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Phone
Chargers
Why Do Phone Chargers Heat Up?
Venkatesh
Vaidyanathan
If you have a mobile device or a laptop, you obviously need to charge
it regularly at frequent intervals.
However, have you ever touched the charger during a charging cycle?
Chances are, if you have, then you undoubtedly noticed that the
charger dissipates a lot of heat, which is quite reasonable and nothing to be
worried about.
Before we get into the reason as to why the chargers heat up so much, we
should provide a technical overview of the inner working of the chargers used
for cell phones and laptops.
Switch Mode Power Supply (SMPS)
The chargers we use either for a mobile device or laptop are not
ordinary wall plugs that provide a connection between the power supply
unit and the device.
This device is known as a Switch Mode Power Supply (SMPS), an electronic power supply that incorporates a
switching regulator used in converting an electric power supply
efficiently.
SMPS are usually used to turn AC or DC power supply into DC loads
(e.g., mobile phones and laptops), while changing the voltage and current
characteristics.
The way it does this is by continually switching between full on and
full off states, hence the name Switch Mode Power Supply.
Now, let’s look at the different stages to determine how the
SMPS converts AC power into usable DC power for an electronic device.
Input Rectifier and Inverter Stage
When the
SMPS receives an AC input from the wall supply, the primary focus is to convert
the input into DC. This process is known as rectitication.
The
rectifier ends up giving an output that is in the form of unregulated
DC voltage. This unrectified DC voltage is then sent to a capacitor.
The
current drawn from the main power supply by the rectifier circuit occurs in
short pulses around the AC voltage peaks.
An SMPS
designed for an AC input can also run from a DC supply, as the DC would pass
through the rectifier unchanged.
The
inverter stage of the process involves the conversion of DC to AC either
directly (if the source is a DC supply) or after the above-mentioned rectifying
stage is completed by running it through a power oscillator.
The power
oscillator consists of a small output transformer that has very few
windings. These windings comprise a frequency of a few tens to hundreds of
kilohertz.
The
frequency selected by default is primarily over 20 Khz.
The
constant switching action is performed by a MOSFET. The
metal-oxide-semiconductor-field-effect transistor (MOSFET, MOS-FET, or MOS FET)
is a type of field-effect transistor (FET) most commonly fabricated by the
controlled oxidation of silicon.
It has an
insulated gate, the voltage of which determines the conductivity of the
device.
This
ability to change conductivity based on the amount of applied voltage can be
used for amplifying or switching electronic signals. It is used as a transistor that
can handle both low voltages and high currents.
Voltage Converter and Output Rectifier
If the
output must be rectified from the input, as is usually the case in main
power supplies, the inverted AC is used to drive the primary winding of a
high-frequency transformer (present in the power oscillator).
This
converts the voltage up or down to the required output level on its secondary
winding.
The
output transformer in the block diagram serves this purpose. If a DC output is
required, the AC output from the transformer (in the power oscillator) must be
rectified.
For output voltages
above ten volts, ordinary silicon diodes will suffice. For lower voltages, Schottky diodes are
used as the rectifier diodes.
Schottky
diodes have the characteristic feature of working in low forward voltage and
they have a very fast switching action. They also have the unique set of
advantages of faster recovery times than silicon diodes and a lower voltage
drop when conducting.
For even
lower output voltages, MOSFETs may be used as synchronous rectifiers; compared
to Schottky diodes, these have even lower conducting state voltage drops.
In the
end, the rectified output is then smoothed by a filter that consists of a
capacitor and inductors.
The Reason for Heating
The heating of the charger occurs primarily as a byproduct of the
power conversion process mentioned above.
A straightforward way to convert power is to rectify the AC wall power
to DC through a diode bridge (which always involves some heat loss) and a
filter (to smooth the ripples from the AC source), and run that into a “linear”
regulator.
A linear regulator works by using feedback to make a transistor act as
a variable resistor.
A resistor is a component that turns power into heat. Your phone would
get the 5V it needs, but the transistor would have to “consume” the other 105V
as heat.
As a result, this is less than 5% efficient, which is totally
impractical for phone use.
The next method one could look into is taking the AC power and running
that into a transformer, which will output a lower voltage.
That lower voltage can get rectified and sent to the same kind of
regulator, which needs to drop only a couple volts.
The transformer is very efficient, while the diodes are a little less
so than with the higher voltage, but the big win is going from a 105V drop in
the regulator to 2-3V or less.
This may therefore be 60-80% efficient. The primary downside is that
transformers can be cumbersome and large if you want them to be
productive.
A final way is to use a switching converter. If you put a voltage into
a switch and regularly switch that switch on and off with an even period,
you’ll find that the average output is half the input.
The only problem is that what you get is a big square wave that goes
from full voltage to zero. However, run that through a good filter, and the
outcome is half the input voltage as DC.
So, in our case, we rectify the input voltage to DC, pass it through a
switch, filter it, and out comes any voltage we want at nearly 100% efficiency,
based on the on vs. off time of the switch.
Of course, a real switch would switch too slow, require a large filter
circuit, and wear out quickly.
Thus, we use an electronic switch, which is where the SMPS proves
to be effective.
Only for a tiny part of the conversion process can a switching
supply be 95% efficient or so, but even in this case, there’s some
inefficiency, which is why some heat is inevitably produced.
Venkatesh is
an Electrical and Electronics Engineer from SRM Institute of Science and
Technology, India. He is deeply fascinated by Robotics and Artificial
Intelligence. He is also a chess aficionado, He likes studying chess classics
from the 1800 and 1900’s. He enjoys writing about science and technology as he
finds the intricacies which come with each topic fascinating.
