Download Fundamentals of Linear Electronics Integrated & Discrete

CHAPTER 2
Diode
Circuits
OBJECTIVES
Describe and Analyze:
• Rectifier Circuits
• Voltage Multiplier
• Clippers & Clampers
• Switching Circuits
• Diode Data Sheet Specs
• Troubleshooting
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 halfwave 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,
quadruplers, etc.
• Voltage multipliers usually supply low currents to a
high-resistance load.
• Output voltage usually drops quickly as load current
increases.
Clippers
Clippers are used to remove portions of an AC signal
Clampers
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.
Troubleshooting
• 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.