JPH0645837A - Protecting circuit - Google Patents

Abstract

PURPOSE:To improve the problem of not effectively functioning the grounding protection circuit of a power amplifier provided with a boot strap capacitor for grounding generated before an amplifier is activated. CONSTITUTION:In the power amplifier provided with an OFF driver circuit 4 to cut off a power transistor Qp1 for output of the power amplifier provided with a bootstrap capacitor C and an activation circuit 13 to hold the potential of an output terminal 8 at a high potential for prescribed after a power source is turned on, and to allow to be an activating state later, a base current supply circuit 9 is provided to supply a current to the base of the power transistor Qp1 for output when the potential of the output terminal 8 is lowered before the activation. Thus, when the potential of the output terminal 8 is lowered even before the amplifier is activated, a large current flows through the power transistor for output, and an over current detection circuit 5 and the OFF driver circuit 4 are worked.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a protection circuit for use in an audio power amplifier or the like, and more particularly to a protection circuit for preventing the output power transistor of a power amplifier having a bootstrap capacitor from being destroyed when a ground fault occurs.

[0002]

2. Description of the Related Art For example, in an audio power amplifier such as a class B pushle system, when an output terminal is short-circuited to a ground potential (hereinafter referred to as a ground fault), an overcurrent flows through the output power transistor and is destroyed. Therefore, a protection circuit is often provided in order to prevent the output power transistor from being destroyed when a ground fault occurs.

A conventional example of the protection circuit is shown in FIG. 5, and its operation waveforms are shown in FIGS.

In FIG. 5, Q _p1 and Q _p2 are output power transistors, respectively, which are arranged in series between the power supply terminal 6 and the ground and perform a class B push-pull operation.

Reference numeral 1 is an input terminal, and 2 is an input stage amplifier.
The input stage amplifier 2 has one end connected to the power supply terminal 6 through the diode D ₁ and the other end connected to the output power transistor Q.
_The upper output transistor Q _p1 is tributed through the driver transistor Q ₁ connected to the base of _p1 .

The input-stage amplifier 2 also tributes the lower output transistor Q _P2 (although the lower driver transistor is included therein), and also the output terminal 8
A class B push-pull operation signal is output to.

The connection point between the diode D ₁ and the driver transistor Q _{1 is} the bootstrap terminal 7, and the bootstrap capacitor C is connected between the bootstrap terminal 7 and the output terminal 8.

When the starting circuit 3 is output in an unstable state immediately after the power source Vcc is turned on, an unpleasant sound is generated. To prevent this, as shown in FIG. 6 (b), the potential of the output terminal 8 is kept constant for a certain period of time. Is kept high and the output power transistors Q _p1 and Q _p2 are controlled via the input stage amplifier 2 in order to start the amplifier at the timing when the circuit state becomes stable.

This power amplifier has an overcurrent detection circuit 5 composed of a transistor Q ₂ and a resistor R ₁ , and an off-driver circuit 4 which turns off the output power transistor Q _p1 in response to the detection signal. The output power transistor Q _p1 is prevented from being destroyed when a ground fault occurs.

The ground fault protection operation will be described below with reference to FIGS. 5 and 6.

The state in which the power supply Vcc is turned on at time t ₁ and the start-up circuit 3 suppresses the start-up for a certain period of time and the start-up starts at time t ₂ when there is no input signal is as follows: a) bootstrap terminal The potential of 7 is high. (See FIG. 6 (a)) b) The potential of the output terminal 8 was held in a high state before the start, but after the start of the start (time t ₂ ), the midpoint potential (about Vc
c / 2) is stable. (See FIG. 6B) c) Therefore, the voltage across the bootstrap capacitor C is V ₁ which is lower for a while than immediately after the power is turned on, and after the start-up, the voltage V ₂ is about half of Vcc. (Refer to Figure (c)) More specifically, if the power supply voltage Vcc is 13.2V,
V ₁ = 0.9 V (voltage between EV of Q _p1 + voltage between CE of Q ₁ )
V ₂ = 5.9 V (voltage of Vcc −D ₁ −midpoint voltage).

In the above state, at time t3,
When the output terminal 8 is grounded due to some abnormality, a large short-circuit current flows through the upper output power transistor Q _P1 . (See FIG. 6D) As a result, the emitter-base voltage VEB of the output transistor Q _p1 increases, and the transistor Q ₂ forming the overcurrent detection circuit 5 is turned on to trigger the off-driver circuit 4.

In the off-driver circuit 4, the PNP transistor Q ₃ and the NPN transistors Q ₄ and Q ₅ are all OFF during normal operation (before occurrence of ground fault). When the transistor Q ₂ is turned on due to the occurrence of a ground fault, a current flows through the route of the power supply terminal 6 → the resistance R ₂ → the transistor Q ₂ → the resistance R ₁ → the output terminal 8, and the power supply has a predetermined value (for example, the output transistor Q for 1 mSec). When the current exceeds the detection current value corresponding to the short-circuit current value at which it breaks if it flows to _p1 , the voltage across the resistor R ₂ turns on the transistor Q ₃ , which turns on the transistor Q ₄ . Transistor Q ₄ is ON
Then, the potential of the base of the transistor Q ₃ decreases,
After that, even if the current of the overcurrent detection circuit 5 disappears, the transistor Q ₃ maintains the ON state. (See Fig. 6 (e) a) When transistor Q ₃ turns on, transistor Q ₅ turns on.
And, to prevent the destruction continues to OFF the output power transistor Q _p1 lowered the base of the potential of the output power transistor Q _p1. (See Figure 6 (d))

However, in the conventional circuit, if a ground fault occurs before the activation of the output terminal, the protection circuit does not operate and the output power transistor Q _P1 is destroyed.

The above problems will be described below with reference to FIG.

If a ground fault occurs before the start-up as shown in FIG. 7 (b), the voltage across the bootstrap capacitor C is as low as about 0.9 V (see FIG. 7 (b)) before the ground fault occurs. The potential of the bootstrap terminal 7 at the moment of occurrence of the fault (time t ₆ ) is
It goes down to 0.9 V and then recovers with a time constant.
(See FIGS. 7 (a) and 7 (C)) Therefore, at time t ₆ , the driver transistor Q ₁ is turned off and the output transistor Q _p1 is also turned off, so that no short-circuit current flows. (See FIG. 7D) Therefore, the overcurrent detection circuit 5 does not function. afterwards,
The recovery of the potential of the bootstrap terminal 7 causes a short current to increase in the output power transistor Q _p1 .
(See FIG. 7 (d)) However, the time required for the current value to reach the level detected by the overcurrent detection circuit 5 usually takes several msec to several msec although it depends on the impedance of the ground fault path.

Normally, the detection level of the overcurrent detection circuit 5 is set to a large current value which destroys the output power transistor Q _P1 when the current is applied to the output power transistor Q _P1 for 1 msec. When it flows for a long time of msec, the power transistor Q _p1 is destroyed. (Fig. 7 (d)
Further, lowering the detection level of the overcurrent detection circuit 5 causes an erroneous operation during a large swing operation in a normal load, which is not preferable.

Therefore, an object of the present invention is to provide a protection circuit capable of protecting the output power transistor even if a ground fault occurs before the output terminal is activated.

[0018]

A first means for solving the above-mentioned problems is to maintain an output power transistor, a bootstrap capacitor, a terminal potential at a high level for a predetermined time after power-on, and then to a start-up state. A startup circuit,
A protection circuit for a power amplifier having an overcurrent detection circuit for detecting an abnormal current of the output power transistor and an off-driver circuit for shutting off the output power transistor in response to a detection signal of the overcurrent detection circuit. So, when the potential of the output terminal drops due to a ground fault before startup,
A protection circuit is characterized in that a base current supply circuit that supplies a current to the base of the output power transistor is provided.

A second means is an output power transistor, a bootstrap capacitor, a starter circuit for holding the output terminal potential at a high level for a predetermined time after power-on and then starting it, and an abnormal current of the output power transistor. Is a protection circuit of a power amplifier having an overcurrent detection circuit for detecting the above, and an off-driver circuit that cuts off the output power transistor in response to a detection signal of the overcurrent detection circuit. The protection circuit is provided with a potential detection circuit that detects a decrease in the potential of the bootstrap terminal or the output terminal and operates the off-driver circuit.

[0020]

According to the first means, when the output terminal is grounded before the start-up, the base current is supplied to the output power transistor by the base potential supply circuit. Therefore, even if the potential of the bootstrap terminal is lowered, it is instantaneously performed. An overcurrent is generated in the output power transistor, and as a result, the overcurrent detection circuit also operates instantaneously, and the off-driver circuit shuts off the output power transistor, thereby preventing the output power transistor from being destroyed.

According to the second means, when the output terminal is grounded before the start-up, the output terminal potential is lowered, and the potential of the bootstrap terminal is instantly lowered by being pulled by the bootstrap capacitor. Since the off-driver is detected and the output power transistor is cut off, the output power transistor is prevented from being destroyed.

[0022]

Embodiments of the present invention will be described with reference to the drawings.

FIG. 1 is a circuit diagram of an embodiment according to the first means, and FIG. 2 shows its operation waveform.

1 is different from the conventional circuit diagram and FIG. 5 in that a base current supply circuit 9 is additionally provided and that the starting circuit 1 is provided.
3 has a function of controlling the base power supply circuit 9 to be inoperative after the amplifier is started, in addition to controlling to start the output terminal 8 after holding the potential of the output terminal 8 at a high level for a predetermined time after turning on the power (Vcc). Since only the points are provided, the same parts are designated by the same reference numerals and the description thereof will be omitted.

An example of the base current supply circuit will be described based on the embodiment shown in FIG. A series circuit of the resistor 5 and the Chener diode D ₂ is provided between the power supply terminal 6 and the ground to serve as a reference potential power supply by the Chener diode D ₂ . The emitter of the NPN transistor Q ₆ whose base is connected to the reference potential and whose collector is connected to the power supply terminal 6 is connected to the base of the output power transistor Q _p1 via the resistor R ₆ .

Then, the reference potential power source (base of the transistor Q ₆ ) is controlled to be connected to the ground via the starting circuit 13 (although not connected before starting) after starting.

The operation will be described.

As shown in FIG. 2 (b), if a ground fault occurs at time t ₆ before activation, the boot transistor terminal 7
The potential is reduced, and then to recover to the output power transistor Q _p1 since the moment of (see FIG. 2 (a)) land絡発students also lowered at the same time the potential of the bootstrap terminal 7 and the lowering of the potential of the output terminal 8 Base current cannot be supplied from the driver transistor Q ₁ .

However, since the potential of the output terminal 8 becomes sufficiently lower than the reference potential of the base current supply circuit 9 (voltage of the Zener diode D ₂ ), the transistor Q ₆ is turned on.
However, a current is supplied from the base current supply circuit 9 to the base of the output power transistor Q _p1 and a large short circuit current flows. (See FIG. 2 (d)) Therefore, the overcurrent detection circuit 5 and the off-driver circuit 4 operate in the same manner as when a ground fault occurs after activation in the conventional circuit diagram 5,
The output power transistor Q _P1 is cut off instantly to prevent destruction. (See Fig. 2 (d) (e)) If there is no ground fault before starting and the amplifier starts normally,
After the start-up, the base current supply circuit 9 is cut off by shorting the base of the transistor Q ₆ to the ground via the start-up circuit 13, so that it has no relation to the operation of the amplifier. Further, with respect to the ground fault generated after the start-up, the conventional circuit operates in the same manner as in FIG. 5 to prevent the output power transistor Q _p1 from being destroyed.

An embodiment of the present invention according to the second means will be described.

FIG. 3 is a circuit diagram of the embodiment, and FIG. 4 shows its operation waveform. In FIG. 3, as compared with the conventional circuit (FIG. 5), a potential detection circuit 10 is additionally provided, and the starting circuit 1
3 is the detection terminal 8 for a predetermined time after turning on the power (Vcc) as in the conventional case
In addition to the function to start after holding the potential of the high potential,
Except for the point of having the function of shutting off the potential detection circuit 10 after the start-up, it is the same as that of FIG.

The potential detecting circuit 10 has a reference potential power source composed of a resistor 7 and a Chener diode D ₃ between the power supply terminal 6 and the ground, the emitter is connected to the reference potential, and the base is booted via the resistor R _8. The collector of the PNP transistor Q ₇ connected to the strap terminal 7 is connected to the collector of the transistor Q ₃ of the off-driver circuit 4 and the transistor Q ₄ ,
It is connected to the connection point of the base resistors R ₃ and R _{4 of} Q ₅ .

Then, the emitter of the transistor Q ₇ is connected to the ground via the starting circuit 13 (although it is not connected before starting), after starting. This starting circuit may be the same as that of the first embodiment.

The operation will be described.

If a ground fault occurs at time t ₆ before activation after power is turned on, the potential of the output terminal drops and is pulled by the bootstrap capacitor C, and the potential of the bootstrap terminal 7 also drops instantaneously. (FIG. 6 (a), (b) refer) Therefore, the potential of the bootstrap terminal 7 than the reference potential of the bootstrap potential detection circuit 10 becomes sufficiently low, the transistor Q ₇ is turned ON between instantaneous.

[0036] Therefore, the transistor Q ₄ is ON OFF driver circuit 4, also ON transistor Q _3, and holds the state.

The transistor Q ₃ is then transistor Q ₅ is also ON when ON, to lower the base potential of the output power transistor Q _p 1, by blocking the output power transistor Q _p1 instantaneously, to prevent destruction . (See FIGS. 4 (d) and 4 (e)) If there is no ground fault before starting, and if starting normally, the starting circuit 13 connects the emitter of the transistor Q ₇ to the ground and shuts off the bootstrap potential detection circuit 10. Therefore, similarly to the conventional circuit (FIG. 5), the output power transistor Q _p1 can be effectively prevented from being destroyed with respect to the subsequent ground fault, and the function of the amplifier is not inconvenient. In this embodiment, the potential drop of the bootstrap terminal is detected,
You may detect the electric potential of an output terminal directly.

In that case, it suffices to connect the base of the transistor Q ₇ to the output terminal 8 via the resistor R ₈ .

[0039]

As described above, according to the present invention, it is possible to effectively prevent the output transistor from being destroyed even against a ground fault that occurs before activation.

[Brief description of drawings]

FIG. 1 is a circuit diagram of an embodiment of the present invention.

[Figure 2] Operation waveform diagram

FIG. 3 is a circuit diagram of another embodiment of the present invention.

[Figure 4] Operation waveform diagram

FIG. 5 Conventional circuit diagram

[Fig. 6] Operation waveform diagram

FIG. 7 is an operation waveform diagram when a ground fault occurs before activation in a conventional circuit.

[Explanation of symbols]

1 Input Terminal 2 Input Stage Amplifier 3,13 Startup Circuit 4 Off Driver Circuit 5 Overcurrent Detection Circuit 6 Power Supply Terminal 7 Bootstrap Terminal 8 Output Terminal 9 Base Current Supply Circuit 10 Bootstrap Potential Detection Circuit Q _p1 , Q _p2 Output Power transistors Q ₁ to Q ₇ transistor D ₁ diode D _2. D ₃ Zener diode R _{1 to} R ₈ resistance C Bootstrap capacitor t ₁ Power-on time t 2 Output start time t 3 Ground fault occurrence time after start t 6 Ground fault occurrence time before start

Claims (3)

[Claims] 1. An output power transistor, a bootstrap capacitor, a starter circuit for holding an output terminal potential at a high level for a predetermined time after power-on and then in a start-up state, and an overcurrent detection circuit for detecting an abnormal current of the power transistor. And an off-driver circuit that shuts off the power transistor in response to a detection signal of the detection circuit, wherein the base of the power transistor has a potential of the output terminal due to a ground fault before starting. A protection circuit comprising a base current supply circuit for supplying a current to the base of the power transistor when the voltage drops. 2. The protection circuit according to claim 1, wherein the base current supply circuit does not apply a base current to the power transistor when the potential of the output terminal decreases due to a ground fault after the activation. 3. An output power transistor, a bootstrap capacitor, a starter circuit that holds the output terminal potential at a high level for a predetermined time after power-on, and then puts it in an activated state, and detects an abnormal current in the output power transistor. A power amplifier protection circuit having an overcurrent detection circuit and an off-driver circuit that cuts off the output power transistor in response to a detection signal of the overcurrent detection circuit, the bootstrap circuit in case of a ground fault before starting. A protection circuit provided with a potential detection circuit for detecting a decrease in the potential of a terminal and activating the off-driver circuit.