JPS6333368Y2 - - Google Patents

Description

[Detailed Description of the Invention] The present invention relates to a circuit for protecting a switching power amplifier used in a solid-state broadcasting device using the PWM method.

The protection device or circuit of conventional solid state broadcasting equipment is
Emphasis was placed on protection against overcurrent, and the only protection against overvoltage was to insert a lightning arrester such as a ball gap between the antenna and the ground, but there were no sufficient measures to protect the amplification element from overvoltage. Broadcasting equipment uses as low a voltage as possible under normal operating conditions, so there is no problem even if there are no protection measures against overvoltage. In some cases, a voltage higher than the rated voltage was applied to the amplifier element, causing the element to break down. The present invention provides a means for preventing this, and will be described in detail below.

The drawing is a block diagram centered on the power amplification section of a PWM broadcasting device that implements the circuit of the present invention. 1 in the figure is a PWM (pulse width modulation) wave generator, which generates a pulse width modulated rectangular wave according to the audio signal input, and this output is amplified by the PWM wave exciter 2 and further modulated. The power is amplified by the amplifier _Q5 , but the harmonic components are removed by the LPF (low pass filter), and the original audio signal wave is superimposed on the DC voltage supplied from the power supply and is then applied to point A. It will be done. _Q1
~ Q ₄ is a field effect transistor (FET) that forms a so-called full-bridge connected switching power amplifier, and the carrier wave input signal from a carrier wave amplifier (not shown) given to each gate is Q ₁ and Q ₄
is in phase, and Q ₂ and Q ₃ are in opposite phase, resulting in polarity as shown. Therefore, in the first half-cycle input, Q ₁ ,
Q ₄ becomes conductive, Q ₂ and Q ₃ are non-conductive, but in the next half cycle, Q ₂ and Q ₃ become conductive and Q ₁ and Q ₄ become non-conductive, which repeats, so the bridge output A current of alternating polarity flows through the transformer T ₁ ,
This output powers a load R ₀ (typically an antenna) through a bandpass filter BPF. The above is PWM
This type of broadcasting equipment is currently used for medium wave output up to 10kW class and is well known.

Now, in the circuit shown in the figure, the voltage applied to point A is
Since this is the power supply voltage of the full bridge type power amplifier consisting of _Q1 to _Q4 , if the voltage at point A changes depending on the audio signal, the output from _T1 changes to follow the audio. The output then passes through the BPF and is supplied to R ₀ as normal amplitude modulated radio frequency power.

Also, R ₁ is the overcurrent detection resistor of the modulator, R ₂ is the overcurrent detection resistor of the power amplifier, and the voltage across these is
It is sent to PWM wave generator (and cutoff controller) 1,
If abnormal current flows in the power supply circuit, PWM
Cuts off the signal rapidly and protects the broadcaster by making _Q5 non-conductive. At this time, an overcurrent usually flows.
The PWM signal is cut off and the PWM signal is generated again after a few seconds, but if an overcurrent flows further, the PWM signal is cut off. Furthermore, if this operation occurs three or four times in a row within a certain period of time (approximately 15 seconds), control is normally made so that no more PWM waves are emitted. This is called notching motion,
Although not related to the present invention, further explanation will be omitted, but the control circuit within the PWM generator 1 has a function of blocking PWM waves at high speed.

This invention not only detects overcurrent using _R1 and _R2 , but also detects abnormalities in the voltage at point A on the power supply side of the switching power amplifier, and points B and C on the amplifier output side, and uses the obtained voltage for switching. Turns on the output of modulation amplifier _Q5 , which controls the power supply voltage of the power amplifier.
It is characterized in that it is supplied to the circuit 1 which performs a control operation of turning off.

Therefore, the operation of the circuit shown in the drawings will be explained. A
The voltage at point is due to R ₃ and R ₄ , and the voltage at point B is due to R ₅ ,
It will be easily understood that the voltage at point C is detected after being divided by R ₆ to an appropriate voltage by R ₇ and R ₈ respectively. Diodes CR1 to CR whose respective anodes are connected to these voltage division points and whose cathodes are commonly connected to the control circuit.
3. Without these, the voltage at any of the three points A, B, and C would be excessive, and if the voltage at another point is low, they cancel each other out, making it impossible to detect the overvoltage. This was established because this may occur. That is, when CR1 to CR3 are used, if the voltage at any one of the three points A, B, and C exceeds the specified value, that voltage will always appear at point D on the negative side of CR1 to CR3. The voltage at point D is Q ₁ ~
If the voltage rises above a value commensurate with the rated voltage of _Q4 , the high-speed cutoff circuit described above will stop the transmission of PWM waves.

Next, before explaining the effects of the present invention, parasitic inductance will be defined. When a switching power amplifier consisting of Q ₁ to _{Q 4} is actually manufactured, its connection wiring always has an inductance component. This exists in all of the power amplifier wiring, and is called the parasitic inductance of the circuit.

Now, the effect of detecting excessive voltage from point A will be explained. For example, when the gate signals of Q ₁ to _{Q 4} disappear, the switching operation of Q ₁ to _{Q 4} is not performed, and the power supply at point A becomes unloaded, and the voltage at point A rises rapidly. This voltage is accompanied by an oscillating voltage due to the parasitic inductance of the circuit, and appears superimposed on the voltage that increases when the output of the modulation amplifier _Q5 becomes unloaded. This voltage with oscillation passes through CR1 and appears at point D, generating a PWM wave.
Q ₁ to _{Q 4} are protected by shutting off in the PWM generator 1 and lowering the voltage at point A.

Next, the effect of detection from point B or point C will be explained.
For example, if there is a sudden change in the impedance on the load side from points B and C in a steady state, such as an open circuit or a short, the BPF will
The voltage at point or point C increases rapidly. The reason for this is that the energy stored in the reactance generates an excessive voltage due to a sudden change in the current flowing through the reactance, and this is well known from the theory of transient phenomena.

Although a considerable protection effect can be obtained even when control is performed using only abnormality detection from points B and C, the control effect is significantly improved by further using detection from point A.

Furthermore, the protection device of the present invention is effective in the event of a lightning strike, etc.; the reason is that a lightning strike disturbs the output impedance of a switching power amplifier, and if the power amplifier is in operation in that state, the output energy of the power amplifier itself is reduced. The switching power amplifier can be protected by cutting off the PWM wave and stopping the operation of the power amplifier for a short period of time at the moment of a lightning strike, in order to destroy the amplifier element (FET).

In broadcasting equipment, the time to stop the PWM wave due to overcurrent detection is usually set to approximately 1 to 2 seconds, but this is because the temperature of the amplifying element rapidly rises due to overcurrent, so the temperature of the element will rise within the stopping time. Since it is desired that the frequency decreases sufficiently and that the broadcasting stop time is as short as possible, the time is set to balance this. This invention determines that the overvoltage generation mode is a transient phenomenon that occurs at the moment of a lightning strike, the moment oscillation stops, or the moment a load abnormality occurs, and the present invention aims to protect against overvoltage. Since this is not as large as when there is an overcurrent, the PWM wave stop time can be selected to be about 0.5 seconds, which has the remarkable effect of shortening the broadcast stop time.

[Brief explanation of the drawing]

The drawing is a diagram showing an example of the configuration of a broadcasting device implementing the device of the present invention. 1...PWM generator, 2...PWM wave exciter,
Q ₁ to _{Q 4} ... Switching power amplifier using FET transistor, Q ₅ ... Modulation amplifier using FET,
R ₀ ...Load (antenna).

Claims (1)

[Scope of utility model registration request] Connected between the power supply and ground of a switching power amplifier composed of a group of field effect transistors connected in a full bridge circuit, and between the two output side terminals of the switching power amplifier and ground in order to extract the voltage between each ground at an appropriate voltage division ratio. It is equipped with a voltage dividing resistor and three diodes whose anodes are connected to the voltage dividing points of each of these voltage dividing resistors and whose cathodes are commonly connected, and each voltage dividing point obtained on the cathode side of the diode group is provided. voltage is supplied to a switching control circuit that controls the power supply voltage of the switching power amplifier, and when an excessive voltage of a certain level or more occurs at any of the voltage division points, the power of the switching power amplifier is turned off. Features a switching power amplifier protection circuit.