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Microelectronics in consumer products > 12. Power > Flashcards

12. Power Flashcards

1
Q

What voltage/how many cells does logic work happily from?

A

3v - 2 cells

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2
Q

Why do you need a power supply unit PSU?

A

because your mc system probably works at 3V or 5V DC but mains is 20V at 50Hz in Europe and 110V at 50Hz in N. America.

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3
Q

What does a PSU do?

A

Converts mains AC to a low voltage DC power.

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4
Q

What is the problem with plug in the wall supplies?

A

The may be very poorly regulated

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5
Q

If the plug in the wall supplies are very poorly regulated what does this mean?

A

This means that the output voltage depends strongly on the load.

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6
Q

When must you therefore provide if using a plug in the wall supply?

A

A regulator to ensure your system sees a steady voltage of 3 or 5V.

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7
Q

What are the two main varieties of PSUs?

A
  1. Linear

2. Switching

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8
Q

What are linear PSUs, adv and disadv?

A

The classical approach, using transformers, rectifiers and smoothing capacitors. Electrically quiet but heavy, bulky and not very efficient.

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9
Q

What are switching PSUs, adv and disadv?

A

Uses power electronics to get a much smaller, lighter and more efficient product. However, they produce a lot of electrical noise.

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10
Q

Applications of switching PSUs?

A

Universal in computers, increasingly common in products such as battery chargers.

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11
Q

Can you draw the diagrams for 5V switching PSU and 5V linear PSU to indicate how much electrical noise they produce?

A

Yes or no

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12
Q

What are the features of mains supplies?

A
  1. Cost - virtually free when compared with batteries
  2. Available readily in buildings, but limited portability, even indoors
  3. Reliable (generally) - don’t run out like batteries but power cuts are not unknown so backup supply may be needed.
  4. High voltage may present potential danger
  5. May be problems if equipment is used abroad
  6. Noisy - transient pulses produced when heavy loads switch on and off and the PSU must filter these out. In more extreme cases e.g. lightning strikes, power surges may occur.
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13
Q

Features of battery supplies?

A
  1. Finite lifetime - run down and need to be replaced
  2. Portable - but may be heavy
  3. Polarity - usually needs protection against reverse polarity (if the user can insert the batteries the wrong way around)
  4. Expensive (very)
  5. Little noise - no transient or switching noise at all from the battery itself (but may get noise within the system)
  6. Characteristics change throughout life - depending on type of battery
  7. Bad for the environment - especially if not rechargeable or recycled
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14
Q

What are the two classes of batteries and examples of each?

A
  1. Primary: zinc chloride, alkaline (manganese), lead (many different types), silver oxide, zinc air
  2. Secondary (rechargeable): Nickel Cadmium (NiCad), Nickel metal hybrid (NiMH), lead acid, lithium ion
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15
Q

Two things to choose a battery depending on?

A
  1. What capacity is needed

2. How portable the product needs to be

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16
Q

What feature should the battery always have?

A

It needs to be easy to replace, so exotic batteries are a nuisance even if they have great electrical properties.

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17
Q

Can you draw the table showing different features/characteristics of different types of batteries?

A

Yes or no

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18
Q

Features of alkaline cells?

A

You can buy them anywhere. At large current drains their capacity is much bigger than cheaper zinc chloride cells but not so much at low currents. Nominally 1.5V but with strongly sloping discharge characteristic.

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19
Q

Features/use of zinc air cells?

A

Used in hearing aids and other applications with a low, steady drain, 1.4V

20
Q

Features of NiMH?

A

Best rechargeable choice at present: much higher capacity, less prone to show memory effects, can be charged more quickly and are less damaging to the environment than the old NiCad.

21
Q

What are lithium ion batteries used in?

A

Laptops

22
Q

What are lead acid batteries used in?

A

Cars (heavy!)

23
Q

How long do secondary (rechargeable) cells last?

A

5 years, 250-1000 charge-discharge cycles.

24
Q

What happens to secondary cells if they are not used?

A

suffer self discharge

25
Q

Can you draw the discharge graph/curves for lead acid, silver oxide, lithium manganese, NimH, and alkaline manganese cells?

A

Yes or no

26
Q

What do the discharge curves show about voltage choice for circuit?

A

That it is not easy to choose the voltage at which the circuit should operate.

27
Q

Alkaline batteries produce 1.5V per cell when new but what happens throughout their life and how much voltage do they produce at the end of their life?

A

This drops steadily throughout their lives. The end point is 0.9V, so 2xAA cells only produce 1.8V at the end of their lives.

28
Q

What voltage do NiMH and NiCad at the end of their lives/ how does their voltage behave at discharge?

A

Produce 1.2V throughout most of their lives, but the voltage plummets when they have nearly completely discharged. 2*AA cells producs 2.4V in this case.

29
Q

What voltage should a well designed product that uses 2xAA cells be capable of operating at?

A

If primary cells are permitted, should be capable of operating at 1.8V.

30
Q

What is the capacity of a cell usually expressed in?

A

Ampere hours or Milliampere Hours (mAh)

31
Q

What are the two things we need power supplies to be able to do?

A
  1. Take an input of variable voltage

2. Produce and output of fixed, regulated voltage that may be lower than the input, higher or the opposite sign

32
Q

From Q31.. How can we do this? (2 methods)

A
  1. Charge pumps - flying capacitors

2. Switching supplies - switch mode power supplies use inductors

33
Q

How does a simple charge pump work? Draw the diagram?

A

Charge two capacitors in parallel… then discharge them in series

34
Q

What happens if there is a power cut and the break in power is really long? What will be lost?

A

The smoothing capacitors in the power supply discharge, so the voltage Vdd at the mc will probably fall so low that the chip will restart when power is restored. All data in RAM will be lost, but at least its clear that a restart has taken place.

35
Q

What is brown out?

A

If the interruption in power is short, it is possible that data may be lost in some parts of the system but not others. The unaffected parts may attempt to use corrupted data from the parts that were affected.

36
Q

Why is brown out serious?

A

Because of corrupt data

37
Q

How is brown out prevented in mcs?

A

Many mcs have brown out detectors (low voltage inhibitors) that force the chip to reset if the voltage falls below a specified value.

38
Q

What does a WDT comprise of and what does it do?

A

Comprises a timer and a counter that run independently of the rest of the chip. If the counter overflows, the watchdog times out and resets the whole mc.

39
Q

How do you avoid WDT resetting the whole chip?

A

In your program you should clear WDT regularly.. or turn WDT off completely

40
Q

What does WDT protect from?

A

Protects the system from infinite loops because if a program gets stuck in an unintentional infinite loop the watchdog will not be cleared and so the counter will eventually overflow, resetting the whole chip.

41
Q

What happens in the mc when sleep mode is entered?

A
  1. The main clock oscillator stops
  2. therefore execution of instructions stops
  3. All data in memory is retained
  4. outputs remain active but of course will not change their values
42
Q

How can mcs be woken up? Typical examples?

A

Woken by either internal or external signals:

  1. When the voltage on an input pin changes logical value
  2. an internal oscillator and counter ‘times out’ after a known delay
43
Q

When the wake-up signal had been detected, why is there a delay?

A

To allow time for the clock oscillator to stabilise.

44
Q

What are the 5 strategies for saving power?

A
  1. Use a slow clock: current is directly proportional to the clock frequency + a constant.
  2. Turn off all peripherals that are not needed: ADCs etc can be switched off when not in use
  3. Ensure all inputs are drive or connected to Vdd or Vss: e.g. dont forget pullups.
  4. Sleep as long as possible, then get processing done as quick as possible.
  5. Use modern components, designed for lower supply voltages.
45
Q

In charge pumps what does the switching and what part provides Vout = -2Vin or -Vin etc?

A

MOSFETs do the switching and can get many different types of pumps to get a desired Vout.

Microelectronics in consumer products (14 decks)

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