CAPACITIVE DISCHARGE IGNITION

1. INTRODUCTION
In the world of small motor, such as mopeds or
lawn movers the ignition system design is based
exclusively on CDI (1). In automobile CDI was in
the past only used in the replacement module market.
Today, due to new standards of pollution control,
the CDI system is becoming one of the most
efficient choice available.
The purpose of this paper is to analyze the behavior
of the CDI, the solutions we propose today in
small motor applications and the state of the art in
automotive.
2. CDI PRINCIPE
The spark necessary to ignite the air/petrol mixture
in the combustion chamber is produced by the CDI
module. This system consists of 7 stages.
HV supply
The HV supply differs from small motors to automobiles.
The small one or two cylinder motors one
or two stroke have a fly-wheel which includes a
supply winding. This coil produces, after rectification,
a positive voltage variable between 100V and
400V.
The HV for an automobile CDI is supplied by a
DC/DC converter. This stage produces generally
400V from the 12V battery voltage.
HV
SUPPLY
SENSOR
CAPACITOR SWITCH IGNITION
COIL
SPARK
PLUG
CONDITIONING
Fig. 1 : General CDI block diagram
(1) CDI : Capacitive Discharge Ignition
1/8
Capacitor
The capacitor between 0.47 and 2mF is used
firstly, to store the charge from the HV supply.
During the second phase of the ignition cycle the
capacitor is discharged through the ignition circuit.
Switch
The switch transfers the energy stored in the capacitor
to the primary of the ignition coil. This function
is carried out by a SCR or a triac. The switch is
generally linked to a diode for the reverse current.
Sensor
The goal of the sensor is to synchronize the spark
with the engine rotation. For the small motor the
sensor detects a bump at each engine revolution.
For car modules the sensor system gives a pulse
for each cylinder ignition point.
Conditioning
The conditioning is a very important stage which
must assume the following functions :
- Optimisation of the SCR gate current for all the
RPM range.
- Filtering of parasitic strikes occurring on the
sensor signal.
- For the most sophisticated small engine and all
the car systems, it has to ensure the correct
lead angle.
This stage is realised using few passive components
for small motor modules, while for automotive
management systems a microprocessor is
needed.
Ignition coil
The ignition coil is a step up transformer which delivers
high voltage to the spark plug. This value can
be between 5 and 20kV depending on the working
conditions.
Spark plug
The spark plug is the final element of the ignition
chain. High engine efficiency and a complete gas
combustion are linked to a good spark quality .
Generally we estimate a minimum of 20 millijoules
is necessary at spark plug.
3. HOW DOES IT WORK ?
Two different topologies are possible.
3.1. First topology
Figure 2 shows the first possibility of discharge
circuit.
Ig
Id
D
SCR
Ith
C
PR
IPR
SE
SPARK
PLUG
HV
SUPPLY
Fig. 2 : First CDI topology

â
APPLICATION NOTE
2/8
When spark is needed a current Ig is injected to the
SCR gate which then fires the SCR. The SCR firing
initiates the capacitor discharge which generates
an alternative current.
The SCR conducts during all the positive phases of
the discharge current while the diode D acts for the
negative parts.
0 -
0 -
0 -
SCR CURRENT
10A/div.
DIODE CURRENT
10A/div.
CURRENT THROUGH
THE IGNITION COIL
PRIMARY
10A/div.
Fig. 3 : Discharge current through the circuit
3.2. Second topology
In the topology shown by figure 4 the SCR acts
during the first part of the current cycle until the capacitor
voltage reverse.
Then the free wheeling diode D conducts as long
as there is energy remaining on the primary coil.
Ig
Id
SCR D
Ith
C PR
IPR
SE
SPARK
PLUG
HV
SUPPLY
Fig. 4 : Second CDI topology
â
APPLICATION NOTE
3/8
0 -
0 -
0 -
SCR CURRENT
10A/div.
DIODE CURRENT
10A/div.
CURRENT THROUGH
THE IGNITION COIL
PRIMARY
10A/div.
Fig. 5 : Discharge current through the circuit
4. SMALL ENGINE SYSTEM
Figure 6 shows the topology we have chosen for
the small motor CDI module.
The supply coil generates an alternating voltage, in
which the positive parts are rectified by D2 and the
negative parts are clamped by D3. This circuit configuration
allows the designer to use 400V
diodes for D2 and D3 instead of 1000V user in
other design. The capacitance C1 is loaded by the
positive rectified current.
FLY WHEEL
D3
D1 D2 C1
SPARK
PLUG
SCR
R
C2 D4
SUPPLY
COIL
SENSOR
STOP
Fig. 6 : Small motor DCI principle circuit
â
APPLICATION NOTE
4/8
0 -
5V/div.
1ms/div.
Fig. 7 : Sensor coil signal
The sensor coil generates, at each engine revolution
a signal as shown in figure 7.
The negative part of this signal is clamped by D4
while the positive part produces a current through
the gate, firing the SCR.
Due to parasitic voltage occurring on the sensor
signal we suggest the use of the conditioning stage
shown in figure 8.
R
R1
C2
Vs C
Fig. 8 : Conditioning stage
This gate drive circuit, using R1 between 1 and
10kW and C1 between 1 and 10mF allows the SCR
to run without problems of parasitic firing.
â
APPLICATION NOTE
5/8
5. SMALL MOTOR APPLICATION CIRCUIT
Figures 9 and 10 show the same application
circuit using both ICC01 and ICC03. In both
cases the conditioning stage limits the current
through the gate, its maximum value being
calculated as follows :
R max = (VS min - VGT max) / 2 IGT max
with Vs = sensor voltage.
FLY WHEEL
D3
D1 D2 C1
SPARK
PLUG
SCR
SUPPLY
COIL
SENSOR
STOP
1
8
4 3
6-7
ICC03
CONDITIONING
Fig. 9 : CDI using ICC03
FLY WHEEL
D3
D1 D2 C1
SPARK
PLUG
SUPPLY SCR
COIL
SENSOR
STOP
2
3-14
12
5 4
ICC01
CONDITIONING
15
6-11
Fig. 10 : CDI using ICC01
â
APPLICATION NOTE
6/8
6. CAR CDI SYSTEMS
Figure 11 shows the general block diagram of
an automobile engine management module. In
such a system the ignition function is linked to
a mC which assumes also the other functions
needed for a correct operation of the engine.
For example the crank shaft angle detector is not
directly connected to the CDI switch but conditioned
by the mC.
+12V
SIGNAL
CONDITIONING
μC
IGNITION
INTERFACE
INJECTION
INTERFACE
CRANK MARKER
KNOCK SENSOR
TEMPERATURE
PRESSURE
PROTECTION VOLTAGE
REGULATOR
Fig. 11 : Engine management module block diagram
â
APPLICATION NOTE
7/8
The circuit of figure 12 is an example of a CDI for
an automotive engine. This uses two SCR directly
triggerable by mC outputs.
+400V
C
DC / DC
CONVERTER
TH
R1 D
R2
TH
R'1 D
R'2
-12V
CRANCK SHAFT
ANGLE SENSOR
μP
Fig. 12 : Example of CDI circuit for automotive
7. CONCLUSION
Capacitive discharge ignition systems are
the only choice for the small engines. They
are also found frequently in racing car engine
management computer. For the future
the CDI could be the solution meet the new
anti-pollution standard requirements.
Information furnished is believed to be accurate and reliable. However, SGS-THOMSON Microelectronics assumes no responsability for the
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in this publication are subject to change without notice. This publication supersedes and replaces all information previously supplied.
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© 1996 SGS-THOMSON Microelectronics - Printed in Italy - All