RU2816509C1 - Low frequency key amplification channel - Google Patents

Abstract

FIELD: amplifier and generator equipment.

SUBSTANCE: used in broadband transmission paths of the audio frequency range for radio broadcasting and hydroacoustic communications. For this purpose, a low-frequency key amplification channel is proposed, containing a pulse-width converter 1 (PW Converter 1), a key power amplifier 2 (KPA 2), a current sensor 3, a threshold feedback circuit 4, an adder 5, a rectifier 6 and a parallel RC circuit 7. In turn, the threshold feedback circuit 4 includes operational amplifiers 4.1 and 4.2, bus of reference voltage 4.3 U_o and diodes 4.4, 4.5.

EFFECT: limiting the amplitude of the output current under overload conditions when operating on a predominantly capacitive load by no more than 20% of the specified limit value with at least a twofold reduction in the harmonic distortion of the amplified signal, which can significantly increase the reliability of operation.

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Description

The invention relates to the field of amplifier and generator technology and can be used in broadband transmission paths of the audio frequency range for radio broadcasting and hydroacoustic communications.

Low-frequency key amplifiers of class D are known, using pulse-width modulation and providing highly efficient amplification of broadband signals in the audio frequency range [Artym A.D. Class D amplifiers and key generators in radio communications and broadcasting. M:. Connection. 1980. p. 207]. The implementation of switching amplification methods favorably distinguishes amplifiers with PWM from linear amplifiers from an energy point of view, especially when operating on a complex load with a low active power factor cos ϕ ≤ 0.5. The formation of pulses modulated in duration depending on the instantaneous values of the amplified signals, for example, as a result of comparison with a reference symmetrical sawtooth voltage, ensures linearity of the amplitude characteristics and optimization of the structure of the pulse voltage spectrum, and, as a consequence, high quality output voltages of a class D amplifier.

Along with high energy efficiency, an additional advantage of broadband key amplifiers is the stability of the output voltage when the load changes over a wide range, which is due to the very low intrinsic resistance of the transistors in the on state.

Modern high-power field-effect transistors make it possible to provide the formation of pulse voltages up to 1000 V with an output current of up to tens of Amperes with an open channel resistance of no more than 0.01-0.1 Ohm. As a rule, in class D amplifiers of hydroacoustic transmitting paths, the inductance value L _f of the low-pass filter is selected from the condition of isolating the excitation voltage of the hydroacoustic emitter at the load in the audio frequency range ( _up to F = 20 kHz) without distorting the amplitude-frequency response (AFC). This is ensured when the inductive reactance Y _L =2πF _in L _f is significantly less than the minimum load resistance Z _n (F _in ).

At the same time, the high stability of the load characteristics of class D amplifiers is associated with the possibility of an almost unlimited increase in the output current with a significant decrease in the load impedance, or with the presence of a peak factor for amplified complex signals. As a result, emergency overload modes may occur, which can lead to failure of the transistors of the final stages of the KUM. The occurrence of such situations is typical for the channels of key amplifiers that provide excitation of broadband hydroacoustic transducers with complex digital message signals, characterized by a significant peak factor and phase shifts under conditions of operation with a capacitive load. An attempt to use negative feedback on the output current, according to a well-known technological solution [Aleksanyan A.A., Alexandrov V.A., Galakhov V.A., London G.F. Copyright certificate of the USSR SU No. 1531186 dated December 23, 19889 Key current generator primarily for geoelectric prospecting. IPC H03K 3/02] puts the class D amplifier into current generator mode. As a result, when the load impedance changes in the frequency band, the spectral structure of the hydroacoustic communication (HA) signal is unacceptably distorted.

If it is necessary to ensure the required stability of the frequency response of the excitation voltage of hydroacoustic transducers (HAT), the safety of the channels of key amplifiers can be ensured by emergency shutdown when the residual voltage on the transistors increases. (Alexandrov V.A., Kiselev P.A., Kurenoy A.V. RF Patent No. 2573229 dated January 20, 2016. Key amplification module. IPC H03F 3/217] or when the amplitude of the output current exceeds the limit value [Alexandrov V.A. ., Mayorov V.A., Polkanov K.I. RF Patent No. 2195687 dated December 27, 2002. Hydroacoustic transmission path. IPC G01S 7/524, 15/00].

The operation of the protection mechanism prevents the failure of transistors by prohibiting the operation of the ASC for the duration of the entire radiation cycle. As a result, there is an unstable mode of digital message transmission, which is unacceptable in the framework of solving the technical and technological problems of hydroacoustic stations.

The closest to the proposed solution in terms of the number of common features is the implementation of a key amplification channel with dynamic limitation of the output current, described in the patent [Alexandrov V.A., Kazakov Yu.V., Kalashnikov S.A. RF Patent No. 2574813 dated 02/10/2016. Multi-channel amplifier class D. IPC H03F 3/217].

The prototype is a low-frequency key power amplification channel according to patent No. 2574813 (Fig. 1), contains a pulse-width converter 1 (PWID 1), a key power amplifier 2 (KUM 2), a current sensor 3, an adder 5 and a threshold feedback circuit 4.

The timing diagrams of the signals presented in Fig. 2 to explain the principle of operation of the prototype device, include voltages:

U - input signal;

U _t - signal from the output of the current sensor;

U _oc - signal at the output of threshold feedback circuit 4; and _p is the difference signal at the output of adder 5;

U _n - signal on the active capacitive load KUM 2, including the low-pass filter choke.

-U _g' +U _g - threshold OS threshold levels (dashed line)

Solid line - without current overload, dotted line - in current overload mode.

The prototype device operates as follows.

The input signal U through the adder 5 is supplied to the input of the PWB 1, where it is converted into a sequence of width-modulated pulses, which are transmitted through the drivers as part of the KUM 2 to control the transistors of the half-bridge final stage of the key amplification. As part of KUM 2 according to the known rules [Artym A.D. Class D amplifiers and key generators in radio communications and broadcasting. M:. Connection. 1980. p. 207] also includes a low-pass filter choke, the inductance L _f of which ensures the limitation of high-frequency components of the output current. In this case, through the current sensor 3 in the load bus, a low-frequency voltage U _n is formed, practically repeating the shape of the input voltage at a given power level. When working on hydroacoustic transducers, the main filtering element of the output voltage is the load capacitance _Сн , the active power factor of which, as a rule, does not exceed cos ϕ=0.4 and at high frequencies in the natural frequency region practically does not exceed cos ϕ _f =0.1. Accordingly, the filter’s quality factor reaches 10, which is associated with a significant increase in the frequency response of the output voltage.

To ensure stability of the output voltage in the operating frequency band, the upper frequency of the operating range of the filter Ω _in is chosen significantly (2-3 times) lower than the natural frequency of the low-pass filter.

As a result, the harmonics of the low-frequency components of the modulated pulse voltage, falling within the region of the low-pass filter’s natural frequency, can be emphasized multiple times, causing an increase in nonlinear distortions, and in some cases causing a dangerous excess in the amplitude of the output current. This circumstance places even more stringent requirements on the quality characteristics of the key amplification path, including PWB 1 and KUM 2, which is achieved by using symmetrical modulation and high-speed field-effect transistors in the final stage of the key amplification circuit.

Thus, in the nominal operating mode, when the value of the output current I _L is less than the achieved value |I _L |≤I _Lд , the required quality of the output voltage is ensured. In this case, the half-waves of the output voltage U _t of the current sensor 3 do not exceed the set values +U _td and -U _td that define the boundaries of comparison of the threshold feedback circuit 4 (FS). Accordingly, the signal U _oc =0 means that there is no feedback on the output current and does not affect the change in the difference signal U _p at the input of PWB 1.

In current overload mode, the output signal U _t of current sensor 3 exceeds the limit values of the threshold circuit 4 OS ±U _td . In this case, the difference signal is received at the second input of adder 5:

As a result, the level and shape of the difference signal U _p at the input of PWB 1 changes. For the same summation coefficients (R ₁ = R ₂ ), the relation Up = U - U _oc is satisfied, which leads to a corresponding change in the low-frequency components of the modulated pulse voltage in KUM 2 and correction output current (uncorrected signals U _t and U _p are shown in Fig. 2 by dotted lines). Due to the corrective effect of the threshold current feedback signal, a limitation of the output current is achieved (U _t - solid line). At the same time, the level and shape of the output voltage changes with a significant deterioration in signal quality. Nonlinear distortions in this case, caused mainly by the third and fifth harmonics, can reach 20-40%. Especially significant distortions occur when operating a predominantly capacitive load, for which the third harmonic can be significantly emphasized by the low-pass filter’s own resonance.

This emphasis on harmonic distortion not only fundamentally worsens the quality of the excitation voltage of the hydroacoustic transducer, but also significantly worsens the stability of the limiting output current of the ASC, the amplitude of which can be more than 1.5 times greater than the set limit value, which affects the reliability of the low-frequency key amplification channel - the prototype .

The objective of the present invention is to increase the reliability of the low-frequency switching power amplification channel while improving the voltage quality in overload modes.

The technical result of the use of the present invention is to limit the amplitude of the output current under overload conditions when operating on a predominantly capacitive load by no more than 20% of the specified limit value with at least a twofold reduction in harmonic distortion of the amplified signal.

The technical result is achieved by the fact that in the known channel of low-frequency switch amplification, containing an adder, the first input of which is connected to the input signal bus, and the second input is connected to the output of the threshold feedback circuit, and the output is to the input of a pulse-width converter connected by the output to the input key power amplifier, the output of which is connected to the load bus through a current sensor, new features have been introduced, namely: a rectifier and a parallel RC circuit connected between the common bus and the input of the threshold feedback circuit, connected through a rectifier to the output of the current sensor.

In turn, the threshold feedback circuit of the low-frequency key gain channel contains the first and second operational amplifiers, and the inverse input of the first operational amplifier is connected to the direct input of the second operational amplifier and connected to the input of the threshold feedback circuit, and the direct input of the first operational amplifier is connected to the inverse input the second operational amplifier and connected to the reference voltage bus, while the outputs of the first and second operational amplifiers, respectively, through positive and negative conductivity diodes, are connected in parallel to the output of the threshold feedback circuit.

Providing a technical result is achieved by modulating the output current limitation when the amplitude value determined as a result of amplitude detection exceeds the set limit value. In this case, a uniform limitation of the half-waves of the low-frequency (LF) modulating (difference) signal is achieved, forming a pulse voltage with PWM and, accordingly, the output voltage and load current. Fixing the amplitude of the rectified signal of the current sensor for the duration of the discharge of the amplitude detector makes it possible to implement a mode of limiting the modulating signal when the current amplitude envelope of the installed sensor is exceeded, which corresponds to a reduction in harmonic distortion of the output voltage while minimizing the excess of the maximum current limit level.

The essence of the invention is illustrated in Fig. 1-4, where in FIG. 1 shows a diagram of the prototype device, Fig. 2 timing diagrams explaining its operation, in Fig. 3 diagram of the claimed device, Fig. 4 timing diagrams explaining the operation of the claimed device.

The block diagram of the claimed low-frequency key amplification channel (Fig. 2) contains a pulse-width converter 1 (PWID 1), a key power amplifier 2 (KPA 2), a current sensor 3, a threshold feedback circuit 4, an adder 5, a rectifier 6 and a parallel RC - circuit 7. In turn, threshold feedback circuit 4 includes operational amplifiers 4.1 and 4.2, reference voltage bus 4.3 U _o and diodes 4.4, 4.5.

The operating principle of the proposed technical solution is illustrated by signal diagrams (Fig. 4), which present the following time dependencies:

U-voltage on the input signal bus for a predominantly capacitive load;

U _t - voltage at the output of the current sensor (the dotted line indicates the signal in overload mode without current limitation);

U _o - threshold voltage level of circuit 5 threshold OS

U _dt - detected voltage level at the output of rectifier 6 loaded onto a parallel RC circuit 7 (the dotted line indicates the signal level without current limitation);

U _p' +U _os' -U _os - limitation levels of the positive and negative half-waves of the input signal, which determine the maximum voltage amplitude U _p at the output of the adder 5 at R ₁ >> R ₂ .;

-U _g' +U _g - threshold OS threshold levels (dashed line)

The proposed device includes blocks made according to known rules in accordance with the stated implementation features, the combined use of which ensures the achievement of a technical result.

PWM 1 is carried out according to the traditional scheme (Artym. A.D.), which implements the formation of a single-channel PWM of the first kind. SHIP-1 includes a sawtooth voltage generator with a clock frequency, significantly (more than 15-30 times) higher than the upper frequency of the amplified signal, and a switch that ensures the formation of a PWM signal as a result of the input action U _p with a sawtooth voltage.

The key power amplifier 2 for the proposed low-frequency key amplification channel is implemented using a half-bridge circuit using field-effect transistors with the required permissible voltage and current values. To control out-of-phase transistors of the half-bridge circuit of the switching amplifier, drivers of pulse signals with floating point are implemented as part of KUM 2, for example, those used in US Pat. RF No. 2573229 (Switch power amplifier module. Published 12/17/2015), or with optoelectronic isolation, according to patent. RF No. 2716041 (High-voltage switching amplifier module. Published 03/06/2020).

Current sensor 3, in relation to the channel of a low-frequency key amplifier with a frequency range of at least 100 Hz, is most simply performed on a current transformer. For example, with one turn of the primary winding and 100 turns of the secondary winding, loaded with a resistance of 100 Ohms, the transmission coefficient of such a current sensor will be K _t = 1V/1A.

Rectifier 6 provides rectification of half-waves of the signal U _t , proportional to the output current U _t =K _t I, performed via a bridge, or (with the secondary winding of a current transformer with a midpoint) via a push-pull circuit using conventional half-voltage diodes.

The RC circuit is designed to perform the function of a detector of the amplitude of rectified rectified voltage U _t . When RC circuit 7 is connected in parallel, a voltage U _dt is generated at the output of the rectifier, the level of which is determined by the maximum signal amplitude U _t with a discharge time constant approximately equal to the half-cycle of the lower frequency range of the amplified signal U.

Threshold feedback circuit 4 is designed to form boundary conditions of positive +U _oc and negative -U _oc polarity to limit the level, respectively, of the positive and negative half-waves of the output signal when the level U _dt is exceeded, the set value of U _o . At the level U _dt <U o _, the boundary values +U _{o c} , -U _{o o} are above the nominal amplitude of the input signal U=U _p and prevent its passage through the adder 6 to the input of the power supply 1. In the current overload mode U _{d t} >U ₀ , which leads to decrease the limit values |+U _oc | and |-U _oc | and, accordingly, limiting the level of the positive and negative half-waves of the amplified signal U _p .

To implement the given function, circuit 4 is performed on two operational amplifiers 4.1 and 4.2 and two diodes 4.4 and 4.5, and also contains a reference voltage bus 4.3 U _o . The intrinsic gain of the operational amplifiers is set to no less than 30, which achieves a limiting depth of the output current of more than 30 dB, corresponding to the formation of the declared technical result.

Within the framework of the problem being solved, adder 5 is designed to limit the level of half-waves of the amplified signal when passing through resistor R ₁ , which is achieved provided that the ratio of resistors in the resistive adder is R ₁ >> R ₂ . For example, for the input resistance of SHIP 1 of at least 10 kOhm, the resistance of the adder resistors can be R ₁ = 1 kOhm, R ₂ = 100 Ohm.

The proposed low-frequency key amplifier channel works as follows.

In the nominal operating mode, the input signal U through the adder 5 is supplied without level limitation to the input power supply 1 with input resistance _Rin =10 kOhm in the form of the resulting voltage U _p =UK _s =UR ₁ / _Rin . In this case, the maximum U _p (for example, U _p =5 V) does not exceed the limit values +U _oc and -U _oc set by the threshold feedback circuit 6. Accordingly, the PWM signal generated by PWM 1 provides modulation of the duration of the control pulses of KUM 2 from the condition that the low-frequency component of the modulated pulse voltage U _n is proportional to the current values of the signal U _p :

U _n (t) = U _p (t)K ₀ = UK _with K ₀ ,

where K ₀ =E/U _pm is the gain of KUM 2 at the power supply voltage ±E.

Next, the modulated pulse voltage is supplied through a choke with inductance L _f as part of KUM 2 and current sensor 3 into the load bus with a significant capacitive component C _f . As a result of the appearance of high-frequency components of the pulse voltage LC low-pass filter of the second order, a low-frequency voltage U _n appears on the load with very insignificant components of high-frequency pulse conversion. So, at the natural cutoff frequency of the filter At the same time, ensuring a given frequency ratio Ω ₀ /ω ₀ with a predominantly capacitive load causes a change in inductance L _f , which corresponds to significant HF components of the inductor current, reaching 0.1 - 0.3 of the rated level of low-frequency load current. It should be noted here that it is precisely this circumstance that limits the depth of the feedback current in the prototype device.

In overload mode associated with load reduction or transient processes when amplifying complex signals, in the proposed low-frequency key amplification channel, the output signal of current sensor 3, supplied through rectifier 7 and RC circuit 8 to the input of circuit 4, exceeds the set limit value U _o . As a result, the levels +U _oc and -U _oc are reduced and limit the amplitude of the negative and positive half-waves of the resulting voltage U _p , which in turn leads to a decrease in the amplitude of the low-frequency component U _{n of} the modulated pulse voltage and limits the maximum value of the load current. Thus, threshold feedback is achieved in the proposed LF channel of the key amplifier. It should be noted that the amplitude value of the signal U _Δt =|U _t | is recorded by an amplitude detector made on the rectifier 6 and RC circuit 7 at the time constant τ=RC. In this case, the boundary values +U _oc and -U _oc have practically no HF components, which achieves a depth of feedback in limiting the output current of more than 30 dB.

When the overload mode is eliminated after a set time interval t _in ≈ (2 - 3)τ, the direction at the input of the threshold feedback circuit 4 is reduced beyond the threshold level U _o , which leads to a decrease in the limitation limits +U _{o c} and -U _{o c} to nominal values. As a result, the limitation of the signal U _p is eliminated, which ensures the transition of the LF key amplification channel to the nominal operating mode.

Thus, the claimed device ensures that the output current is limited during an overload with a depth of more than 30 dB, which more than halves the maximum amplitude of the output current exceeding the established nominal value compared to the prototype device. At the same time, the limitation of the shape of the output LF voltage corresponding to the overload mode is associated with minimal harmonic voltages (no more than 10%) compared to the prototype device, where distortion with a significantly capacitive component of the load can exceed 30%.

As a result, the proposed solution makes it possible to significantly increase the reliability of the operation of hydroacoustic emitting paths in the GL and SV modes while eliminating disruptions in the emission cycles of complex signals under conditions of load changes within wide limits.

The company has developed an experimental sample of the claimed low-frequency channel of the key amplifier, the results of which confirmed its advantages compared to technical analogues that implement a technical solution based on the low-frequency channel of the key amplifier - the prototype. Based on the results of experimental testing, it was proposed to implement the proposed technical solution in new developments of generator devices of hydroacoustic stations.

Claims (1)

A low-frequency key amplification channel containing an adder, the first input of which is connected to the input signal bus, the second input is connected to the output of the threshold feedback circuit, and the output is connected to the input of a pulse-width converter, the output of which is connected to the output of a key power amplifier connected by the output through the sensor current to the load bus, characterized in that it includes a rectifier and a parallel RC circuit connected between the common bus and the input of the threshold feedback circuit connected through the rectifier to the output of the current sensor, while the feedback circuit contains the first and second operational amplifiers, wherein the inverse input of the first operational amplifier is connected to the direct input of the second operational amplifier and connected to the input of the threshold feedback circuit, and the direct input of the first operational amplifier is connected to the inverse input of the second operational amplifier and connected to the reference voltage bus, wherein the outputs of the first and second operational amplifiers respectively, through diodes of positive and negative conductivity, they are connected in parallel to the output of the threshold feedback circuit.