Chapter 1 Common Diode Applications Basic Power Supply Circuits

Power Supply A group of circuits used to convert ac to dc. Rectifier – Converts ac to pulsating dc. Filter – Reduces variations in the rectifier output. Voltage regulator – Maintains a constant dc output voltage.

Transformers Transformers are described in terms of the relationship between secondary voltage (V S ) and primary voltage (V P ). Step-up – V S is greater than V P. Step-down – V S is less than V P. Isolation – V S is approximately equal to V P.

Transformer Voltage and Current Ratios

Half-wave Rectifiers Half-wave rectifier – A diode placed in series between a transformer (or ac line input) and its load.

Positive Half-wave Rectifiers This circuit converts an ac input to a series of positive pulses.

Average Load Voltage and Current Average voltage (V ave ) – The dc equivalent of a voltage waveform. Average current (I ave ) – The dc equivalent of a current waveform. For the output from a half-wave rectifier:

Negative Half-wave Rectifiers This circuit converts an ac input to a series of negative pulses.

Peak Inverse Voltage (PIV) Peak inverse voltage (PIV) – The maximum diode reverse bias produced by a given circuit. For the diode in a half-wave rectifier:

Full-wave Rectifier

Full-wave Rectifier Operation Diodes conduct during alternate half cycles of the input signal. V L(pk) is approximately half the value of V S(pk). The circuit produces two positive half-cycles for each input cycle.

Average Load Voltage and Current Average voltage (V ave ) – The dc equivalent of a voltage waveform. Average current (I ave ) – The dc equivalent of a current waveform. For the output from a full-wave rectifier:

Peak Inverse Voltage (PIV) Peak inverse voltage (PIV) – The maximum diode reverse bias produced by a given circuit. For the diode in a full-wave rectifier:

Negative Full-wave Rectifiers The negative full-wave rectifier converts an ac input to a series of negative pulses.

Full-Wave Bridge Rectifiers The most commonly used because: It does not require the use of a center-tapped transformer. It can be coupled directly to the ac power line. It produces a higher dc output than a comparable full-wave center-tapped rectifier.

Bridge Rectifier Operation Conduction alternates between two diode pairs.

Calculating load voltage and current relationships

Working with Rectifiers Rectifiers are high current circuits, so I F R B can have a significant impact on diode forward voltage (V F ). Cooling is often used to minimize the effects of high power rectifier reverse current. Components may be cooled using a fan and/or a heat sink. High transformer tolerances can introduce significant discrepancies between calculated and measured current and voltage values.

Integrated Rectifiers Integrated Rectifier – A rectifier circuit that is etched on a single piece of silicon (housed in a single case). IC rectifiers are: Cheaper to produce Easier to troubleshoot The diodes in an IC rectifier operate at the same temperature, so they have equal values of forward voltage and leakage current. IC rectifier cases are designed to be attached easily to a heat sink.

Filters A filter reduces the voltage and current variations in the output from a rectifier.

Ripple Voltage (V r ) Ripple voltage (V r ) – The variation in the filter output voltage.

The Basic Capacitive Filter The capacitor: charges through the rectifier diode. discharges through the load.

Surge Current The high initial current through a power supply. Surge current lasts only long enough for the filter capacitor to charge.

Limiting Surge Current Surge current can be limited by: Inserting a current limiting resistor between the filter capacitor and the rectifier. Using a low-value filter capacitor (which shortens the duration of the surge). Using an inductive filter.

Filter output voltages

Filtering and Half-wave Rectifier PIV

LC Filters

Voltage Regulators Voltage regulator – The final circuit in the dc power supply.

Zener Voltage Regulators

Regulator Response to Load Variations

Zener Reduction of V r

A Basic DC Power Supply