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Author Topic: THE SWITCH MODE POWER SUPPLY  (Read 4712 times)
K0YNE
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« on: July 16, 2009, 09:52:46 PM »

SWITCH MODE POWER SUPPLY
 
A SHORT DISSERTATION ABOUT THE SWITCH MODE POWER SUPPLY.
 
SINCE THE ADVENT OF COMPUTERS, POWER SUPPLIES HAVE CHANGED DRAMATICALLY TO SUPPLY
A PURE DC VOLTAGE WITHOUT THE 60 OR 120 CYCLE AC RIPPLE THAT WAS A PROBLEM IN THE PAST.
THE SWITCH MODE METHOD, REDUCED THE WEIGHT AND SIZE OF THE POWER SUPPLY SIGNIFICANTLY
WHERE IT MADE THE PC MUCH LIGHTER IN  WEIGHT AND SIZE.
THE ELECTRICAL DESIGN  WAS SIMILAR TO OLD CAR RADIOS WHEREAS A 6 VOLT DC BATTERY VOLTAGE
WAS TRANSFORMERED UP TO 300 DC VOLTS TO OPERATE TUBE RADIOS WITH A "VIBRATOR" IN LINE
TO PROVIDE A 300 TO 400 CPS FREQUENCY AND VERY LITTLE FILTERING NEEDED.
 
THE SWITCH MODE SUPPLIES CAN BE FOUND IN SMALL PACKAGES TO PLUG IN THE
WALL AND RECHARGE ALL TYPES OF RECHARGEABLE BATTERIES IN CAMERAS FOR
EXAMPLE AND CHARGE THEM QUICKLY.
 
THE SWITCH MODE POWER SUPPLIES MOST OFTEN ARE DIVIDED INTO TWO SECTIONS. THE FIRST
SECTION  WOULD BE WHERE THE LINE VOLTAGE OF 120 VAC AT 60 HZ PER SECOND WOULD ENTER
THE BOARD AND WOULD BE RECTIFIED WITH A BRIDGE RECTIFIER  ARRANGEMENT.  FROM THE 120V AC
YOU WOULD HAVE A RECTIFIED VOLTAGE OF APPROXIMATLEY 169 VOLTS PLUS OR MINUS. THIS IS CALCULATED
BY USING 1.414 TIMES THE AC VOLTAGE OF 12O VOLTS. THIS VOLTAGE MAY INCORPORATE A FILTER
CAPACITOR, BUT A LOT OF RIPPLE CAN BE TOLERATED AS IT IS FED TO THE SWITCHING TRANSITOR.
THE SWITCHING TRANSISTOR  HAS THREE CONNECTIONS. THE TRANSISTOR IS FED THE 169 DC VOLTS IN
ONE CONNECTION, AND AN OUTPUT CONNECTION IS HOOKED TO THE THE PRIMARY OF A SPECIAL
TRANSFORMER. THE OTHER LEAD OF THE PRIMARY IS HOOKED TO GROUND ON THE AC SIDE OF THE
BOARD. NOW THAT THE TRANSISTOR IS IN LINE TO THE TRANSFORMER, AND IT HAS THE 169 VOLTS
SITTING ON ONE OF ITS CONNECTIONS. NOW THE THIRD CONNECTION OF THE TRANSISTOR WOULD
BE ITS BASE. THE BASE WOULD BE USED TO PROVIDE THE THE NECESSARY VOLTAGE PULSES
THAT WILL TURN THE TRANSISTOR OFF AND ON RAPIDLY, AT A FREQUENCY OF 400 CYLES PER SECOND.
 
NOW THAT 169 VOLTS DC IS BEING PULSATED INTO A FERRITE TRANSFORMER, WE NOW ARE SEEING
THE SECOND SIDE OF THE BOARD AS THE SECONDARY VOLTAGE IS BEING TRANSFORMERED DOWN TO A
LOWER DC VOLTAGES THE COMPUTER WILL USE. THE TRANSFORMER IS SPECIAL, AND IS A FERRITE
MATERIAL THAT WILL HANDLE HIGH FREQUENCY CURRENT EFFICIENTLY. IN THE PAST, IRON CORE
TRANSFORMERS WERE USED, BUT HUGE CAPACITORS OF THOUSANDS OF MICROFARDS WERE NEEDED
TO PROVIDE ADEQUATE FILTERING.
.
NOW CONSIDER THAT WE HAVE STEPPED DOWN  THE VOLTAGE FROM THIS TRANSFORMER AND DERIVING
3 OR MORE DIFFERENT WINDINGS TO GIVE US DIFFERENT VOLTAGES. EACH WINDING WILL RECEIVE
RECTIFICATION , FILTERING AND REGULATION ON ITS OUTPUT.  ANOTHER BIG BENEFIT IS THE USE OF SMALL
CAPACITORS TO DO THE FILTERING. TAKING UP VERY LITTLE SPACE. SINCE THE FREQUENCY IS SO FAST, THE TIME
FOR CAPACITORS TO CHARGE AND DISCHARGE IS SHORT, SMALLER VALUE CAPACITOR ARE NEEDED DURING THE
CHARGE AND DISCHARGE TIME PERIOD TO DUMP THEIR CHARGES INTO THE LOAD.  THE SWITCH MODE POWER
IN SHORT, CONVERTS LINE AC VOLTAGE TO DC, PULSATES THAT VOLTAGE THRU A STEP DOWN TRANSFORMER,
AND RECTIFIES IT AGAIN BECOMING PURE DC FOR THE COMPUTERS USE. COMPUTERS AND OTHER DIGITAL
DEVICES CANNOT TOLERATE ANY AC RIPPLE. THE RIPPLE INTERFERES WITH THE SQUARE WAVES AND WOULD
MAKE PROCESSING DATA NEARLY IMPOSSIBLE. A COMPUTERS VOLTAGES  ARE MINUS AND PLUS 5, 12 AND
MORE, BUT I AM NOT SURE OF HOW MANY SOURCES THERE ARE IN TOTAL.
 
THE OBJECTIVE OF THE PRESENTATION IS MERELY TO MAKE YOU AWARE OF SOME OF THE ASPECTS
OF THE POWER SUPPLY AND I DID NOT COVER SOME OF THE OTHER VOLTAGE REGULATION PROPERTIES.
THANKS FOR VIEWING .ALSO SWITCH MODES MAY NOT OPERATE AT 400CPS.   

PAUL
KYNE
« Last Edit: July 17, 2009, 12:06:12 PM by kbasq » Logged
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