Circuit Design Flashcards

1
Q

What are 7 advantages to using a modular design?

A

> Re-usability of sub-system/sub-circuits across multiple products
Much shorter development cycle and time-to-market
Ability to spread the design effort among multiple designers
Ability to outsource the manufacture of sub-systems entirely
Ability to upgrade sub-system components without a major product re-design
Better immunity to component obsolescence
Systematic approach to testing

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2
Q

What are 3 industries where modularity is used massively?

A

> Avionics
Automotive industry
Consumer electronics

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3
Q

What is the main requirement when making a modular system?

A

Well-defined interfaces between individual circuits and sub-systems

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4
Q

What are 4 aspects of modular interfaces?

A

> Electrical nature of analogue or digital circuits
Physical connector
Digital information protocols
Power supplies

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5
Q

Define “Failure mode”

A

The possible fault conditions that may arise with a components or system and the consequences of each failure.

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6
Q

Define “Fail-safe design”

A

A design approach which considers various failure modes within a proposed circuit or system. If the failure occurs, the consequences are not dangerous.

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7
Q

What is a big conflict in fail-safe design?

A

Conflict between safety and security/economoy

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8
Q

What are preventative measures that can be done to help a circuit fail-safe?

A

> Use of over-voltage and over-current protection
Use of over-rated components (where appropriate)
Avoiding polarised capacitors (where possible)
Using isolation where hazardous voltages may be present
Ensure power-failure or disconnection produces safe input and output conditions

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9
Q

What are 3 options for power supply failure on critical circuits?

A

> Battery backup (Nor suitable for long term)
Switching to another power source
Warning state is the default action when power is lost

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10
Q

How is over-voltage protection implemented?

A

Using crow-bar circuits
> Uses a thyristor to short circuit the supply if the voltage rises above a certain threshold.
> This normally also triggers an over-current protection condition also.

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11
Q

What can happen if over-voltage occurs?

A

It can destroy multiple components simultaneously

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12
Q

What is the main protection required for batteries?

A

Over-current protection

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13
Q

What is required for power supplies to protect them from over-current situations?

A

Fuses

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14
Q

What are the properties and function of a polymeric positive-temperature-coefficient fuse?

A

> Natural fuse
Changes resistance when heated by a current
Resistance can change rapidly from fairly low ( a few ohms) to very high when a certain threshold current is exceeded.
After a short time of cooling the process is reversed. The fuse automatically resets

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15
Q

What are the disadvantages of a polymeric positive-temperature-coefficient fuse?

A

Disadvantage:
> Takes several milliseconds to trip
> On resetting, the resistance does not return to the initial state immediately.

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16
Q

What is SOA?

A

> Safe-operating-area

> The SOA is the maximum power dissipation that is permitted for a safe junction temperature.

17
Q

What happens if SOA is exceeded?

A

• Internal protection circuits to avoid over-heating cause the device to shutdown

18
Q

What could cause an old device fail?

A

> Contacts can oxidise in harsh conditions
PCB are prone to dry solder joints
Fatigue joints which eventually become disconnected due to repeated thermal cycling.

19
Q

What are examples of processor, logic or control failures?

A

> Bugs
Multiple inputs are pressed at the same time
Inputs fail
Errors

20
Q

What can be done to prevent processor, logic or control failures?

A

> Use watchdog timers
If inputs fail, outputs fail safe
Error checking

21
Q

How do capacitors fail?

A

> Short-circuiting can destroy the dielectric
Capacitors can explode when subjected to over-voltage
- Reverse polarity destroys the thin, insulating aluminium oxide layer on the anode
- A large current flows through the electrolyte gel and the heat turns it to gas. This rapidly builds-up and the pressure causes a rupture to the casing.

22
Q

How can you avoid capacitor failure?

A

> Use a capacitor with a breakdown voltage well above the highest voltage likely to occur in the circuit
Avoiding using polarised electrolytic capacitors

23
Q

How can resistors fail?

A

> Excessive power dissipation can cause failure

24
Q

How can you avoid resistor failure?

A

> Use resistors rated such that the maximum possible dissipation is below the resistor power rating. Unless a current limiter is used.

25
How can semiconductor devices fail?
> Bond-wires that connect the wafer die to the package pins can detach > Insulating layers breakdown and can lead to failure of a component.
26
What are other methods of circuit component failure?
> Electro-static discharge (ESD) > Electrical stressing (reverse-biased breakdown) > Thermal stress > Mechanical stress
27
How can CMOS fail?
Latch-up is a common failure in CMOS logic
28
What are 2 load faults?
> Short circuit | > Low-resistance
29
Why are low-resistance faults worse than shorts?
They are harder to detect
30
What is the solution to short/low-resistance faults?
Foldback current limits
31
What is human interaction failure?
This is what could go wrong when a human interacts with the device incorrectly.
32
What are the 3 methods to combat polarity correction/detection?
> Diode in series > Rectifier > Shunt diode and over-current protection
33
How does a rectifier combat reverse polarity?
> This allows the polarity to not matter > It uses diodes to correct it > Two voltage drops (across each diode)
34
How does a shunt diode and over current protection combat reverse polarity?
Over current protection stops current flow if reverse bias occurs.
35
How can control applications be changed to protect against abuse?
> It may be undesirable to have an output permanently on (e.g. for applications like hand tools or cars) > They are usually designed such that pressure must be continually applied to activate the output. > These are often called a dead man’s switch.
36
What is an interlock? What does it protect?
> This is when a circuit is designed to overcome an external control failure. Example: > A electric gate to a home. > Controlled with a remote control > This control is overridden if an obstruction is detected which would prevent it from closing.