Download AC/DC power amplifier dedicated for immunity testing of electronic

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Measurement Automation Monitoring, Oct. 2016, no. 10, vol. 62, ISSN 2450-2855
Damian GONSCZ
INSTITUTE OF MEASUREMENT SCIENCE, ELECTRONICS AND CONTROL, SILESIAN UNIVERSITY OF TECHNOLOGY
10 Akademicka street, 44-100 Gliwice
AC/DC power amplifier dedicated for immunity testing
of electronic devices
Abstract
The paper deals with theoretical and technical aspects of construction of
laboratory AC/DC amplifier. The work includes project, theoretical
analysis, computer simulations and testing of prototype power amplifier.
The block diagram of prototype device is shown in Fig. 3. The amplifier
converts the DC component with value 12 V or 24 V and AC component
(voltage disturbing signal). The amplifier consists of two sections and each
section contains two component amplifier units working in the parallel
structure. Developed device will be used in the laboratory stand for
immunity testing of vehicle on-board electronic devices against selected
conducted disturbances. The results of theoretical analysis are discussed in
the chapter 3. The amplifier is characterized by good metrology
parameters and has wide frequency range (Fig. 11 and Fig. 12). The test
results of prototype amplifier are presented in the chapter 5.
generators. These type of disorders are attributed to the specific
car vehicle models. The applicable ISO standards contain the
general disturbance models that complement the model disorders
defined by manufacturers [2], [3], [4], [5]. The view of the
exemplary laboratory stand is shown in Fig. 2.
Keywords: laboratory power amplifiers, immunity tests against conducted
disturbances, signal processing.
1. Introduction
The power amplifiers are an important part of research
equipment of the current scientific and service laboratories from
the electromagnetic compatibility (EMC) area. Laboratory
measuring amplifiers are usually characterized by very good static
and dynamic parameters. In the EMC research the power
amplifiers are used e.g. in the immunity tests of different electrical
devices against conducted disturbances. A specific area are the
EMC testing of devices of the automotive industry. The EMC
testing of vehicles comprises different special measurements of
levels of the conducted and radiated emission. The EMC analysis
in automotive area also requires carrying out immunity tests of
electronic components against selected conducted and radiated
disturbances. This type of testing is particularly very important
among all tests for prototype cars and new vehicles. The electronic
modules of modern cars are much more sensitive to conducted
disturbances in their power and signal circuits. In the ideal case
the parameters and shape of voltage in electric power supply lines
should be nominal. Many dysfunctions of prototype electronic
equipments are caused by disturbances in the supply lines both
during normal vehicle work and a during temporary specific
operating conditions. There is a special processing chain in the
laboratory stand for immunity tests against conducted disturbances
in electric power lines of cars. Integral part of this chain (Fig. 1) is
arbitrary waveform generator. The generator reconstructs typical
and real course of the voltage car supply 12 VDC or 24 VDC
together with real disturbances.
Fig. 2. Laboratory stand for immunity tests against conducted disturbances
The newly developed device must obtain positive test result for
each required of test (ISO standards and models defined by car
manufacturers). Formal aspects relating to electromagnetic
compatibility of cars and other vehicles are included in the
Directive 2004/104/EC [6].
2. Design and structure of power amplifier
The AC/DC power amplifier was developed and built by the
author of this paper [1]. The block diagram of the internal
structure of amplifier is presented in Fig. 3.
Voltage gain: 20 (26 dB)
OUT
IN
0,1
/5W
Channel A (positive)
(P)
- 35 V
+ 35 V
Voltage gain: 20 (26 dB)
IN
OUT
0,1
RL(1)
/5W
Channel B (positive)
RL(3)
Voltage gain: 20 (26 dB)
OUT
IN
0,1
/5W
RL(2)
Channel A (negative)
- 35 V
+ 35 V
Fig. 1. Processing chain for immunity tests against conducted disturbances
IN
A device under test (DUT) must be supplied by the voltage
recorded before in the real conditions (selected power supply
connector in the car). This is first case of laboratory testing [1].
A case of immunity testing against real disturbances is required in
the EMC research of a new product dedicated to a specific vehicle
model. Many vehicle manufacturers create also its own models of
conducted disturbances. This is second case of laboratory testing.
The models are an alternative to real disorders and are often
implemented into libraries of the control software of EMC
(N)
Voltage gain: 20 (26 dB)
OUT
0,1
/5W
Channel B (negative)
+ 15 V
-1
+ 15 V
- 15 V
10 k
1
- 15 V
Fig. 3. Block diagram of the AC/DC laboratory power amplifier
Measurement Automation Monitoring, Oct. 2016, no. 10, vol. 62, ISSN 2450-2855
The amplifier consists of two independent amplifying parts
(Fig. 3). The first part amplifies the voltage input signal with no
sign change and the second part amplifies this signal with
a negative sign (phase reversal). This operation of signal
processing is necessary for work of the amplifier in a bridge
configuration. Positive part (Fig. 3) contains two component
amplifier units (A and B) working in the parallel structure.
Similarly constructed is part negative. All component amplifiers
are built on the basis of integrated circuits LM3886. Electric
diagram for a single amplifying block is shown in Fig. 4.
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3. Some results of simulation tests of power
amplifier
In order to verify the correctness of amplifier project, the author
has made the selected simulation tests of amplifier in the computer
program LTspice. The view of structure of one channel of power
amplifier executed in the LTspice software is shown in the Fig. 5.
4700 F/50
100 nF
3A
+ 35 V
470 
0,7 H
10
1
5
IN
22 k
9
7
3A
4
4,7 / 1W
3
LM3886
8
Cx
100 nF
OUT
100 k
47 k
- 35 V
Rx
10 / 1W
4700 F/50
100 nF
5,1 k
100 F/50
Fig. 4. Electric diagram for a single amplifying block of power amplifier
The LM3886 is a high-performance audio power amplifier
capable of delivering 68 W of continuous average power to a 4 Ω
load and 38 W into 8 Ω with 0,1% THD+N [7]. This work
configuration of amplifier LM3886 is one of the typical structures
recommended by the manufacturer but the used structure is
modified by the author in order to capabilities conversion of
AC/DC signals. The amplifier voltage gain is equal to 20
(100 kΩ / 5.1 kΩ - resistors marked in the Fig. 4). The maximal
value of the output current of the integrated amplifier can be up to
4 A [7].The current value depends on the impedance and the type
of the used load, so it is assumed that the output current will be
lower than 4 A. For increase the current capacity, in the project
there were used two amplifiers working in parallel configuration
(an increase in power up to 136 W). The induction choke with
value 0.7 H is a typical component of the structure for protection
against undesirable oscillations of amplifier from output side.
High frequency oscillations (over acoustics) can heat the amplifier
and consequently damage this component. These oscillations can
occur for some loads and they can’t be easily identified. Very
important is the circuit RxCx at the amplifier output (Fig. 4). This
circuit protects the amplifier before the subsistent excitation (over
acoustics frequencies). The reason for the excitation of amplifier
can be a load RL (a large inductive reactance) and a work of
system with higher frequencies of signals.
In the structure of the preamplifier are used operational
amplifiers OP07. The total processing range of the preamplifier is
equal ± 15 V (Fig. 3).
In the immunity tests, there can be used one of the independent
channels: “Positive” or “Negative” (Fig. 3). In this case, the load
is RL(1) or RL(2). The bridge configuration uses the work of two
channels at the same time. The load for this case is RL(3). In this
configuration, the power of amplifier system increases
theoretically four times (because current and voltage rise twice) in
relation to the power of a single channel. In practice, we cannot
exceed the limit value of the current for a single channel because
the integrated circuits may be thermally damaged. The research
confirms that the maximum amplifier power is equal to 300 W,
because constant component of processing signal causes
significant heating of the integrated amplifiers. A wider range of
the output voltage (bridge configuration) let us properly generate
most surge disturbances used in the immunity tests of automotive
devices.
Fig. 5. The view of one channel of amplifier from LTspice simulation software
This channel in the simulation process can be taken as channel
positive or negative. The working validation of system was done
in the first stage. For this process the author assumed that the input
voltage signal has constant component equal 12 V (nominal
voltage of car installation) with imposed sinusoidal interferences.
Exemplary input voltage signal is presented in Fig. 6. The courses
of the output voltage and output current for single channel (Fig. 5)
are shown in Fig. 7.
Fig. 6. The portion of the waveform of input signal for first part of simulation tests
Fig. 7. The waves of voltage and current signals on the output of single channel of
simulated power amplifier
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Measurement Automation Monitoring, Oct. 2016, no. 10, vol. 62, ISSN 2450-2855
In the above analysis the system has been connected to
impedance with value 2 Ω. In this case should be established the
gain of amplifier so that the levels of output voltages were the
same as the levels of input voltages.
In the next stage the analysis of amplifier frequency band was
made. The analysis of the amplifier frequency bands must be
carried out for the typical setup of amplifier. The output of each
amplifier channel must be connect to resistance load with value
8 Ω. A voltage sinusoidal signal with the frequency 1 kHz and
voltage value 1 VRMS should be applied to the input of the
amplifier channel. At output of the amplifier channel we must
have the power equal to 1 W [8]. For this purpose, we must set the
appropriate value of the amplifier gain. The results of analysis are
shown in Fig. 8.
Fig. 8. The frequency band and a shifts of phase for one channel of amplifier
The simulation analysis confirms correct working of power
amplifier based on integrated circuits LM3886. The integrated
circuits work in class AB and they are made in bipolar technology.
4. Prototype of the laboratory power amplifier
The internal arrangement of the electronic modules in the casing
of the amplifier prototype is shown in Fig. 9.
Very important in prototype is efficient cooling system of
integrated circuits. A DC power supply with a power about 500 W
is used in the prototype device.
5. Test results of prototype amplifier
The basic properties and functionality of the amplifier are
analyzed in laboratory tests. Before use, the amplifier prototype
should be calibrated (multi-turn potentiometer GAIN from Fig. 3
and Fig. 9). The calibration process of the amplifier is simple. This
is a necessary process because the amplifier must cooperate on
laboratory stand with the arbitrary waveform generator
(AutoWave – Fig. 2). This type of generator has input voltage
range equal ±100 V (alternative option of measurements of real
disturbances) but the range of the output voltage signals from the
instrument generation part is only ±10 V. So the output signals are
always rescaled to a range ±10 V. The output signal of generator
is rescaled too during use in immunity tests the typical disturbance
signals included in the library of generator. The easiest way for
fast calibration process is setting a reference signal (e.g. 10 VDC)
in the measuring software of generator AutoWave. The next step
is setting the amplifier input divider together with measuring
process of the DC voltage at the amplifier output. The real
measured value of the DC output voltage of the amplifier must be
in accordance with the set value in the signal generator. In our
case, it is 10 VDC. For calibration of the amplifier we can use the
digital oscilloscope. It follows that the amplifier must allow to the
amplification of voltage. For the amplifier project the maximally
amplification can be equal 20. It should be recalled that in the one
of the simulation test the amplifier has had voltage amplification
equal 1 (Fig. 6 and Fig. 7).
The laboratory generator type (AutoWave) consists of
a 2-channel voltage recorder and a 4-channel arbitrary waveform
generator. The samples of voltage signals are measured with an
accuracy better than 0.2%. The characteristics of generator is
included in the paper [1]. The exemplary work effect of the
amplifier for an selected immunity test is presented in below
Fig 10.
a)
b)
2 volts /div.
12 volts
Fig. 10. The signal from waveform generator – measuring software (a) and a voltage
signal at the amplifier output for channel "positive" (b)
Fig. 9. A view of internal electronic modules of the amplifier prototype
Measurement Automation Monitoring, Oct. 2016, no. 10, vol. 62, ISSN 2450-2855
The exemplary shape of curve from the Fig. 10a has created by
the author in the generator control software. In real conditions the
amplifier must also provide the required level of the output current
in order to perform immunity testing of electronic devices with
higher power rating. So in the tests the amplifier output was
connected to an electronic device with a power of 20 W. The work
effect of the amplifier for bridge configuration was tested too [1].
The studies of the parameters of amplifier were performed using
the system Audio Precision SYS-2722 [8]. The Audio Precision
device comprises a configurable signal generation module and
a module for precise analysis of the output signals. Similarly as in
the simulation tests of the amplifier the laboratory tests were
performed without a constant component too. In this case for the
analysis we must set the switch to position AC (Fig. 9). The
frequency bands of the amplifier for the channel "Positive" and the
channel "Negative" are shown in Fig. 11.
a)
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These values concern only non-linear distortions of amplifier
because input signal was sinusoidal. The power range is only
10 W because maximally power of the output resistor is equal 5 W.
6. Conclusions
The constructed amplifier is characterized by good dynamic
parameters and has the frequency response to approx. 100 kHz.
The total processing error of amplifier is estimated for a few
percents. The study confirmed the convergence of simulation tests
and laboratory tests.
A DC component in the signal processing causes significant
electrical losses of the amplifier. It is necessary to include an
effective cooling system in the construction. The integrated
modules LM3886 have many protection circuits (against shortcircuit and overvoltage and against overheating). The power
amplifier is suitable for immunity tests of automotive electrical
equipment with less power against conducted disturbances in
electric power lines.
The developed design of amplifier allows you to do basic
immunity tests and is a cheap alternative to expensive commercial
solutions.
7. References
b)
Frequency bands of the amplifier prototype for channel “positive” (a)
and “negative” (b) for the following standard parameters:
Input of amplifier: sinusoidal voltage signal with frequency 1 kHz and
voltage value 1 VRMS.
Output of amplifier: the output of channel was connected to non-inductive
power resistor with a value 8 Ω.
Selection of gain factor: at output of the amplifier we must obtain the power
equal to 1 W.
[1] Gonscz D.: Generator of conducted disturbance for immunity tests of
electric automotive equipment, Measurement Automation Monitoring
May 2015, vol. 61, no. 05.
[2] Rybak T., Steffka M.: Automotive Electromagnetic Compatibility
(EMC), Kluwer Academic Publishers, Dordrecht, 2004, TLFeBOOK.
[3] Rodriguez V. (ETS-Lindgren L.P.): Automotive Component EMC
Testing: CISPR 25, ISO 11452-2 and Equivalent Standards.
Automotive EMC, Safety & EMC 2011, http://www.semc.cesi.cn.
[4] Henry W.: Electromagnetic Compatibility Engineering, Wiley, USA,
2009.
[5] Spriessler R., Fuhrer M.: Load Dump Pulses According to Various
Test Requirements: One Phenomenon – Two Methods of Generation –
A Comparison, Automotive EMC Conference 2006.
[6] Commission Directive 2004/104/EC, Official Journal of the European
Union, October, 2004.
[7] Datasheet of integrated amplifier LM3886, Texas Instruments,
www.ti.com, 2013.
[8] The technical data of the laboratory instrument type Audio Precision
SYS-2722, http://www.ap.com/download/manuals.
_____________________________________________________
Received: 28.07.2016
Paper reviewed
Fig. 11. The frequency bands of the power amplifier prototype
THD + N [%]
Values of total harmonic distortion and noise for channels:
"positive" and "negative" are shown in Fig. 12.
3,2
3
2,8
2,6
2,4
2,2
2
1,8
1,6
1,4
1,2
1
0,8
0,6
0,4
0,2
0
Channel P
Channel N
Damian GONSCZ, PhD
He received a PhD in the discipline of electrical
engineering in 2003. He currently works at the Faculty
of Electrical Engineering of Silesian University of
Technology. He is assistant professor in the Institute of
Measurement Science, Electronics and Control. Author
is a member of Polish Society for Theoretical and
Applied Electrical Engineering and Metrology
Commission of Polish Academy of Sciences too. Author
carries out scientific research in the area of
electromagnetic compatibility.
e-mail: [email protected]
0,01
0,02
1
2
0,05
3
0,1
4
0,2
0,5
17
5
6
Output power[W]
28
59
10
10
Fig. 12. Values of total harmonic distortion and noise for channels: "positive"
and "negative" of amplifier
Accepted: 01.09.2016