Component Selection

By

Prof. Bitar

Power Supply Example

• Input:• 120VAC / 60Hz (nominal)

• Output:• 5V +/- 5% (4.5 to 5.5 V)• 100mA (max)

Possible Full-Wave Topologies(Assuming Linear Voltage Regulators)

D1

D2

C1

+

12

0V

AC

, 6

0H

z

INGND

OUT

U1

LoadC2

Vout

N1N2

N3

TR1

N1 N2

TR1

D1

C1

+

12

0V

AC

, 6

0H

z

INGND

OUT

U1

LoadC2

Vout

D4

D3

D2

Typical Voltage WaveformsT

Secondary Voltage (A)

Secondary Voltage (B)

After Rectifying and Filtering Ripple VoltageAfter Regulator

Time (s)

0.00 10.00m 20.00m 30.00m 40.00m 50.00m

Out

put

-10.00

-5.00

0.00

5.00

10.00

V drop across regulator

Ripple VoltageAfter Regulator After Rectifying and Filtering

Secondary Voltage (B)

Secondary Voltage (A)

Begin Distributor Part Searchfor “Voltage Regulators”

Yikes! Only 41,864 Possibilities…

41,864 Possibilities!

Narrow the Search!

In Stock !!!

6,241 Linear

Further Search Criteria• Current (100mA)• Package / Case (TO-92 Thru-Hole!)• Packaging (for shipping)• Topology (Positive Fixed)• Voltage (5V)

Down to Mfg & Pricing(Note Sorting Criteria)

• Fairchild Semiconductor $0.32(More $, but better spec sheets & support.)

• Micro Commercial Co:$0.20(Not as well supported, but CHEAP!)

Selecting Part-Number Link Yields More Information Including Price Breaks!

Finally a Data Sheet!

Component Data Sheet• Absolute Maximum Ratings

(Not to be exceeded !!)• Input Voltage Max (30V)• Operating Temp (-20 to 120 C)• Storage Temp (-55 to 150 C)

• Electrical Characteristics

(Under very specific test conditions!)• Output Voltage

(4.8 to 5.2 V @ T = 25 C)

• Load Regulation (ΔVOUT / Δ ILOAD)

(60mV MAX from 1 to 100mA)

• Line Regulation (ΔVOUT / Δ VIN)

(150mV MAX, Vin from 7 to 20V)

• Quiescent Current (5.5mA MAX)

• Output Noise Voltage (40uV TYPICAL)

• Ripple Rejection (41dB MIN)

• Dropout Voltage (1.7V TYPICAL)

Q: Does this Voltage Regulator Meet our Specifications? YES!

• OK, Repeat this Process for Every Component…

• Power Transformer• Diodes• Filter Capacitors

Selecting a Power Transformer

• Questions:1. What voltage do we need?

2. What is the maximum current?

3. What configurations do we need?

4. What configurations are available?Single or Dual Primary

Single or Dual Secondary

1. What Voltage Do We Need?

• Desired DC Output = 5V• Minimum Drop Across Regulator = 1.7V (spec)• Maximum Ripple Voltage = 1V (guess)• Drop Across Rectifier Diodes = 0.7 or 1.4V• Therefore, for C.T. Transformer:

• Vpk = 5+1.7+1+0.7 = 8.4V = 5.9VRMS

• Or for Full-Wave Bridge:• Vpk = 5+1.7+1+1.4 = 9.1V = 6.4VRMS

• OK, Looking for 6.4VRMS @ 100mA (min)

Parts Distributor Search

Possible Choices

Tamura 3FS212 : 6.3VAC, 180mA Transformer $3.75

Transformer Spec Sheet

Electrical Specifications

Mechanical Specifications

Hole Pattern Spacing

Configuration Options

Available Voltages / Currents