INTRODUCTION This note describes cost switching power supply applicati

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OFF-LINE FLYBACK SWITCHING REGULATOR
INTRODUCTION This note describes cost switching power supply applications requiring multiple output voltages, e.g. personal computers, instruments, etc. discontinuous mode flyback regulator used this application provides good voltage tracking between outputs, which allows primary side voltage sensing. This sensing technique reduces costs eliminating need isolated secondary feedback loop. cost, pin) UC1842 current mode control chip employed this power supply provides performance advantages such Fast transient response Pulse pulse current limiting Stable operation simplify drive circuit requirements, TO-220 power SGSP369 utilized power switch. This switch driven directly from output control chip. POWER SUPPLY SPECIFICATIONS Input voltage 95VAC 130VAC Hz/60 Output voltage load Ripple voltage load Ripple voltage load Ripple voltage Line Isolation 3750 Switching Frequency Efficiency Full Load BASIC CIRCUIT OPERATION 117VAC input line voltage rectified smoothed provide operating voltage circuit. When power initially applied circuit, capacitor charges through When voltage across reaches level output enabled, turning power During time energy stored transformer (inductor) this time polarity output windings such that output rectifiers reverse biased energy transferred. Primary current sensed resistor, R10, compared fixed reference inside IC1. When this level reached, turned polarity transformer windings reverses, forward biasing output rectifiers. energy stored transferred output capacitors. Many cycles this store/release action needed charge outputs their respective voltages. Note that must have enough energy stored initially keep control circuitry operating until charged level approximately voltage across through voltage divider error amplifier (pin compared internal reference. Energy stored leakage inductance causes voltage spike which added normal reset voltage across when turns off. clamp consisting limits this voltage excursion from exceeding BVDSS rating addition, turn-off snubber made keeps power dissipation delaying voltage rise until drain current decreased from peak value. This snubber also damps ringing which occur parasitics. Less than line load regulation achieved loading output control winding with This resistor dissipates leakage energy associated with this winding. Note that must isolated from with diode otherwise could charge necessary initial start-up. small filter inductor secondary added reduce output ripple voltage less than This inductor also attenuates high frequency noise.
AN247/1188
Figure Off-line Flyback Regulator.
Notes resistors unless noted. appendix construction details. Ferroxcube 204T50-3C8 (Toroid), turns Wire Gauge AWG).
Figure Block Diagram UC1842 Current Mode Controller.
TYPICAL SWITCHING WAVEFORMS
Upper trace Gate source voltage Lower trace Gate Current
TYPICAL SWITCHING WAVEFORMS
Upper trace Drain source voltage Lower trace Primary current PERFORMANCE DATA
Conditions Line VAC) Nominal Line (120 VAC) High Line (130 VAC)
Upper trace charging current Lower trace output ripple voltage
5.211 4.854 5.199 4.950 5.220 4.875 5.208 4.906 5.207 4.855 5.200 4.902
12.05 12.19 11.73 11.68 12.07 12.23 11.73 11.67 12.06 12.21 11.71 11.66
12.01 12.14 11.69 11.63 12.03 12.18 11.68 11.62 12.02 12.15 11.67 11.61
Overall Line Load Regulation
APPENDIX POWER TRANSFORMER
Core Ferroxcube EC-35/3C8 0.25mm. each outer Note reduced center only. 0.5mm.
TRANSFORMER CONSTRUCTION
Information furnished believed accurate reliable. However, SGS-THOMSON Microelectronics assumes responsibility consequences such information infringement patents other rights third parties which result from use. license granted implication otherwise under patent patent rights SGS-THOMSON Microelectronics. Specification mentioned this publication subject change without notice. This publication supersedes replaces information previously supplied. SGS-THOMSON Microelectronics products authorized critical components life support devices systems without express written approval SGS-THOMSON Microelectronics.
1995 SGS-THOMSON Microelectronics Printed Italy Rights Reserved
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INTRODUCTION This note describes cost switching power supply applicati

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