What are the functions of the resistors and capacitors on both sides of the
crystal oscillator?
Scanner power adapter background knowledge
The crystal oscillator refers to a slice (referred to as a wafer) cut from
a piece of quartz crystal at a certain azimuth angle. The quartz crystal
resonator is referred to as quartz crystal or crystal or crystal oscillator; and
the crystal element that adds IC inside the package to form an oscillator
circuit is called Crystal oscillator. The charger is generally packaged in a
metal shell, but also in a glass shell, ceramic or plastic package.
The general applications of crystal oscillators are:
1. A general-purpose crystal oscillator, used in various circuits to
generate oscillation frequency.
2. Quartz crystal resonators for clock pulses are used in conjunction with
other components to generate standard pulse signals, which are widely used in
digital circuits.
3. Quartz crystal resonators for microprocessors.
4. CTVVTR uses quartz crystal resonator.
5. Quartz crystal oscillators for watches.
Today's problem
In the daily charger circuit design, we often use crystal oscillators, and
we often see capacitors and resistors at both ends of the crystal oscillator,
such as the two pictures below. So what are the functions of such capacitors and
resistors? How are their values selected? You can combine theory and practical
projects to give your own views.
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Inductance and chokes in chargers
The charger manufacturer introduced the following winding components
(inductance and holding ring)
1. Simple inductance (no DC current through)
2. Common-mode line filter inductance (especially dual-winding inductance
with large symmetrical power frequency current)
3. Series line filter inductor (inductance carrying large and asymmetric
power frequency current)
4. Flow coil (inductance wrapped around a ferrite core with an air gap and
carrying a large DC bias current)
5. Rod-shaped flow ring (a flow-throwing ring wound on a ferrite core or a
rod-shaped iron powder core)
In order to facilitate the discussion, the "inductance" we are talking
about here refers to the winding components without DC current, and the "choke
coil" refers to the winding components with large bias current and relatively
small AC ripple.
The design and material selection of winding components must fully consider
the place of application. In addition, the design must be repeated many times,
and the relationship between some interconnected but opposite variables must be
coordinated.
If the engineer can fully understand and grasp the theoretical and actual
specifications required for the optimal design of various winding components in
the charger, then the design skills he possesses will be precious and
unique.
The design method used here is mainly based on its scope of application,
focusing on the three aspects of cost, size and loss. The final design can only
be a compromise solution. Since these three main aspects are contradictory, it
can only be used. A compromise solution. The task of the designer is to obtain
the best compromise.
In charger applications, inductors without DC bias are generally limited to
low-pass filters used in the charger circuit. Here, their main function is to
prevent high noise from being transmitted back to the charger circuit. For this
type of application, we should choose a core material with a high
conductivity.
Chokes (inductors carrying large bias DC currents) are used in
high-frequency power output filters and continuous buck-boost converter
"transformers". In these applications, low permeability and high frequency
should be preferred Magnetic core material with low magnetic loss.
In order to reduce the number of turns and reduce the copper loss, the most
ideal core material should have high permeability and small magnetic loss.
Unfortunately, in the design of the current-carrying circle, the existence of
large DC components and the actual use The limited saturation magnetic flux
density of magnetic materials makes us have to choose low-conductivity materials
or introduce air into the core. However, due to the too low effective
permeability, more windings are required to achieve the required inductance.
value. Therefore, in the choke coil design, in order to pass a larger DC
current, both low copper loss and high efficiency must be taken into
account.
Simple inductor
In the application field of charger, pure inductance (cannot carry DC
current component or forced AC high current component) is rare. Unlike the
common-mode filter inductors that will be introduced below, since no air gap is
required, the value of this inductance can be obtained directly from the given
core inductance A value, so the design is relatively simple, and this will not
be described. But it must be remembered that the size of this type of inductance
is proportional to the square of the number of turns. Therefore, A1 must be
given for 1 turn (as shown in the following formula), or A1 for multiple turns
must be given. At this time, the value of A should be reduced to the value of 1
turn by dividing by the square of the number of turns.
L=N2AL
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