Presentation on power supplies

Prince kumar kushwaha 0902EC101053

Power Supplies

• Power supplies provide the necessary power, voltage and current requirements for electronic devices.

• They usually change ac to dc voltage.– For example, 120 volts ac is changed to 13.8 volts

dc.

Power SuppliesConsist of:

1. Transformer – steps ac voltage up or down.

2. Rectifier Diodes – change ac to “bumpy” dc.

3. Filter Network – includes capacitors and inductors, smooths out the bumps.

4. Voltage Regulator – keeps the voltage constant.

5. Protection – usually a crowbar circuit.

Power Supply SpecificsHalf Wave – Full Wave

Rectifier

Power Supply SpecificsBridge Rectifier

Rectifier Comparison

Rectifiers – Half Wave• Rectifiers are semiconductor diodes that conduct in

only one direction. Today, most rectifier diodes are made of silicon.

Diode Ratings

• Increase current capacity by putting additional diodes in parallel.

• Increase voltage capacity by putting additional diodes in series.

• Voltage rating 1.4 RMS minimum.

Power Supply Filters

• Use large coils (called chokes) and capacitors to “fill in” the gaps between ripples in the incoming pulsating dc.

• AC “hum” results from improper filtering.– Pulling too much current from power supply.– Failed component (particularly capacitor).

• “Wall-Warts” generally do not produce quality dc.– Generally produce pulsating dc to charge

batteries.

Filter Network

Capacitors and inductors are used in power supply filter networks. The capacitors and inductors smooth out the “bumpy” ac to dc.

Important Concepts• Half Wave Rectifier:

– Simplest.– Hard to filter well.

• Full Wave Rectifier:– Easier to filter.– Requires transformer with center-tap.– Transformer secondary must be twice intended voltage.

• Bridge Rectifier:– Easier to filter (just like full wave).– Center-tap transformer not required.– Transformer secondary same as intended voltage.– Higher parts count.

Power Supply Safety

Power Supply Safety

Power Supply Safety

• Grounding is important.• Connection integrity is important.

– Somewhere neutral and ground are connected together.

• Miss-wired “hot” lead will lead to a hot chassis someplace.

Power Supply Safety

• Filter capacitors hold a charge a long time.• They charge to the output voltage of the

power supply.• Bleeder resistors across the filter

capacitors “bleed off” charge when supply is turned off.

Bleeder Resistors• A bleeder resistor is installed across the filter

capacitors as a safety feature to make sure that the capacitors are discharged.

Switching-Mode Power Supplies

Switching Power Supplies

Switching Power Supplies

• The advantage of a switching-mode power supply is that the relatively high frequency oscillator allows the use of small, lightweight and low-cost transformers.

• This makes them considerably smaller and lighter than linear power supplies. Almost all modern powers supplies, including those in PCs, are switching mode power supplies. Their disadvantages are complexity and RF egress (interference).

Switching Power SuppliesIn a switched mode power supply, the first step in converting 120 V ac to a 12 V dc output is to rectify and filter the 120 V.