Ignition Coils and Drivers
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Ignition
coils are devices used to generate the high voltages needed to create a
spark that ignites fuel in an internal combustion engine, and for the
most part they're found in older cars, trucks and smaller low cost
vehicles like lawn tractors.
Fig. 1:
Ignition
Coil
For the purposes of this
discussion, the
term ignition coil specifically refers to three terminal devices
consisting of two coils of wire with a common connection which are
wound on a magnetic core.
Fig. 2: Ignition
Coil Schematic
These devices are particularly useful for creating very high voltages
and very long arcs; unfortunately, there's a lot of bad information on
them out on the internet and I hope to clear some of that up here. On
the other hand, there's also a lot of good information out there too,
and I'll try to avoid repeating too much of it. Hopefully, this
discussion will give you enough information to understand exactly how
ignition coils work as well as a few ideas and projects to get you
started.In automotive
applications a common
ignition coil circuit consisted of
a voltage supply in series with a mechanical switch across the '+' and
'-' terminals, with the switch being opened and closed by a cam.
Fig. 3: Automotive
Application Circuit, Switch Closed
When the cam
closes the switch, current flows through the "low voltage"
winding, which has relatively few turns, and slowly increases the
magnetic flux in the core (stores energy.) When the cam opens the
switch, the current quickly decreases, causing the magnetic flux in the
core to rapidly decrease as well. Since the high voltage winding (which
has many more turns than the low voltage winding) is wound on the same
core, it sees the same rapid decrease in magnetic flux. You may
remember from earlier that the voltage across a winding is given by
Eq. 1: Voltage Across a
Winding With Changing Magnetic Flux
Where VHV
is
the voltage across the high voltage coil, NHV
is the number of turns in the high voltage coild and Φ is the magnetic
flux in the core. Since the voltage of the coil is related to the rate
of change in magnetic flux, and the magnetic flux is changing rapidly,
a large voltage is created at the "HV" terminal.
Let's look at this
process in more detail. First, consider the moment
at which the switch closes. It's fair to assume that at that moment the
magnetic core will be completely demagnetized and the flux will be
zero. Closing the switch will apply a voltage to the coil which will
cause a magnetic flux to build up over time.
Eq.2 :Magnetic Flux
Resulting from Low Voltage
Winding
Where Φ is the magnetic flux in the core, VLV is the voltage across the
low voltage winding, and NLV is the number of turns in the
low voltage winding.
At some point in time
the cam will open the switch again, and this is
where things get a little more interesting. At this moment there is
some quantity of magnetic flux stored in the magnetic core which
represents some stored energy, and therefore cannot simply dissappear.
Rather, there must be some mechanism for dissipating this energy and
reducing the flux back to zero. In order to accomplish this, a large
negative voltage appears across the low voltage winding, which by Eq. 2
indicates a decreasing magnetic flux over time. The key to the
operation of the ignition coil is in the fact that the large negative
voltage which appears at the low voltage winding results in a very
rapid decrease in flux.
Since the high voltage
winding is on the same core, it will experience
the same rapid decrease in magnetic flux. Substituting Eq. 2, which
gives the change in flux over time, into Eq. 1, which gives the voltage
of the high voltage winding as a function of flux, will give the
voltage across the high voltage winding due to the voltage applied to
low voltage winding. This substitution give the following result.
Eq. 3: Transformer
Action Between Low and High
Voltage Windings
Notice that
this
equation describes transformer action, where the
voltage that appears at the high voltage winding is equal to the
voltage across the low voltage winding multiplied by the turns ratio of
the two windings. This has the effect of multiplying the large voltage
that appears at the low voltage winding in order to reset the core by
the turns ratio of the two windings. With a turns ratio of 100:1 and a
reset voltage at the low voltage winding of 300V, a voltage of 30 kV
will appear across the high voltage winding.
Fig. 4: Automotive
Application Circuit, Switch Open
I have not yet
seen a
good explanation elsewhere as to what factors
control the low voltage winding reset voltage. I threw out 300 V just
to put some numbers on the problem, but how is that voltage actually
determined? The short answer is that when the low voltage winding is
disconnected the voltage across it, and consequently the voltage across
the high voltage winding, will increase until the energy finds
somewhere to go. The particulars of the voltage that this happens at,
the path the energy takes and the duration of the event are all
dependent on the circuitry that's connected to the coil. Here are a few
examples of some of the more common circuits.
Let's start with the
circuit shown in Fig. 3 and Fig. 4. There is an
important omission that differentiates this circuit with those found in
automobiles which drastically reduces the performance of the circuit as
shown. At the moment when the switch starts to open (as in Fig. 4) an
increasing voltage potential occurs across the switch, which quickly
breaks down to form an arc at a relatively low voltage. The arc voltage
then becomes the force causes the magnetic flux to fall, and at a rate
much lower than we would like. Of course, this causes the output
voltage to be much lower as well, and a weak arc (or no arc at all)
occurs.
Fig. 5: Low Performance
From Ignition Coild Due to
Switch Arcing
The actual
circuit used
in automotive ignition systems includes a
capacitor across the switch in order to prevent arcing and increase
performance. While the switch is closed there is very little voltage
across it (perhaps a few milivolts) which causes the capacitor to
completely discharge. At the moment when the switch begins to open the
capacitor forces the voltage across it to remain at zero (or very
nearly zero) volts, preventing an arc from forming. The capacitor
provides another important function in this circuit by providing a path
for the low voltage winding current to flow when the switch opens.
Fig. 6: Improved
Performance with Capacitor Across
Switch
So how
exactly, besides
preventing the switch from arcing, does the
capacitor increase the voltage at the low voltage winding? When the
switch opens all of the current flowing in the low voltage winding
begins to flow through the capacitor, until all of the energy stored in
the inductor is transfered to the capacitor. The equation for the
energy stored in each components is given below.
Eq. 4: Energy Stored in a A) Capacitor and
B)
Inductor
These
equations can be
used to estimate the maximum voltage achieved
across the low voltage winding. Assume that the low voltage winding
inductance is 5 mH, the peak current in the low voltage winding is 5 A,
the inductor does not saturate, and we'd like 300 V across the low
voltage winding. Using equation 4B we can calculate that 5 mH carrying
5 A stores 62.5 mJ of energy. If this energy is completely transfered
to the capacitor, we can use equation 4A to calculate that we need a
capacitance of 1.39 uF to reach 300 V. The actual voltage will be
limited if either the low voltage or high voltage windings are able to
create an arc before the capacitor is fully charged.
That largely covers how
an ignition coil operates in a vehicle, but
chances are that most non-vehicular applications would preferably not
include a cam and mechanical switch. The switch's function can easily
be replaced by a transistor or MOSFET, but some care is required in
designing the circuit so as not to damage any of the relatively
delicate semiconductor devices. I've seen a few places on the internet
where the author declares semiconductor switches unreliable for
ignition coil applications, but this is not so. There is no reason why
a semiconductor switch can't be used reliably in this application, and
it's more likely that their circuit was poorly designed.
First, observe what
happens when the mechanical switch is simply
replaced with a MOSFET. The switch is moved to the low side of the
coil, but this doesn't change the operation of the circuit, it just
makes it easier to drive the MOSFET's gate.
Fig. 7: Ignition Coil
With Mosfet Switch
That's simple
enough -
the switch is replaced by a MOSFET (with the
body diode and parasitic capacitor shown,) but how is it different than
using a mechanical switch? There are two main differences. First, the
capacitance across the MOSFET is tiny compared to capacitors typically
used with mechanical switches. A 400 V 10 A MOSFET might have a
capacitance on the order of 100 pF, or about 10,000 times smaller than
the capacitor in the earlier automotive example. The practical
implication of this is that the voltage across the MOSFET will increase
much more quickly, and to a much higher voltage than in the automotive
application. Using the example of 5 mH and 5 A, the voltage across the
MOSFET would theoretically rise to 1.25 GV (1,250 Million Volts!) Now,
obviously those sort of outrageous voltages would never be reached.
Something else in the circuit would give way well before the voltage
reached that high (I'm looking at you, MOSFET.) The other difference,
which seems pretty obvious is that there is no longer a mechanically
opening contact. In a mechanical system with uncontrolled voltage
build-up an arc would form across the switch and dissipate all of that
energy. There is no such safety mechanism with the MOSFET.
So what would happen in
the MOSFET circuit? Well, there are two
possibilities. The best case would be that an arc would jump between
two points in the circuit that are close together before the voltage
was able to build up to a level where anything in the circuit was
damaged. More likely, and more damaging, would be that the voltage
across the MOSFET would build up to the breakdown voltage and the
MOSFET would begin to conduct in an event called "avalanche." A large
enough avalanche will cause significant damage to a MOSFET, or destroy
it completely in one shot. Despite the potential damage caused, many
MOSFETs can withstand some level of avalanche safely, and if the
circuit is designed correctly, may be able to withstand repetitive
avalanches.
The key parameters to
designing a circuit that can withstand avalanche
are usually listed on a MOSFET's datasheet as "Single Pulse Avalanche
Energy" and "Repetitive Avalanche Energy" (if the MOSFET is avalanche
rated at all.) These parameters specify the maximum energy the MOSFET
can safely absorb without damage. A datasheet might list the single
pulse avalanche energy for a particular switch to be 500 mJ. In this
case, equation 4B would be used to compute the the amount of energy
stored in the inductance that will be switched off and compared to the
datasheet value. If this hypothetical device were used to switch the 5
mH, 5 A inductor in the earlier example (which we calculated to be 62.5
mJ) we would expect the switch to survive a single avalanche. This same
device might have a repetitive avalanche energy of 15 mJ, in which case
there would be no garauntee that the device would survive or operate
correctly after multiple avalanches. In either case, the MOSFET will be
absorbing the energy of the avalanches, and may heat up substantially.
Care must be taken to ensure that the avalanche energies do not exceed
the datasheet specifications AND that the device will stay within the
datasheet temperature specifications.
Of course, allowing the
MOSFET to operate in avalanche mode may not be
the best option, and there are other ways to limit the voltage it
experiences. The most obvious method is to add an external capacitance
across the MOSFET, just like with the mechanical switch. The
capacitance can be sized, as before, to ensure a certain voltage or
prevent the circuit from exceeding a certain voltage. A more robust
solution is to add a capacitor in series with a resistor (typically
called a "snubber.") This
allows the energy absorbed by the
capacitor
to be dissipated in the external
resistor, rather than the MOSFET
itself. It also prevents a large current spike from the capacitor
through the MOSFET when it first switches on. There are other types of
snubber circuits as well, some of which various combinations of
resistors, capacitors, and diodes.
Those
are the basics of ignition coils
that I wanted to discuss. Here
are some additional pages with more detailed information on several
ignition coil related subjects.
How
do you figure out what's inside your ignition coil without cutting it
in half and counting wires? This is another topic I've seen a lot of
bad information on. Correctly measuring the various parameters of an
ignition coil is a bit tricky, but it's not very complicated as long as
you have a good idea how to interpret the measurements you take.
Here's
a no-frills ignition coil driver project based on a simple MOSFET
circuit. A design spreadsheet is included if you'd like to modify it
for your own purposes. Schematics, PCB layouts, and a parts list are
also included.
This project is somewhat
more involved than the simple project. It also features better
performance and more flexible control.A design spreadsheet is
included if you'd like to modify it for your own purposes. Schematics,
PCB layouts, and a parts list are also included.
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Questions?
Comments? Suggestions? E-Mail me at MyElectricEngine@gmail.com
Copyright 2007-2010 by Matthew Krolak - All Rights
Reserved.
Don't copy my stuff without asking first.
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