JPH0965652A - Dc-dc converter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the boosting ability of a transformer by sufficiently bringing out the performance of the transformer. SOLUTION: A battery terminal 32 and a transistor 34 are connected in series with the primary coil 33a of a transformer 33 and, at the same time, a capacitor 36 is connected to the secondary coil 33b. A microcomputer 4 turns on the transistor 34 by outputting a charge signal to the transistor 34. When the transistor 34 is turned on, a current detecting circuit 40 detects the primary current flowing to the primary coil 33a and, when the primary current is made to become a set value by means of a comparator 42, an OR gate 43, a flip flop 44, and an AND gate 45, turns off the transistor 34. In addition, the microcomputer 4 detects the voltage of a battery and, when the voltage is low, lowers the charging frequency of the transistor 34.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a DC-DC converter for boosting voltage using a transformer.

[0002]

2. Description of the Related Art There is a method of using a transformer as a DC-DC converter for boosting. This is to charge the capacitor with the current generated on the secondary side by switching the primary current of the transformer. An example of this method is the method disclosed in Japanese Patent Laid-Open No. 3-36958. This is because the maximum value of the transformer primary current, which is usually a constant value, is obtained in order to obtain stable boosting characteristics regardless of fluctuations in the power supply voltage, and the maximum primary current of the transformer when the power supply voltage is high when the power supply voltage drops. The value is reduced compared to the value.

[0003]

However, even in this method, the boosting time (the time required to charge and boost the capacitor to a predetermined voltage) due to the voltage drop of the power supply (battery) for the primary side current, for example, is required. There was a problem that it would significantly increase.

SUMMARY OF THE INVENTION Therefore, an object of the present invention is to provide a DC-DC converter which can bring out the transformer performance sufficiently and improve the boosting performance.

[0005]

According to a first aspect of the present invention, there is provided a transformer in which a DC power source and a switching element are connected in series to a primary coil, and a capacitor is connected to a secondary coil, and the switching element. A switching element driving means for outputting a driving signal to turn on the switching element, a primary current detecting means for detecting a primary current flowing through the primary coil when the switching element is turned on, and a primary current detecting means. When the primary current by the means reaches a set value, drive stop means for turning off the switching element, power supply voltage detection means for detecting the voltage of the DC power supply, and voltage of the DC power supply by the power supply voltage detection means A frequency for changing the drive frequency of the switching element in the switching element drive means based on The DC-DC converter with a changing unit as its gist.

According to a second aspect of the invention, the frequency changing means in the first aspect of the invention lowers the drive frequency of the switching element when the voltage of the DC power supply is low compared to when the voltage is high. The gist is a DC-DC converter that has been designed.

According to a third aspect of the present invention, a transformer in which a DC power source and a switching element are connected in series to a primary coil and a capacitor is connected to a secondary coil,
Switching element driving means for outputting a driving signal to the switching element to turn on the switching element; primary current detecting means for detecting a primary current flowing through the primary coil when the switching element is turned on; When the primary current by the primary current detection means reaches a set value, drive stopping means for turning off the switching element, capacitor voltage detection means for detecting the voltage of the capacitor, and voltage of the capacitor by the capacitor voltage detection means. Based on the above, the gist is a DC-DC converter provided with frequency changing means for changing the driving frequency of the switching element in the switching element driving means.

According to a fourth aspect of the present invention, the frequency changing means in the third aspect of the invention increases the drive frequency of the switching element when the voltage of the capacitor is low compared to when the voltage is high. The DC-DC converter that has been adopted is the gist. (Operation) According to the invention described in claim 1, in the transformer, the DC power source and the switching element are connected in series to the primary coil, and the capacitor is connected to the secondary coil. A drive signal is output to the switching element, and the drive signal turns on the switching element. With the ON operation of this switching element, a primary current flows through the primary coil. This primary current is detected by the primary current detecting means. The drive stopping means turns off the switching element when the primary current detected by the primary current detecting means reaches a set value. When the switching element is turned off, that is, the primary current is cut off, a secondary current flows through the secondary coil and is stored in the capacitor. The voltage is boosted by repeating such operations.

On the other hand, the power supply voltage detecting means detects the voltage of the DC power supply, and the frequency changing means changes the drive frequency of the switching element in the switching element driving means based on the voltage of the DC power supply.

Considering the amount of energy E charged in the capacitor per unit time, the set value of the primary current is I _1max , the drive frequency is f, and the primary side inductance of the transformer is L ₁ . When

[0011]

[Equation 1] Becomes Here, the period t (= 1 / f) of the drive signal is the _sum of the time t1 when the primary current I ₁ reaches the set value I _1max and the period t2 when the secondary current flows and energy is transferred to the capacitor after that. I need time. Regarding t1, considering the coil resistance of the transformer,

[0012]

[Equation 2] Becomes However, VB is the voltage of the DC power supply. Also, in general, t1 >> t2,

[0013]

(Equation 3) Becomes Substituting equation (3) into equation (1),

[0014]

(Equation 4) Becomes Therefore, by determining the drive frequency f by the DC power supply voltage VB as in the formula (3), the energy E charged in the capacitor per unit time is proportional to the DC power supply voltage VB as in the formula (4). It will be done. That is, when the DC power supply voltage VB decreases, E also decreases in proportion.

On the other hand, in the prior art, the primary current set value I _1max is changed by the DC power supply voltage. For example, in order to keep the drive frequency f constant, the DC power supply voltage V
If the primary current setting value I _1max is changed according to B,

[0016]

(Equation 5) Then, substituting this into equation (1),

[0017]

(Equation 6) Becomes As described above, in the conventional technique, the DC power supply voltage VB
As E decreases, E decreases with the square.

Therefore, as is clear from the comparison of the equations (4) and (6), the present invention reduces the degree of decrease in the amount of energy per unit time when the voltage of the DC power supply decreases, that is, the boosting. This is advantageous in shortening the time. In other words, the charging energy to the capacitor per switching is determined by the square of the primary current setting value I _1max , and the total charging speed is determined in proportion to the charging frequency (driving frequency). Efficient boosting can be realized by changing the charging frequency without reducing the primary current setting value I _1max as in the present invention.

According to the second aspect of the present invention, the first aspect is provided.
In addition to the effect of the invention described in (1), the frequency changing means lowers the drive frequency of the switching element when the voltage of the DC power supply is low compared to when the voltage is high, so that a more preferable drive frequency is set.

According to the third aspect of the present invention, in the transformer, the DC power source and the switching element are connected in series to the primary coil, and the capacitor is connected to the secondary coil. A drive signal is output to the switching element, and the drive signal turns on the switching element. With the ON operation of this switching element, a primary current flows through the primary coil. This primary current is detected by the primary current detecting means. The drive stopping means turns off the switching element when the primary current detected by the primary current detecting means reaches a set value. When the switching element is turned off, that is, the primary current is cut off, a secondary current flows through the secondary coil and is stored in the capacitor. The voltage is boosted by repeating such operations.

On the other hand, the capacitor voltage detecting means detects the voltage of the capacitor, and the frequency changing means changes the driving frequency of the switching element in the switching element driving means based on the voltage of the capacitor.

That is, when the switching element is turned on before the secondary current becomes "0" and the driving frequency of the switching element is constant regardless of the charging voltage of the capacitor, as the charging voltage of the capacitor rises. The rate of decrease of the secondary current accompanying the switching off of the switching element gradually increases, and the secondary current reaches “0” every time. Then, the primary current accompanying the turning on of the switching element starts to flow from "0" and does not reach the set value of the primary current. As can be seen from the equation (1), one switching operation of the switching element (on / off The energy for charging the capacitor due to the operation) becomes small, and the boosting performance deteriorates. On the other hand, by changing the drive frequency based on the voltage of the capacitor, even if the rate of decrease of the secondary current accompanying switching off of the switching element gradually increases as the voltage of the capacitor at the start of charging rises from a low voltage to a high voltage. Since the charging frequency is changed, the primary current accompanying switching on of the switching element does not start to flow from "0", and the set value of the primary current is reliably reached. Therefore, efficient DC-DC conversion is performed.

According to the invention of claim 4, claim 3
In addition to the effect of the invention described in (1), the frequency changing means increases the drive frequency of the switching element when the voltage of the capacitor is low compared to when the voltage is high, so that a more preferable drive frequency is set.

[0024]

BEST MODE FOR CARRYING OUT THE INVENTION

(First Embodiment) A first embodiment of the present invention will be described below with reference to the drawings.

The present embodiment is embodied as a fuel injection control device for a diesel engine. In Figure 1,
1 shows an overall configuration of a fuel injection control device for a diesel engine.

A fuel injection control device for a diesel engine includes a pump 1 for pressurizing fuel, a common rail 2 for storing high-pressure fuel pressurized by the pump 1, and a common rail 2.
Fuel injection valve 3 for injecting high-pressure fuel from the
And a solenoid valve drive circuit 5 that opens and closes the fuel injection valve 3 in response to a command from the microcomputer 4.

A drive shaft 7 of the pump 1 is rotatably supported in a housing 6, and the drive shaft 7 is drivingly connected to an output shaft of a diesel engine. A cam 8 is eccentrically fixed to the drive shaft 7, and a piston 9 is attached to the cam surface (outer peripheral surface) of the cam 8.
Are contacted by the spring 10. Piston 9 is cylinder 1
It is slidably supported in 1 and slides up and down by the rotation of the cam 8 accompanying the rotation of the drive shaft 7. At this time,
The fuel is introduced from the fuel tank 12 into the cylinder 11 by the downward movement of the piston 9, and the fuel in the cylinder 11 is pressurized by the upward movement of the piston 9, and the pressurized fuel flows through the check valve 13. It is supplied to the common rail 2 via. Further, the microcomputer 4 is a pressure sensor 1 provided on the common rail 2.
While monitoring the fuel pressure in the common rail 2 at 4, the solenoid valve 15 provided in the pump 1 is controlled to open and close to spill the pressurized fuel to the tank 12 side so that the fuel pressure in the common rail 2 becomes constant. I am supposed to keep it.

The fuel injection valve 3 comprises a fuel injection valve body 16 and a three-way solenoid valve 17, and high-pressure fuel from the common rail 2 is supplied to the fuel injection valve body 16 and the three-way solenoid valve 17. The fuel injection valve body 16 has a fuel chamber 18
A needle valve 19 is arranged inside, and the needle valve 19 is connected to a piston 20 and is biased by a spring 21 in a direction to close the injection port. The three-way solenoid valve 17 includes a housing 22, and an outer valve 23 is vertically slidably supported in an inner hole 22a of the housing 22. The housing 22 has a first port (fuel intake port) 24, a second port 25,
A drain port 26 is provided. The first port (fuel intake port) 24 is the common rail 2 and the second port 2
5 is connected to the upper space of the piston 20, and the drain port 26 is connected to the drain tank 27. The outer valve 23 has a port 28 corresponding to the first port 24.
And a port 29 corresponding to the second port 25.

A drive solenoid 30 is provided above the housing 22. When the drive solenoid 30 is in the non-energized state, the outer valve 23 is urged downward by the spring 31 so that the first port 24 of the housing 22 and the port 28 of the outer valve 23 communicate with each other and the second port 25 of the housing 22 and the outer valve 23. Communicates with the port 29. Therefore, the high-pressure fuel from the common rail 2 is applied to the upper surface of the piston 20 through the ports 24, 28, 29, 25 and the inner hole 23a of the outer valve 23, and this pressure pushes the needle valve 19 downward to open the injection port. close.

When the drive solenoid 30 is energized, the outer valve 23 is suctioned up and the port 24 is closed so that the port 25 and the port 26 communicate with each other, so that the pressure applied to the piston 20 passes through the port 25 and the port 26 and the drain tank 2
It leaks to the 7 side (low pressure side). Therefore, the needle valve 19
Rises by the high pressure fuel in the fuel chamber 18 and opens the injection port to inject the fuel.

When the drive solenoid 30 is not energized, the outer valve 23 has a downward force FF = πPc (D1 ² -D2 ² ) / 4 + Fs, where Pc is the common rail pressure D1 is the diameter D2 of the inner hole 23a of the outer valve 23. Since the urging force of the spring 31 acts on the diameter Fs of the port 25, the port 25 and the port 26 communicate with each other to inject the high-pressure fuel in the common rail 2 by absorbing only the downward force F of the outer valve 23. Not only energy is required, but also when the operation of sucking the outer valve 23 is not performed at high speed, the ports 24 and 26 communicate with each other, and the fuel escapes to the drain tank 27. Furthermore, the suction operation of the outer valve 23 must be performed at an extremely high speed in order to precisely control the injection start timing. Therefore, the higher the common rail pressure Pc, the larger the suction energy of the three-way solenoid valve 17 required.

Therefore, in order to quickly operate the three-way solenoid valve 17 against the fuel pressure that reaches a maximum of 150 MPa, the solenoid valve drive circuit 5 applies the voltage (12 V) of the battery, which is the DC power source for the vehicle, to the vehicle. A high voltage that exceeds is generated and discharged when the drive solenoid 30 of the three-way solenoid valve 17 is energized to supply a high voltage to the three-way solenoid valve 17. Due to the sudden rising current that accompanies the supply of such a high voltage, the magnetic flux rapidly increases, enabling a high response even under a high fuel pressure.

FIG. 2 shows a specific structure of the solenoid valve drive circuit 5, the microcomputer 4 and the three-way solenoid valve 1 in the circuit 5.
7 shows a connection state with the drive solenoid 30 of No. 7. The electromagnetic valve drive circuit 5 uses a feedback type separately excited DC-DC converter.

A primary coil 33 of a transformer 33 is connected to a battery (battery terminal 32 in FIG. 2) which is a DC power source.
a and the transistor 34 as a switching element are connected in series. Secondary coil 33b of transformer 33
The diode 35 and the capacitor 36 are connected in series. At a connection point a between the diode 35 and the capacitor 36, a diode 37, a drive solenoid (load) 30 for the three-way solenoid valve 17, and a transistor 38 are connected in series. The base terminal of the transistor 38 is connected to the microcomputer 4 by a drive signal line. A constant current circuit 47 is connected to the battery terminal 46, and the constant current circuit 4
The constant current generated in 7 is supplied to the drive solenoid 30 via the diode 48.

On the other hand, the connection point b between the capacitor 36 and the diode 35 is connected to the voltage detection circuit 39, and the potential (capacitor voltage V _C ) at the point b is detected. or,
The emitter terminal of the transistor 34 is connected to the current detection circuit 40 as the primary current detection means, and the primary coil 33a is connected.
The primary current I ₁ flowing through the sensor is detected. As a more specific configuration of the current detection circuit 40, a current detection resistor may be connected to the emitter terminal of the transistor 34 and the terminal voltage of the current detection resistor may be detected.

The non-inverting input terminal of the comparator 41 is connected to the voltage detection circuit 39, and the inverting input terminal is connected to the microcomputer 4. The non-inverting input terminal of the comparator 42 is connected to the current detection circuit 40, the inverting input terminal is connected to the microcomputer 4, and the primary current setting value I _1max is connected to the same terminal.
A constant voltage corresponding to is supplied. Comparator 4
The output terminal of 1 and the output terminal of the comparator 42 are connected via an OR gate 43 to a set / reset flip-flop (R
S flip-flop) 44 is connected to the reset terminal R. The charge signal line (driving signal line) extending from the microcomputer 4 is connected to the set terminal S of the RS flip-flop 44 and the input terminal of the AND gate 45. The output terminal Q of the RS flip-flop 44 is connected to the other input terminal of the AND gate 45. The output terminal of the AND gate 45 is connected to the base terminal of the transistor 34.

In the present embodiment, the comparator 4
2, the OR gate 43, the RS flip-flop 44, and the AND gate 45 constitute drive stopping means for turning off the transistor 34 when the primary current I ₁ reaches the set value I _1max .

Next, the basic operation of the solenoid valve drive circuit 5 configured and connected as described above will be described with reference to the time chart of FIG. The microcomputer 4 as a switching element driving means, a power supply voltage detecting means, and a frequency changing means outputs a charge signal (driving signal; charge pulse) having a frequency f shown in FIG. 3 using a charge signal line. This charge signal causes transistor 34 to turn on. That is, when the charge signal level at the set terminal S of the RS flip-flop 44 becomes H (timing T1 in FIG. 3), the RS flip-flop 44 is set and the output Q
Becomes H level, the transistor 34 is turned on through the AND gate 45, and the primary current I ₁ starts to flow from the battery to the primary coil 33a of the transformer 33.

The primary current I ₁ is converted into a voltage by the current detection circuit 40 and input to the non-inverting input terminal of the comparator 42. When the primary current I ₁ is smaller than the primary current setting value I _1max , the output of the comparator 42 is L level,
The output Q of the RS flip-flop 44 remains at the H level and the transistor 34 is kept on (T1 to T in FIG. 3).
2 period). Then, the primary current I ₁ is the primary current setting value I
_{When 1max} is reached (timing T2 in FIG. 3), the output of the comparator 42 becomes H level and the RS flip-flop 44 is reset, so that the output Q becomes L level,
Transistor 34 turns off.

Further, the primary current I ₁ is the primary current set value I _1max
Even if the charge signal does not reach the level, the AND gate 45 causes the transistor 34
Turns off. Therefore, the charge signal is
Determines the maximum on-time of. Usually, the duty value of the charge signal is set to a value of about 90% in consideration of a period in which a secondary current I ₂ described later flows.

When the transistor 34 is turned off (T in FIG. 3)
2 timing), the secondary current I ₂ of the transformer 33 has an initial value I _1max / n (n: winding ratio of the transformer 33) due to the magnetic energy of the transformer primary current I ₁ that has been flowing.
Flow through the diode 35, and the charge is charged in the charging capacitor 36 through the diode 35. As a result, the capacitor 36 is charged.
Voltage rises (the period from T2 to T3 in FIG. 3).

By repeating the above operation, the voltage of the capacitor 36 gradually rises. The voltage V _{C of the} capacitor 36 is monitored by the voltage detection circuit 39, and this capacitor voltage V _C is input to the non-inverting input terminal of the comparator 41. On the other hand, the inverting input terminal of the comparator 41 is supplied with a constant voltage as the charging voltage setting value of the capacitor 36 from the microcomputer 4 to compare the two. When the voltage V _{C of the} capacitor 36 reaches the set voltage (T in FIG. 3).
4 timing), the output Q of the RS flip-flop 44
Becomes L level, the transistor 34 is turned off regardless of the charge signal, and at that time, the charge to the capacitor 36 is stopped.

When the fuel injection valve 3 is subsequently opened, when the transistor 38 is turned on by the drive signal (drive pulse) from the microcomputer 4 (timing of T5 in FIG. 3), the diode 37 and the inductive load are generated. The electric charge accumulated in the capacitor 36 is discharged at once through the drive solenoid 30 of the three-way solenoid valve 17 (period T5 to T6 in FIG. 3). In this way, the transistor 38 is turned on and the drive solenoid 30 is energized. At this time, since the applied voltage is boosted higher than the battery voltage, the drive solenoid 3
It is possible to realize a high-speed response even under a high fuel pressure by applying a large current to zero. That is, the valve opening operation is promptly performed by the current from the capacitor 36 at the time of valve opening.

After discharging, a constant current flows from the constant current circuit 47 to the drive solenoid 30 through the diode 48 (the period of T7 to T8 in FIG. 3). Thus, during the valve opening operation of the fuel injection valve 3, the valve open state is maintained by the constant current from the constant current circuit 47. Further, while the transistor 38 is on, the charge signal is at the L level in order to prevent the secondary current of the transformer 33 from flowing through the drive solenoid 30.

Although the fuel injection valve 3 is opened in this way, the microcomputer 4 operates the accelerator opening signal,
The engine speed signal, water temperature signal, etc. are input and the operating state of the diesel engine is detected by these. The microcomputer 4 drives and controls the transistor 38 according to the operating state of the diesel engine to control the fuel injection amount and the fuel injection timing. That is, as shown in FIG. 3, the fuel injection amount and the fuel injection timing are controlled by adjusting the injection start timing T5 and the injection end timing T8 from the reference crank position.

Further, as shown in FIG.
Built-in converter (ADC) 50 to convert battery voltage VB to A
The frequency f of the charge signal is converted according to the value of the D conversion.
Change.

Assuming that the charge frequency f and the primary current set value I _1max are fixed, if the battery voltage VB drops, the primary current I ₁ rises slowly and the charge signal level becomes H. During that period, the primary current I ₁ does not reach the primary current set value I _1max . For this reason, the charging energy per switching is small, and the boosting performance is significantly reduced. Therefore, in the present embodiment, the primary current set value I _1max is a fixed value, and the frequency f of the charge signal is changed according to the battery voltage VB.

When considering the amount of energy E charged in the capacitor 36 per unit time, the above (1)
This is as described using equations (6). That is, as is clear from the comparison between the above equations (4) and (6), the degree of reduction in the energy amount per unit time when the battery voltage drops is smaller in the present embodiment, that is, the boosting This is advantageous in shortening the time. In other words, the charging energy to the capacitor 36 per switching is determined by the square of the primary current setting value I _1max , and the total charging speed is determined in proportion to the charging frequency (charging frequency). As in the present embodiment, it is better to reduce the charging frequency without reducing the primary current setting value I _1max ,
This means that efficient boosting can be realized.

A specific method of changing the charge frequency will be described with reference to the flowchart of FIG. FIG.
This is a charge frequency calculation routine, which is started, for example, at regular intervals. The microcomputer 4 is step 301
Then, the battery voltage VB is taken in through the AD converter 50. Then, the microcomputer 4 determines the charge frequency f from the battery voltage VB fetched in step 302.
The charge frequency is determined in step 302 using, for example, the characteristic line L1 in FIG. The data regarding this characteristic line L1 is stored in advance in the memory 51 of FIG. In FIG. 5, the characteristic line L1 is an upward-sloping line,
The charge frequency is set to be lower as the battery voltage VB is lower. In addition, the battery voltage is to some extent (Fig.
Then, the charging frequency f is set to a constant value when the voltage is 12 V or more. This is because the charging frequency becomes too high,
This is to prevent the switching loss of the charging transistor 34 from becoming excessive.

The effect of this embodiment over the prior art will be further described with reference to FIG. In FIG. 6, the battery voltage is maintained at 12 V before the timing of T9, but the battery voltage is reduced after the timing of T9.

In the prior art, when the battery voltage VB decreases, the charge frequency is constant, so the primary current I
₁ does not reach the primary current setting value I _1max at the normal battery voltage, and the charge signal becomes L level (in the figure, T10, T11, T12, T13, T14, T1
5, T16), the primary current is cut off. Since the primary current value I _1max 'at this time is smaller than the value I _1max at the normal battery voltage, the energy for one charge is small. Therefore, the voltage rise of the capacitor 36 also becomes slower than the normal battery voltage. As a result, the voltage charged in the capacitor 36 at the timing T17 at which the solenoid 30 as a load is energized and driven becomes lower than the normal battery voltage by ΔV, and the capacitor 36
The peak of the load current due to the discharge is also reduced by ΔI. Therefore, in the conventional technique, the load (solenoid) current changes as the battery voltage drops, which affects the injection characteristics.

On the other hand, in the present embodiment, the battery voltage V
The charge frequency is changed by B (t _cha2 ＞
t _cha1 ). Therefore, even when the battery voltage VB is low, the primary current setting value I _1max at the normal battery voltage is reached, and the energy that moves in one charge is the same as that at the normal battery voltage. Therefore, when the battery voltage VB is lowered. It is possible to reduce the influence of the load current on the load current.

As described above, in the present embodiment, the microcomputer 4 monitors the battery voltage and lowers the drive frequency of the transistor 34 when the battery voltage drops. That is, the charging energy to the capacitor 36 by one switching operation (on / off operation) of the transistor 34 is determined by the square of the set value I _1max of the primary current, and the charging speed is 1 at the charging frequency (driving frequency f). Charge frequency (driving frequency) by battery voltage VB
When the battery voltage becomes low by changing the
It is possible to perform more efficient DC-DC conversion as compared with the conventional method in which the set value of the primary current is changed. That is, by changing the drive frequency, the performance of the transformer 33 can be sufficiently brought out, and the increase of the boosting time when the battery voltage drops can be minimized. As shown in FIG. 5, a more specific method of changing the drive frequency is such that the charge frequency is set lower as the battery voltage is lower, and the frequency is set more finely.
According to the equation (3), the drive frequency is lowered proportionally to the battery voltage. (Second Embodiment) Next, a second embodiment will be described focusing on differences from the first embodiment.

As shown in FIG. 7, in the present embodiment, the frequency f of the charge signal (driving signal) is changed by the charging voltage of the capacitor 36 regardless of the battery voltage. Therefore, in order to monitor the charging voltage of the capacitor 36, the voltage detection circuit 39 as the capacitor voltage detection means.
Is also input to the AD converter (ADC) 50.

In the first embodiment, as shown in FIG. 3, the secondary current I ₂ is set to "0" at each switching, but when the capacity of the capacitor 36 is large or when the transformer 3 is used.
When the secondary side inductance of 3 is large, there may be a design method in which the switching of the next primary side starts before the secondary current I ₂ becomes “0”. The phenomenon in this case will be analyzed.

Consider the booster power supply with an equivalent circuit as shown in FIG.
I will decide. As a hypothesis, the coil resistance of the transformer and the
The wiring resistance and the loss of the transistor are set to "0".
You. Analyze one on / off of transistor 34
To do. Further, in FIG. 9, the primary current I ₁(T) and secondary current
I₂The change in (t) is shown.

Regarding the circuit on the primary side,

[0058]

(Equation 7) And the initial condition is given by I ₁ (t) = I _1o .

Regarding the circuit on the secondary side,

[0060]

(Equation 8) And the initial conditions are

[0061]

[Equation 9] However, V _o is given by the charging voltage of the capacitor 36.

By solving these, the following is obtained. 0 <t
When ≦ t1 (transistor 34 is on),

[0063]

(Equation 10) Becomes When t1 <t (transistor 34 is off),

[0064]

[Equation 11] Becomes Here, the number of turns of the transformer 33 is set to the primary side n ₁ ,
Assuming that the secondary side is n ₂ , the primary current I ₁ flows linearly (increases) with a value obtained by multiplying the final value of the secondary current during the previous switching operation by n ₂ / n ₁ as the initial value I _1o. , Secondary current I
₂ is the primary side current value I ₁ (t
The value of n ₁ / n ₂ times that of 1) is _used as the initial value I _2o and flows (decreases) in the cos curve. Then, as the boosting proceeds and the charging voltage of the capacitor 36 increases, the value of θ in the equation (7) increases, and the time until the secondary current I ₂ becomes “0” becomes shorter.

This state is shown in FIGS. When the battery voltage VB is high, the result is as shown in FIG. 10, and when the battery voltage VB is low, the result is as shown in FIG. As the charging voltage of the capacitor 36 increases, the decreasing rate of the secondary current I ₂ gradually increases. When the battery voltage VB is low as shown in FIG. 11, the secondary current I ₂ reaches “0” every time when the charging voltage of the capacitor 36 rises. Then, after that, the primary side current I
₁ starts to flow from “0”, and the primary current setting value I _1max is not reached. For this reason, as in the case of the first embodiment, the charging energy per switching is small and the boosting performance is significantly reduced.

Therefore, in the present embodiment, the capacitor 3
The charge voltage of 6 is monitored by the microcomputer 4, and the charge frequency is changed according to the value. A more specific method of changing the charge frequency is to make the charge frequency higher as the capacitor charge voltage is lower.
The characteristic line L2 indicated by is stored in the memory 51 in advance. In FIG. 13, the horizontal axis represents the capacitor voltage V _C and the vertical axis represents the charge frequency, and the characteristic line L2 is a line descending to the right. The frequency is set to be higher as the capacitor voltage V _C is lower. As shown in FIG. 14, the microcomputer 4 fetches the capacitor voltage through the AD converter 50 in step 401 at regular time intervals, for example, and in step 4
The drive frequency f is determined based on the characteristic line L2 from the capacitor voltage captured in 02.

As a result, as shown in FIG. 12, even when the battery voltage VB is low, the primary current I ₁ reaches the primary current setting value I _1max at each switching, so that the capacitor 36 of FIG. The voltage rises quickly, and it is possible to suppress the decrease in boosting ability.

As described above, in the second embodiment,
The microcomputer 4 monitors the voltage of the capacitor 36 and increases the drive frequency of the transistor 34 when the capacitor voltage is low. Therefore, as the voltage of the capacitor 36 at the start of charging becomes higher from the time when it is lower, the charging frequency becomes higher even if the rate of decrease of the secondary current accompanying the turning off of the transistor 34 gradually increases. The primary current does not start flowing from "0", and the set value of the primary current is reliably reached. In this way, by changing the charging frequency (driving frequency), the performance of the transformer 33 can be sufficiently brought out in the process of charging the capacitor 36, and the increase of the boosting time when the battery voltage drops can be minimized. This is an effective method not only when the battery voltage drops, but also when the drive frequency is high.

As a method of changing the driving frequency, FIG.
As shown in, the charge frequency is set higher as the capacitor voltage is lower, and the frequency is set more finely. The first embodiment and the second embodiment have been described above, but the charge frequency may be changed by both the battery voltage and the capacitor charging voltage by combining these two embodiments. That is, as indicated by the one-dot chain line in FIG. 7, in order to monitor the battery voltage VB and the charging voltage of the capacitor 36, the output of the voltage detection circuit 39 and the battery voltage application line are connected to an AD converter (AD).
C) It may be input to 50 and the frequency of the charge signal may be changed not only by the battery voltage but also by the charging voltage of the capacitor 36.

When it is detected that the battery voltage or the charging voltage of the capacitor 36 is in the decreasing direction or the changing amount per predetermined time, the frequency of the charge signal is changed (lower or higher). ) May be.

It is needless to say that the present invention can be embodied not only in the fuel injection control device (fuel injection valve drive device) of the diesel engine but also in the booster circuit for ignition of the internal combustion engine.

[0072]

As described above in detail, according to the invention described in claim 1, by changing the charging frequency (driving frequency), the performance of the transformer is sufficiently brought out and the boosting capability is improved when the power supply voltage drops. Exerts an excellent effect that can be achieved.

According to the invention described in claim 2, according to claim 1
In addition to the effect of the invention described in (1), the frequency can be set more finely. According to the third aspect of the present invention, by changing the charging frequency (driving frequency), it is possible to sufficiently bring out the performance of the transformer in the process of charging the capacitor and to improve the boosting ability.

According to the invention of claim 4, claim 3
In addition to the effect of the invention described in (1), the frequency can be set more finely.

[Brief description of drawings]

FIG. 1 is an overall configuration diagram of a fuel injection control device for a diesel engine.

FIG. 2 is a configuration diagram of an electromagnetic drive circuit according to the first embodiment.

FIG. 3 is a time chart for explaining the operation.

FIG. 4 is a flowchart for explaining the operation.

FIG. 5 is a map for determining the charge frequency.

FIG. 6 is a time chart for explaining the operation.

FIG. 7 is a configuration diagram of an electromagnetic drive circuit according to a second embodiment.

FIG. 8 is an equivalent circuit diagram for explaining a primary current and a secondary current in a transformer.

FIG. 9 is a time chart showing changes in primary current and secondary current.

FIG. 10 is a time chart for explaining the operation.

FIG. 11 is a time chart for explaining the operation.

FIG. 12 is a time chart for explaining the operation.

FIG. 13 is a map for determining the charge frequency.

FIG. 14 is a flowchart for explaining the operation.

[Explanation of symbols]

4 ... Switching element driving means, power supply voltage detecting means, microcomputer as frequency changing means, 32 ... Battery terminal,
33 ... Transformer, 33a ... Primary coil, 33b ... Secondary coil, 34 ... Transistor as switching element,
36 ... Capacitor, 39 ... Voltage detecting circuit as capacitor voltage detecting means, 40 ... Current detecting circuit as primary current detecting means, 42 ... Comparator forming drive stopping means, 43 ... OR gate forming drive stopping means, 44 ...
RS flip-flops constituting drive stop means, 45 ...
An AND gate that constitutes the drive stopping means.

Claims (4)

[Claims] 1. A transformer in which a DC power supply and a switching element are connected in series to a primary coil and a capacitor is connected to a secondary coil; and a drive signal is output to the switching element to turn the switching element. A switching element driving means for turning on, a primary current detecting means for detecting a primary current flowing through the primary coil when the switching element is turned on, and a switching operation when the primary current by the primary current detecting means reaches a set value. Drive stopping means for turning off and turning off the element; power source voltage detecting means for detecting the voltage of the DC power source; and drive frequency of the switching element in the switching element driving means based on the voltage of the DC power source by the power source voltage detecting means. And frequency changing means for changing C-DC converter. 2. The DC-DC according to claim 1, wherein the frequency changing unit lowers the drive frequency of the switching element when the voltage of the DC power source is low, compared with when the voltage is high.
converter. 3. A transformer in which a direct current power source and a switching element are connected in series to a primary coil and a capacitor is connected to a secondary coil; and a drive signal is output to the switching element to turn the switching element. A switching element driving means for turning on, a primary current detecting means for detecting a primary current flowing through the primary coil when the switching element is turned on, and a switching operation when the primary current by the primary current detecting means reaches a set value. Driving stop means for turning off and turning off the element, capacitor voltage detecting means for detecting the voltage of the capacitor, and changing the driving frequency of the switching element in the switching element driving means based on the voltage of the capacitor by the capacitor voltage detecting means. Frequency changing means for DC-DC converter, wherein the door. 4. The DC-D according to claim 3, wherein the frequency changing means increases the drive frequency of the switching element when the voltage of the capacitor is low compared to when the voltage of the capacitor is high.
C converter.