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APPLICATION NOTE
CAPACITIVE DISCHARGE IGNITION
A. BREMOND P. MERCERON
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.
Fig. 1 : General CDI block diagram
HV
SUPPLY
CAPACITOR
SWITCH
IGNITION COIL
SPARK PLUG
SENSOR CONDITIONING
(1) CDI : Capacitive Discharge Ignition
AN819/0695 1/8
APPLICATION NOTE
Capacitor The capacitor between 0.47 and 2F 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. Fig. 2 : First CDI topology 3. HOW DOES IT WORK ? Two different topologies are possible. 3.1. First topology Figure 2 shows the first possibility of discharge circuit. - 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.
C Ith HV SUPPLY SCR Ig Id D IPR
PR
SE
SPARK PLUG
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APPLICATION NOTE
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. Fig. 3 : Discharge current through the circuit The SCR conducts during all the positive phases of the discharge current while the diode D acts for the negative parts.
00-
SCR CURRENT 10A/div. DIODE CURRENT 10A/div.
CURRENT THROUGH THE IGNITION COIL PRIMARY 10A/div. 0-
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.
Fig. 4 : Second CDI topology
C Ith HV SUPPLY SCR Ig
D
PR Id IPR
SE
SPARK PLUG
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APPLICATION NOTE
Fig. 5 : Discharge current through the circuit
0-
SCR CURRENT 10A/div.
DIODE CURRENT 10A/div. 0-
CURRENT THROUGH THE IGNITION COIL PRIMARY 10A/div.
0-
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.
Fig. 6 : Small motor DCI principle circuit
STOP
D1
D2
C1
SUPPLY COIL
SPARK D3 SCR PLUG
FLY WHEEL
R SENSOR C2 D4
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APPLICATION NOTE
Fig. 7 : Sensor coil signal
5V/div. 01ms/div.
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. Fig. 8 : Conditioning stage
Due to parasitic voltage occurring on the sensor signal we suggest the use of the conditioning stage shown in figure 8.
C2 R R1 Vs C
This gate drive circuit, using R1 between 1 and 10k and C1 between 1 and 10F allows the SCR to run without problems of parasitic firing.
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APPLICATION NOTE
5. SMALL MOTOR APPLICATION CIRCUIT Figures 9 and 10 show the same application circuit using bot h ICC01 and ICC03. In both cases the conditioning stage limits the current through the gate, its maximum value being calculated as follows : Fig. 9 : CDI using ICC03 R max = (VS min - VGT max) / 2 IGT max with Vs = sensor voltage.
STOP 1 D1 8 SUPPLY COIL D2 6-7 SPARK PLUG
ICC03
C1
D3 4 3
SCR
FLY WHEEL
SENSOR
CONDITIONING
Fig. 10 : CDI using ICC01
STOP 2 D1 15 SUPPLY COIL 12 D3 5 4 6-11 SCR D2 3-14 SPARK PLUG
ICC01
C1
FLY WHEEL
SENSOR
CONDITIONING
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APPLICATION NOTE
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 C 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 theC.
Fig. 11 : Engine management module block diagram
+12V
PROTECTION
VOLTAGE REGULATOR
KNOCK SENSOR
CONDITIONING
SIGNAL
C
INJECTION
INTERFACE
TEMPERATURE PRESSURE
IGNITION
CRANK MARKER
INTERFACE
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APPLICATION NOTE
The circuit of figure 12 is an example of a CDI for an automotive engine. This uses two SCR directly triggerable by C outputs. Fig. 12 : Example of CDI circuit for automotive
+400V DC / DC CONVERTER C TH R1 R2 D
-12V
CRANCK SHAFT ANGLE SENSOR
P
TH R'1 D
R'2
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 consequences of use of such information nor for any infringement of patents or other rights of third parties which may result from its use. No license is granted by implication or otherwise under any patent or patent rights of SGS-THOMSON Microelectronics. Specifications mentioned in this publication are subject to change without notice. This publication supersedes and replaces all information previously supplied. SGS-THO MSON Microelectronics products are not authorized for use as critical components in life support devices or systems without express written approval of SGS-THOMSON Microelectronics.
(c) 1996 SGS-THOMSON Microelectronics - Printed in Italy - All rights reserved. SGS-THOMSON Microelectronics GROUP OF COMPANIES Australia - Brazil - Canada - China - France - Germany - Hong Kong - Italy - Japan - Korea - Malaysia - Malta - Morocco The Netherlands - Singapore - Spain - Sweden - Switzerland - Taiwan - Thailand - United Kingdom - U.S.A. 8/8
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