advertisement No Design Offline Power Supply – Design Note 62 Anthony Bonte and Ron Vinsant Offline Switcher Eliminates Optocoupler Feedback. Low Cost, Simple, 50W, Universal Input Power Supply. Linear Technology has broken through the “buy-vs-build” barrier for offline power supplies. The new LT1105 1 current-mode PWM control IC is used to make a simple, triple output power supply (Figure1). The circuit features low cost, high reliability and customizable footprint. It accepts a universal input of 85VAC-270VAC while providing isolated and regulated output voltages of 5V at 5A, 12V at 1.5A and –12V at 0.5A. MTBF is calculated at >100k hours for full load at 25°C ambient. The power supply contains all necessary components including an input EMI filter. All outputs have continuous short-circuit protection. Figure 2 indicates 5V load regulation performance as a function of input line voltage. The LT1105 eliminates optocoupler feedback by regulating the flyback voltage of the bootstrap bias winding. This reduces the number of components crossing the isolation barrier to one: the transformer. The transformer is designed to meet international safety standards and is subject to a set of compromises involving efficiency, maximum power output, size, coupling, leakage inductance, interwinding capacitance and ultimately cost. A unique sampling error amplifier incorporated into the LT1105 allows operation in spite of the resultant transformer limitations. The error amplifier provides a feedback term allowing load regulation performance to be set with one external resistor. Thus, ±1% line and load regulation performance is achievable for single output voltage power supplies operating in either continuous or discontinuous mode2. LTC has simplified the magnetics design task by creating a series of off-the-shelf transformers for a variety of applications. New transformer design continues as an area of development. Transformers in power levels of 50W and 100W are presently available and meet international safety standards UL1950 and IEC950. Completed transformers are available from Coiltronics at 305-781-8900. 09/92/62 The LT1105’s totem-pole output drives the gate of external high-voltage FET switch Q1. R10 controls switching transition speed. Transition speed is a trade-off between minimizing switch dV/dt common mode current contributions vs minimizing switching losses. FET conduction losses are set by the values of switch “on” resistance and primary current. The FET voltage rating must exceed the sum of the maximum rectified DC input voltage plus the leakage inductance spike. Finally, the external FET is protected from insufficient or excessive gate drive voltage with a drive protection circuit built into the LT1105. Short-circuit protection is provided by bootstrap operation of the LT1105. Shorting an output results in switch duty cycle “on” time being limited to 500ns. The transformer cannot store sufficient energy to maintain a regulated bias winding voltage. The LT1105 senses this condition and shuts down the power supply. The power supply then returns to start-up mode. Trickle resistor R11 charges input bypass capacitor C8 to the LT1105 start threshold voltage. If the output remains shorted, the LT1105 starts and stops again. This “burp” mode protects the power supply from overload or indicates an incomplete power loop. Sense resistor R22 sets the maximum switch current available. To guarantee “burp” mode operation under fault conditions, C8 must be prevented from peak-detecting the large leakage inductance spike during maximum switch current cycles. Otherwise, the bootstrapped supply voltage would increase under a fault condition thereby leading to catastrophic failure. Resistor R3 along with C8 forms an R-C filter which prevents the diode D2/C8 combination from peak detection. This ensures well defined start cycles. 1. Data Sheet, LT1103/LT1105 Offline Switching Regulator, Linear Technology Corporation, Milpitas, CA., March 1992 2. Bonte, A. and Vinsant R., “Offline Switching Regulators Achieve ±1% Regulation in a Flux-Sensed Converter”, Seventh Annual Applied Power Electronics Conference, IEEE-7803-0485392, p 513-516, 1992 NEU MOLEX3 E1 C1 0.1µF 250V MP3-X2 EARTH GND BALUN RT1 D502EL C3 4700pF 250V MP3-Y C2 0.1µF 250V MP3-X2 C4 4700pF 250V MP3-Y R1 1M 1/2W 5 C5 6800pF 250V MP3-Y 2 5 R17 45.3k 1% 3 3 R16 2k 1 R6 3.92k 1% R5 12k 1% R4 499 1% 2 C10 0.047µF C11 390pF C7 220µF 385V C9 1000pF 5 + D4 1N4148 3 D3 BAV21 BR1 GBU4K 1 FB 5 OSC 2 ILIM 3 2 3 1 GND VIN 6 4 15V VSW VC 5 8 4 7 LT1105 R11 204k 1W C14 0.1µF R8 330 1 C13 1µF 4.75 4.80 4.85 4.90 4.95 0 DN62 • F02 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5V LOAD (A) 270VAC 220VAC 110VAC 5.00 85VAC 5.05 12V LOAD = 0.75A (50%) –12V LOAD = 0.25A (50%) 5.10 5.15 5.20 5.25 C8 39µF 35V C15 0.047µF R9 30k Q1 R10 800V, 3Ω 10 1 + D2 BAV21 R3 100 D6 MUR1100E D7 300V 5W 4 R22 0.15Ω 2W 4 HS1 T2 3• 2 5 6• 3 11T 60T C27 1000pF C21 1000µF 35V C26 1000pF D10 MUR410 R27 20 HS2 D8 MBR1645 R26 10 D9 MUR410 C25 1000pF + + + C19 1000µF 35V C18 1000µF 35V + C17 1000µF 35V 5V @ 5A C6 4700pF 250V MP3-Y C20 1000µF 35V COM –12V @ 0.5A + 12V @ 1.5A MOLEX4 J4 4 3 2 1 MOLEX4 J3 4 3 2 1 MOLEX4 J2 4 3 2 1 COM = SECONDARY SIDE RETURN = PRIMARY SIDE RETURN = EARTH GROUND DN62 F01 1. ALL RESISTANCES ARE IN OHMS, 1/4W, 5% 2. ALL GROUNDS MEET AT LT1105. MAIN GROUND RETURN IS FROM LT1105 TO C7 AND C5 3. MBR1645 MOUNTING TAB IS TIED TO THE DEVICE'S CATHODE INTERNALLY 4. DO NOT SUBSTITUTE COMPONENTS WITHOUT COMPLETE EVALUATION 5. ALL 1% RESISTORS ARE METAL FILM 6. R16 OUTPUT VOLTAGE ADJUSTMENT = ±0.5V ON 5V OUTPUT NOTE UNLESS OTHERWISE SPECIFIED: TRANSFORMER: COILTRONICS CTX02-11090-1 7T • 10 9 13 3T • 14 11 4T • 12 1 R25 20 Figure 2. 5V Load Regulation vs Line Voltage 5V OUTPUT VOLTAGE (V) Figure 1. LT1105 Fully Isolated, Offline Flyback, 100kHz, 50W Converter with Load Regulation Compensation C17, C18, C19, C20, C21 = UNITED CHEMICON LXF35VB10212.5X30LL C25, C25, C27 = WIMA FKP 2, 100VDC, POLYPROPYLENE FILM C8 = PANASONIC ECA1VFQ390 C9, C11 = WIMA FKC 2, 250VDC OR 100VDC, POLYCARBONATE FILM C10, C13, C14, C15 = WIMA MKS 2, 63VDC, METALLIZED POLYESTER R11 = FOUR 51k, 1/4W IN SERIES R16 = BOURNS 3266W-1-202 R22 = R.G. ALLEN MICRON MCP70 HS1, HS2 = THERMALLOY HEATSINK 7020U-MT Q1 = PHILIPS BUK426-800A (FULLY INSULATED F-PAK) D2, D3 = PHILIPS BAV21 D7 = GENERAL INSTRUMENT 1.5KE300A MUR410, MBR1645, MUR1100E = MOTOROLA C1, C2 = WIMA MP3-X2, METALLIZED PAPER C3, C4, C5, C6 = WIMA MP3-Y, METALLIZED PAPER R1 = 1MEG, 0.5W, CARBON COMPOSITION RT1 = MIDWEST COMPONENTS NTC THERMISTOR D502EL BALUN = COILTRONICS 1212-01103 BR1 = GENERAL INSTRUMENTS GBU4K C7 = SPRAGUE 82D221M385KC2D PARTS LIST: HOT J1 1 2 F1 2A SLOW-BLOW ALL TESTING PERFORMED ON AN OFFLINE CIRCUIT MUST BE DONE WITH AN ISOLATION TRANSFORMER CONNECTED BETWEEN THE OFFLINE CIRCUIT'S INPUT AND THE AC LINE. USERS AND CONSTRUCTORS OF OFFLINE CIRCUITS MUST OBSERVE THIS PRECAUTION WHEN CONNECTING TEST EQUIPMENT TO THE CIRCUIT TO AVOID ELECTRIC SHOCK. REPEAT: AN ISOLATION TRANSFORMER MUST BE CONNECTED BETWEEN THE CIRCUIT INPUT AND THE AC LINE IF ANY TEST EQUIPMENT IS TO BE CONNECTED. BEFORE PROCEEDING ANY FURTHER, THE READER IS WARNED THAT CAUTION MUST BE USED IN THE CONSTRUCTION, TESTING AND USE OF OFFLINE CIRCUITS. HIGH VOLTAGE, AC LINE-CONNECTED POTENTIALS ARE PRESENT IN THESE CIRCUITS. EXTREME CAUTION MUST BE USED IN WORKING WITH AND MAKING CONNECTIONS TO THESE CIRCUITS. REPEAT: OFFLINE CIRCUITS CONTAIN DANGEROUS, AC LINE-CONNECTED HIGH VOLTAGE POTENTIALS. USE CAUTION. DANGEROUS AND LETHAL POTENTIALS ARE PRESENT IN OFFLINE CIRCUITS! WARNING! • (408) 432-1900 ● FAX: (408) 434-0507 ● TELEX: 499-3977 • Linear Technology Corporation For literature on our DC to DC Converters, call (800) 637-5545. For applications help, call (408) 432-1900, Ext. 456 LT/GP 0992 190K 1630 McCarthy Blvd., Milpitas, CA 95035-7487 LINEAR TECHNOLOGY CORPORATION 1992