JPH07222482A - Saturation-proof circuit of output transistor - Google Patents

Abstract

PURPOSE:To detect the saturation more accurately by a method wherein a first transistor which has a base to which an input signal is applied is provided and the level of the input signal is so controlled as to prevent an output transistor from the saturation in accordance with the ratio of the base current of the first transistor to an output current. CONSTITUTION:The output current of a power amplifier 8 is inputted to a first current miller circuit 14 which is composed of transistors Q1, Q2 and Q3. The output current I1 of the first current miller circuit 14 from the transistor Q2 is made to flow into a ground through a resistor R1. The output current IbE2292.70of the first current miller circuit 14 from the transistor Q3 is applied to the bases of a transistor Q6 and an output transformer T2. The ratio of the base current to collector current of the transistor Q6 is monitored and, if the ratio exceeds a predetermined value, the gain of a variable gain type amplifier 8 is turned down to reduce the base current.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a circuit for preventing saturation of an output transistor which supplies a current to a load.

[0002]

2. Description of the Related Art FIG. 6 shows a conventional output transistor saturation prevention circuit in a motor drive circuit. Reference numeral 61 is a motor drive signal source, and the motor drive signal from the motor drive signal source is added to power amplifiers 62, 63 and 64. 3 motors
Since it is a phase, three power amplifiers 62, 63, 64 are provided. 65, 66, 67 are power amplifiers 6
2, 63 and 64 are NPN type output transistors, whose output currents are output terminals 68, 69 and 7
It is supplied to the coil of the motor connected to zero.

Output transistors 65, 66 and 67 represent lower output transistors in the push-pull output circuit, and PNP type output transistors are connected to their collectors. FIG. 7 shows one of them (one phase). 71 is a PNP type output transistor, the emitter is connected to the power supply V _CC , the collector is connected to the output terminal 68 together with the collector of the output transistor 65, and the base is connected to the power amplifier 62.

In the positive half wave of the sine wave drive signal output from the power amplifier 62, the output transistor 65 is turned on,
The output transistor 71 is turned off, and in the negative half-wave, the output transistor 65 is turned off.
Turns on. L1 indicates a motor coil as a load.

Each output transistor 65, 66, 6 of FIG.
As can be seen from FIG. 4, which shows the static characteristic, when the collector-emitter voltage V _CE becomes small, 7 enters the saturation region, and in this saturation region, the output current I ₀ changes linearly according to the base current. Since it does not, the shape of the sine wave as the drive signal is lost, which hinders the drive of the motor. In FIG. 4, I _{B1 to} I _B4 represent base currents as parameters, and I _B1 <I _B2 <I _B3 <I _B4 .

In the conventional circuit of FIG. 6, the output transistor 6
The output voltages of 5, 66 and 67 are detected so that this voltage does not fall below a predetermined value V _CE0 (reference voltage) (does not enter the saturation region). Therefore, the output transistor 6
The collectors of 5, 66 and 67 are connected to the inverting terminal (−) of the comparator 76 via the resistors 72, 73 and 74.
The non-inverting terminal of the comparator 76 (+), the applied voltage I _S · R _S caused by the current I _S that is proportional to the driving current I from the current source 75 in conjunction with the drive signal source 61 and the resistor R _S is as the reference voltage To be done.

When the voltage input to the inverting terminal (-) of the comparator 76 (corresponding to V _CE of the output transistor) exceeds this reference voltage, the power amplifiers 62, 63 and 64 are controlled to output transistors 65 and 66. , 67 to reduce the base current so that V _CE does not _drop below I _S · R _S. This circuit has three output transistors 65, 6
Responds to the lowest voltage of V _CE of 6,67.

[0008]

In the above-mentioned conventional circuit,
Since the output characteristics (a) of the output transistors 65, 66, 67 shown in FIG. 8 are different, the reference voltage is set to I _S · R _S by guaranteeing a region that is not saturated even if the transistors have variations. This is a characteristic as shown in FIG. 8B, which is far from the actual output characteristic (A) of the output transistor.

As described above, the reference value of V _CE is set higher than the actual value in order to give priority to not to saturate. Therefore, V _{CE of} the output transistor increases.
Since V _CE of the output transistor becomes a reactive voltage for the load, power loss occurs and the torque of the motor decreases. Further, this is not preferable because the power consumption in the IC is increased when the output transistor is composed of the IC.

The present invention has been made in view of the above circumstances, and an object thereof is to provide a saturation prevention circuit for an output transistor, which has improved detection accuracy without increasing unnecessary reactive voltage. .

[0011]

In order to achieve the above object, the present invention provides a saturation prevention circuit for an output transistor that supplies a current to a load, the base of which is connected to the base of the output transistor, A first transistor for receiving the input signal is provided, and a control means for controlling the level of the input signal is provided based on the ratio of the base current and the output current of the first transistor so that the output transistor is not saturated.

[0012]

With this structure, the saturation due to the decrease in V _CE of the output transistor appears in the ratio of the base current of the first transistor to the output current (collector current), so that when this ratio reaches a predetermined value. , The gain of the amplifier circuit is reduced, and the output transistor is reliably prevented from entering the saturation region. That is, since it is detected that the output transistor is about to enter the saturation region and is blocked, V _CE of the output transistor can be reduced.

[0013]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 5 shows a saturation prevention circuit 1 for an output transistor according to the present invention.
The motor drive circuit which incorporated 1,12,13 is shown. In the figure, L1, L2, and L3 are coils of a three-phase brushless motor, which are arranged at phase positions of 0 °, 120 °, and 240 °, respectively. Hall elements 1, 2, and 3 are arranged at positions deviated from the coils L1, L2, and L3 by 30 °, and output a signal indicating the rotor position (hereinafter referred to as "hall signal"). Each Hall signal is input to the Hall amplifiers 4, 5, 6 in the form of a differential signal.

The Hall signals amplified by the Hall amplifiers 4, 5 and 6 are subjected to matrix and amplitude limiting actions in the next circuit block 7. Here, in the matrix, since the Hall signal is shifted by 30 ° with respect to the coils L1, L2, and L3,
It is an operation of matching the phases (that is, eliminating the shift of 30 °) so as to be suitable for driving the coils L1, L2 and L3. The output of the circuit block 7 is the power amplifiers 8, 9, 10
And output transistor saturation prevention circuits 11, 12, 13
Through the coils L1, L2, L3.

FIG. 3 shows the saturation prevention circuits 11, 12, and 13.
Of these, the saturation prevention circuit 11 is shown. T1 and T2 are a PNP type output transistor and an NPN type output transistor which form a push-pull type output circuit, and the transistor T2 is turned on in the positive half wave of the sine wave drive signal output from the power amplifier 8. , In the negative half-wave, the transistor T1 is turned on to drive the coil L1 alternately.

Reference numeral 11u is a saturation prevention circuit for the upper output transistor T1, and 11d is a lower output transistor T1.
2 is a saturation prevention circuit. Upper output transistor T1
2 shows the details of the saturation prevention circuit 11u of FIG. 1, and details of the saturation prevention circuit 11d of the lower output transistor T2 are shown in FIG.
Is shown in.

First, FIG. 1 will be described. The output current of the power amplifier 8 is input to the first current mirror circuit 14 including the transistors Q1, Q2 and Q3. This first
The output current of the current mirror circuit 14 is the transistor Q2.
And Q3 respectively. The output current I ₁ of the transistor Q2 flows to the ground through the resistor R ₁ , and the output current I _b of the transistor Q3 flows into the bases of the transistor Q6 and the output transistor T2.

The emitter of the transistor Q6 is connected to the ground similarly to the emitter of the output transistor T2, and the collector is connected to the second current mirror circuit 15. The second current mirror circuit 15 is composed of transistors Q4 and Q5, and a current I output from the transistor Q5.
₂ flows to ground through resistor R ₂ .

The non-inverting terminal of the voltage on (c) point I ₁ · R ₁ and voltage I ₂ · R ₂ each operational amplifier 16 that occurs (d) point (+) and the inverting terminal (-) is applied to the . Here, I ₁ = I _b / α and I ₂ = I _C / β are selected. However, α and β are constants, I _b is the output current of the transistor Q3, and I _C is the collector current of the transistor Q6.

The emitter area of the output transistor T2 is selected to be r times the emitter area of the transistor Q6.
Here, r is selected within the range of 200 to 600. When the output transistor T2 reaches the saturation region, the transistor Q6
The collector current I _{C of the same} also increases. V _{CE0 at} the boundary between the saturation region and the linear region of the transistor T2 is a point where the ratio of the base current and the collector current of the transistor Q6 becomes a predetermined value. The transistor Q6 has its collector connected to the power supply V _CC through the current mirror circuit 15 and operates, so that it does not saturate.

In this embodiment, the ratio of the base current to the collector current of the transistor Q6 is monitored, and when it exceeds a predetermined value, the gain of the variable gain amplifier 8 is reduced to reduce the base current. Now, the collector current I of the transistor Q6
Suppose that the saturation prevention is activated when the ratio of _C to the base current becomes larger than x. This x can be determined as follows.

First, the base current of the transistor Q6 is I
_{Let b1 be} the base current of the transistor T2, and _Ib2 be I
_b2 = rI _b1, therefore, it becomes _{I b = I b1 + I b2} = (1 + r) I b1 ····· (1). Further, x = I _C / I _b1 (2) From the expressions (1) and (2), I _b · x / (1 + r) = I _C (3) I _C = I _Since it is ₀ / r, I _b · x · r / (1 + r) = I ₀ (4) On the other hand, I ₁ · R ₁ = I ₂ · R ₂ , I ₁ = I _b / α, From I ₂ = I _c / β and I _C = I ₀ / r I _b · R ₁ / α = I ₀ · R ₂ / β (5) From equations (4) and (5), x = {(1 + r) β / α} × R ₁ / R ₂ (6)

X may be set based on the equation (6). When the ratio of the collector current I _C of the transistor Q6 to the base current I _b1 becomes x, an output is generated from the operational amplifier 16 and acts to reduce the gain of the amplifier 8. This prevents the base current of the output transistor T2 from decreasing and V _{CE from} falling below V _CE0 in FIG.

Next, the saturation prevention circuit for the upper output transistor T1 shown in FIG. 2 includes a transistor Q12 corresponding to the transistor Q6 in FIG. 1, a current mirror circuit 18 corresponding to the current mirror circuits 14 and 15 in FIG. 19 is formed of a transistor having a conductivity type opposite to that of FIG. 1, and resistors R3 and R4 corresponding to the resistors R1 and R2 of FIG.
Is realized by the same configuration as that of FIG. 1 except that is connected to the power supply line 20. Reference numeral 17 is an amplifier corresponding to the operation amplifier 16 in FIG.

Transistors Q6 and Q in FIGS.
The base current and the collector current of 12 are detected by passing a current which is a constant fraction of the base current and the collector current as a detection current through the resistors R1, R2, R3 and R4 to determine the ratio of the base current and the collector current inside the IC. Resistance ratio R1 /
It can be detected by R2 and R3 / R4, and the variation of the setting ratio can be made extremely small.

[0026]

As described above, according to the present invention, the output current of the first transistor, which depends on the V _CE of the output transistor, is set to the ratio of the base current without setting the reference voltage of the V _CE of the output transistor in a pseudo manner. Since it is monitored by, more accurate saturation detection can be performed. Therefore, the increase of the reactive voltage can be suppressed to a minimum, the torque of the motor can be sufficiently secured, and when the output transistor is formed by the IC circuit, the heat generated by the reactive power in the IC is reduced, so that the package size and the mounting board are reduced. There is a thermal design margin in the material of the product. Further, since the ratio of the base current of the first transistor and the output current thereof is compared, it is easy to set the operation start of the saturation prevention.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing an output transistor saturation prevention circuit embodying the present invention.

FIG. 2 is a circuit diagram showing another example of a saturation prevention circuit for an output transistor embodying the present invention.

FIG. 3 is a diagram showing a push-pull output circuit having the prevention circuits of FIGS. 1 and 2;

FIG. 4 is a diagram showing static characteristics of an output transistor with a base current as a parameter.

FIG. 5 is a diagram showing a motor drive circuit including a saturation prevention circuit of the present invention.

FIG. 6 is a circuit diagram showing a conventional saturation prevention circuit for output transistors.

FIG. 7 is a circuit diagram showing an output circuit using the output transistor.

FIG. 8 is an explanatory diagram of a conventional example.

[Explanation of symbols]

 1, 2, 3 Hall element 8 Variable gain amplifier 11, 12, 13 Output transistor saturation prevention circuit L1, L2, L3 Motor coil T1, T2 Output transistor Q6 transistor 14 First current mirror circuit 15 Second current mirror circuit R1, R2 resistance 16 operation amplifier

Continuation of the front page (51) Int.Cl. ⁶ Identification code Office reference number FI Technical display location H03F 3/26 8839-5J H03G 3/30 B

Claims (3)

[Claims] 1. A variable gain amplifier, a first current mirror circuit driven by the output of the amplifier, a first transistor which receives an output current of the first current mirror circuit as a base, and the first current mirror. An output transistor having a base receiving an output current of the circuit and having an emitter connected to the emitter of the first transistor and a collector connected to a load; a second current mirror circuit driven by an output current of the first transistor; First for detecting the output current of the first current mirror circuit
Detection means and a second means for detecting an output current of the second current mirror circuit
A saturation prevention circuit for an output transistor, which comprises: detection means; and gain control means for controlling the gain of the amplifier to be narrowed when the detection value of the second detection means exceeds a predetermined value with respect to the detection value of the first detection means. . 2. A saturation prevention circuit for an output transistor that supplies a current to a load, the base of which is connected to the base of the output transistor, the base of which is provided with a first transistor for receiving an input signal. A saturation prevention circuit for an output transistor, comprising control means for controlling a level of the input signal so that the output transistor is not saturated based on a ratio of a base current and an output current. 3. The saturation prevention circuit for an output transistor according to claim 1, wherein the load is a motor coil.