Ignition system for internal combustion engines

Abstract: An ignition control apparatus comprises: an overvoltage protection circuit (10) having a thyristor (11) connected to an input terminal (B), for preventing an overvoltage from passing to subsequent circuits by shutting off the thyristor (11) when an overvoltage is input thereto; a DC-DC converter (20A) having a field effect transistor (22) and a gate drive circuit (23A) for driving the gate thereof, connected in series to the overvoltage protection circuit (10), for stepping up and outputting the output voltage from the overvoltage protection circuit (10); a parallel connection section (50) comprising a diode (51) and a resistor (52) for connecting the input terminal (B) and the gate drive circuit (23A), but not through the overvoltage protection circuit (10); a capacitor (30) which is charged by an output from the DC-DC converter (20A); and a thyristor (40) for discharging an electrical load charged into the capacitor (30), in accordance with a trigger pulse from an external section.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an ignition system for internal combustion engines ideally suited for use with small size vehicles such as motor cycles. 2. Description of the Related Art Heretofore, for the ignition control apparatus of an internal combustion engine (referred to hereunder as an engine) of a motor cycle, an AC-capacitor discharge ignition (referred to hereunder as an AC-CDI) has been widely used. With capacitor discharge ignition, the charge of a capacitor is discharged rapidly, and the charge current input to the primary winding of an ignition coil to thereby generate a high voltage in the secondary winding to cause a spark at the spark plug. With an AC-CDI, the high voltage for charging the capacitor is obtained from an AC voltage generated in an excitor coil housed in an AC generator. This AC generator is provided for supplying power to the battery and other electrical loads, and is driven by the engine crank shaft. With the recent advances in semiconductor technology however, a DC-CDI (DC-capacitor discharge ignition) which increases the voltage of the battery power source, for example a 12V DC power source, using a DC-DC converter to thus produce a high voltage to charge the capacitor, can be used instead of the AC-CDI. If a DC-CDI is used, the excitor coil becomes unnecessary, as does the means for taking out the output from the excitor coil, and hence an improvement in reliability, and miniaturization of the system is possible compared to when an AC-CDI is used. FIG. 4 is a circuit diagram showing a configuration for a conventional DC-CDI and related parts, applicable to small size motor cycles such as power assisted bicycles. This circuit was designed by the present applicant to assist in explaining the problems to be solved by the present invention. The apparatus shown in FIG. 4, incorporates a DC-CDI 1, an AC generator 2 (only the windings shown in FIG. 4) which is connected to an engine (not shown in FIG. 4), a voltage regulator 3 (REG-REC) for rectifying and voltage regulating an output from the AC generator 2, a battery 4 (BAT) connected to an output from the voltage regulator 3, an on-off switch 5 with one terminal connected to an output terminal from the voltage regulator 3, and which turns on and off according to the operation of a brake pedal or a brake lever, a stop lamp 6 (S/L), an ignition coil 7 with a primary winding 7a connected to an output terminal I from the DC-CDI 1, and a spark plug 8 connected to a secondary winding 7b of the ignition coil 7. The output terminal from the voltage regulator 3, the positive terminal of the battery 4, and the one terminal of the switch 5, are connected to an input terminal B of the DC-CDI 1. Furthermore, the AC generator 2, the battery 4, the stop lamp 6, the ignition coil 7 and the spark plug 8 have their respective other terminals connected to earth. The DC-CDI 1 comprises; an overvoltage protection circuit 10, a DC-DC converter 20, a thyristor 40, and a capacitor 30. In operation, a DC voltage input from the input terminal B is stepped up and charges the capacitor 30. The thyristor 40 is then fired in accordance with a trigger signal supplied to a gate terminal 40 G from an external section (not shown in FIG. 4), thereby causing a discharge current to flow in the primary winding 7a of the ignition coil 7 connected to the output terminal I. The overvoltage protection circuit 10 comprises for example as shown in FIG. 4: a thyristor 11 with the anode connected to the terminal B; a resistor 12 with one end connected to the terminal B; a zener diode 13 with the cathode connected to the other end of the resistor 12 and the anode connected to earth; a diode 14 with the anode connected to the cathode of the zener diode 13 and the cathode connected to the gate of the thyristor 11; a resistor 15 connected between the gate and the cathode of the thyristor 11; and an electrolytic capacitor 16 with a positive electrode connected to the cathode of the thyristor 11, and a negative electrode earthed. The overvoltage protection circuit 10 is for protecting the circuits in subsequent stages from an overvoltage, normally around 80 - 100 volts, due for example to load dump surge and other such spike voltages generated by the AC generator 2, when for example the positive terminal of the battery 4 is disconnected. In operation, when the terminal voltage of the electrolytic capacitor 16 becomes greater than the sum of the zener voltage of the zener diode 13, the forward voltage of the diode 14, and the forward voltage between the gate and the cathode of the thyristor 11, then the thyristor 11 is switched off. In general, with the overvoltage protection circuit 10, the constants for the various elements are set so that the thyristor 11 goes off when a voltage in excess of around 20V is input. The DC-DC convertor 20 comprises: a step-up transformer 21 comprising a primary winding 21a and a secondary winding 21b; an FET (field effect transistor) 22 with the drain connected to one terminal of the primary winding 21a, and the source earthed; a gate drive circuit 23 for high frequency drive control of the gate of the FET 22; a resistor 24 connected between the gate of the FET 22 and the terminal of the primary winding 21a which is not connected to the FET 22 and the cathode of the thyristor 11 of the overvoltage protection circuit 10; and a diode 25 with the anode connected to one terminal of the secondary winding 21b. The other terminal of the secondary winding 21b is earthed, while the cathode of the diode 25 is connected to the anode of the thyristor 40 and to one terminal of the capacitor 30. The gate drive circuit 23 comprises an oscillator 231 and an overvoltage protection zener diode 232. The oscillator 231 generates a continuous pulse of a predetermined frequency, and applies this between the gate and the source of the FET 22 to thereby control the on/off switching of the FET 22. In this case, the gate of the FET 22 is always pulled up by the DC voltage output from the overvoltage protection circuit 10 via the resistor 24. Therefore, the FET 22 comes on when for example, the output level of the oscillator 231 is in a high impedance (off) condition, and goes off when the output level of the oscillator 231 is in a low impedance (on) condition. The oscillator 231 can be constructed for example as a self-excitation type circuit using a compound winding in the step-up transformer 21 (not shown in the figure), or as a separate excitation type circuit using a separate CR oscillator. Moreover, commutation failure in the firing period for the thyristor 40 and in the period for the thyristor 40 to go fully off (commutation turn off time), can be prevented by pausing the oscillator 231. With the DC-DC converter 20 constructed as described above, the FET 22 is switchingly driven so that a current in the form of a pulse train flows in the primary winding 21a of the step-up transformer 21, and a stepped-up AC voltage is generated between the terminals of the secondary winding 21b. The output from the secondary winding 21b is then half wave rectified by the diode 25, and the half wave rectified current then charges the capacitor 30. A description will now be given concerning the minimum drive voltage for the above-mentioned DC-CDI 1 shown in the FIG. 4, that is to say the minimum input voltage required to produce a spark at the spark plug. The following description is given with the terminal T1 of the thyristor 11 in FIG. 4 as the anode terminal, and the terminal T2 as the cathode terminal. The minimum voltage required to operate the DC-DC converter 20 is determined by the ON voltage existing between the gate and the source of the FET 22. In the case where general circuit components are used, then the voltage between the terminal T2 including the resistor 24, and the ground is approximately 1.5V. On the other hand, the minimum voltage necessary to operate the overvoltage protection circuit 10 is determined for the thyristor 11 to go from off to on at start-up, by the forward voltage for the diode 14, the gate voltage, and the resistance values of the resistors 12 and 15, and is typically around 2.7V between the terminals T1 and T2. However, once the thyristor 11 has started, then the voltage between the terminals T1 and T2 becomes equal to the ON voltage of the thyristor 11 at around 0.8V. Consequently, at start-up a voltage of approximately 4.2V (1.5V + 2.7V) between the terminal T1 and the ground, that is the terminal voltage of the input terminal B, becomes the minimum operating voltage for the DC-CDI 1. With a small size motor cycle, normally a kick pedal as well as a self starter is provided as means for starting the engine. Therefore, with the circuit shown in FIG. 4, under conditions for example where the battery 4 has become discharged, has deteriorated, or the terminal of the battery 4 has become disconnected, then the self starter motor will not operate, and hence the rider uses the kick pedal to start the engine. At this time, instead of the output from the battery 4, the output from the AC generator 2 which is rotated with pushing down on the kick pedal, is supplied as the DC input power supply to the DC-CDI 1. When the engine is started using the kick pedal, in most cases the rider pushes down on the kick pedal while gripping the brake lever (with the switch 5 on). Consequently, when the AC generator 2 is driven by the kick pedal operation and starts generating power, the stop lamp 6 becomes an electrical load connected to the output from the AC generator 2. In this case, the output from the AC generator 2 cannot rise sufficiently for the input voltage to the DC-CDI 1 to attain the before-mentioned minimum operation voltage, thus resulting in the situation where the engine cannot be started. The following Table 1 shows examples of actual measured values for the input voltage to the DC-CDI 1, in relation to the operating force on the kick pedal, and the capacity of the stop lamp. The measured values in Table 1 were measured with the battery 4 removed. The downward force on the kick pedal is shown as medium kick for the average value for a woman, and as strong kick for the average value for a man. With the conventional DC-CDI as described above, if the input voltage at the time of pushing down on the kick pedal is insufficient, the various internal circuits of the DC-CDI cannot be started. Consequently, a spark cannot be produced at the spark plug, and hence the engine cannot be started. SUMMARY OF THE INVENTION In view of the above background, it is the object of the present invention to provide an internal combustion engine ignition control apparatus whereby starting is possible at a lower input voltage than has heretofore been possible with conventional apparatus. According to the present invention as claimed in claim 1, there is provided an internal combustion engine ignition control apparatus comprising: an overvoltage protection circuit having a power source input section for inputting power from an external section with a first semiconductor switching element connected thereto, which shuts of the first semiconductor switching element when an overvoltage is input thereto; a voltage step-up circuit having a second semiconductor switching element and a drive circuit for the second semiconductor switching element, for stepping up an output voltage from the overvoltage protection circuit; a connection section for connecting the power source input section and the drive circuit of the second semiconductor switching element in parallel with the overvoltage protection circuit but not via the first semiconductor switching element; a charging element which is charged by an output from the voltage step-up circuit; and a discharge circuit for discharging an electrical load charged into the charging element. Furthermore, according an embodiment of the present invention, there is provided an internal combustion engine ignition control apparatus comprising: an overvoltage protection circuit having a first semiconductor switching element connected to a power source input section for inputting power from an external section, which shuts off the first semiconductor switching element when an overvoltage is input thereto, so that the overvoltage does not pass; a voltage step-up circuit having a second semiconductor switching element and a drive circuit for the second semiconductor switching element, connected in series with the overvoltage protection circuit, for stepping up and outputting an output voltage from the overvoltage protection circuit; a parallel connection section for connecting the power source input section and the drive circuit for the second semiconductor switching element of the step-up circuit but not through the first switching element; a charging element which is charged by an output from the voltage step-up circuit; and a discharge circuit for discharging an electrical load charged into the charging element, according to instructions from an external section. Moreover, according to a further embodiment of the present invention there is provided an internal combustion engine ignition control apparatus comprising: an overvoltage protection circuit having a first thyristor connected to a power source input section for inputting power from an external section, for protecting subsequent circuits from an overvoltage by shutting off the first thyristor when an overvoltage is input thereto; a DC-DC converter having a voltage step-up transformer connected in series to an output terminal of the overvoltage protection circuit, and an FET gate drive circuit, for stepping up a DC voltage output from the overvoltage protection circuit and outputting this as a DC voltage; a parallel connection section constructed without a semiconductor switching element, for connecting the power source input section and the gate drive circuit, but not through the first thyristor; a capacitor which is charged by an output from the DC-DC converter; and a second thyristor which is fired in accordance with a control signal from an external section, to thereby discharge an electrical load of the capacitor. With the above construction, since the parallel connection section is connected to the power source input section and the drive circuit for the second semiconductor switching element, but not through the first semiconductor switching element, then the voltage step-up circuit can be started with a lower voltage than for the conventional arrangement. Moreover, in this embodiment, since the parallel connection section constructed without a semiconductor switching element is connected to the power source input section and the gate drive circuit but not through the first thyristor, and the DC-DC converter is constructed using a FET, then the value for the current necessary for the gate drive circuit to drive the gate can be reduced. Consequently the DC-DC converter can be started at a lower voltage than for the conventional arrangement, and structural miniaturization of the parallel connection section can be simplified. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a circuit diagram showing a circuit configuration for a DC-CDI and related parts, according to a first embodiment of the present invention; FIG. 2 is a circuit diagram showing a circuit configuration for a DC-CDI and related parts, according to a second embodiment of the present invention; FIG. 3 is a circuit diagram showing a circuit configuration for a DC-CDI and related parts, according to a third embodiment of the present invention; and FIG. 4 is a circuit diagram showing a circuit configuration for a conventional DC-CDI and related parts, considered by the present applicant. DESCRIPTION OF THE PREFERRED EMBODIMENTS A first embodiment of the present invention will now be described with reference to FIG. 1, which is a circuit diagram showing a configuration of a DC-CDI (internal combustion engine ignition control apparatus) according to a first embodiment of the present invention. In FIG. 1 parts corresponding to the respective parts in FIG. 4, are indicated by the same symbol and description is omitted. The DC-CDI 1A, DC-DC converter 20A and gate drive circuit 23 shown in FIG. 1 have respectively the same functions as the DC-CDI 1, the DC-DC converter 20 and the gate drive circuit 23 shown in FIG. 4. A parallel connection section 50 shown in FIG. 1 is newly provided in the DC-CDI 1A, in accordance with the present invention, in place of the resistor 24 shown in FIG. 4. The parallel connection section 50 comprises a diode 51 and a resistor 52 connected in series, and connects directly the input terminal B and the gate of the FET 22 and the gate drive circuit 23A, and not via the overvoltage protection circuit 10. As a result, with the present embodiment, the parallel connection section 50 is provided in parallel with the overvoltage protection circuit 10, and hence power source voltage is supplied directly to the gate drive circuit 23A from the input terminal B and not via the thyristor 11. Consequently, with the present embodiment, when the voltage at the input terminal B is greater than the total of the forward voltage of the diode 51 (voltage between terminals T1 and T3) of approximately 0.65 volts and the voltage between the terminal T3 and the ground of approximately 1.5 volts (the same as the voltage between the terminal T2 and the ground in FIG. 4), that is, approximately 2.15V, then the DC-DC converter 20A is ready to operate, that is to say, the FET 22 can conduct. The minimum power source voltage at the input terminal B necessary for start-up of the DC-CDI 1A, is thus that determined by the voltage necessary for starting the overvoltage protection circuit 10, that is to say, the minimum required power source voltage at input terminal B at the time of start-up of 2.7V. The above voltage values are examples of typical values at normal temperature. With the parallel connection section 50 in this case, the diode 51 is for protecting the gate drive circuit 23A and the gate of the FET 22 from an external surge of negative polarity. The resistor 52 operates together with the zener diode 232 inside the gate drive circuit 23A to protect the oscillator 231 and the gate of the FET 22 from an overload voltage of positive polarity. Since the resistor 52 provides the resistance for when the before-mentioned overvoltage of approximately 100V is absorbed by the zener diode 232, then preferably this has a relatively large resistance value in order to lower the necessary allowable surge rating for the zener diode 232. On the other hand, since the power for driving the gate of the FET 22 is supplied via the resistor 52, then if the resistance value is increased, there will be a problem with reduced switching speed for the FET 22. However, since the FET 22 is voltage driven type switching element, then even if the resistance value is relatively large there is no problem with the on/off operation itself. In absorbing a negative polarity surge it is also possible to use the forward characteristics of the zener diode 232. Moreover, with respect to the overload voltage between the drain and the source of the FET 22, since the protection given by the overvoltage protection circuit 10 works the same as with the conventional arrangement, then an element having the same specifications as that for the conventional arrangement can be used for the FET 22. With the present embodiment as described above, since the parallel connection section 50 is provided in parallel with the overvoltage protection circuit 10, then the minimum power source voltage required at the time of starting the DC-CDI 1A can be approximately 2.7V, and can thus be lower than the 4.2V for the conventional example described with reference to FIG. 4. Consequently, the DC-CDI 1A can be started, and a charging current output from the output terminal I, even under the conditions given in Table 1 with an electrical load applied to the AC generator 2 and the kick pedal pressed down with a medium force. As a result, in this case also a spark can be produced by the spark plug 8, and hence the engine can be started. A description of a second embodiment according to the present invention will now be given with reference to FIG. 2. In FIG. 2, a DC-DC converter 20B, a gate drive circuit 23B, and a parallel connection section 50B respectively corresponding to the DC-DC converter 20A, the gate drive circuit 23A, and the parallel connection section 50 shown in FIG. 1, constitute the feature of this embodiment. Other parts are constructed the same as those indicated with the same symbol in FIG. 1. The second embodiment differs from the first embodiment in that: the input side connection point for the parallel connection section 50B is a connection point inside the overvoltage protection circuit 10 between the resistor 12 and the zener diode 13; the parallel connection section 50B is made up of a resistor 52B only; and the gate drive circuit 23B is made up of the oscillator 231 only. That is to say, compared to the first embodiment, the respective surge absorbing diodes are omitted from the gate drive circuit 23B and the parallel connection section 50B. The resistance value for the resistor 52B may be the same as that for the resistor 52 of the first embodiment. Since the zener diode 13 normally has a sufficient allowable surge rating to meet the requirement for operating in the overvoltage protection circuit 10, then with the above-mentioned construction, both the positive and negative polarity surges can be absorbed by the zener diode 13, and hence the respective surge absorbing elements provided in the first embodiment can be omitted. With the present embodiment, compared to the first embodiment, the minimum drive voltage at the time of starting the gate drive circuit 23B is increased by the voltage drop across the resistor 12 due to power supply to the gate drive circuit 23B via the resistor 12. However, by omitting the diode from the parallel connection section 50B, then the minimum drive voltage at the time of starting the gate drive circuit 23B is reduced by the voltage drop for the diode. As a result, the minimum voltage required for the overall DC-CDI 1B at the time of starting is still approximately 2.7 V at the input terminal B. Next is a description of a third embodiment of the present invention with reference to FIG. 3. The difference of the DC-CDI 1C shown in FIG. 3 to the DC-CDI 1B shown in FIG. 2 is that the input side of the parallel connection section 50B is connected to the cathode side of the diode 14 inside the overvoltage protection circuit 10. With this embodiment, compared to the second embodiment, the voltage drop from the input terminal B to the gate drive circuit 23B is increased by the voltage drop for the diode 14. Hence the voltage required for the overall DC-CDI 1C at the time of starting is greater than for the first and second embodiments, at around 3.5V. However in this case also, operation can still be started at a lower voltage than the 4.2V required with the conventional apparatus described with reference to FIG. 4. The arrangement of the respective surge absorption elements in the first through third embodiments described with reference to FIGS. 1 through FIG. 3, is not necessarily limited to the above described arrangement. For example modifications are also possible such as; eliminating the diode 51 in FIG. 1, adding the zener diode 232 to the circuit examples in FIG. 2 and FIG. 3 in the same way as in FIG. 1, and adding elements such as capacitors to the respective portions.

Claims: An internal combustion engine ignition control apparatus comprising: overvoltage protection means (10) having a power source input section (B) for inputting power from an external section with first semiconductor switching means (11) connected thereto, which shuts off said first semiconductor switching means when an overvoltage is input thereto; voltage step-up means (20A) having second semiconductor switching means (22) and drive means (23A) for said second semiconductor switching means, for stepping up an output voltage from said overvoltage protection means (10); charging means (30) which is charged by an output from said voltage step-up means; discharge means (40) for discharging an electrical load charged into said charging means; characterized in that it further comprises connection means (50) for connecting said power source input section (B) and the drive means (23A) for said second semiconductor switching means in parallel with the overvoltage protection circuit (10) but not via said first semiconductor switching means (11). An internal combustion engine ignition control apparatus according to claim 1, wherein said voltage set-up means is connected in series with said overvoltage protection means (10); and wherein said discharging in the discharge means (40) is performed according to instructions from an external section. An internal combustion engine ignition control apparatus according to claim 2, wherein said parallel connection means (50) comprises at least one resistance element (52). An internal combustion engine ignition control apparatus according to claim 2, wherein said parallel connection means (50) comprises at least one resistance element (52), and reverse flow prevention means (51). An internal combustion engine ignition control apparatus according to claim 2, wherein said overvoltage protection means (10) also has a resistance element (12) with one end thereof connected to said power source input section (B), and an overvoltage absorbing means (13) connected to an other end of said resistance element, and wherein said parallel connection means (50) is connected to said power source input section (B) via said resistance element (12), and to the drive means (23A) of said second semiconductor switching element (22) but not through said first switching means. An internal combustion engine ignition control apparatus according to claim 1, wherein said first semiconductor switching means is a first thyrister (11) which is for protecting subsequent circuits from an overvoltage; wherein said voltage step-up means is a DC-DC converter (20A) having a voltage step-up transformer (21), which outputs the stepped up voltage as a DC voltage, connected in series to an output terminal of said overvoltage protection means (10); wherein said second semiconductor switching means is a field effect transistor (22); and wherein said drive means is a field effect transistor gate drive means (23A).

Inventors: SASAKI SATORU, YANASE ATSUSHI, SASAKI, SATORU, YANASE, ATSUSHI
Applicants: MITSUBA CORP CO LTD, MITSUBA CORPORATION CO., LTD.
Application year: 1996
Main classification: F02P3
IPC codes: F02P15, F02B61, F02P11, F02P1, F02P3