JPS5820544B2 - transistor chopper control device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Application of the Invention] The present invention relates to a transistor chopper control device, and particularly to a control device suitable for controlling the base current of a power transistor that controls the load current thereof.

[Prior art]

As a method of controlling a DC motor using a transistor chopper, a circuit as shown in FIG. 1, for example, is generally used.

In Figure 1, B is a battery as a power source, M is a DC motor with a series field winding F, D is a flywheel diode, 1
turns on the current IC flowing from battery B to DC motor M.
, a transistor chopper that controls OFF, and 2 a base current control circuit that controls the base current IB of the transistor chopper 1.

In a transistor chopper, an excessive base current flows because it is necessary to sufficiently saturate the collector-emitter voltage of the transistor when the chopper is turned on.

However, since the value of this base current is too excessive, the loss of the base current control circuit becomes extremely large.

Accurate control of the base current is required to reduce this loss.

FIG. 2 shows the base current control characteristics of the conventional method, and FIG. 3 shows a specific example of the conventional control force method.

In the circuit configuration shown in FIG. 1, the conventional method controls the relationship between the load current IM (motor current) and the base current IB to have the characteristics shown in FIG. 2 when controlling the ON/OFF state of the transistor chopper 1.

That is, the first method is to flow a constant base current IBmax, which is necessary to drive the maximum motor current IMmax, regardless of the load current, as shown in the characteristic a in FIG. 2.

The second method is to control the current IB in proportion to the current 1M, as shown in the characteristics of FIG.

As a specific circuit for controlling such characteristics, there is a method as shown in FIG. 3, for example.

In Figure 3, R is the load and O of the power transistor T.
The load current IC is controlled by N, OFF operation.

TA is a drive transistor. To obtain the characteristics shown in Figure 2a, the drive transistor TA must be turned ON and OFF.
It is sufficient to operate the F transistor and supply a base current of lB=π to the power transistors VB1 and T.

Moreover, in order to obtain the characteristic b in FIG. 2, another transistor is inserted in series with the driver transistor TA in FIG. 3, and the base current of the additional transistor is adjusted according to the magnitude of the current LM. It is sufficient to continuously control the current value of the pulsed base current IB applied to the power transistor T to a value proportional to the magnitude of 1M.

The drawback of such a conventional method is that the loss of the drive circuit that flows the base current IB increases, and the drive transistor TA
is to generate heat.

In particular, in method a, the base current is constant even when the load current LM is small, so the loss in the base resistor RB is large.

b; In formula 15, the base current IB becomes smaller as the current of IM becomes smaller than in the method a, so the loss of the base resistor RB becomes smaller.

However, since two transistors are provided in series, the current loss due to these transistors is large.

In this way, high loss in the base drive circuit is undesirable from the standpoint of efficiency of the transistor chopper.)
.

Another disadvantage is that cooling is required to sufficiently cope with heat generated by the drive transistor due to loss.

[Purpose of the invention]

An object of the present invention is to provide a control device that can reduce loss in a base current control circuit of a power transistor in a transistor chopper.

[Summary of the invention]

In order to reduce loss in a base current control circuit in a transistor chopper, the present invention receives a signal value generated from the base current of the power transistor in accordance with the value of the current flowing through the power transistor.

It is characterized in that it is controlled by an output signal of a function generator generated based on an inverse function of the DC amplification factor of the power transistor.

[Embodiments of the invention]

FIG. 4 shows an embodiment of a transistor chopper to which the present invention is applied.

In FIG. 4, 1 is a power transistor for a transistor chopper, and 2 is a base current control circuit composed of a base current control transistor 2a and a function generator 2b.

3 is a current detector.

The function of the emitter current IE, collector current IC, and base current IB of the power transistor 1 is expressed by the following formula (1) Also, the direct current amplification factor hFE of the transistor is IC hFE=□
...B2) IB.

In addition, when using composite connection transistors, etc.,
Since a large amount of hFE can be obtained, less IB is required.

Here, the relationship between IE and IC can be viewed as the relationship expressed by the following equation.

1E-=IC (3) Therefore, by detecting the current on the output side of the transistor chopper 1, that is, the emitter current IB of the power transistor, as shown in FIG. Detection is possible.

In FIG. 4, base current control circuit 2 is supplied with current from the positive end of battery B. In FIG.

Further, the base current IB of the power transistor is controlled based on the value of the emitter current IEO of the power transistor detected by the current detector 3.

FIG. 5 shows h for the collector current IC of the transistor chopper power transistor 1 in the circuit of FIG.
This represents the relationship between the general characteristics of FE and the base current IB required to saturate the collector-emitter voltage of the power transistor 1 based on the relationship between hFE and IB shown in equation (2).

Moreover, FIG. 6 is a diagram explaining the base current control characteristics of the present invention.

As shown in Figure 5, the hPEi properties of a transistor generally increase slightly as the collector current increases from zero, and range from about 1/2 to 3/2 of the maximum collector current 1cmax.
It reaches a maximum at about 1/200%, and thereafter decreases as the collector current increases.

Therefore, the base current required to saturate the collector-emitter voltage in this case has the characteristics of the base current IB shown in FIG. 5 from equation (2).

That is, by adopting such a base current characteristic, it is possible to operate the transistor in the saturation region while minimizing loss, that is, heat generation, in the base current control circuit 2.

In particular, when a transistor chopper employing the present invention is used in an electric vehicle, etc., the current of the transistor that is often used at all times except during acceleration such as startup is Since the current is approximately 1 ca, which is less than half, the effect of reducing the current loss of the base current control circuit due to the reduction in the base current IB is noticeable.

FIG. 6 shows the base current IB flowing through the power transistor according to the collector current IC (ie, IE) of the power transistor.

In FIG. 6, the characteristic A is based on the conventional method, and the characteristics B and C% are based on the present invention. In particular, the C% characteristic is expressed approximately and is the characteristic when it is easier to implement.

In the case of the approximation method, the minimum required constant base current IBmin is up to 1ca, which is often used during chopper operation.
When the collector current exceeds lca, the base current IB
is increased in proportion to the collector current IC.

When such a method is used, it is possible to drive the base current in the vicinity of the lca, which is always used, with a value sufficiently smaller than that of the conventional method.

The base current control circuit that controls the base current 1B in this way includes the base current control transistor 2a and the function generator 2.
It consists of b.

The characteristic B or C in FIG. 6 is generated by the function generator 2b.

The current detector 3 detects the emitter current I of the power transistor.
E is detected and output voltage VIE is generated.

Depending on the value of VIE, the function generator 2b generates an output voltage VK having the characteristic of, for example, C shown in FIG.

That is, when the emitter current IE is smaller than Ica, the base current IBmin is the lowest, and l
As the collector current IC increases beyond ca, I
The base current control transistor 2a is controlled by outputting an output voltage VK that increases B.

By controlling as described above, the base current IB of the power transistor can be reduced when the motor is under a light load.

Therefore, the loss in the base current control circuit of the power transistor is reduced and the efficiency of the chopper is improved.

Furthermore, since the heat loss of the transistors and the like of the base current control circuit 2 is reduced structurally, the cooling structure is significantly simplified.

In addition, although the method of detecting the collector current (substantially the same as the emitter current) of a transistor and controlling the base current IB has been described above, the detected through current may be the motor current LM.

Further, the present invention can be applied even if an average value or an instantaneous value is used as the detection current.

Note that when using the average value of the collector current 1c, IB may be controlled in consideration of the relationship of the chopper conduction rate α.

Other effects when the present invention is applied will be explained.

Figure 7 shows a transistor chopper circuit with multiple power transistors connected in parallel, and Figure 8 shows transistors TI and T2.
FIG. 9 shows the relationship between the peak value of the collector turn-off current when the conventional method and the present invention are applied.

In FIG. 7, TI and T2 are power transistors whose collectors and emitters are connected in parallel, respectively, and RBl and RB2.
is a balance resistance for each base current IB1.1B2.

When such a parallel connection of transistors is used, for example, during the turn-off operation of the transistor, there is a large difference in the turn-off current icp due to the difference in the characteristics of the elements, as shown in the characteristics a and b shown in FIG.

To prevent this, the usual method is to use an element whose turn-off time matches the maximum current of the transistor, or to use a circuit whose turn-off time matches.

In this case, in the conventional method that does not control the base current, for example, as shown in FIG.
As is reduced, the peak current icp at turn-off increases, as in characteristic a, which is unfavorable in terms of turn-off current balance.

This is because the turn-off operation is delayed due to excessive base current flowing.

Note that the b characteristic is a collector current waveform of an element with no delay in turn-off operation.

In this way, if the present invention is applied to the unbalance of turn-off current in parallel operation of transistors,
The a% characteristic shown in FIG. 9 becomes the characteristic of a', and the unbalance of the turn-off current is reduced, resulting in good characteristics.

This is because as the collector current decreases, the base current also decreases, so that the delay in turn-off time due to excessive base current of the transistor is reduced.

〔Effect of the invention〕

As described above, the present invention reduces power consumption and heat generation in the base current control circuit of a power transistor.

Furthermore, the transistor parallel connection type has an effect on the balance of turn-off current.

[Brief explanation of drawings]

Fig. 1 shows a chopper circuit to which the conventional method is applied, Fig. 2 shows the base current control characteristics of the conventional method, Fig. 3 shows the base current control circuit of the conventional method, Fig. 4 shows the chopper circuit to which the present invention is applied, and Fig. 5 shows the chopper circuit to which the present invention is applied. The figure shows hFE and IB% characteristics diagram of 1 transistor, 6th
Figure 7 shows the base current control characteristics of the present invention and the conventional method, Figure 7 shows the transistor chopper circuit with multiple power transistors connected in parallel, Figure 8 shows the transistor turn-off operation, and Figure 9 shows the characteristics of the present invention and the conventional method. It is a comparison diagram of effects. 1...Transistor chopper, 2...
Base current control circuit, 3... Current detector.

Claims (1)

[Claims] 1. A power transistor connected in series with a DC motor is provided for a DC power source, and the current flowing through the DC motor is controlled by controlling the application time of a base current for making the power transistor conductive. In what you do,
Current detection means generates an electric signal according to the load current flowing through the power transistor, and a control signal is generated from the output signal of the current detection means based on a function approximated to an inverse function of the DC amplification factor of the power transistor. A transistor chopper control device comprising a function generator and base current control means for controlling the base current of the power transistor according to a control signal from the function generator. 2. In claim 1, in a region where the load current is small, a constant minimum base current necessary to saturate the collector-emitter voltage of the power transistor is passed, and when the load current exceeds a certain value, A transistor chopper control device characterized by increasing base current in proportion to load current.