and test equipment ground paths. Note: The term "hot ground" is really a misnomer, since the circuit ground is not at any ground potential. Instead, think of the "hot ground" as "hot common" meaning that it is the common reference point for making voltage measurements in the electronic system. 32 AC Direct Off-Line Power Supplies

7 digital display indicating the potential safety leakage current that may be produced from each exposed metal part. The reading should be less than 500 microamps on each part. A readings considerably higher than this indicates a high leakage current potential and shock hazard. Note: Any metallic part that reads overrange on the CURRENT/POWER LCD (blank display with a digit "1" on the left) is connected directly to one side of the AC line. AC Line-Derived Power Supply Voltage & Waveform Measurements Determining if the AC line-derived power supply is functioning properly requires voltage measurements at key input and output circuit points. To analyze an AC li ne-derived power supply you must test the input AC voltage and the output DC voltage. The input AC voltage to the bridge rectifier is applied to the bridge rectifier at the junction of D2 and D3 and junction of D4 and Dl as shown in figure 24. At these junctions the anode of a diode connects with a cathode of another. These points may be identified with a sine wave symbol on a bridge rectifier module or IC. On the power supplies used in this course, the input ACV test points are identified as "ACV H" (hot) and "ACV N" (neutral) as shown in figure 24. Test Equipment Input ACV Measurement To isolation Transformer D4 ACV N Cl HOT Ground Test Equipment Output DCV, VPP Measurement 0 Fig. 24: Test lead connections to measure input and output voltages and waveforms. To measure the input AC volts to a power supply, connect the test instrument leads as shown in figure 24. Note that when connecting a test instrument for input AC voltage measurements you do not connect to the common or hot ground of the power supply. Therefore, connecting a second ground shared by the same instrument or another instrument shunts DI producing high input currents. AC Direct Off-Line Power Supplies 37

8 CAUTION When connected for AC input voltage measurements, as in figure 24, do not connect a second test instrument ground or ground lead from the same instrument. Doing so shunts D1 causing high AC line current that may damage power supply components. Figure 25 shows the typical scope waveform you should observe at the AC input voltage test points to an AC line-derived power supply. The waveform peaks may be flattened slightly due to inductance of the isolation transformer and input AC filter coils. This may lower the measurements from the nominal values. Some typical voltage readings with the PR570 set to output 120 V are shown in figure 25. The time of one AC waveform cycle is approximately milliseconds and the frequency is 60 Hz. Volts/DIV = 50 Time/DIV = 2 ms VRMS = Approx. 115 V Vpeak = Approx. 167 V VPP = Approx. 335 VPP milliseconds 1 ti me = 60 Hz Fig. 25: Input AC voltage waveform and approximate measurement values. The output voltage of the AC line-derived power supply is developed at the positive terminal of the filter capacitor in respect to the common or hot ground of the circuit. The negative terminal of the capacitor connects to the common or hot ground. Connecting to the terminals of the large filter capacitor near the bridge rectifier provides the proper circuit points for outputmeasurement. In the power supplies in this course. TP1 corresponds to the positive capacitor terminal and HOT GND to the negative capacitor terminal of Cl as shown in figure 24. The positive terminal of the capacitor feeds the regulator or remainder of the switch mode power supply circuitry. Initially these circuits are the load on the filter capacitor. When the capacitor is loaded it discharges between AC alternations resulting in a variation to the output DC voltage. 38 AC Direct Off-Line Power Supplies

9 This voltage variation appears as a repeating sawtooth waveform of approximately 120 Hz when viewed on a scope. The sawtooth waveform is commonly referred to as "AC ripple." Analyzing the output should include a measurement of the DC voltage and the peak-to-peak amplitude of the AC ripple. When the filter capacitor is unloaded or has no discharge path, the output DC voltage has little if any ripple. The DC output voltage reads near the peak of the AC voltage or approximately 169 volts. A voltage near 169 volts with little or no ripple indicates the raw DC supply is not loaded. As the capacitor is loaded by the remainder of the circuits the AC ripple increases. The increased load and ripple peak-to-peak volts results in a reduced DCV measurement. The ripple peak-to-peak volts rarely exceeds 10% of the DC value. For example, if a ripple greater than 16VPP is measured, the load may be excessive, the filter capacitor defective, or a bridge diode open. To determine if a bridge rectifier diode has opened, measure the frequency of the ripple waveform. A frequency of 60 Hz indicates an open diode. Volts/DIV = 1 or.5 Time/DIV = 5 ms No Load = Approx 169 V 0V Time Ripplet VPP - - Med. Load = Approx 160 V 0V )1.- Time Ripple VPP Heavy Load = Approx 150 V 0V ) Time Fig. 26: AC ripple waveform at output of AC line-derived power supply to hot ground. AC Direct Off-Line Power Supplies39