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

