JPS6159003B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to protection of power transistors in circuits using power transistors such as audio power amplifiers, motors, and drive control circuits.

When an abnormal situation occurs in an audio power amplifier or the like, such as a load short circuit or the application of an abnormally high voltage, the active elements such as transistors forming the circuit become overloaded, exceeding their durability limits and may be destroyed. Conventionally, various protection circuits have been devised in preparation for such situations. In particular, many protection circuits have been proposed to protect output stage power transistors that are prone to overload. The effectiveness of the protection circuit will vary depending on the specific protection method, but
Recently, so-called operating range limiting type protection circuits have become common, which limit the moving range of the operating point of an output transistor so that the operation of a power amplifier does not go out of a predefined operating range.

In such a case, the current of the output transistor is detected using the potential difference generated in a resistor inserted between the emitter of the output transistor and the output or between the emitter and ground, and the collector is detected via the resistor connected to the collector of the output transistor. Generally, a method was adopted in which the voltage was detected, the potential difference and the collector voltage were added together, and the result was applied to the control transistor. However, even if the resistor inserted into the emitter of the output transistor (hereinafter referred to as emitter resistor) has a small value, it causes a relatively large voltage drop due to the output current, resulting in poor power utilization and a decrease in maximum output. For this reason, it is mounted on a car.
In power amplifiers used in so-called on-vehicle equipment that use a 12V battery power supply, the emitter resistor of the output transistor is usually omitted in order to output as much power as possible. In such a case, the collector current is detected indirectly by detecting an increase in the voltage between the base and emitter of the output transistor, and the detected voltage is applied to the control transistor.
In such a protection circuit, when the collector voltage becomes extremely high, the base current flows into the output transistor, which should be cut off, through the resistor connected to the collector and the resistor connected between the base and emitter of the output transistor. The inventor's experiments have revealed that as a result of the output transistor becoming conductive in a high voltage range, so-called secondary breakdown occurs, leading to destruction.

The first object of the present invention is to provide a protection circuit that can prevent the above situation from occurring and provide complete protection.

The second objective is to provide a means for achieving the first objective simply and reliably at a lower cost without adding any particularly large-scale circuit functions.

FIG. 1 shows an embodiment of a protection circuit according to the present invention, and is an embodiment applied to an audio power amplifier circuit.
In the figure, an input capacitor C ₁ is connected to the base of a PNP type transistor Q ₁ biased by resistors R ₁ and R ₂ , and a resistor R ₃ is connected to the collector of the transistor Q ₁ . Capacitor C ₂
is provided for the purpose of removing power supply ripples, one end is connected between resistors _R6 and _R1 , and the other end is grounded. A feedback signal from the output OUT is applied to the emitter of the transistor _Q1 by a resistor _R5 and a division ratio formed by a series connection of a capacitor _C3 and a resistor _R4 . NPN transistor Q ₂ is transistor Q ₁
It further amplifies the output signal and sends the output signal generated at the resistors R ₇ and R ₁₂ connected to the collector side to the next class B push-pull power amplification stage. Capacitor _C4 is inserted between the collector and base of transistor _Q2 for phase compensation.

Diodes D ₁ to D ₃ are located between the collector of transistor Q ₂ and resistor R ₇ , and are connected to an NPN type transistor Q ₄ for positive half-cycle amplification, and an output transistor.
Q ₆ , a PNP transistor Q ₇ for negative half-cycle amplification, an NPN transistor Q ₉ , and a bias diode for providing an appropriate current during no signal to the output transistor Q ₁₁ . The collector of the transistor _Q4 is connected to the resistor _R12 , and the emitter is connected to the base of the output transistor _Q6 and to the midpoint P via the resistors _R9 and _R10 . The emitters of transistors Q ₆ and Q ₇ and the collectors of transistors Q ₉ and Q ₁₁ are connected to the midpoint P at the same time, and their potential is kept at Vcc/2, half of the power supply voltage Vcc, by design. In addition, the midpoint P reaches the output terminal OUT via the output coupling capacitor _C5 , and the load
R ₁₇ is connected. Further, a capacitor _C6 is connected between the output terminal OUT and the resistor _R12 for a puto strap. The emitter of the transistor Q ₉ is connected to the base of the output transistor Q ₁₁ , and is also connected to a series circuit of resistors R ₁₄ and R ₁₅ and grounded.

Since the operating principle of a power amplifier circuit having such a configuration is generally well known, the explanation thereof will be omitted.

FIG. 1 further includes, as a protection circuit, a control transistor Q ₃ whose base is connected to the connection point between resistors R ₉ and R ₁₀ that constitute the first resistance circuit R _A1 ;
A control transistor Q ₃ whose base is connected to the connection point of resistors R ₁₄ and R ₁₅ that constitute the first resistance circuit R _A2
are shown, and resistors R ₈ and R ₁₃ are connected to the bases of these transistors to detect the collector voltages of output transistors Q ₆ and Q ₁₁ , respectively. The circuit including resistors R ₈ and R ₁₃ constitutes second resistance circuits R _B1 and R _B2 . The collector of transistor Q ₃ is the base of transistor Q ₄ , the transistor
The collector of Q ₈ is connected to the base of transistor Q ₂ . These two protection circuits have traditionally been considered useful because they have a small number of elements and can easily provide protection functions.

Regarding the operation of such a protection circuit, first, the protection operation for the output transistor Q ₆ for positive half-cycle amplification is to create a voltage according to the collector current of the transistor Q ₆ using resistors R ₉ and R ₁₀ , and to generate a voltage corresponding to the collector current of the transistor Q 6 using the resistor R ₈ . This is accomplished by detecting the collector voltages of transistor _{Q 6} and controlling transistor Q ₃ and the input to transistor Q ₅ with each of these voltages. The protective operation for the output transistor Q ₁₁ for negative half-cycle amplification is to create a voltage according to the collector current of the transistor Q ₁₁ using resistors R ₁₄ and R ₁₅ , detect the collector voltage using the resistor R ₁₃ , and convert these voltages. This is achieved by controlling transistor _Q8 by controlling the input to transistor _Q2 .

The operation of this protection circuit will be explained in more detail with reference to FIG. Now, in Figure 1, B is centered on the midpoint P.
It is convenient to think of the class push-pull circuit as being divided into an upper and a lower half.

Figure 2 shows the load curve of the output transistor _Q6 when considered separately in this way. Now, in the normal operation state of the amplifier circuit where the protection circuit does not operate, the load characteristics of the transistor _Q6 are as shown by the curve R _L , and the DC potential at the midpoint P is
Centering around Vcc/2, the left side is conductive and the right side is disconnected. Here, Vcc/2 of load curve R _L
The bulge on the left side is due to an operating state with an input signal relatively lower than 1KHz, and is caused by a phase difference between the collector voltage and collector current due to the reactance of the output coupling capacitor _C5 .

Now, if the load is short-circuited, the resistance R ₁₇ = 0, and in the circuit shown in Figure 1, the output coupling capacitor C ₅ is the only apparent load, so the phase difference between the collector voltage and collector current reaches almost 90°. result,
In the absence of a protection circuit, the load curve R′ _L is equal to the supply voltage.
Rising from Vcc, collector-emitter voltage V _CE
It should draw a circular trajectory leading to =0. However, in reality, the transistor often enters the secondary breakdown region SB soon after rising from the power supply voltage Vcc, or the transistor itself is destroyed due to excessive collector current. In FIG. 1, the control transistor _Q3 serves to limit the operating range of the output transistor _Q6 to prevent destruction.

Now, if the load is short-circuited while there is an input signal, the signal voltage between the base and emitter of transistor _Q6 increases and a large current attempts to flow through the collector. However, the rise in the base-emitter voltage of transistor Q ₆ appears as a voltage divider at the connection point of resistors R ₉ and R ₁₀ , and the collector voltage is detected by resistor R _8. For example, in FIG. 2, transistor Q ₆ The collector-emitter voltage is Vcc/2
If it is on the right side, that is, before the load is short-circuited, the operating point passes through the power supply voltage Vcc point, and the current suddenly tries to flow from here, but the resistance
_R8 detects that the collector voltage is Vcc, and the control transistor _Q3 becomes conductive, causing the transistor
Short-circuit the Darlington-connected base and emitters of Q ₄ and Q ₆ to keep the transistors Q ₄ and Q ₆ in the damped state.

This state continues from voltage Vcc to point B. The straight line including points B, C, and D is on the axis of the collector-emitter voltage _VCE , but in FIG. 2, it is shown floating above the axis for ease of explanation. Now, the operating point that has returned to point B takes the form of a straight line I that rises from there toward point A.

The slope and position of this straight line I are transistor Q ₃ ,
It is determined by the base-emitter voltage-collector current characteristics of Q ₆ and the values of resistors R ₈ , R ₉ , and R ₁₀ .

The lower protection circuit, centered around control transistor _Q8 , operates to cut off transistor _Q2 , which ultimately causes the output transistor to
It is the same that Q ₁₁ is cut off and protected.
Although such so-called operating area limiting type protection circuits are well known and used, the one that detects the voltage between the base and emitter of the output transistor as shown in Figure 1 has been found to have no problems through experiments conducted by the present inventor. point was confirmed.

Now, when used in in-vehicle equipment, an abnormally high surge voltage may be superimposed on the original battery voltage (for example, 12V). This can be caused by a car's headlights turning on and off, a generator malfunctioning,
This may occur due to poor contact with the battery terminal, turning the starter on or off during startup, etc.
It is known that it can reach close to 100V.

In such a case, of course, the control transistor _Q3 operates to cut off the transistor _Q4 so as to force the operating point of the transistor _Q6 into the straight line region between points B and D in FIG.

In this case, the voltage applied from the power supply voltage Vcc to the transistor Q ₃ through the resistor R ₈ also reaches the base of the transistor Q ₆ through the resistor R ₉ because the transistor Q ₄ is interrupted.

According to experiments conducted by the present inventors, it was found that the base-emitter voltage of transistor _Q6 easily exceeds 1V, although it depends on how the power amplifier is designed. Due to this voltage, current begins to flow through transistor _Q6 again, and the protection circuit connects point B → point C →
The operating point, which is supposed to move like point D, will move in the direction shown by the dotted line and arrow from point C on the way.
It is known that in such a high voltage region, secondary breakdown may occur when the collector current is relatively small, and in that case, the output transistor _Q6 will be destroyed.

In view of this point, the present invention provides a second cut-off circuit to prevent the output transistor Q ₆ from becoming conductive again when an abnormally high voltage is applied to the power supply, and acts directly on the base of the output transistor Q ₆ to prevent this. It's something to break down. That is, the portion surrounded by dotted lines 1 and 2 in FIG. 1 is the portion that characterizes the present invention.

The emitters of control transistors Q ₅ and Q ₁₀ are connected to the emitters of output transistors Q ₆ and Q ₁₁ , respectively.
The collectors of transistors Q ₅ and Q ₁₀ are connected to the bases of output transistors Q ₆ and Q ₁₁ , respectively, and the bases of transistors Q ₅ and Q ₁₀ are connected to resistors R ₁₁ and
It is connected to the connection point between resistors R ₉ and R ₁₀ and the connection point between resistors R ₁₄ and R ₁₅ via R ₁₆ respectively.

Now, the operation of the complete protection circuit with the additional protection circuits connected within the dotted lines 1 and 2 will be explained with respect to the upper circuit. As mentioned above, when a power surge such that the operating point is on a straight line in Fig. 2 is applied to the upper transistor _Q6 , the driver transistor _Q4
The surge voltage then turns on the control transistor Q ₃ through the resistor R ₈ and is also applied to the base of the output transistor Q ₆ through the resistor R ₉ . At this time, a surge voltage is also applied to the base of the second control transistor _Q5 through the resistor _R11 , so the transistor _Q5 becomes conductive and the transistor _Q6
Therefore, in Figure 2, the operating point _of transistor _Q6 does not rise in a straight line, but rises from point B to point C as in a straight line. → Proceed in the direction of point D. In general, this type of protection circuit protects transistors that are exposed to high voltages from a base-emitter short circuit, which has the highest collector-emitter breakdown voltage (V _(BR)CES) It is protected by keeping it in the same condition and providing a margin for pressure resistance. For this reason,
The protection circuit only has a protective function up to point D of this voltage V _(BR)CES .

However, the base-open collector-emitter breakdown voltage V _{(B
R) It is more than twice as high as CEO} and provides sufficient protection against surge voltage.

It can also be seen that the lower protection circuit, dotted line 2, has a similar function and protects the transistor _Q11 .

In terms of circuit configuration, it is also related to the design of the protection circuit centered on transistors Q ₃ and Q ₈ , which is the first protection circuit, but the base of the control transistors Q ₅ and Q ₁₀ , etc., which is the second protection circuit, is also related to the design. The connection point is the connection point of resistors R ₉ and R ₁₀ and the connection point of resistors R ₁₄ and R ₁₆ as shown in Figure 1.
without being limited to the connection point of resistor R ₉ or
It may be connected to the dividing point of R ₁₀ or to the dividing point of resistors R ₁₄ or R ₁₅ by an appropriate design.

Specifically, as shown in Figure 3a, for example, three resistors R ₉ , R ₁₉ , and R ₁₀ are connected in series between the base and emitter of transistor Q ₆ , and the junction of resistors R ₉ and R ₁₉ is connected to the transistor Q ₅ . The connecting portions of the resistors R ₁₉ and R ₁₀ may be connected to the base of the transistor Q ₃ , respectively, and each connecting point may be a dividing point of the resistive element as shown in FIG. 3b. In Figure 3b, the connection point between resistance elements R ₉ and R ₁₉ is at the base of transistor Q ₃ , and the connection point between resistance elements R ₁₉ and R ₁₀ is at the base of transistor Q 3.
Each is connected to the base of _Q5 . Furthermore, both the resistors R ₁₁ and R ₁₆ in the embodiment shown in FIG. 1 and the resistor R ₁₁ in the embodiment shown in FIG. 3 a can be omitted.

As described above, according to the present invention, current detection of the output power transistor is performed using the base-emitter voltage of the output power transistor, and the protection circuit is configured with a relatively simple circuit, so it is inexpensive. Moreover, it has the effect of realizing complete protection against power surges without causing any output power loss.

[Brief explanation of the drawing]

Fig. 1 is an electrical connection diagram showing one embodiment of the circuit of this invention, Fig. 2 is an operation explanatory diagram, and Figs. 3a and 3b are electrical connection diagrams showing main parts of another embodiment of the circuit of this invention. . In the figure, Q ₆ and Q ₁₁ are output transistors, Q ₄ and Q ₉
is a drive transistor, R _A1 and R _A2 are first resistance circuits, R _B1 and R _B2 are second resistance circuits, Q ₃ and Q ₈ are first protection transistors, and Q ₅ and Q ₁₀ are second protection transistors. It is a transistor. Note that the same reference numerals in the figures indicate the same or corresponding parts.

Claims (1)

[Scope of Claims] 1: at least one output transistor; a drive transistor for driving the output transistor; connected between the base and emitter of the output transistor, generating a divided voltage according to the collector current of the output transistor; a first resistor circuit connected to the collector of the output transistor;
a second resistor circuit that derives a detection voltage according to the collector voltage; a first protection transistor whose bases are connected to the first and second resistor circuits and which controls the input to the drive transistor; A transistor protection circuit comprising a second protection transistor having a collector circuit connected between a base and an emitter of the output transistor, the base of which is connected to the first and second resistor circuits. 2. The output transistor includes a pair of output transistors for performing push-pull operation, and the first and second resistance circuits and the first and second protection transistors correspond to each of the output transistors. provided,
A transistor protection circuit according to claim 1. 3. The first resistance circuit includes first to third resistance elements, connection portions of the first to second resistance elements are connected to the base of the first protection transistor, and the first and second resistance elements are connected to the base of the first protection transistor. a connecting portion of the resistive element is connected to the base of the second protection transistor;
A transistor protection circuit according to claim 1 or 2.