CHAPTER 2: Diode Circuits potx

• Half-wave rectifiers cause DC current to flow in the AC source.. Diodes Used As Switches A small AC signal can’t forward-bias a diode.. Diode Data Sheet Some important diode specifica

CHAPTER 2

Diode

Circuits

Intro to Rectifiers

• The job of a rectifier circuit is to produce a

DC output from an AC input

• Rectifiers are in power supplies where they convert 60 Hz AC into “raw” DC

• Rectifiers are in AM radios where they are demodulators: they convert a radio signal into a DC level that varies with the audio

signal

Half-Wave Rectifier

The most basic rectifier circuit Note that the DC output

is not steady as a battery is It’s pulsating DC

Half-Wave Rectifier

• Half-wave rectifiers are not efficient for

converting 60 Hz AC into DC Half the input never makes it to output

• Half-wave rectifiers cause DC current to flow

in the AC source If the source is a

transformer, DC current could damage it

• The demodulator in an AM radio is a

half-wave rectifier

Full-Wave Rectifier

Uses all the AC input Requires a transformer

Full-Wave Bridge Rectifier

Requires 4 diodes, but does not require a transformer

Filter Capacitor

Capacitor required to convert “raw” DC to usable DC Filtered DC still has a small AC ripple on top of the DC

Ripple

Full-wave ripple frequency is twice AC frequency

Power Supply System

Regulator removes most ripple & keeps DC level fixed

Voltage Doubler

On negative half-cycle, D1 charges C1 to Vp

On positive half-cycle D2 adds AC peak to Vp on C1

and transfers it all to C2

Voltage Doubler

• Voltage doublers allow you to develop higher

voltages without a transformer

• Stages can be cascaded to produce triplers,

Clippers

Clippers are used to remove portions of an AC signal

Clampers are used to add a DC level to an AC signal

Diodes Used As Switches

A small AC signal can’t forward-bias a diode When a

DC forward-bias is applied,the small AC signal can pass through the diode’s low internal resistance

Diode Data Sheet

Some important diode specifications:

VRRM: Peak repetitive reverse voltage Higher voltage will cause reverse breakdown in diode

IO: Average forward current The maximum DC current that diode can conduct More current can burn up diode

IFSM: Peak surge current Maximum current the diode can conduct for a few milli-seconds, such as when it charges the filter capacitor in a power supply

Diode Data Sheet (cont)

VF: Forward voltage drop Maximum voltage across

diode when conducting Usually specified at IO

Typically about 0.7 Volts for silicon

IR: Reverse current Maximum leakage current in a

reverse-biased diode Usually specified at some

temperature

trr: Reverse recovery time How long it takes for a

diode to stop conducting after a reverse bias voltage

is applied Important for rectifiers in switching power supplies which operate at frequencies from 20 kHz

to 200 kHz to 1 MHz or higher

Checking a Diode

Using a meter set to Ohms, you can separate the live

ones from the dead ones

Troubleshooting

• When a piece of electronic equipment fails, the first suspects are the components under high stress

Stresses are high current, high voltage, and high

temperature Power supplies can have all three

ingredients

• Diodes can “pop”, often from too much surge current into the filter capacitor

• Aluminum electrolytic filter capacitors can dry out

over time, and occasionally spring a leak Capacitors have ESR: equivalent series resistance It can

increase with age, and causes ripple to increase

Troubleshooting

CAUTIONS:

• A power supply that puts out only 5 Volts DC can

have 120 Volts AC or more on the rectifier diodes

and filter capacitors That’s the case in “off-line”

switching power supplies which, today, are the most commonly used supplies in electronic equipment

• If you wear a ring, and you grab the top of a large

filter capacitor charged to only 5 Volts, the ring could get hot enough to burn your finger badly if it hits both the (+) and (-) terminals at the same time