3. Practical Drivers Flashcards
Why might you hear ‘virtual earth’ or ‘virtual ground’ in relation to an inverting amplifier circuit?
As the non-inverting terminal is connected to 0V, the voltage on the inverting terminal is also at 0V or very nearly so
Book 4 page 37
What is a decoupling capacitor used for?
Makes an extra (albeit short-term) source of current available just as it is needed to cope with sudden demands
Can also temporarily sink current resulting from spikes of excessive voltage
Book 4 page 45
Why might PWM be more efficient than controlling the motor power by varying the voltage across the motor with an analogue power amplifier?
Even when fully on, the motor will only have the small saturation voltage across the switching transistors
As opposed to the latter case where the amplifiers would carry the motor current as well as a large fraction of the supply voltage
Book 4 page 46
What situations lead to loss of energy in PWM when controlling a motor?
- Energy is lost at each switching transition, so more energy is lost with increasing frequency
- Inductance in the windings causes the motor current to reduce as the frequency increases
Book 4 page 46
How does the power consumed by a MOSFET transistor compare with a bipolar as part of a circuit for driving a motor?
Bipolar - Saturation voltage between collector and emitter remains quite low over a range of collector current values
MOSFET - Behaves approximately like a resistor, provided that the current is not too large, and the ON resistance of the MOSFET is low enough, then the saturation voltage can be much less than a bipolar transistor. At high currents though, the power consumed could be much higher
Book 4 page 47
Under what conditions can a half-bridge circuit drive a DC motor in either direction?
If a split (double) supply is available
Book 4 page 51
What does a half-bridge consist of?
Two transistors in a series arrangement
Book 4 page 51
Where might you see a similar arrangement to a half-bridge circuit used to drive a DC motor in either direction?
Push-pull configuration for analogue amplifiers
Book 4 page 51
What type of transistor circuit can be used to drive a DC motor in either direction with a single supply?
A full or H-bridge circuit
Book 4 page 51
What can a full or H-bridge circuit also provide apart from just driving a motor in either direction?
Motor braking capability
Book 4 page 51
Why are extra components required in a full or H-bridge circuit?
To protect against voltage and current surges due to inductance, back emf and other causes
Book 4 page 51
Which type of transistor combines some of the advantages of MOSFET and bipolar transistors?
Insulated gate bipolar transistor (IGBT)
Book 4 page 47
