Linear Power Supply

Outline

• Linear Power Supply– Poor man’s cell phone charger– Small signal resistance

• Adjustable power supply• Rectifier Circuit– Half-wave rectifier circuit– Full-wave rectifier circuit– Full-wave rectifier circuit as a Frequency mixer

• Filter Circuit• Rectifier Circuit

Outline

• Large-Signal and Small-Signal Operation

• Application of Diodes (Up to 3.5.1)

How does a small increase in Vad affect Vout?

Drop in Output Voltage

(Slope)

Small Signal Resistance

Small Signal Resistance: ID/VT

I-V Characteristic of a 1n4001 Diode

Export I-V Data to a Table

Export Table

Small Signal Resistance

VF (V) IF (Computed)

0.748 67.57 mA

0.740 57.89 mA

Dynamic Resistance from the measurement:(0.748-0.740)/(67.57 mA-57.89 mA)= 0.826 Ohms

Using Small Signal Model in a Small Signal Calculation

Small Signal Model of Adaptor

Change in Vad

How should a load be modeled?

A laptop consumes an average power of 25 W with a supply voltage of 3.3 V. Determine the average current drawn from the batteries or the adaptor.

P=VII= 7.56 A

If the laptop is modeled by a resistor, then RL is 0.436 Ohms.

You Can’t Power a Laptop with a Circuit Like this!

How should a Cellphone load be modeled?

A cellphone consumes an average power of 100 mW with a supply voltage of 2.4 V. Determine the average current drawn from the batteries or the adaptor.

P=VII= 41.6 mA

If the cellphone is modeled by a resistor, then RL is 79 Ohms.

Use a 79 Ohm Resistor to Model a Cell Phone Load

Dual-Output Adjustable Linear Regulated Power

Supply

Block Diagram

Simple Description

• Transformer: “Downconvert” the AC line voltage to a smaller peak voltage Vm, usually about 2-3 Volts larger than the ultimately desired DC output.

• Diode Rectifier Circuit: produce a waveform with large DC component.

• Filter: smooth out the rectified sinusoid. • Regulator: eliminate residual ripple

c03f27

Half-Wave Rectifier

Assume constant voltage model for diodes

Voltage offset

Diode is off for Vin < VD,on

On for Vin less than –VD,on

Ideal Diode Model

An AC-DC Converter Circuit

An AC-DC Converter Circuit

Constant Voltage Diode Model

Issue: It remains to be seen whether it can provide a current to a load

How should a load be modeled?

A laptop consumes an average power of 25 W with a supply voltage of 3.3 V. Determine the average current drawn from the batteries or the adaptor.

P=VII= 7.56 A

If the laptop is modeled by a resistor, then RL is 0.436 Ohms.

Rectifier Driving a Resistive Load

Vin begins to fall after t1 because RL providesa discharge path for C1.C1 must be sufficient large so that the currentdrawn by RL does not reduce Vout significantly.

The diode is turned on again

ripples

C1 is called smoothing capacitor

Tin

Magnitude of the Ripple

(Vp-VD,on)/RL is the currentdischarged through RL

(basic form of this equation:Q=CV)

The Maximum Forward Bias Current

Maximum Forward Current Occurs at the point when the diode turns on (t1)1.C1dVout/dt is maximum2.The current supplied to RL is (Vp-VD,on)/RL

Screen Shots

Smoothed Capacitor