4.1 Semiconductions - Diode Characteristics Flashcards
how are schottky diodes constructed?
metal-to-n junction
what are schottky diodes known as?
hot-carrier diodes
what are schottky diodes characterised by? (3)
-fast switching times
-low forward voltage drop
-low junction capacitance
what range are schottky diodes use for?
high frequency
200 Khtz to 2 Mhtz
where are schottky diodes used? (4)
-logic gates,
-switch mode power converters
-RF detectors and mixers
-clamping
what are the disadvantages of schottky diodes? (2)
-low reverse voltage rating (50v and below)
-high reverse leaking current
in rectifier circuits, which part of the waveform affects load resistance in a half-wave rectifier?
on the respective half waves
in rectifier circuits, in a half-wave rectifier, if you were to use a charging capacitor, what conditions change?
the diode is not conductive for the entire positive half wave but only opens when the transformer voltage is greater than the voltage
in rectifier circuits, what is the opening time expressed as? what is its value?
as an angle
between 30 and 60 degrees
in rectifier circuits, what is often placed in front of a diode to protect it? why?
protective resistor
to limit the incoming current and to stop from peaking in the diode
in rectifier circuits, in a full-wave recitfier, which part of the waveform has an effect on work load?
both half waves in the same direction
in rectifier circuits, in a full-wave rectifier with a charging capacitor, what happens to the hum voltage?
only half as large but twice the frequency of the power grid
what is the disadvantage of a full wave rectifier?
a transformer with a double coil or tapped coil is required
what is a bridge circuit?
a full-wave rectifier without the need for a tapped transformer
called a WYE rectifier
what is the advantage of a bridge circuit when subjected to capacitive load?
each diode only has to handle a maximum sticking voltage of Vtr because at any given time two diodes are in sequence
what type of circuit is used to rectify type 3-phase AC?
six diode circuit
how does an 3-phase rectifier work?
3 stators are individually in a negative state where current flows through a diode, the load resistor and another diode, then on to the next phase
what is produced as a result of 3-phase rectification?
a pulsating DC voltage
in a 3 phase rectifier circuit, what is used in order to produce a DC voltage?
a charging capacitor laid in parallel
what is used to level out the direct current voltage at the charging capacitor?
filter circuit
in the filtering of rectifier circuits, what is used to remove the ripple remaining after the reservoir capacitor?
which is more efficient?
LC or RC
LC
what are the advantages and disadvantages of LC Filtering?
Ad:
only a small loss of DC
Dis:
expensive inductance
how can Zener diodes be used to stabilise voltage output?
by passing a small voltage from a voltage source, via a current limiting resistor
what are cascades often used for?
creation of high voltage necessary for cathode ray tubes, Geiger-muller counters or igniter boxes
why are cascades more advantageous than high voltage transformers
low current consumption
how does a delon circuit work? (voltage doubler)
half cycles aren’t sent to the consumer together
capacitors are charged, two half cycles are added together and the two capacitor voltages are the new output voltage
how does a villard circuit work? (voltage doubler)
based on a one-way rectifier
during the negative half cycle, the capacitor is charged.
during the positive half cycle, the transformer voltage and capacitor voltage are added
diode now in conducting direction and the second capacitor is charge of the transformer and capacitor
what is the principle of a cascade circuit?
any voltage can be created but the current capacity is very low
where are clamping circuit used? and why?
used in radar and comms equipment
to change the reference level of a waveform without altering its amplitude
what are clipping circuit’s purpose?
they remove undesirable characteristics from a waveform but cutting off part of the voltage waveform
in clipping circuits, what types of limiters are there? and their purposes (5)
positive limiter - removes all parts above the waveform
negative limiter - removes all parts below the waveform
series limiter - the diode is in series between the input and low
parallel limiter - diode is parallel between the input and load
combined limiter - uses positive and negative limiter in same circuit
what does a zener diode have to protect and how is it done?
steep breakdown in reverse biased direction
minimal changes in voltages leads to large current changes
in a zener diode, how can the amount of breakdown be influenced?
by the amount of dopant introduced to the material during manufacture
in a zener diode, what is zener breakdown?
where valance electrons are torn out of their lattice structure, increasing free electrons, increasing conductivity of the material
what effects diodes that use the zener effect?
temperature (negative), leading to decrease in voltage
zener diodes which the avalanche effect is the main cause, are affected by temperature how?
positively therefore higher voltage
what does the avalanche effect allow charge to transport through?
the barrier layer
which bias and condition does a zener diode operate in?
reverse bias
breakdown
what must be taken into consideration regarding to a zener diodes maximum power rating?
careful selection of value of resistor
why is the load connected in parallel to the zener diode?
so that the voltage across the resistor is always the same as the zener diode
what must be higher than the zener diode?
the voltage of the supply
what can be generated from a zener diode stabiliser circuit, and what could be used to overcome this?
electrical noise
addition of a large value decoupling capacitor
when connecting several zener diodes in series with regular diodes, what must the supply voltage be?
higher than the larger output reference voltage
in a stabilisation circuit with zener diodes, how does the stabilisation improve?
stabilisation increases as input voltage increases, but power dissipation increases at a higher rate
in a stabilisation circuit with zener diodes, what compromise should be taken to gain good stabilisation and acceptable power?
suggested input voltage of 2-4 times the value of the output voltage
