JPH04156104A - Transistor protecting circuit for high frequency power amplifier - Google Patents

Abstract

PURPOSE:To resolve the overload state of a transistor TR for the stop of a PWM(pulse width modulation) signal by connecting a discharge circuit to a power supply terminal and controlling this discharge circuit to short-circuit the power supply terminal in case of abnormality. CONSTITUTION:A discharge circuit 4 is connected between the power input terminal of a high frequency power amplifier 5, namely, the output side of a low pass filter 2 and the ground, and this circuit 4 is so constituted that it is turned on by the signal from a control circuit 7 in case of abnormality. This discharge circuit 4 consists of a thyristor. If an abnormality detecting circuit 6 detects abnormality, the PWM signal is stopped by the control circuit 7 and the discharge circuit 4 is turned on. Thus, power supply to the high frequency power amplifier 5 through a TR 1 and the low pass filter 2 is cut off, and the electric charge stored in the low pass filter 2 is quickly discharged through the discharge circuit 4.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a transistor protection circuit, and more particularly to a transistor protection circuit for a high frequency power amplifier using a pulse width modulation method.

[Prior Art] FIG. 3 shows an example of a transistor protection circuit of a pulse width modulation high frequency power amplifier conventionally used in medium wave broadcasting equipment. In the figure, reference numeral 5 denotes a high-frequency power amplifier with a push-pull configuration, which is composed of four final-stage transistors 8, each of which amplifies and outputs high-frequency inputs having opposite phases. A transistor 1 controlled by a PWM (pulse width modulation) signal and a low pass filter (LPF) 2 are connected to the power supply circuit of this high frequency power amplifier, and a voltage corresponding to the PWM signal is supplied to the power amplifier 5. Note that 3 is a flywheel diode.

On the other hand, power is supplied to the high frequency power amplifier 5.

An abnormality detection circuit 6 is provided to detect an abnormality caused by a change in current, etc., such as a change in load impedance or a lightning surge, and a control circuit 7 controls the transistor 1 based on a signal detected by the abnormality detection circuit 6. PW to be input
It is configured to stop the M signal.

This control signal turns off the transistor 1 that stops the PWM signal, cuts off the DC voltage supplied to the high frequency power amplifier 5, and causes the final stage transistor 8 of the high frequency power amplifier 5 to become overloaded. We are trying to protect it by preventing it from happening.

[Problem to be solved by the invention]

Such a transistor protection circuit stops the PWM signal in response to an abnormal plagiarism signal and cuts off the power supply to the high-frequency power amplifier 5, but at this time, the electric charge stored in the low-pass filter 2 is transferred to the high-frequency power amplifier 5. It will be supplied and discharged by the final stage transistor 8. For this reason,
There is a problem that until the electric charge stored in the low-pass filter 2 is discharged, the final stage transistor 8 becomes overcharged and there is a risk that the transistor will be damaged.

An object of the present invention is to provide a transistor protection circuit that eliminates an overcharged state of a transistor when a PWM signal is stopped.

[Means for Solving the Problems] The transistor protection circuit of the present invention connects a discharge circuit to the power supply end of the high frequency power V amplifier, and controls this discharge circuit to short-circuit the power supply end in the event of an abnormality. It is configured as follows.

For example, the discharge circuit is constituted by a thyristor, and this thyristor is inserted between the direct load I side and the low potential side of the high frequency power amplifier, and the thyristor is turned on when an abnormality is detected.

[Effect]

According to the present invention, when an abnormality is detected, the power supply to the high-frequency power amplifier is stopped, and the power supply terminal is short-circuited by the discharge circuit, so that overcurrent to the transistor due to the accumulated charge existing at the power supply terminal is prevented. to prevent its damage.

〔Example〕

Next, the present invention will be explained with reference to the drawings.

FIG. 1 is a circuit diagram of a first embodiment of the present invention, and this example shows an example applied to a power amplifier grounded type series modulation system. In the same figure, 5 is a high frequency power amplifier composed of four final stage transistors 8, and as in the conventional case, PWM
DC potential V based on power supply potential V0 via transistor 1 and low-pass filter 2 controlled by a signal,
is being supplied. Further, 6 is an abnormality detection circuit that detects abnormalities caused by changes in load impedance, lightning surges, etc., and the control circuit 7 controls the PW
M signal can be stopped.

A discharge circuit 4 is connected between the power supply input terminal of the high-frequency power amplifier 5, that is, the output side of the low-pass filter 2, and the ground, and is turned on in the event of an abnormality by a signal from the control circuit 7. It is composed of This daytime +-2 circuit 4 is constructed of a thyristor here.

Note that 3 is a flywheel diode.

According to this configuration, when the abnormality detection circuit 6 detects an abnormality, the control circuit 7 stops the PWM signal and turns on the discharge circuit 4.

As a result, transistor 1 and low-pass filter 2
The supply of potential to the high-frequency power amplifier 5 through the low-pass filter 2 is cut off, and the charges stored in the low-pass filter 2 are quickly discharged through the discharge circuit 4.

Therefore, the stored charge in the low-pass filter 2 is not applied as an overcharge to the final stage transistor 8 of the high-frequency power amplifier 5 in the event of an abnormality, making it possible to prevent damage to these transistors.

FIG. 2 is a circuit diagram of a second embodiment in which the present invention is applied to a modulation amplifier grounded series modulation system, and the same parts as in FIG. 1 are given the same reference numerals.

In this embodiment, a discharge circuit 4 is connected between one end of a low-pass filter 2 provided on the ground side and the power supply side, and is turned on by a control circuit 7 operated based on the output of an abnormality detection circuit 6. By doing so, it is possible to prevent the charges stored in the low-pass filter 2 from being applied to the high-frequency power amplifier 5 during an abnormality, and damage to the transistor 8 can be effectively prevented.

〔Effect of the invention〕

As explained above, the present invention, when an abnormality is detected,
It is configured to short-circuit the power supply end by controlling the discharge circuit connected to the high-frequency power amplifier, so at that time, the power supply to the high-frequency power amplifier is stopped, and the power supply end is connected to the high-frequency power amplifier by the discharge circuit. This has the effect of short-circuiting and preventing overcurrent to the transistor due to the pre-storage current present at the power supply end, thereby preventing damage to the transistor.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram of a first embodiment of the present invention, FIG. 2 is a circuit diagram of a second embodiment of the present invention, and FIG. 3 is a circuit diagram of a conventional transistor protection circuit. 1...Transistor, 2...Low pass filter, 3
... Flywheel diode, 4... Discharge circuit (thyristor), 5... High frequency power amplifier,
6... Abnormality detection circuit, 7... Control circuit, 8... Final stage transistor. Figure 1

Claims (1)

[Claims] 1. In a high-frequency power amplifier having a transistor configuration, which supplies power using a pulse width modulation method and stops the supply of power in the event of an abnormality, a discharge circuit is connected to the power supply end, A transistor protection circuit for a high frequency power amplifier, characterized in that the discharge circuit is controlled to short-circuit a power supply end. 2. The discharge circuit is constituted by a thyristor, the thyristor is inserted between the high potential side and the low potential side of the high frequency power amplifier, and the thyristor is configured to be turned on when an abnormality is detected. A transistor protection circuit for a high frequency power amplifier according to item 1.