SMPSWhat is SMPS?SMPS means Switch Mode PowerSupply. This is used for D.C to D.Cconversion. This works on the principle ofswitching regulation. The SMPS system ishighly reliable, efficient, noiseless andcompact because the switching is done atvery high rate in the order of several KHzto MHz
NecessityThe SMPS regulators are used to provide regulated voltage for various equipments like PC, CFL, TV, etc.
A pulse train drives the base of ‘switchingor pass transistor’. When the voltage to thebase is high, the transistor saturates, whenthe voltage is low, the transistor turns off.Here the transistor functions as a switch.When the transistor is ON, load current isdrawn through the transistor and choke L.When the transistor is OFF the loadcurrent is maintained by the energy storedin the choke L. The current flows throughearth, Diode D, choke, load and earth.Hence this diode is called ‘Retrieval Diode’.
Duty cycle of the Transistor D = On Time On Time + Off Time (one cycle time) The output voltage = Input voltage x D
For example If I/P voltage is 200 volts and D=0.25 O/P voltage = 200 x 0.25 = 50V. Regulation is achieved by modifying the Duty cycle. Duty cycle depends on onetime of transistor, which in turn depends on the width of the pulse applied to the base of the transistor, which is controlled by ‘Pulse width modulation’ by regulator circuit
Regulation The relaxation oscillator produces a square wave. The square wave is integrated to get a triangular wave, which drives the non-inverting input of a triangular to pulse converter. The pulse train out of this circuit then drives the Pass Transistor. The output is sampled by a voltage divider and fed to a comparator. The feed back voltage is compared with a reference voltage. The output of the comparator then drives the input of the triangular to pulse converter
RegulationIf the output voltage tries to increasethe comparator produces a higheroutput voltage which raises thereference voltage of the triangular-to pulse converter. This makes thepulse that drives the base of theswitching transistor narrower. Thatmeans duty cycle is reduced. Sincethe duty cycle is lower the outputbecomes less which tries to cancelalmost all the original increase inoutput voltage.
Regulation Conversely, if the regulated output voltage tries to decrease, the output of the comparator decreases the reference voltage of the triangular -to pulse converter. This makes the pulse wider and the transistor conducts for larger time and more voltage comes out of the L.C.filter. This cancels out the original decrease in output voltage
Input SectionBlock 1 EMIBlock 2 Current limiterBlock 3 FloateBlock 4 Power factor corrector 100khzBlokck 6-9 DC-DC converter
TELECOMMUNICATION ENGINEERING CENTRE (DOT) for the S.M.P.S. BASED POWER PLANT GENERIC REQUIREMENTS (No. G7SMP/-01/01 JULY 04) Primary application of the rectifiers SMPS 48V- 5600W are in the supply of Telecom equipment. The convection cooled unit may be operated up to 60oC ambient air temperature. The rectifier operates from a nominal 3 X 230 Vac rms (with neutral wire) source. The mains frequency may vary from 45 Hz to 65 Hz. Total harmonic distortion (THD) of the input current wave form is below 5%.
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