Transcript Diode Circuits Outline • Large-Signal and Small-Signal Operation • Application of Diodes (Up to 3.5.1)

Diode Circuits
Outline
• Large-Signal and Small-Signal
Operation
• Application of Diodes (Up to 3.5.1)
How does a small increase in Vad affect Vout?
(Slope)
Small Signal Resistance
Small Signal Resistance: ID/VT
Measured Diode Voltage
15
Measured Data
Barrier Potential is ~ 665 mV
Diode Current (mA)
12
9
6
3
0
400
440
480
520
560
600
640
Diode Voltage
680
720
760
800
Small Signal Resistance
VF (V)
IF (Computed)
0.70
0.748
5.74 mA
13.8 mA
Dynamic Resistance from the measurement:
(0.748-0.70)/(13.8 mA-5.74 mA)= 48 mV/8.06 mA =5.95 Ohms
From the manufacture’s specification=8.33 Ohms, using data from 0.7V
and 0.725 V in Figure 4.
Using Small Signal Model in a
Small Signal Calculation
Small Signal Model of Adaptor
Change in Vad
Change in Vout
due to a drop in
c03f26
c03f27
Half-Wave Rectifier
Voltage offset
Diode is off for
Vin < VD,on
Assume constant voltage model for diodes
On for Vin less than –VD,on
An AC-DC Converter Circuit
Ideal Diode Model
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
conceptualized?
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=VI
I= 7.56 A
If the laptop is modeled by a resistor, then RL is 0.436 Ohms.
Rectifier Driving a Resistive Load
The diode is
turned on again
ripples
Vin begins to fall after t1 because RL provides
a discharge for C1.
C1 must be sufficient large so that the current
drawn by RL does not reduce Vout significantly.
C1 is called smoothing capacitor
Magnitude of the Ripple
Tin
The Maximum Forward Bias Current
Maximum Forward Current Occurs at the point
when the diode turns on (t1)
1. C1dVout/dt is maximum
2. The current supplied to RL is (Vp-VD,on)/RL
Screen Shots
Smoothed Capacitor