Why Electronics Need Cooling | transistor heat sink
Learn why electronic component cooling is critical to the design of
electronic circuit boards. We’ll look at the different options available
as well as how to simulate and virtually test the performance of cooling systems, using computational fluid dynamics.
All of our electrical devices are built by combining different
electronic components together. Each component has a specific function.
Take this very simple lighting circuit for example. The battery provides
the electrical energy, the LED produces light from the energy supplied
by the battery and the resistor protects the LED by reducing the current
in the circuit. If we remove the resistor the LED will instantly burn
out.
Because the resistance of the circuit reduced, meaning it becomes
much easier for more electrons to flow from the battery and through the
LED component which you can only see under a microscope.
They can only cope with a certain amount of electrons flowing through
them, otherwise they will burn out. On the other hand an electrical
cable is much thicker so it can handle far more current flowing through
it. That’s why we have different sized cables, to handle different
amounts of electrical current.
Coming back to the resistor, this is essentially adding a restriction
to the flow of electrons. It’s like having a kink in a water pipe, the
kink restricts how much water can flow through the pipe and the water is
now colliding with the pipe wall so it wastes energy and will result in
a pressure drop. As we know, pressure is like voltage and the resistor
is like the kink in the pipe. So, when we add a resistor to the circuit,
we restrict the current or the amount of electrons flowing and we get a
voltage drop.
Why Do We Get a Voltage Drop?
Well if we look at a normal copper wire, this is made from millions
and millions of copper atoms. Copper is a conductor which means the
copper atoms have an eletron which is free to move around between other
atoms. They do move naturally to other atoms, but randomly in any and
all directions. If we apply a voltage difference across the wire, the
voltage difference or pressure of the battery will force electrons to
flow through it.
But with a resistor, the material is less conductive and creates a
harder path for the electrons to flow through. The electrons are going
to collide and as they collide their energy is converted into heat. So
the energy of the battery is really being wasted and turned into heat.
Because the energy of the battery is being removed by the resistor, we
get a voltage drop.
That’s why when we look at the resistor through a thermal imaging camera, we can see it is generating heat.
Some components such as mosfets and igbt’s will produce a lot of heat.
Take this cheap bench power supply for example.
It has 4 mosfets inside, 2 here:
And 2 here:
If we remove the heat sink, we’ll look at that part a little later,
and then power a small DC circuit with about 1.2A we can see with the
thermal imaging camera that these components very quickly reach 45
degree Celsius and that’s with the fan on. We cut the power here because
we don’t want to damage them.
All electronic components have a thermal limit or a maximum operating
temperature. When they reach or exceed this certain temperature, they
will breakdown and potentially destroy the circuit board.
For some components like a fuse, this is desirable because the
material breaks and this instantly cuts the power to the circuit, which
does help to prevent component damage but it also completely stops the
circuit board from working until the fuse is replaced.
With components like an IGBT, the build-up of heat isn’t a good thing
because as they increase in temperature, they become unreliable and the
current passing through them increases. This additional current creates
more heat which in turn allows more current to flow, so the component
reaches thermal runaway and will eventually just destroy itself. So, to
increase the life span of the components and circuit board, as well as
keep the component operating in a stable, reliable condition, we need a
way to remove the thermal energy it generates.
How to Remove Thermal Energy From Electronic Components?
Some components such as this simple resistor LED circuit will operate
fine in ambient conditions, they do not produce much heat and any heat
they do produce will dissipate into the ambient air.
When the heat starts to increase, we can use a simple fan to blow air
across the component. The moving air will pick up and carry more heat
away. This is the method used on PC’s and that’s why there is a fan
inside to literally remove heat from the internal components.
But there is a problem with this method, we’re blowing the heat off
one component and this hot air then passes across other components, so
we are cooling down one component but if we’re not careful with the
design we will heat up others.
We usually need a more effective way to get the heat out of the
component and a popular method is to use a heat sink to provide passive
cooling. This heat sink is usually an aluminium or “Aluminum” (US) block
which has lots of fins. The fins help to increase the surface area of
the component to allow exposure to more ambient air. The heat sink is
made from metal because it conducts heat well, much better than air. So,
by making it easier for more heat to escape and increasing the exposure
to air, we effectively cool down the component. There’s a limit to how
much we can remove with this method though.
The next stage is to attach the component to a heat sink and then use
a fan to blow ambient air over the component and heat sink, to increase
the heat removal. That’s exactly the method used in this DC bench power
supply. The fan and heat sink are combined to remove the excess heat.
You can see the heat is dissipating out through the heat sink and when we cut the power but leave the fan running, it drops in temperature very quickly.
Another method that’s most commonly used in laptops is to use a heat
pipe. That’s the strange orange bar that you’ll see inside your laptop
running between the processor and the fan.
Inside this is a small amount of liquid and a wick. The heat of the
processor is absorbed into the pipe and this heat causes the liquid
inside to boil and evaporate, the vapour moves towards the opposite end
which is cooler because the fan is blowing air across the surface and
this removes the heat from the heat pipe. This removal of heat causes
the vapour to condense back into a liquid and this liquid flows back
along the wick to pickup more heat, and so the cycle repeats.
These again have a performance limit and to increase the heat removal
we have to start using these huge units, which take up a lot of space
and again blow the heat over other components.
The next stage for maximum cooling is to use water, or liquid
cooling. You may have seen many high spec gaming computers now start to
use a water-cooling system to remove heat from their CPU and GPU.
We basically have a small pump which cycles water between the heat
exchanger of the CPU, known as the water block, and the radiator, which
is a heat exchanger with some fans. Again the fans will blow air across
the heat exchanger and remove the unwanted heat from the water, so the
water picks up the unwanted heat from the chip, carries this to the
radiator and then flows through the heat exchanger of the radiator. As
it flows through, the fans blow air across the outside which removes the
unwanted heat. The water therefore leaves cooler and returns to the
chip to pick up more heat.
The reason this method is so efficient is because water has a
substantially higher heat capacity than air. So, it can pick up more
heat. Rather than pushing air across the fins and blowing the heat
across other components, the water-cooled system is collecting the heat
and moving this away, then rejecting it completely from the system.
This method is increasingly used in power electronics especially in
high power applications where we often find these banks of IGBT’s. These
generate vast amounts of heat and need to operate reliably for long
periods of time.
As we saw with the bench power supply the IGBT’s were spaced out and
take up a lot of room. So instead, what we can do is mount these to a
thermal block which is basically a heat sink or heat exchanger that
water flows through instead of air. As the IGBT’s generate heat, this
will pass through the block and into the water. Between the IGBT’s and
the thermal block we have a thick layer of thermal paste which just
helps to increase the transfer of heat. Inside the block we have these
fins to help increase the surface area of the heat exchanger and
maximise the exposure to the cooling water to remove the heat.
We want to ensure that these particular IGBT’s do not exceed 90 degrees Celsius (194 F).
Thanks for reading this far.
Happy Repairing!
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