A power amplifier for a hand-held mutual inductor tester

Abstract

A kind of power amplifier of handheld mutual inductor tester, comprising: PWM modulator, switching power amplifier circuit and filter circuit;Input signal waveform is converted into SPWM wave by PWM modulator, after being amplified by switching power amplifier circuit, using filter circuit to restore the amplified SPWM wave into analog waveform;Switching power amplifier circuit includes two isolation components, two drive chips, four MOS tubes, four diodes, four freewheeling diodes, two bootstrap capacitors and four resistors, every two MOS tubes are connected in series to form a bridge arm, a freewheeling diode is connected between the drain and source of each MOS tube, the positive and negative power supply ends of a bridge arm are connected with positive supply voltage and power ground respectively, and the positive and negative power supply ends of another bridge arm are connected with power ground and negative supply voltage respectively.The utility model avoids the high voltage peak of power amplifier, thereby eliminating arc and most electromagnetic interference.

Description

Technical Field

[0001] This utility model belongs to the field of current transformer technology, and more specifically, relates to a power amplifier for a handheld current transformer tester. Background Technology

[0002] Instrument transformers are indispensable electrical components in power transmission and supply systems, and also a very important part of power system relay protection. Their performance directly affects the stable operation of the power system and the correct operation of relay protection.

[0003] The testing method for instrument transformers directly affects the operation of relay protection devices. Traditional testing instruments are gradually being phased out due to their large size, heavy weight, and cumbersome measurement process. With the development of technology, handheld instrument transformer testers are becoming increasingly popular.

[0004] The research and application of handheld current transformer testers hinges on the study of their power amplifiers. Improving the efficiency and reducing power consumption of the power amplifiers while preventing damage from accidental disconnections during testing is of paramount importance. Because the secondary windings of current transformers often have high inductance, a sudden disconnection during testing can generate a high surge voltage, potentially damaging the power amplifier. Utility Model Content

[0005] To address the shortcomings of existing technologies, this utility model provides a power amplifier for a handheld current transformer tester.

[0006] The present invention adopts the following technical solution.

[0007] The first aspect of this utility model discloses a power amplifier for a handheld current transformer tester, comprising: a PWM modulator, a switching power amplifier circuit, and a filter circuit, specifically: The PWM modulator converts the input signal waveform into an SPWM wave. Both output terminals of the PWM modulator output two SPWM waves, which are complementary SPWM waves with dead time. The switching power amplifier circuit includes two isolation components, two driver chips, four MOSFETs, four diodes, four freewheeling diodes, two bootstrap capacitors, and four resistors. Each pair of MOSFETs is connected in series to form a bridge arm. The drain of each MOSFET is connected to the cathode of a freewheeling diode, and the source of the MOSFET is connected to the anode of the corresponding freewheeling diode. The positive and negative power supply terminals of one bridge arm are connected to the positive power supply voltage and power ground (PGND), respectively, while the positive and negative power supply terminals of the other bridge arm are connected to power ground (PGND) and the negative power supply voltage, respectively. Each resistor and a diode are connected in parallel to form a parallel unit. The two outputs of the PWM modulator are each connected to the control signal input HIN of a driver chip through an isolation element; the high-level output of each driver chip is connected to the gate of a MOSFET in a bridge arm through a parallel unit, and the low-level output is connected to the gate of another MOSFET in the corresponding bridge arm through a parallel unit; wherein, the anode of the diode in all parallel units is connected to the gate of the corresponding MOSFET, and the cathode is connected to the high-level output or low-level output of the corresponding driver chip; The high-side floating ground (VS) terminal of each driver chip is connected to the midpoint of the corresponding bridge arm. The high-side floating power supply (VB) terminal of the driver chip is connected to the high-side floating ground (VS) terminal through a bootstrap capacitor. The midpoints of the two bridge arms are respectively connected to the two input terminals of the filter circuit. The two output terminals of the filter circuit are the two output terminals of the power amplifier.

[0008] Preferably, the filter circuit includes a first inductor L1, a second inductor L2, a third inductor L3, a fourth inductor L4, a fifth capacitor C5, a sixth capacitor C6, a seventh capacitor C7, an eighth capacitor C8, and a sixth resistor R6. The first input terminal of the filter circuit is connected to the first output terminal of the filter circuit through the first inductor L1 and the third inductor L3. The second input terminal of the filter circuit is connected to the second output terminal of the filter circuit through the second inductor L2 and the fourth inductor L4. One end of the sixth resistor R6 and one end of the eighth capacitor C8 are both connected to the connection point of the first inductor L1 and the third inductor L3. The other end of the sixth resistor R6 is connected to one end of the seventh capacitor C7. The other ends of the seventh capacitor C7 and the eighth capacitor C8 are both connected to the connection point of the second inductor L2 and the fourth inductor L4. The connection point of the second inductor L2 and the fourth inductor L4 is connected to the reference ground GND. The fifth capacitor C5 and the sixth capacitor C6 are connected in parallel between the first output terminal and the second output terminal of the filter circuit.

[0009] Preferably, the magnetic cores of the first inductor L1, the second inductor L2, the third inductor L3, and the fourth inductor L4 are all made of iron-silicon-aluminum magnetic rings with core losses less than a set loss threshold. When the current changes, the change in the inductance value of the first inductor L1, the second inductor L2, the third inductor L3, and the fourth inductor L4 is always less than 10%. The inductance values ​​of the first inductor L1 and the second inductor L2 are equal, the inductance values ​​of the third inductor L3 and the fourth inductor L4 are equal, and the capacitance values ​​of the fifth capacitor C5 and the sixth capacitor C6 are equal.

[0010] Preferably, the values ​​of the first inductor L1, the second inductor L2, the third inductor L3, the fourth inductor L4, the fifth capacitor C5, the sixth capacitor C6, the seventh capacitor C7, and the eighth capacitor C8 are as follows: A reference filter circuit is set up, which has the same structure as the filter circuit, and the transfer function of the reference filter circuit is the set transfer function. The inductance value of each inductor in the filter circuit is equal to the value of the corresponding inductor in the reference filter circuit multiplied by the characteristic impedance coefficient and then divided by the cutoff frequency coefficient. The capacitance value of each capacitor in the filter circuit is equal to the value of the corresponding capacitor in the reference filter circuit divided by the characteristic impedance coefficient and then divided by the cutoff frequency coefficient. The characteristic impedance coefficient is the characteristic impedance of the filter circuit divided by the characteristic impedance of the reference filter circuit. The cutoff frequency coefficient is the cutoff frequency of the filter circuit divided by the cutoff frequency of the reference filter circuit.

[0011] Preferably, the isolation element is an isolation optocoupler, and the two output terminals of the PWM modulator are respectively connected to the anode of an isolation optocoupler. The anode of the isolation optocoupler is the anode of the light-emitting diode inside the isolation optocoupler. The output terminal of the isolation optocoupler is connected to the control signal input terminal HIN of a driver chip. The output terminal of the isolation optocoupler is the collector of the phototransistor inside the isolation optocoupler.

[0012] Preferably, the PWM modulator uses a dual PWM control chip UC3637.

[0013] The beneficial effects of this utility model are as follows, compared with the prior art: This invention adds freewheeling diodes to the drain and source terminals of the power MOSFET. This prevents the loop current from immediately dropping to zero upon disconnection, instead allowing it to slowly decay along the freewheeling path. The magnetic energy is converted into heat and dissipated in the loop impedance and freewheeling diodes, avoiding high-voltage spikes and eliminating arcing and most electromagnetic interference. The induced voltage is also clamped at the diode's forward voltage plus the power supply voltage, a value far below the power MOSFET's breakdown voltage, thus protecting the MOSFET and preventing damage to the power amplifier from accidental disconnection during testing. This invention employs a four-inductor, four-capacitor filter circuit to convert the signal from an SPWM wave back to an analog signal, with optimized parameters to reduce harmonic distortion, improve filtering, reduce fundamental attenuation, refine the restored signal waveform, and minimize interference. This invention uses two push-pull bridge arms, with each arm having half the power, and includes a bootstrap capacitor for drive control. The floating power supply uses a bootstrap circuit with a high high-side operating voltage and low static power consumption. Attached Figure Description

[0014] Figure 1 This is a structural diagram of a handheld current transformer tester. Figure 2 This is the circuit diagram of the power amplifier of this utility model. Detailed Implementation

[0015] To make the objectives, technical solutions, and advantages of this utility model clearer, the technical solutions of this utility model will be clearly and completely described below with reference to the accompanying drawings of the embodiments. The embodiments described in this application are merely some embodiments of this utility model, and not all embodiments. Based on the spirit of this utility model, all other embodiments obtained by those skilled in the art without creative effort are within the protection scope of this utility model.

[0016] like Figure 2 As shown, Embodiment 1 of this utility model proposes a power amplifier for a handheld current transformer tester, comprising: a PWM modulator, a switching power amplifier circuit, and a filter circuit, specifically: The PWM modulator converts the input signal waveform into an SPWM wave. Both output terminals of the PWM modulator output two SPWM waves, which are complementary SPWM waves with dead time. The signals output from the two output terminals are used as the first drive signal 1 and the second drive signal 2, respectively. The switching power amplifier circuit includes two isolation components, two driver chips (first driver chip U1 and second driver chip U2), four MOSFETs (first MOSFET V1, second MOSFET V2, third MOSFET V3 and fourth MOSFET V4), four diodes (second diode D2, third diode D3, fifth diode D5 and sixth diode D6), four freewheeling diodes (first freewheeling diode D11, second freewheeling diode D12, third freewheeling diode D13 and fourth freewheeling diode D14), two bootstrap capacitors (first bootstrap capacitor C2 and second bootstrap capacitor C11), and four resistors (first resistor R1, second resistor R2, third resistor R3 and fourth resistor R4). Two MOSFETs are connected in series to form a bridge arm. The drain of each MOSFET is connected to the cathode of a freewheeling diode, and the source of the MOSFET is connected to the anode of the corresponding freewheeling diode. The positive and negative power supply terminals of one bridge arm are connected to the positive power supply voltage V and the power ground PGND, respectively. The positive and negative power supply terminals of the other bridge arm are connected to the power ground PGND and the negative power supply voltage -V, respectively. Each resistor and a diode are connected in parallel to form a parallel unit. It should be noted that the maximum absolute value of the current allowed to be provided by the positive and negative power supply voltages is 15A, and the maximum power provided by the main power supply providing both the positive and negative power supply voltages is 3000W.

[0017] The two outputs of the PWM modulator are each connected to the control signal input HIN of a driver chip through an isolation element; the high-level output of each driver chip is connected to the gate of a MOSFET in a bridge arm through a parallel unit, and the low-level output is connected to the gate of another MOSFET in the corresponding bridge arm through a parallel unit; wherein, the anode of the diode in all parallel units is connected to the gate of the corresponding MOSFET, and the cathode is connected to the high-level output or low-level output of the corresponding driver chip; The high-side floating ground (VS) terminal of each driver chip is connected to the midpoint of the corresponding bridge arm. The high-side floating power supply (VB) terminal of the driver chip is connected to the high-side floating ground (VS) terminal through a bootstrap capacitor. The midpoints of the two bridge arms are respectively connected to the two input terminals of the filter circuit. The two output terminals of the filter circuit are the two output terminals of the power amplifier.

[0018] It should be noted that the handheld instrument transformer tester is a device capable of testing the current and voltage transformer's volt-ampere characteristics, 5% error curve, 10% error curve, DC resistance, transformation ratio, and polarity according to national standards. Its structural diagram is shown below. Figure 1 As shown, the handheld current transformer tester of this embodiment includes a central processing unit 11, a human-machine interaction control unit 12, a peripheral unit 13, a signal unit 14, a power amplifier power supply unit 15, a power amplifier unit 16, and a voltage output unit 17; the signal unit 14 includes a DAC digital-to-analog converter 141 and a signal filtering circuit 142; the power amplifier power supply unit 15 includes a lithium battery pack 151 and a boost power module 152; Human-machine interaction is achieved through the human-machine interaction control unit 12. Controlled by the central processing unit 11, the control signal is input to the DAC digital-to-analog converter 141 to generate different signal waveforms. In this embodiment, the signal waveform is a voltage signal. This signal waveform is processed by the signal filtering circuit 142 to remove various interference signals, especially the common-mode induced voltage generated by high voltage and high current in the field. Then, the signal waveform is amplified by the power amplifier 16 before being output, simulating the voltage waveform required for testing the various functional indicators of the instrument transformer. This waveform is then output to the voltage output unit 17 for subsequent detection processing. The power amplifier 16 is the power amplifier of the handheld instrument transformer tester described in Embodiment 1. The peripheral unit 13 is used for signal transmission and display. The handheld current transformer tester has a maximum voltage of 130V and a maximum current of 15A. The lithium battery pack in this embodiment uses a power battery pack that can provide 16V. It incorporates various protection measures, including temperature protection, output current overcurrent protection, hardware short-circuit protection, overcharge protection, and over-discharge protection. The boost power module 152 can boost the 16V voltage to ±200V, which serves as the positive and negative power supply terminals of the power amplifier 16, respectively.

[0019] It should be noted that this utility model converts the input signal waveform into an SPWM wave through a PWM modulator. After being amplified by a switching power amplifier circuit, the amplified SPWM wave is restored to an analog waveform using a filter circuit. This analog waveform is the signal after the input signal is amplified according to the set gain. Preferably, in this invention, the isolation element is an isolation optocoupler, and the two output terminals of the PWM modulator are respectively connected to the anode of an isolation optocoupler. The anode of the isolation optocoupler is the anode of the light-emitting diode inside the isolation optocoupler. The output terminal of the isolation optocoupler is connected to the control signal input terminal HIN of a driver chip. The output terminal of the isolation optocoupler is the collector of the phototransistor inside the isolation optocoupler.

[0020] Specifically, the isolation optocoupler is an HCPL-3180 isolation optocoupler.

[0021] Preferably, the filter circuit of this utility model includes a first inductor L1, a second inductor L2, a third inductor L3, a fourth inductor L4, a fifth capacitor C5, a sixth capacitor C6, a seventh capacitor C7, an eighth capacitor C8, and a sixth resistor R6. The first input terminal of the filter circuit is connected to the first output terminal of the filter circuit through the first inductor L1 and the third inductor L3. The second input terminal of the filter circuit is connected to the second output terminal of the filter circuit through the second inductor L2 and the fourth inductor L4. One end of the sixth resistor R6 and one end of the eighth capacitor C8 are both connected to the connection point of the first inductor L1 and the third inductor L3. The other end of the sixth resistor R6 is connected to one end of the seventh capacitor C7. The other ends of the seventh capacitor C7 and the eighth capacitor C8 are both connected to the connection point of the second inductor L2 and the fourth inductor L4. The connection point of the second inductor L2 and the fourth inductor L4 is connected to the reference ground GND. The fifth capacitor C5 and the sixth capacitor C6 are connected in parallel between the first output terminal and the second output terminal of the filter circuit.

[0022] Preferably, in this invention, the magnetic cores of the first inductor L1, the second inductor L2, the third inductor L3, and the fourth inductor L4 are all made of iron-silicon-aluminum magnetic rings with core losses less than a set loss threshold. When the current changes, the inductance values ​​of the first inductor L1, the second inductor L2, the third inductor L3, and the fourth inductor L4 always change by less than 10%. The inductance values ​​of the first inductor L1 and the second inductor L2 are equal, the inductance values ​​of the third inductor L3 and the fourth inductor L4 are equal, and the capacitance values ​​of the fifth capacitor C5 and the sixth capacitor C6 are equal.

[0023] The values ​​of the first inductor L1, the second inductor L2, the third inductor L3, the fourth inductor L4, the fifth capacitor C5, the sixth capacitor C6, the seventh capacitor C7, and the eighth capacitor C8 are as follows: A reference filter circuit is set up, which has the same structure as the filter circuit, and the transfer function of the reference filter circuit is the set transfer function. The inductance value of each inductor in the filter circuit is equal to the value of the corresponding inductor in the reference filter circuit multiplied by the characteristic impedance coefficient and then divided by the cutoff frequency coefficient. The capacitance value of each capacitor in the filter circuit is equal to the value of the corresponding capacitor in the reference filter circuit divided by the characteristic impedance coefficient and then divided by the cutoff frequency coefficient. The characteristic impedance coefficient is the characteristic impedance of the filter circuit divided by the characteristic impedance of the reference filter circuit. The cutoff frequency coefficient is the cutoff frequency of the filter circuit divided by the cutoff frequency of the reference filter circuit. The specified transfer function is the transfer function of the Butterworth filter. The formula is:

[0024] in, For the Laplace operator; The values ​​of the first inductor L1, the second inductor L2, the third inductor L3, the fourth inductor L4, the fifth capacitor C5, the sixth capacitor C6, the seventh capacitor C7, and the eighth capacitor C8 are determined by the following formula:

[0025]

[0026] in, , These are the characteristic impedance coefficient and the cutoff frequency coefficient, respectively. , These are the inductance values ​​of the first inductor L1, the second inductor L2, the third inductor L3, and the fourth inductor L4, respectively. These are the corresponding values ​​of the first inductor L1, the second inductor L2, the third inductor L3, and the fourth inductor L4 in the reference filter circuit, respectively. , These are the capacitance values ​​of the fifth capacitor C5, the sixth capacitor C6, the seventh capacitor C7, and the eighth capacitor C8, respectively. These are the corresponding values ​​of the fifth capacitor C5, the sixth capacitor C6, the seventh capacitor C7, and the eighth capacitor C8 in the reference filter circuit, respectively. Specifically, in this embodiment, the inductance values ​​of the first inductor L1 and the second inductor L2 are 16uH, the inductance values ​​of the third inductor L3 and the fourth inductor L4 are 12uH, the capacitance value of the eighth capacitor C8 is 0.69uF, the capacitance values ​​of the fifth capacitor C5 and the sixth capacitor C6 are 0.11uF; C7 is a 250V / 2.2uF capacitor, and R6 is a 10Ω resistor.

[0027] Preferably, the PWM modulator of this invention uses a dual PWM control chip UC3637.

[0028] The UC3637 contains a high-speed, 1MHz bandwidth, low-impedance error amplifier that can function as both a general-purpose fast operational amplifier and a feedback compensation operational amplifier. The UC3637 implements the functions of two PWM comparator circuits, as well as features such as undervoltage lockout and 2.5V threshold control. The UC3637 chip can not only generate high-frequency sawtooth waves but also internally modulate and control the dead time, improving system stability.

[0029] This invention adds freewheeling diodes to the drain and source terminals of the power MOSFET. This prevents the loop current from immediately dropping to zero upon disconnection, instead allowing it to slowly decay along the freewheeling path. The magnetic energy is converted into heat and dissipated within the loop impedance and the freewheeling diodes, avoiding high-voltage spikes and eliminating arcing and most electromagnetic interference. The induced voltage is also clamped at the diode's forward voltage plus the power supply voltage, a value far below the power MOSFET's breakdown voltage, thus protecting the MOSFET and preventing damage to the power amplifier from accidental disconnection during testing. Furthermore, this invention employs a four-inductor, four-capacitor filter circuit to convert the signal from an SPWM wave back to an analog signal, with optimized parameters to reduce harmonic distortion and improve filtering efficiency. The results are better, with less fundamental attenuation, a more delicate restored signal waveform, and less interference. This invention uses two push-pull bridge arms, with the power on each arm effectively halved. It includes drive control via a bootstrap capacitor, and the floating power supply uses a bootstrap circuit. Its high-side operating voltage can reach 600V, and its static power consumption at 15V is only 116mW. The output power supply terminal (pin 3Vcc, i.e., the gate drive voltage of the power device) voltage ranges from 10 to 20V; the logic power supply voltage range (pin 9VDD) is 3.3 to 20V, which can be easily matched with TTL or CMOS levels, and a ±5V offset is allowed between the logic power ground and the power ground; the operating frequency is high, up to 100kHz; the turn-on and turn-off delays are small, at 120ns and 94ns respectively; the totem-pole output peak current is 2A. The advantage in preventing power amplifier damage lies in the small turn-on and turn-off delays, which make it easier to control the dead time and thus avoid the situation where the upper and lower arm power transistors conduct simultaneously, burning out the MOSFET.

[0030] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of this utility model and not to limit it. Although the utility model has been described in detail with reference to the above embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the specific implementation of this utility model. Any modifications or equivalent substitutions that do not depart from the spirit and scope of this utility model should be covered within the protection scope of the claims of this utility model.

Claims

1. A power amplifier for a hand-held mutual inductor tester, comprising: The PWM modulator, switching power amplifier circuit, and filter circuit are characterized by: The PWM modulator converts the input signal waveform into an SPWM wave. Both output terminals of the PWM modulator output two SPWM waves, which are complementary SPWM waves with dead time. The switching power amplifier circuit includes two isolation components, two driver chips, four MOSFETs, four diodes, four freewheeling diodes, two bootstrap capacitors, and four resistors. Each pair of MOSFETs is connected in series to form a bridge arm. The drain of each MOSFET is connected to the cathode of a freewheeling diode, and the source of the MOSFET is connected to the anode of the corresponding freewheeling diode. The positive and negative power supply terminals of one bridge arm are connected to the positive power supply voltage and power ground (PGND), respectively, while the positive and negative power supply terminals of the other bridge arm are connected to power ground (PGND) and the negative power supply voltage, respectively. Each resistor and a diode are connected in parallel to form a parallel unit. The two outputs of the PWM modulator are each connected to the control signal input HIN of a driver chip through an isolation element; the high-level output of each driver chip is connected to the gate of a MOSFET in a bridge arm through a parallel unit, and the low-level output is connected to the gate of another MOSFET in the corresponding bridge arm through a parallel unit; wherein, the anode of the diode in all parallel units is connected to the gate of the corresponding MOSFET, and the cathode is connected to the high-level output or low-level output of the corresponding driver chip; The high-side floating ground (VS) terminal of each driver chip is connected to the midpoint of the corresponding bridge arm. The high-side floating power supply (VB) terminal of the driver chip is connected to the high-side floating ground (VS) terminal through a bootstrap capacitor. The midpoints of the two bridge arms are respectively connected to the two input terminals of the filter circuit. The two output terminals of the filter circuit are the two output terminals of the power amplifier.

2. The power amplifier of the handheld current transformer tester according to claim 1, characterized in that: The filter circuit includes a first inductor (L1), a second inductor (L2), a third inductor (L3), a fourth inductor (L4), a fifth capacitor (C5), a sixth capacitor (C6), a seventh capacitor (C7), an eighth capacitor (C8), and a sixth resistor (R6). The first input terminal of the filter circuit is connected to the first output terminal of the filter circuit via the first inductor (L1) and the third inductor (L3). The second input terminal of the filter circuit is connected to the second output terminal of the filter circuit via the second inductor (L2) and the fourth inductor (L4). One end of the sixth resistor (R6)... One end of the eighth capacitor (C8) is connected to the connection point of the first inductor (L1) and the third inductor (L3). The other end of the sixth resistor (R6) is connected to one end of the seventh capacitor (C7). The other ends of the seventh capacitor (C7) and the eighth capacitor (C8) are connected to the connection point of the second inductor (L2) and the fourth inductor (L4). The connection point of the second inductor (L2) and the fourth inductor (L4) is connected to the reference ground (GND). The fifth capacitor (C5) and the sixth capacitor (C6) are connected in parallel between the first output terminal and the second output terminal of the filter circuit.

3. The power amplifier of the handheld current transformer tester according to claim 2, characterized in that: The magnetic cores of the first inductor (L1), the second inductor (L2), the third inductor (L3), and the fourth inductor (L4) are all made of iron-silicon-aluminum magnetic rings with core losses less than a set loss threshold. When the current changes, the change in the inductance value of the first inductor (L1), the second inductor (L2), the third inductor (L3), and the fourth inductor (L4) is always less than 10%. The inductance values ​​of the first inductor (L1) and the second inductor (L2) are equal, the inductance values ​​of the third inductor (L3) and the fourth inductor (L4) are equal, and the capacitance values ​​of the fifth capacitor (C5) and the sixth capacitor (C6) are equal.

4. The power amplifier of the handheld current transformer tester according to claim 3, characterized in that: The values ​​of the first inductor (L1), the second inductor (L2), the third inductor (L3), the fourth inductor (L4), the fifth capacitor (C5), the sixth capacitor (C6), the seventh capacitor (C7), and the eighth capacitor (C8) are as follows: A reference filter circuit is set up, which has the same structure as the filter circuit, and the transfer function of the reference filter circuit is the set transfer function. The inductance value of each inductor in the filter circuit is equal to the value of the corresponding inductor in the reference filter circuit multiplied by the characteristic impedance coefficient and then divided by the cutoff frequency coefficient. The capacitance value of each capacitor in the filter circuit is equal to the value of the corresponding capacitor in the reference filter circuit divided by the characteristic impedance coefficient and then divided by the cutoff frequency coefficient. The characteristic impedance coefficient is the characteristic impedance of the filter circuit divided by the characteristic impedance of the reference filter circuit. The cutoff frequency coefficient is the cutoff frequency of the filter circuit divided by the cutoff frequency of the reference filter circuit.

5. The power amplifier of the handheld current transformer tester according to claim 1, characterized in that: The isolation element is an isolation optocoupler. The two output terminals of the PWM modulator are respectively connected to the anode of an isolation optocoupler. The anode of the isolation optocoupler is the anode of the light-emitting diode inside the isolation optocoupler. The output terminal of the isolation optocoupler is connected to the control signal input terminal HIN of a driver chip. The output terminal of the isolation optocoupler is the collector of the phototransistor inside the isolation optocoupler.

6. The power amplifier of the handheld current transformer tester according to claim 1, characterized in that: The PWM modulator uses a dual PWM control chip UC3637.

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