9/27/2004 EE 42 fall 2004 lecture 12 1

Lecture #12 Circuit models for Diodes, Power supplies

Reading: Malvino chapter 3, 4.1-4.4

Next: 4.10, 5.1, 5.8

Then transistors (chapter 6 and 14)

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Circuit models

• Now that we have studied the physics underlying how a diode works, we are going to hide all of it in a circuit model

Why?

• If we create a circuit model, then we can draw and analyze electronic circuits without getting lost in the details.

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IV curve for an ideal diode

• The IV curve for a ideal diode is to have zero current in the reverse direction, and no resistance when forward biased

Voltage →

Current

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Real diode IV curve

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Idealized devices

• We have encountered the idea of ideal devices before:

• A voltage source is like a battery, but produces a perfect voltage regardless of current:

And the ideal current

source, a current

regardless of voltage~

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The ideal diode

• We now add another ideal device, the ideal diode.

A real diode drawn as the same symbolsometimes in a circle tomake it clear that it is nota ideal diode

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The ideal diode as a switch

• The ideal diode behaves as a switch:

• If current is being pushed through in the forward direction the switch is closed.

• If a reverse bias voltage is applied, the circuit is closed.

Reverse Bias:

Forward Bias:

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Ideal diode vs real diode IV curve

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Ideal diode vs real diode IV curveWe could improve our model for real diode by not closing the switch until the voltage gets about 0.7 volts into the forward bias. We can do this in a circuit by making a circuit model

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The ideal diodeTo make a somewhat better model of a real diode:

We use an ideal diode in series

with an ideal voltage source

~+

0.7 volts

-

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Ideal diode vs real diode IV curveWe could improve our model further by sloping the IV curve for the region where forward current is flowing

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Improved diode modelTo make an even better model of a real diode:

We use an ideal diode in serieswith an ideal voltage source anda resistor. The resistance needed forthe model is given by the inverse’of the slope of the IV curve

~+

0.7 volts

-

R

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Key point: the model can change

• Which model you use for a device can change depending on – What the mode of operation of the device is– how accurately you need to model the device

For example: A hand analysis of a power supply would probably use an ideal diode, and then break the problem into two time periods– When the diode is forward biased– When the diode is reverse biased

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Higher accuracy models

• If a diode was to be used at high frequencies (hundreds of megahertz or higher) then the model would have to account for the movement of charge in and out of the depletion zone, a capacitive effect.

It is important to use a model which is accurate enough to account for the necessary effects, without using so complicated a model that it is difficult to understand what is going on!

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Applications

• Applications of diodes include• Power supply rectifiers• Demodulators• Clippers • Limiters • Peak detectors• Voltage references• Voltage multipliers

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Half-wave rectifier

• A single diode can be used to take an alternating current, and allow only the positive voltage swing to be applied to the load

~ R

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An AC input is sinusoidal

-1.5

-1

-0.5

0

0.5

1

1.5

0 5 10 15 20

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The diode blocks the negative voltages

-0.2

0

0.2

0.4

0.6

0.8

1

1.2

0 5 10 15 20

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Full-wave rectifier

• If we add an additional diode, it does not pass current at the same time as the first diode, but the load is now disconnected during the negative half cycle.

• What if we could flip the connection and use the negative half wave?

~ R

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Full-wave rectifier

• The result is called a full wave rectifier

~ R

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Full-wave rectified voltage

0

0.2

0.4

0.6

0.8

1

1.2

0 5 10 15 20

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Transformers

• In order to use a full wave rectifier, the source and the load must be able to float with respect to each other

• One way to isolate AC power is to use a transformer. A transformer is a couple of coils of wire which transfer power by a changing magnetic field.

• By having different numbers of windings, or turns of wire, a transformer can step up or step down an AC voltage.

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Transformers

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• The voltage across the secondary of the transformer (the output windings) is:

• But this only works for changes in the voltage—and therefore for AC only

11

22 V

N

NV

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Filtering

• A transformer and a full wave rectifier will produce a voltage which is always positive, but varies with time

• In order to power electronic devices, we need to smooth out the variations with time.

• Another way to look at this is that we need to store energy temporarily while the input voltage changes sign.

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Power supply filter capacitor

• If we add a capacitor in parallel with the load, it will charge up when power is available from the voltage source, and then it will slowly discharge through the load when the diodes are off.

~ R

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Full wave rectified, with filtering

0

0.2

0.4

0.6

0.8

1

1.2

0 5 10 15 20

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Ripple

0

0.2

0.4

0.6

0.8

1

1.2

0 5 10 15 20

The result is a DC voltage, with some residual variations at twice the

frequency of the AC power. The variation is called ripple.