9/29/2004ee 42 fall 2004 lecture 131 lecture #13 power supplies, dependent sources, summary of ideal...
Post on 20-Dec-2015
216 views
TRANSCRIPT
9/29/2004 EE 42 fall 2004 lecture 13 1
Lecture #13 Power supplies, dependent sources, summary of ideal components
Reading: Malvino chapter 3, 4.1-4.4
Next: 4.10, 5.1, 5.8
Then transistors (chapter 6 and 14)
9/29/2004 EE 42 fall 2004 lecture 13 2
Topics
Today:
• Inductors and transformers
• Power supplies
• A new type of ideal device, the dependent source.
• Summary of ideal devices
9/29/2004 EE 42 fall 2004 lecture 13 3
Inductors
• Any coil of wire will produce a magnetic field when current flows through it
• The magnetic field holds energy.• If the current is changed, the magnetic field will
change, and therefore the energy stored will change.
• Since the power must come from the circuit, this creates a voltage:
dt
dILV
9/29/2004 EE 42 fall 2004 lecture 13 4
Symbol for and ideal inductor
• The symbol for an inductor is similar to that of a resistor, but with loops
dt
dILV
9/29/2004 EE 42 fall 2004 lecture 13 5
Transformers
• A transformer is a couple of coils of wire which transfer power from one to the other 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.
9/29/2004 EE 42 fall 2004 lecture 13 6
Transformers
9/29/2004 EE 42 fall 2004 lecture 13 7
Transformer symbols
• The symbol for a transformer is a pair of the same loopy lines used for inductors, but close together. If the inductor has a core of a magnetic material, it is shown as a couple of lines between the coils.
The number of turns in the coils willBe written nearby.
9/29/2004 EE 42 fall 2004 lecture 13 8
Ideal transformer
• 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
9/29/2004 EE 42 fall 2004 lecture 13 9
Ideal transformer
• The transfer of power can increase or decrease the voltage, and the current changes as well.
• Remember:
so if it is ideal (lossless)) 11VIP
2211 VIVIP
2
1
2
1
N
N
V
V
9/29/2004 EE 42 fall 2004 lecture 13 10
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
9/29/2004 EE 42 fall 2004 lecture 13 11
An AC input is sinusoidal
-1.5
-1
-0.5
0
0.5
1
1.5
0 5 10 15 20
9/29/2004 EE 42 fall 2004 lecture 13 12
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
9/29/2004 EE 42 fall 2004 lecture 13 13
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
9/29/2004 EE 42 fall 2004 lecture 13 14
Full-wave rectifier
• The result is called a full wave rectifier
~ R
9/29/2004 EE 42 fall 2004 lecture 13 15
Full-wave rectified voltage
0
0.2
0.4
0.6
0.8
1
1.2
0 5 10 15 20
9/29/2004 EE 42 fall 2004 lecture 13 16
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.
9/29/2004 EE 42 fall 2004 lecture 13 17
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
9/29/2004 EE 42 fall 2004 lecture 13 18
Full wave rectified, with filtering
0
0.2
0.4
0.6
0.8
1
1.2
0 5 10 15 20
9/29/2004 EE 42 fall 2004 lecture 13 19
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.
9/29/2004 EE 42 fall 2004 lecture 13 20
Regulated power supply
• In order to produce a power supply without ripple, we will need to use an active device called a regulator
~ RConstant voltagepower supplyto load
9/29/2004 EE 42 fall 2004 lecture 13 21
Ideal devices
Wire:Current in =current outNo voltage differences
Resistor
IRV
dt
dVCI
9/29/2004 EE 42 fall 2004 lecture 13 22
Ideal devices 2
Inductor:
dt
dILV
Ideal diode:Reversed bias no current, open circuitForward bias no voltage drop, just like a wire
9/29/2004 EE 42 fall 2004 lecture 13 23
Ideal devices 3
Transformer
2
1
2
1
N
N
V
V
+
V1
-
+
V2
-
9/29/2004 EE 42 fall 2004 lecture 13 24
Ideal devices 4
~Voltage source:Voltage given, current can be anything
Note: the voltage could be given as A function of time
)(tV
Current sourceCurrent given, voltage can be anything
Note: the current could be given as A function of time
)(tI
9/29/2004 EE 42 fall 2004 lecture 13 25
Ideal devices 5
~Dependent Voltage source:Voltage given as a multiple of another Voltage or a current, current can be anything
12 KVV
Dependent Current sourceCurrent given as a multiple of a differentcurrent or voltage, voltage can be anything)(tI
+
V1
-
+
V2
-
+
V1
- 12 GVI