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ELEC2130 - Circuit Design > Power Sources > Flashcards

Power Sources Flashcards

1
Q

What does a power supply provide?

A

Both voltage and current

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2
Q

What are the properties of an unregulated voltage source?

A

> The output voltage is dependent on the load current which is dependent on the source resistance.

> The only exception is if the source resistance is much lower than any possible load resistance.

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3
Q

What are the properties of a regulated voltage source?

A

> Maintains the supply voltage irrespective of the load current.

> Up to a predefined current compliance.

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4
Q

What are the two important protections that power supplies need? How can they be implemented?

A

> Over-voltage protection

  • Clamp diodes
  • Crow-bar circuit that effectively short-circuits the power supply in the event of excess voltage

> Over-current protection

  • Mandatory for almost every circuit
  • Fuses
  • PCB mounting
  • Resettable fuses
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5
Q

When should batteries be used?

A

Batteries are only used when there is a clear mobility advantage

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6
Q

When are batteries beneficial?

A

Batteries are only used when there is a clear mobility advantage

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7
Q

When is mains beneficial?

A

Mains devices have much better long term reliability

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8
Q

Is a linear power supply regulated or unregulated?

A

The output of the circuit is unregulated.

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9
Q

What is the circuit for a linear power supply / transformer-based power supply?

A

[Picture51]

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10
Q

What are the 3 main aspects of the linear power supply / transformer-based power supply? What do they do?

A

> Full bridge rectifier

> Mutual-inductance impedance transformer

  • High input voltage
  • Low output voltage
  • Specific VA power rating
  • Ideal transformer

> Smoothing capacitor

  • Energy storage element
  • Resistance for charge is small so τ is small for charging
  • Resistance for discharge is large so τ is large for discharge
  • The larger the capacitor the better the DC smoothing
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11
Q

What is the circuit for a switching power supply?

A

[Picture52]

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12
Q

What are the benefits of a switching power supply over a transformer based power-supply?

A

> More efficient

> More compact

> Better for high power

> Can adapt to various input voltages to create flexible power supplies

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13
Q

What are the disadvantages of a switching power supply over a transformer-based power supply?

A

> Quality of the DC output is generally worse

> Less reliable

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14
Q

What is the operation of a switching power supply?

A
  1. Incoming AC voltage is rectified to a HIGH DC voltage (at peak value of the AC voltage = 235V in the UK)
  2. A capacitor smooth’s the high DC voltage
  3. Rapid switching MOSFET (driven by PWM) chops the high voltage at a high frequency
  4. A high-frequency transformer reduces the voltage
  5. Rectifier rectifies the signal again.
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15
Q

What are the properties of the PWM chopping circuit?

A

> Output voltage is controlled by the duty cycle

> Feedback loop can control the PWM signal to deliver the exact power required for a given load

> Typical PWM frequency between 10-500 KHz

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16
Q

Why have a high frequency transformer?

A

> This transformer can be made much smaller

> Inductance reduces with increased frequency

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17
Q

How does a battery work? What does this mean for the output voltage?

A

> Stores energy in chemical form.

> Output voltage is not constant

> Different battery technologies have different discharge characteristics.

> It is important to look at the discharge curves for batteries.

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18
Q

What are the two main types of batteries?

A

> Single-discharge (Primary)

> Rechargeable (Secondary)

19
Q

When are single-discharge batteries a good use?

A

> Low power consumption applications

> P < 1mW

20
Q

What are 3 different types of rechargeable batteries and their properties?

A

NiMH (Nickel metal-hydride):

  • General applications

Lead acid:

  • Heavy duty applications Li-ion (Lithium ion)
  • Used in smartphones
  • Good energy to weight ratio.
  • Susceptible to damage from:

> Extreme temperatures

> Excessive discharge

> Suffer a destructive failure mode known as ‘venting with flame’

21
Q

How are batteries charged?

A

Charging methods are quite different and usually involve controlled sequences of constant currents and constant voltages.

22
Q

What are the important protections that batteries need? Why?

A

> Over current protection is very important for battery powered circuits.

> Batteries need to be treated with respect and can cause substantial fires.

23
Q

Name the two types of linear voltage regulators

A

> Fixed voltage

> Variable voltage

24
Q

What type of package do linear voltage regulators come in?

A

> 3 terminals

> May look like transistors

25
Q

What is the circuit for a linear voltage regulator?

A

[Picture53]

26
Q

How is the transistor in a linear voltage regulator operated?

A

> Operated in the linear region

> Adjusted to vary the output voltage

> Behaves as a Current-controlled/Voltage-controlled resistor

> It is dissipating energy as it is in the linear region so gets warm.

27
Q

How does the feedback control loop work for a linear voltage regulator ?

A

Output is continually compared with the voltage reference to help regulate the output

28
Q

Define “Headroom voltage”

A

The minimum difference between the unregulated input voltage and the output voltage.

29
Q

What is a typical headroom voltage?

A

2V

30
Q

What is the equation for the power dissipation of a voltage regulator?

A

Preg = (Vin - Vout) × Iload

31
Q

What might have to be done to a voltage regulator when it dissipates a lot of power?

A

May require thermal management

32
Q

What is the simplest voltage reference solution?

A

Zener diode

33
Q

How is a zener diode used as a voltage reference? What is the circuit?

A

[Picture54]

> Uses the zener effect to maintain a stable voltage drop

> Used in reverse biased mode

> As long as the supply voltage

> Zener voltage then it will have a consistant voltage drop over a wide range of currents

34
Q

What are the problems with zener diodes?

A

> Voltage precision is poor (due to tolerances)

> Voltage drifts with temperature.

35
Q

What are the main types of devices that need thermal management?

A

Linear devices

36
Q

How can heat be transferred? What are the properties of each way?

A

> Radiate

  • Need a matt black surface

> Conduct

  • More efficient
  • Heatsinks increase surface area
37
Q

What is TjMax?

A

Maximum internal junction temperature

38
Q

What is thermal resistance? What are the units?

A

> How efficiently thermal energy flows between one body to another

> How much the temperature increases per watt of power dissipated

> °C / W

39
Q

What is the equation to calculate the thermal resistance between the junction and the ambient air?

A

> Between the junction and the outer casing: RθJC

> Between the outer casing and the ambient air: RθCA

> Junction to ambient temperature thermal resistance: RθJC

> RθJC = RθJC+RθCA

40
Q

How can the junction temperature be calculated if RθJC and RθCA and P and TA is known?

A

TJ = P(RθJC + RθCA) + TA

41
Q

When is a heatsink required?

A

TJ > TJMax

42
Q

How would you mathematically work out the thermal resistance of the heatsink required?

A

(TJ - TA) / P - RθJC = RθSA

43
Q

What are the two important things to remember when using heat sinks?

A

> Use heatsink compound to aid heat transfer

> The metallic tab for heat-sink mounting on transistors is often electrically floating and not grounded

ELEC2130 - Circuit Design (15 decks)

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