Table of contents

1. Introduction

2. Technical Specifications

3. Front Panel Controls

4. Patch Connection Details

5. Operating Instructions

6. Circuit Description

IC BASED BUCK CONVERTERUSING PWM

INTRODUCTION

A DC-to-DC converter is a device that accepts a DC input voltage and produces a DCoutput voltage. Typically the output produced is at a different voltage level than the input.

In addition, DC-to-DC converters are used to provide noise isolation, power busregulation, etc. This is a summary of some of the popular DC-to-DC converter topolopgies

A buck converter is a step-down DC to DC converter. Its design uses two switches (atransistor and a diode) and an inductor and a capacitor.

The simplest way to reduce a DC voltage is to use a voltage divider circuit, but voltagedividers waste energy, since they operate by bleeding off excess voltage as heat; also,output voltage isn’t regulated (varies with input voltage). A buck converter, on the otherhand, can be remarkably efficient (easily up to 95% for integrated circuits) and self-regulating, making it useful for tasks such as converting the 12-24V typical battery voltagein a laptop down to the few volts needed by the processor.

TECHNICAL SPECIFICATIONS

BUCK CONVERTER

INPUT : 20V TO 26V

OUTPUT : +5V DC

USING UC3843 CURRENT MODE CONTROLLER IC or LM3525

SWITCHING FREQUENECY :60KHZ

INDUCTANCE VALUE : 500µH, 2A

MOSFET :IRF540N

Build in input varriable voltage sourceto study the input line varriations : Varriable from 20v to 24v

Bulid in varriable outputloading arrangement : Varriable in 5 Step of 10 ohms, 15 ohms, 47 ohms

100 ohms and 220 ohms

Front panel controls

Power : Rocker switch selects instrument switch on.

Input voltage source. : Varriable from 20 to 26 v for buck converter input.

output loading arrangment: selectable fixed resistors for output loading for the converters.

Buck Converter Circuit using UC3843 current mode pwm switching ic.Input: 20 to 26v. Output: +5v DC.

Voltage monitoring I/P and O/P terminals and Gate PWM , Inductor-Mosfet drain Signal terminals.

Patch Connection Details

1. Connect the input voltage source (20v to 26v) to input of BUCK circuit and also to input DC voltmeter of 30V range or 50V as per dotted patch connections.

2. Connect the output of BUCK converter to another DC voltmeter of 10v or 15v Range .

T o study the input line voltage regulation.Plot the graph input voltage vs output voltage.

Set the input voltage such that say 20v and observe the output voltage , it should be +5v.and varry the input voltage with different settings ie, 20v, 21v, 22v, 23v and 24v.The output must be +5v DC constant. This is input line regulation.

Observe the Gate PWM PULSE in CRO and Measure the PWM Duty cycle ON Time in milli Seconds

SL.NO: INPUT VOLTAGE OUTPUT VOLTAGE PWM GATE PULSE Duty cycle On Time

1. 20V .................. ....................... 2. 21V ................... ....................... 3. 22V .................. ....................... 4. 23V .................. ....................... 5. 24V .................. .........................

To study the output load regulation, Plot the graph output voltage vs output current.

Set the input voltage to say 24v. Now the output is +5v. Then connect the output to Varriable load throughammeter of 100mA Range.

Now varry the load in 5 steps of 220E, 100E, 47E 15E and 10E Ohms and observe the output voltage andcurrent. plot the graph.

INPUT VOLTAGE: 24V DC.

Observe the Gate PWM PULSE in CRO. The square wave Duty cycle Pulse width ON Time will varryaccording to the load to main the output voltage constant 5VDC. The ON time will increase when load current increases.

Operating instructions

SL.NO: OUTPUT LOAD OUTPUT VOLTAGE PWM GATE PULSE Duty cycle On Time

1. 220E .................. ....................... 2. 100E ................... ....................... 3. 47E .................. ....................... 4. 15E .................. ....................... 5. 10E .................. .........................

Here in the buck converter circuit, PWM Current mode IC UC3843 IS USED.

Input voltage is connected to switching mosfet IRF540N and pin 7(vcc) of ic.

Pin 8 is ref output, 5v will come out. This ref 5v, resistor Rosc and Capacitor Cosc at pin 4 of ic determines the oscillator switching frequency.

Pin 3 is used for current sensing.

Pin 1 is for componsation.

Output voltage is sampled by pin 2 with voltage divider which determines the output pulse width at pin 6.

The output pulses from pin 6 drives the gate of mosfet IRFZ540N. The mosfet switchs the inductance500µH.

The voltage switching spikes at the coil is rectified by high frequency schotty diode FR304 and filtered bycapacitor

The buck converter is introduced in this page using the evolutionary approach. Let us considerthe circuit in Fig. 1, containing a single pole double-throw switch.

IC BASED BUCK CONVERTERUSING PWM

Manual for

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