Power Sources Flashcards
What does a power supply provide?
Both voltage and current
What are the properties of an unregulated voltage source?
> 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.
What are the properties of a regulated voltage source?
> Maintains the supply voltage irrespective of the load current.
> Up to a predefined current compliance.
What are the two important protections that power supplies need? How can they be implemented?
> 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
When should batteries be used?
Batteries are only used when there is a clear mobility advantage
When are batteries beneficial?
Batteries are only used when there is a clear mobility advantage
When is mains beneficial?
Mains devices have much better long term reliability
Is a linear power supply regulated or unregulated?
The output of the circuit is unregulated.
What is the circuit for a linear power supply / transformer-based power supply?
[Picture51]
What are the 3 main aspects of the linear power supply / transformer-based power supply? What do they do?
> 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
What is the circuit for a switching power supply?
[Picture52]
What are the benefits of a switching power supply over a transformer based power-supply?
> More efficient
> More compact
> Better for high power
> Can adapt to various input voltages to create flexible power supplies
What are the disadvantages of a switching power supply over a transformer-based power supply?
> Quality of the DC output is generally worse
> Less reliable
What is the operation of a switching power supply?
- Incoming AC voltage is rectified to a HIGH DC voltage (at peak value of the AC voltage = 235V in the UK)
- A capacitor smooth’s the high DC voltage
- Rapid switching MOSFET (driven by PWM) chops the high voltage at a high frequency
- A high-frequency transformer reduces the voltage
- Rectifier rectifies the signal again.
What are the properties of the PWM chopping circuit?
> 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
Why have a high frequency transformer?
> This transformer can be made much smaller
> Inductance reduces with increased frequency
How does a battery work? What does this mean for the output voltage?
> 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.
What are the two main types of batteries?
> Single-discharge (Primary)
> Rechargeable (Secondary)
When are single-discharge batteries a good use?
> Low power consumption applications
> P < 1mW
What are 3 different types of rechargeable batteries and their properties?
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’
How are batteries charged?
Charging methods are quite different and usually involve controlled sequences of constant currents and constant voltages.
What are the important protections that batteries need? Why?
> Over current protection is very important for battery powered circuits.
> Batteries need to be treated with respect and can cause substantial fires.
Name the two types of linear voltage regulators
> Fixed voltage
> Variable voltage
What type of package do linear voltage regulators come in?
> 3 terminals
> May look like transistors
What is the circuit for a linear voltage regulator?
[Picture53]
How is the transistor in a linear voltage regulator operated?
> 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.
How does the feedback control loop work for a linear voltage regulator ?
Output is continually compared with the voltage reference to help regulate the output
Define “Headroom voltage”
The minimum difference between the unregulated input voltage and the output voltage.
What is a typical headroom voltage?
2V
What is the equation for the power dissipation of a voltage regulator?
Preg = (Vin - Vout) × Iload
What might have to be done to a voltage regulator when it dissipates a lot of power?
May require thermal management
What is the simplest voltage reference solution?
Zener diode
How is a zener diode used as a voltage reference? What is the circuit?
[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
What are the problems with zener diodes?
> Voltage precision is poor (due to tolerances)
> Voltage drifts with temperature.
What are the main types of devices that need thermal management?
Linear devices
How can heat be transferred? What are the properties of each way?
> Radiate
- Need a matt black surface
> Conduct
- More efficient
- Heatsinks increase surface area
What is TjMax?
Maximum internal junction temperature
What is thermal resistance? What are the units?
> How efficiently thermal energy flows between one body to another
> How much the temperature increases per watt of power dissipated
> °C / W
What is the equation to calculate the thermal resistance between the junction and the ambient air?
> 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
How can the junction temperature be calculated if RθJC and RθCA and P and TA is known?
TJ = P(RθJC + RθCA) + TA
When is a heatsink required?
TJ > TJMax
How would you mathematically work out the thermal resistance of the heatsink required?
(TJ - TA) / P - RθJC = RθSA
What are the two important things to remember when using heat sinks?
> Use heatsink compound to aid heat transfer
> The metallic tab for heat-sink mounting on transistors is often electrically floating and not grounded
