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Transient PSpice Analysis (7.4)PowerPoint Presentation

Transient PSpice Analysis (7.4)

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Transient PSpice Analysis (7.4)

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Transient PSpice Analysis (7.4)

Dr. Holbert

April 26, 2006

ECE201 Lect-23

- What is the voltage as a capacitor discharges to zero?
- What is the voltage as a capacitor charges from one voltage (often zero) to another constant voltage?
- How does the current through an inductor increase from zero to a final value?
- How does the current through an inductor decrease from an initial value to zero?

ECE201 Lect-23

- What are the transient and AC steady-state responses of an RC circuit to a sinusoidal source?
- What are the transient and AC steady-state responses of an RL circuit to a sinusoidal source?

ECE201 Lect-23

- Changes in capacitor voltages and inductor currents from one value to another are easily solved.
- Changes in other voltages or currents in the circuit may or may not be easy to solve directly; they are all easy to solve using Laplace transforms (EEE 302).

ECE201 Lect-23

- Steady-state responses to sinusoidal sources are easy to find using AC steady-state analysis.
- Transient responses to sinusoidal sources are hard to find directly; they are easier to find using Laplace transforms.

ECE201 Lect-23

- Computer RAM
- Refresh time
- Write time

- Stator coil on a motor
- Response to a step in current

ECE201 Lect-23

3.3V

Precharge

Q1

Sense Amp

+

Data

Vout

Q2

C

–

ECE201 Lect-23

- When the Precharge line is high (> 3V) and the Data line is low (~0V), transistor Q1 is on and the capacitor charges up to 3V.
- If the Data line goes high after the capacitor is charged, then Q2 turns on and the capacitor discharges.

ECE201 Lect-23

- With Q1 and Q2 off, the capacitor holds a charge that represents the stored data bit.
- This charge leaks through Q2, the input of the sense amplifier, and the capacitor.
- To determine the time before a refresh is necessary, we can use a simple equivalent circuit.

ECE201 Lect-23

+

1MW

1000pF

–

The 1MW resistor models the parallel combination of the off resistance of Q2, the input resistance of the sense amplifier, and the leakage resistance of the capacitor.

v(t)

ECE201 Lect-23

What is the time constant for this circuit?

ECE201 Lect-23

- The RAM discharge time is the time required for the capacitor to discharge to a given voltage from an initial voltage of 3V.
- What is the initial voltage?
- What is the DC steady state (final) voltage?
- What does the capacitor voltage v(t) look like?

ECE201 Lect-23

v(t) = 3Ve-t/RC

ECE201 Lect-23

Suppose we must refresh before v(t) drops below 1.5V. How long can we wait before a refresh?

t = 0.693ms

ECE201 Lect-23

- With Q2 off, Q1 is turned on to charge the capacitor.
- The current to charge the capacitor comes through Q1.
- To determine the time necessary to precharge the capacitor, we use a simple equivalent circuit.

ECE201 Lect-23

The 10 W resistor models the “on” resistance of Q1.

+

10W

+

–

v(t)

3.3V

1000pF

–

ECE201 Lect-23

What is the time constant for this circuit?

ECE201 Lect-23

- The RAM precharge time is the time required for the capacitor to charge to a voltage of 3V from an initial voltage of 0V.
- What is the initial voltage?
- What is the DC steady state (final) voltage?
- What does the capacitor voltage v(t) look like?

ECE201 Lect-23

v(t) = 3.3V(1-e-t/RC)

ECE201 Lect-23

Suppose we must precharge the capacitor to 3V. How long does this take?

t = 24.0ns

ECE201 Lect-23

PSpice Defibrillator Example

- Start PSpice and enter circuit diagram
- Set capacitor and inductor initial conditions
- Setup Transient analysis, 0.01 ms step to 15 ms end
- Run simulation; Probe starts automatically
- Plot: (1) 50 resistor voltage, (2) capacitor voltage, and (3) clockwise inductor current
- Find peak heart voltage and current
- Determine charging time constant ()

ECE201 Lect-23

50 mH

t=5ms

t=5ms

20

30 µF

+

–

6000 V

50

ECE201 Lect-23