Electronic
devices are inseparable from the power adapter. The power adapter supplies the
energy required by the electronic device, which determines the importance of
the power adapter in the electronic device. The quality of the power adapter
directly affects the working reliability of the electronic device, so the
requirements of the electronic device for the power adapter are also
increasing. Existing power adapters are mainly composed of two main categories:
linear regulated power adapters (referred to as linear power adapters) and switching regulated power adapters (referred to as power adapters). These two
types of power adapters are widely used due to their unique characteristics.
Linear power adapter
The composition block
diagram of the linear power adapter is shown in the figure.
Figure Block diagram of linear power
adapter
The linear power adapter is generally
composed of four parts: voltage transformation, rectification, filtering and
voltage stabilization.
Transformer-turns the AC grid voltage
into the desired AC voltage. The transformation process is usually completed by
a transformer, and some use capacitors for voltage reduction.
Rectification-to convert AC voltage to
DC voltage. Rectifier circuits usually have half-wave rectifier circuits,
full-wave rectifier circuits, bridge rectifier circuits, etc. Bridge rectifier
circuits are more commonly used.
Filtering—filter out the AC components
in the pulsating direct current (regular change in size) obtained by rectification.
Commonly used filter circuits include capacitor filter circuit, inductance
filter circuit and resistance-capacitance filter circuit.
Voltage stabilization-Stabilize the DC
voltage output by the filter circuit, even if the output DC voltage does not
change with changes in the grid voltage and load. Voltage stabilizing function
can be completed by voltage stabilizing diode, DC/DC converter, series voltage
stabilizing circuit, etc.
The advantages of the
linear power adapter are good stability, high reliability, high output voltage
accuracy, and low output ripple voltage. Its disadvantage is that it requires
the use of power frequency transformers and filters. Their weight and volume
are large, and the power consumption of the adjustment tube is large, which
greatly reduces the efficiency of the power adapter. Under normal
circumstances, the efficiency of the power adapter will not More than 50%.
However, its excellent output characteristics make it widely used in occasions
where the performance requirements of the power adapter are high.
Power Adapter
The power adapter is
named because its control device works in the ON and OFF state. Its essence is
to change the size of the output voltage by changing the on time of the control
device in the circuit to achieve the purpose of maintaining the stability of
the output voltage The schematic diagram of the power adapter and the
input/output waveforms are shown in the figure.
The schematic diagram of the power
adapter and the input/output waveform diagram in the figure, Ui is the unstable
DC voltage after rectification; Uo is the chopped output voltage; S is the
switch control device; RL is the load; T is the switch on and off cycle; Ton is
The switch closing time is the on-time; Toff is the switch off time, the off
time.
Compared with the
linear power adapter, the power adapter can meet the requirements of modern
electronic equipment. Since the invention of the power adapter in the mid-20th
century, it has been widely used in computers, communications, aerospace,
office and electrical equipment due to its outstanding advantages It is likely
to replace the linear power adapter.
The power adapter has the following
characteristics.
1. efficient.
The adjustment tube of the power adapter
works in the switching state, and the size of the output voltage can be changed
by changing the ratio of the conduction and cut-off time of the adjustment
tube. When the adjustment tube is saturated and conductive, although there is a
large current flowing through, the saturation tube voltage drop is very small;
when the adjustment tube is cut off, the tube will withstand a higher voltage,
but the current flowing through the adjustment tube is basically equal to zero.
It can be seen that the power consumption of the adjustment tube working in the
switching state is very small. Therefore, the efficiency of the power adapter
is relatively high, generally up to 65% to 90%.
2. Small size and light weight.
Because the power
consumption of the adjusting tube is small, the radiator can also be reduced
accordingly. Moreover, the power adapter can also remove the 50Hz power
frequency transformer, and the switching frequency is usually tens of
kilohertz, so the capacity of the filter inductor and capacitor can be greatly
reduced. Therefore, the power adapter is much smaller in size and weight than
the linear power adapter of the same power.
3. Low requirements on grid voltage.
Because the output voltage of the power
adapter is related to the ratio of the turn-on and cut-off time of the
regulating tube, and the amplitude change of the input DC voltage has little
effect on it, therefore, a large fluctuation of the grid voltage is allowed.
The general linear voltage stabilizing circuit allows the grid voltage to
fluctuate by ±10%, while the switching voltage stabilizing circuit can still
work normally when the grid voltage is 140-260V and the grid frequency changes
by ±4%.
4. The control circuit of the adjusting
tube is more complicated.
In order to make the adjustment tube
work in the switching state, a control circuit needs to be added. The pulse
waveform output by the adjustment tube needs to be filtered by LC before being
sent to the output end. Therefore, compared with the linear power adapter, its
structure is more complicated and debugging is more troublesome.
5. Large ripple and noise components in
the output voltage.
The adjustment tube works in the
switching state, which will generate spike interference and harmonic signals.
After rectification and filtering, the ripple and noise components in the
output voltage are still larger than the linear power adapter.
In the future, the
development of switching power supply adapters, in addition to continuing to
maintain the existing advantages, is mainly to adopt new technologies and new
technological measures to overcome some of its own shortcomings.