JPS6029223Y2 - Overvoltage protection circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an overvoltage protection circuit for a solid state amplifier, and particularly to an overvoltage protection circuit that is effective for monolithic integration.

Solid-state audio amplifiers are vulnerable to overvoltages such as surge voltages superimposed on the power supply.

Therefore, the solid state amplifier is provided with an overvoltage protection circuit.

FIG. 1 is a circuit diagram of a conventional monolithically integrated audio solid-state amplifier with a protection circuit.

The amplification function of the circuit is to amplify the input signal at the preamplification stage A1, and then use the diodes D2. D3, resistor R4, transistor T
3. T.

Power is converted by a so-called quasi-complementary 5EPP circuit composed of T, and the power is supplied to the load resistor RL.

Here, CO is a coupling capacitor for cutting direct current.

In FIG. 1, a diode D1 for constant voltage generation, a transistor T1. T2, resistors R1, R2°R3 constitute an overvoltage protection circuit.

The setting conditions for resistors R2 and R3 are 3 R2-4-R,” Vc, ≦VDI + VBEI
When set to "" (1), the transistor T□ becomes cut-off.

However, in the above formula (1), ■. . 1 is the power supply voltage during normal operation, ■o1 is the Zener voltage of diode D, Vag
□ is the base-emitter voltage of the transistor T□.

If the transistor T□ is cut off, the transistor T2 is also cut off in a chain manner, so this overvoltage protection circuit has no effect on the amplification function.

On the other hand, if abnormal voltage such as surge voltage is added to the power supply voltage ■.

. When the following conditions are satisfied for a power supply voltage of 2, transistors T1 and T2 turn on and transistor T3
The base potential of the amplifier is clamped to zero potential to stop the amplification function and protect the amplifier circuit from overvoltage.

Generally, the withstand voltage of a transistor is BVCBO=BVoms
>BVoEs, and with the above protection, the withstand voltage of the output transistor increases to BVoEs.

R... (2) Suspension support'VCC2>VDI + VBEIHowever, in the above conventional circuit, the operation start voltage, which is the power supply voltage at which the protection circuit starts operating, and the operation end voltage, at which the protection circuit stops operating, are the same voltage. , after the power supply voltage rises and the protection circuit operates,
If the power supply voltage becomes even slightly lower than the operation start voltage, the protection circuit will no longer operate.

Therefore, when the power supply voltage slightly fluctuates up and down near the operation start voltage, the protection circuit repeatedly operates and deactivates, and the potential of the collector of transistor T2 connected to the base of transistor T3 is amplified in a short time. The potential repeatedly fluctuates greatly between the potential when the circuit performs the amplification function and the zero potential when the protection circuit operates.

In other words, when the power supply voltage rises to a critical voltage near the protection circuit's operation start voltage, the continuous operation and non-operation of the amplifier circuit causes oscillation as seen from the output side of the amplifier circuit. It becomes unstable.

The present invention is a new invention to improve the above-mentioned drawbacks of the conventional technology, and its purpose is to create an overvoltage protection circuit for solid-state amplifiers that operates stably near the operation start voltage, which is the power supply voltage at which the protection circuit starts operating. It is about providing.

In order to achieve this purpose, the overvoltage protection circuit of the present invention uses a series circuit in which a first load is interposed between the base-emitter junction of a first transistor having two collectors and a low voltage generation circuit as a power supply. inserted between said first
The emitter of the first transistor is connected to the high potential side through means for making the potential of the emitter of the transistor follow the fluctuation of the potential on the high potential side of the power source, and the base of the first transistor and the low power source of the first transistor are connected to each other. A second load and a second
Connect the collector and emitter of the transistor in series,
The collector output of the first transistor causes the second
The base of the transistor is controlled so that the off voltage of the first transistor when the power supply voltage goes from a high potential to a low potential is lower than the on voltage of the first transistor when the power supply voltage goes from a low potential to a high potential. The second embodiment is characterized in that the operation/non-operation of the amplifier is controlled by the output of the other collector of the first transistor.Examples will be described in more detail below.

FIG. 2 is a circuit diagram of a solid state amplifier equipped with a monolithically integrated overvoltage protection circuit according to the present invention, and the same parts as in FIG.

In Figure 2, T1' is a PNP transistor with two collector electrodes in its collector region. One collector is connected to the base of transistor T2, and the other collector is connected to the base of transistor T6. .

Connection point between resistor R1 and diode D1 and transistor T1
A resistor R5 is connected between the bases of the transistor T
The base of □' and the collector of transistor T6 are connected to resistor R6.
connected via.

Note that the emitter of the transistor T6 is grounded.

Next, the operation of the circuit shown in FIG. 2 will be explained.

Power supply voltage ■.

0 is low, transistors T1' and T6 are both cut-off, so transistor T
2 also acts as a chain cutoff and does not affect the amplification system.

The condition for transistor T1' to turn on is lX
Vcct'=VDt+VBEI'""(
3) R2+R.

becomes.

However, vcct' is the power supply voltage at which the protection circuit in FIG. 2 starts to operate, that is, the on-voltage of the transistor T□', and 8.1' is the base-emitter voltage of the transistor T1'.

When an overvoltage is applied to the circuit and transistor T1' is turned on, transistor T2 is turned on and transistor T3 is turned on.
The operation of clamping the base of the amplifier to protect the amplifier circuit is the same as the conventional one.

As described above, when transistor T1' is turned on, transistor T6 is also turned on.

For this reason, a current path of resistor B□→resistor R6→resistor R6→transistor T6 is created, and a voltage drop occurs due to resistor R5.

Therefore, the base of the transistor T1' is biased in the forward direction, so that the ON condition of the transistor T2 is accelerated and instantaneously becomes saturated.

On the other hand, the power supply voltage ■.

In a state where C drops from the high voltage side, transistor T1'
■ The power supply voltage that transitions from on to off.

. 2', that is, the off-state voltage of transistor T1' is Xxv
o.

2'≦VDI +VBEI ''...(4)
-to- R2+ R3R3+ R6.

As is clear from equations (3) and (4) above, the power supply voltage ■.

The operation end voltage, at which the power supply voltage starts to drop from the high power supply voltage side and the overvoltage protection circuit becomes inoperable, is lower than the operation start voltage, at which the overvoltage protection circuit begins to operate, when 0 begins to rise from its normal value, and the overvoltage protection circuit becomes inoperable. The overvoltage protection circuit according to the invention has hysteresis characteristics in the power supply detection voltage.

Therefore, according to the overvoltage protection circuit according to the present invention, even if the power supply voltage is around the critical voltage near the operation start voltage of the overvoltage protection circuit, after the protection operation starts, the power supply voltage is lower than the operation start voltage. Since the protection operation continues until the output of the amplifier circuit drops to 1000 Ω, the output of the amplifier circuit does not become unstable with repeated large fluctuations in a short period of time, as occurs when the output of the amplifier circuit oscillates.
The output of the amplifier circuit can be stably obtained.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram of a solid state amplifier equipped with a conventional overvoltage protection circuit, and FIG. 2 is a circuit diagram of a solid state amplifier equipped with an overvoltage protection circuit according to the present invention. In the figure, T□', T2', and T6 are transistors, Dl is a diode, and R1, R5, and R6 are resistors.

Claims (1)

[Claims for Utility Model Registration] The base of the first transistor having two collectors.
A series circuit with a first load interposed between the emitter junction and the constant voltage generation circuit is inserted between the power supplies, and the emitter potential of the first transistor is changed to a potential on the high potential side of the power supply. the emitter is connected to the high potential side via means for causing the emitter to follow the voltage, and a second load and a collector and emitter of the second transistor are connected between the base of the first transistor and the low potential side of the power supply. connected in series, the base of the second transistor is controlled by the collector output of the first transistor, and the power supply voltage is higher than the on-voltage of the first transistor when the power supply voltage goes from a low potential to a high potential. An overvoltage protection circuit characterized by lowering the off-voltage of a first transistor when going from a potential to a low potential, and controlling operation/non-operation of an amplifier by the other collector output of the first transistor.