Circuit Design Flashcards
What are 7 advantages to using a modular design?
> 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
What are 3 industries where modularity is used massively?
> Avionics
Automotive industry
Consumer electronics
What is the main requirement when making a modular system?
Well-defined interfaces between individual circuits and sub-systems
What are 4 aspects of modular interfaces?
> Electrical nature of analogue or digital circuits
Physical connector
Digital information protocols
Power supplies
Define “Failure mode”
The possible fault conditions that may arise with a components or system and the consequences of each failure.
Define “Fail-safe design”
A design approach which considers various failure modes within a proposed circuit or system. If the failure occurs, the consequences are not dangerous.
What is a big conflict in fail-safe design?
Conflict between safety and security/economoy
What are preventative measures that can be done to help a circuit fail-safe?
> 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
What are 3 options for power supply failure on critical circuits?
> Battery backup (Nor suitable for long term)
Switching to another power source
Warning state is the default action when power is lost
How is over-voltage protection implemented?
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.
What can happen if over-voltage occurs?
It can destroy multiple components simultaneously
What is the main protection required for batteries?
Over-current protection
What is required for power supplies to protect them from over-current situations?
Fuses
What are the properties and function of a polymeric positive-temperature-coefficient fuse?
> 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
What are the disadvantages of a polymeric positive-temperature-coefficient fuse?
Disadvantage:
> Takes several milliseconds to trip
> On resetting, the resistance does not return to the initial state immediately.
What is SOA?
> Safe-operating-area
> The SOA is the maximum power dissipation that is permitted for a safe junction temperature.
What happens if SOA is exceeded?
• Internal protection circuits to avoid over-heating cause the device to shutdown
What could cause an old device fail?
> Contacts can oxidise in harsh conditions
PCB are prone to dry solder joints
Fatigue joints which eventually become disconnected due to repeated thermal cycling.
What are examples of processor, logic or control failures?
> Bugs
Multiple inputs are pressed at the same time
Inputs fail
Errors
What can be done to prevent processor, logic or control failures?
> Use watchdog timers
If inputs fail, outputs fail safe
Error checking
How do capacitors fail?
> 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.
How can you avoid capacitor failure?
> Use a capacitor with a breakdown voltage well above the highest voltage likely to occur in the circuit
Avoiding using polarised electrolytic capacitors
How can resistors fail?
> Excessive power dissipation can cause failure
How can you avoid resistor failure?
> Use resistors rated such that the maximum possible dissipation is below the resistor power rating. Unless a current limiter is used.
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.
What are other methods of circuit component failure?
> Electro-static discharge (ESD)
Electrical stressing (reverse-biased breakdown)
Thermal stress
Mechanical stress
How can CMOS fail?
Latch-up is a common failure in CMOS logic
What are 2 load faults?
> Short circuit
> Low-resistance
Why are low-resistance faults worse than shorts?
They are harder to detect
What is the solution to short/low-resistance faults?
Foldback current limits
What is human interaction failure?
This is what could go wrong when a human interacts with the device incorrectly.
What are the 3 methods to combat polarity correction/detection?
> Diode in series
Rectifier
Shunt diode and over-current protection
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)
How does a shunt diode and over current protection combat reverse polarity?
Over current protection stops current flow if reverse bias occurs.
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.
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.
