ch-3 wkg principle of smps

CHAPTER THREE

WORKING PRINCIPLE OF SMPS POWER PLANT

SMPS

What is SMPS ?

SMPS means Switch Mode Power Supply. This is used for D.C to D.C conversion.

This works on the principle of switching regulation. The SMPS system is highly reliable,

efficient, noiseless and compact because the switching is done at very high rate in the order

of several KHz to MHz.

Necessity

The SMPS regulators are used in B.S.N.L at various locations like CDOT, E10B

Transmission systems etc.

Principle of Switching Regulator

A pulse train drives the base of ‘switching or pass transistor’. When the voltage to the

base is high, the transistor saturates, when the voltage is low, the transistor turns off. Here the

transistor functions as a switch. When the transistor is ON, load current is drawn through the

transistor and choke L. When the transistor is OFF the load current is maintained by the

energy stored in the choke L. The current flows through earth, Diode D, choke, load and

earth. Hence this diode is called ‘Retrieval Diode’.

Duty cycle of the Transistor = On Time = D On Time + Off Time (one cycle time)

The output voltage = Input voltage x D

RTTC NAGPUR Page 21 of 35

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.

Principle of 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.

If the output voltage tries to increase the comparator produces a higher output voltage

which raises the reference voltage of the triangular- to pulse converter. This makes the pulse

that drives the base of the switching transistor narrower. That means duty cycle is reduced.

Since the duty cycle is lower the output becomes less which tries to cancel almost all the

original increase in output voltage.

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

RTTC NAGPUR 2 of 35

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.

For maximum efficiency the duty cycle should be less than 0.5. As long as the

triangular voltage exceeds the reference voltage, the output is high. Since Vref is adjustable,

we can vary the width of the output pulse and hence the duty cycle.

Switching regulators are more efficient than conventional regulators as the power loss in the

switching element is reduced to minimum as it conducts only for a fraction of a cycle.

Now a days SMPS technology is extended to power plants also. Power plants upto 2000A

capacity have been developed using SMPS principle.

Specification of SMPS Power Plant

1) Input Voltage 320 V to 480 V

Frequency 45 Hz TO 65 Hz

2) Output Voltage

in Float Mode -54.0 ± 0.5 V. adj range -48 V to -56V

in charge mode : -55.2 V ± 0.5V

3) Input power factor >0.95 Lag with 25% to 100% load at nominal input.


ITI’s 50V – 2000A POWER PLANT (Multi Rack Type)Suitable for VRLA Batteries with 100A SMPS Rectifier Modules

IntroductionThe power system is intended primarily to provide uninterrupted DC power to

Telecom equipments and current for charging the batteries in the presence of AC Mains. The system works from commercial AC mains which is rectified and regulated to –50V DC and is fed to the equipment (exchange). The system has provision to connect three sets of VRLA batteries and facility to charge them simultaneously to ensure that uninterrupted DC power supply is always available to the exchange.

The power system –50V, 2000A has the following features :

(a) Multi-rack configuration.

(b) Facility to parallel a maximum of 21 nos. (or 22 nos.) of 100A (5600W) Rectifier modules operation from three phase, 400V, 50Hz AC input.

(c) Termination for three sets of VRLA batteries and exchange.

(d) System input : Three phase, 4-wire, 50 Hz supply.

The power system has a single DC bus called auto float/charge bus. Depending upon the status of the batteries, the output DC voltage is maintained at 54.0 + 0.5 V under auto float condition. During auto charge the maximum DC voltage reached across the bus is 55.2 volts. The exchange battery and rectifier modules are connected in parallel.

The system employ natural convection cooling and has AC input distribution, DC output distribution, protection and alarm circuitry for rectifiers, battery and equipment.

Technical Specification

For Module

(1) Input Voltage :(a) 320V to 480V r m s three phase (Nominal Voltage – 400V).(b) Frequency : 45 Hz …. 65 Hz.

(2) Output Voltage :Float mode :Nominal voltage : -54.0 + 0.5V, Adjustment range : -48.0 to –56.0 V

Charge mode Voltage : -55.2 + 0.5 V


(3) Rated current : 100 Amps.

(4) Psophometric noise : Less than 4 mV without battery floated.Less than 2 mV with battery floated.

(5) Input power factor :Greater than 0.95 lag with 25% to 100% load at nominal input.

(6) Efficiency :Greater than 90% at full Load and nominal input.

(7) Protection :(a) Short circuit protection.(b) Input over/under voltage protection.(c) Output over voltage protection.(d) Constant current features settable from 80 Amps. to 110 Amps. In auto

float/charge mode.

(8) Alarms and indicating lamps :(a) FR/BC on Auto Float/Charge : Green LED(b) Rectifier module over voltage : Red LED(c) DC output fail/Under voltage : Red LED(d) FR/BC Over Load (Voltage Drop): Amber/Yellow LED(e) Mains Available : Green LED

For System

(1) Input Voltage :(a) 3 Phase, 4 Wire, 50 Hz (Range – 320V to 480V RMS)(b) Frequency : 45 Hz …. 65 Hz.

(2) Output Voltage :Float mode Voltage : -54.0 + 0.5VCharge mode Voltage : -55.2 + 0.5V

(3) Rated Current :Equipment : 1100 Amps.Batteries : 300 Amps. Each

(4) Protection (a) Short circuit/Over load protection.(b) Input over/under voltage protection.(c) Battery/Equipment over voltage protection.


(5) Alarms and indicating lamps :(a) Load Voltage High - Red LED(b) Load Voltage Low - Red LED(c) Fuse Fail - Red LED(d) FR/BC Fail - Red LED

(FR/BC No Output MCB Trip)(e) Mains available - Green LED(f) Mains out of range - Red LED(g) Mains Fail - Red LED(h) System (Exchange) Overload - Red LED(i) FR/BC Float/Charge Mode - Green LED(j) Mains “ON”/Battery Discharge - Red LED

Functional description of power systemThis Power System is of multi rack type and consists of the following :

(a) Eight racks – One main, one auxiliary and six extension racks.

(b) AC Distribution module in each rack.

(c) Rectifier modules (A maximum of three modules in extension rack and two each in main rack and auxiliary rack).

(d) DC distribution module in each rack.

(e) Metering in each rack.

(f) Power system controller in main rack.

(a) Rack :The rack is made of mild steel profiles with hinged front door. The door

accommodates display and alarm enunciator. The rack is convection cooled and has ventilator slots in the front and sides. The rear panel is screw type and can be dismantled. The cabinet accommodates 19” subsystems. Air baffles are provided for better heat transfer. Depending upon the load requirement (Equipment and Batteries), additional modules can be added. The bottom and top also have ventilator features. The DC power termination and distribution is done at the top. The AC power termination and distribution is done at the bottom.

(b) AC Distribution Panel :The AC input to the rack is terminated at the bottom of the rack on screw type

terminals. Individual AC circuit breakers are provided for each module. The line, neutral and earth are terminated on moulded plug which is fixed to the respective sockets on the rectifier module. To monitor AC input current, 3 nos. of single phase AC current transformers are mounted on the panel of main rack. A small signal transformer is mounted on the PSC panel to provide AC input to power system controller card.


(c) Rectifier module :The SMPS rectifier module – 50V, 5600 watts works on 400V AC input and provides

– 50V DC for system. The input is through 9 pin AC socket and the DC output is through terminals. The module has front panel to indicate status and faults in the module. The control signal is taken through 8 pin telephone jack and is terminated on to the power system controller card.

The rectifier modules are convection cooled and can be jacked in and out of the cabinet easily. The DC output from each module is terminated on the respective DC bus bar mounted on the DC distribution panel.

(d) DC Distribution Panel :This panel is mounted at the top of the cabinet. The panel incorporates the following :

(1) Input from individual rectifier modules terminated on cabinet.

(2) DC shunts to monitor current in various paths.

(3) Termination of battery 1, 2 and 3.

(4) Termination of equipment positive and negative.

(5) Fuses for battery 1, 2 and 3.

(e) Metering :The front panel of main rack consists of two AC meters to monitor individual line to

line voltage and current. The selector switch selects the relevant phases. The DC meters monitor both voltage and current of batteries and exchange.

(f) Power system controller :The Power system controller card consists of an electronic circuit which monitors the

state of each rectifier module and display their status. It also controls the operation of the module so as to make it work in auto float or auto charge mode. The current signals are monitored continuously to ensure equal sharing of current. In case of faults, the same is displayed and for faults like input voltage beyond limits, DC output over voltage, over load etc. it shuts off the module. The various alarms as per following details are displayed on the front panel with audible alarm.

(1) Mains out of range : Red(2) Load voltage high (above 57V) : Red(3) Load Voltage low (below 42V) : Red(4) Mains fail : Red(5) System overload : Red(6) Mains available : Green(7) System over load : Red(8) Mains on battery discharge : Red(9) FR/BC in Float-charge mode : Green


(10) FR/BC Fail : Red

Functional Description of RectifierThe SMPS 50V-5600W rectifier is a state-of-the-art switch-mode power conversion

equipment. The unit consists of two cascaded power converters performing power factor correction and DC/DC conversion. The power stages are synchronized and working with constant switching frequency of 100 kHz.

The rectified AC mains voltage is processed first in the power factor corrector circuit which is based on a boost topology. The boost converter has the inherent advantage of continuous input current waveform which relaxes the input filter requirements. The performance of the basic boost cell is improved by a proprietary snubber circuit which reduces the switching losses of the power semiconductors due to non-zero switching times. Furthermore, the snubber circuit also decreases the electromagnetic interference (EMI) generated primarily during the turn-off process of the boost diode. The output of the boost converter is a stabilized 400V DC voltage.

Further conversion of the stabilized high voltage output of the power factor corrector circuit is necessary to generate the isolated low voltage output and to provide the required protection functions for telecommunication application. These tasks are achieved in the DC/DC converter circuit which is based on full-bridge topology. The full-bridge circuit is operated by phase-shift pulse width modulation with current mode control. This control method provides zero voltage switching condition for all primary side power semiconductors effectively reducing switching losses and electromagnetic interference, an advanced solution reduces the stresses of the output rectifier diodes.

Proper operation of the power converters is managed by individual controller circuits and supervised by the housekeeping electronics.

Remote commanding and monitoring of the modules are possible through a power system controller housed in the system.

Functional Description of Power System ControllerPower system controller is designed to control the modes of operation of rectifiers,

acknowledge and displays the status of rectifiers and system and controls parameters of rectifiers.

The controller accepts signal from individual rectifiers through 8 pin telephone jack and controls the operation of each individual rectifiers.

The mode of operation of rectifier modules depends on the coded signal M1 and M2 from the controller. Depending on the state of batteries, the ATM circuit either gives a signal for float or charge. These signals are encoded by an encoder to obtain suitable coded signals M1 and M2.


Depending upon the mode of operation of Rectifier modules, they acknowledge coded signals S1 and S2. These signals are decoded to display whether the modules are in auto float/charge or fail condition.

The total battery current can be suitably programmed to limit the current supplied from the modules through current programming pin in modules.

SMPS 48V – 5600W - IntroductionThe SMPS 48V-5600W is a three-phase, unity power factor power supply with a wide

input voltage range of 3 X 185 Vac to 275 Vac (with neutral wire) and with a useful output power of 5600W delivered to the load. This unit has been developed for cost effective but highly intelligent modular telecommunication power systems. It fulfills the specification of

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%.

The output of the rectifier conforms to the generic requirements of telecommunication power supplies in terms of noise, voltage programmability, as well as over voltage, overload and short-circuit protection. The rectifier SMPS 48V-5600W can be set in the 3 modes ‘auto float’, ‘auto charge’ and ‘manual boost’ by the power system controller.

For detailed technical data, please refer to Chapter 2.


General description of operationThe SMPS 48V-5600W rectifier is a state-of-the-art switch-mode power supply. It is

composed of 3 identical single-phase sub-modules (R, S and T) as shown in the block diagram :

The sub-modules are connected between neutral and one of the phases (R, S or T) on the input, and in parallel on the output. All ‘-‘ wires are protected by circuit breakers, which are mechanically coupled.

The interface card IFC 52 provides :

1. all reference voltages and protections to the sub-modules.

2. Signalization and manual interface (adjustment potentiometers and test jacks) for the whole unit, and

3. Communication with power system controller ‘ITI’.

Each of the sub-modules consists of two cascaded power converters performing over factor correction and dc/dc conversion. The power stages are synchronized and working with constant switching frequency of ~100 kHz. The rectified ac mains voltage is processed first in the power factor corrector circuit which is based on a boost topology. The boost converter has the inherent advantage of continuous input current wave form which relaxes the input filter requirements. The performance of the basic boost cell is improved by a proprietary snubber circuit which reduces the switching losses of the power semiconductors due to non-zero switching times. Furthermore, the snubber circuit also decreases the electromagnetic


interference generated primarily during the turn-off process of the boost diode. The output of boost converter is a stabilized 400 Vdc voltage.

Further conversion of the stabilized high voltage output of the power factor corrector circuit is necessary to generate the isolated low voltage output and to provide the required protection functions for telecommunication application. These tasks are achieved in the dc/dc converter circuit which is based on a full-bridge topology. The full-bridge circuit is operated by phase-shift pulse-width modulation with current-mode control. This control method provides zero voltage switching conditions for all primary side power semiconductors effectively reducing switching losses and electromagnetic interference. An advanced solution reduces the stresses on the output rectifier diodes.

Proper operation of the power converters is managed by individual control circuits and supervised by the housekeeping electronics.

Remote commanding and monitoring of the modules are possible through a power system controller.

Block diagram of a single sub-module R, S or TThis chapter gives more detailed information about the technical merit of single sub-

module based on the functional blocks shown in the diagram below (R, S and T).

Input SectionBlock 1 of the drawing presented above is the input EMI filter of the rectifier. The

fixed frequency, synchronized operation of the different circuits allowed to optimize the filter’s performance. It has only one differential and one common mode filter stage.

Block 2 represents the Inrush Current Limiter circuit which consists of series combination of surge rated power resistors and fuse. The circuit limits the input current of the rectifier during the initial charging of the energy storage capacitors connected to the output of the boost power factor corrector circuit. In normal operation the current limiting components are by-passed through relay which is controlled by the housekeeping electronics.


A general purpose full-wave Bridge Rectifier circuit forms Block 3. It is directly mounted on the heat sink.

Power factor correctorThe power stage of the Power Factor Corrector is a boost converter represented by

Block 4. The circuit operates with 100 kHz constant frequency in continuous inductor current mode. Because of the relatively high switching frequency a loss-less snubber has been added to the basic boost converter to reduce switching losses and semiconductor stresses.

When the boost transistor conducts the energy being stored in the boost inductor increases. During the off-state of the transistor energy is transferred from the inductor to the output capacitor through the boost diode. The inductor current is measured with a sense resistor and it is forced to follow the input voltage wave form. The technical literature refers to this technique as the resistor emulation mode which is the most preferred load by the utility companies.

The output Capacitor of the boost converter is marked by number 5 in the block diagram. This capacitor is used for low-frequency energy storage as well. Due to the nature of ac sources the energy absorbed at the input of the unit varies according to the mains cycle. In order to deliver constant power at the output energy must be stored inside the unit. Therefore, high voltage 450V electrolytic capacitors are used at the output of the boost converter to provide cost and volume effective energy storage.

Block 12 is the controller of the Power Factor Corrector. It uses the UC3854B integrated circuit which had been developed to control boost converters in power factor corrector applications. This integrated solution takes care about all sensing, controlling and protection functions which are necessary to achieve proper input current wave form and to stabilize the output voltage of the power factor corrector circuit. The control principle implemented in the UC3854B is average current mode control.

DC/DC ConverterThe heart of the module is the dc/dc Converter shown in Blocks 6-9. Block 6 shows

the primary arrangement of the full-bridge power converter employing a safety isolated high-frequency transformer. Because of its important role in providing safety isolation between the input and the output of the module, the transformer coupling is emphasized in Block 7. The secondary side of the dc/dc stage provides rectification (Block 8) and filtering (Block 9) functions which are realized using current-doubler topology. Particularity of the implemented solution is integration of two inductors on a common ferrite core.

The full-bridge converter takes energy from its input when two diagonally located switches are turned on at the same time. This energy is transferred to the output through the transformer immediately. The energy will be stored in the output filter inductor showed in Block 9 and transferred to the output capacitor of the dc/dc converter during the passive interval when energy is not absorbed from the source. This sequence can be achieved by different ways depending on the implemented control strategy.


The dc/dc Controller, shown in Block 13, is using the phase-shift pulse width modulation technique which provides loss-less, zero voltage turn-on condition for the primary side semiconductors. Further benefit is the greatly reduced electromagnetic interference generated by the converter. The control principle is peak current mode control.

Like the power stage, the controller circuit of the dc/dc converter is also divided between the primary and the secondary side of the rectifier. Communication between the separated parts are realized using optical isolators marked by number 14.

Major part of the dc/dc controller is referred to as Secondary Controller in Block 15. The secondary side controller is responsible for output voltage and current regulation functions.

Output SectionBlock 10 forms the physical Output Section of the sub-module. It is a shielded,

common-mode, low-pass filter stage to reduce conducted electromagnetic interferences to the required level.

HousekeepingThe name Housekeeping refers to the auxiliary power supply and to all internal

primary side supervisory functions necessary for the operation of the unit. Besides the auxiliary power converter (current-mode controlled fly-back converter), Block 11 also includes the master clock, under- and over-voltage lock-out, and start-up sequence generator.

Output CharacteristicsThe power system controller can set the rectifier into the 3 modes of operation, i.e.

‘auto float’, ‘auto charge’ and ‘manual boost’.

The output characteristic is different for these 3 modes as shown below :



56

48

0 80 110 lout(A)(

‘auto float’ mode

Vout (V)

65

56

025 50 lout(A)

(

‘manual boost’ mode

Vout (V)

66

54

0 80 110 lout(A)(

‘auto charge’ mode

Vout (V)


1800

ch-3 wkg principle of smps

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