How to use the oscilloscope to effectively assist the design of switching power supply (1)
Q1: The ripple of the output voltage of the switching power supply is an important indicator. How to measure this indicator correctly with an oscilloscope?
A1: Ripple is defined as a clutter signal with periodic and random components attached to the DC level. It is called PARD (Periodic And Random Deviation) in English. It is defined as the peak-to-peak value of the clutter. Points to note when measuring ripple:
The ground wire of the oscilloscope probe will bring a lot of ripple, you should unplug the ground wire and use the ground wire inside the probe to measure. Of course, the best measurement method is to use a 50-ohm terminating resistor and connect it directly to the oscilloscope with a BNC cable. Here, it should be noted that the 50-ohm resistor should consider power consumption, and may require a high-power resistor.
Related standard requirements, such as whether to separate periodic power frequency ripple and switching ripple, high frequency noise, etc. As another example, whether the measurement frequency should be limited below 20MHz.
Q2: The switching power supply always has electromagnetic radiation, and the more likely it is to be interfered by other electrical equipment. What can be done to achieve the period of being free from interference from other electrical appliances and effectively radiating outwardly from local appliances?
A2: Because the switching power supply works in the switching state of high voltage and large current, the electromagnetic compatibility problems caused by it are quite complicated. From the perspective of electromagnetic compatibility of the whole machine, there are several types of common impedance coupling, line-to-line coupling, electric field coupling, magnetic field coupling and electromagnetic wave coupling. The three elements of electromagnetic compatibility are: interference source, propagation path and interfered body. Common impedance coupling is mainly because there is a common electrical impedance between the interference source and the victim, through which the interference signal enters the victim. The coupling between lines is mainly the mutual coupling caused by parallel wiring of wires or PCB lines that generate interference voltage and interference current. The electric field coupling is mainly due to the existence of potential difference, the coupling of the induced electric field to the victim. The magnetic field coupling is mainly the coupling of the low-frequency magnetic field generated near the pulsed power line of a large current to the object of interference. The electromagnetic wave coupling is mainly due to the high-frequency electromagnetic wave generated by the pulsating voltage or current, radiating outward through the space, and coupling the corresponding interfered body. In fact, each coupling method cannot be strictly distinguished, only the emphasis is different.
From the three elements of electromagnetic compatibility, to solve the electromagnetic compatibility of switching power supply, we can start from three aspects. 1) Reduce the interference signal generated by the interference source; 2) Cut off the propagation path of the interference signal; 3) Enhance the anti-interference ability of the victim. When solving the internal electromagnetic compatibility of the switching power supply, the above three methods can be used comprehensively.
The premise is cost-effectiveness and ease of implementation. External interference to the switching power supply, such as power line harmonic current, power line conduction interference, electromagnetic field radiation interference, etc., can only be solved by reducing the source of the interference. On the one hand, it can enhance the design of the input and output filter circuit, improve the performance of the active power factor correction (APFC) circuit, reduce the voltage and current change rate of the switch tube and the rectifier freewheeling diode, and adopt various soft switching circuit topologies and control methods . On the other hand, strengthen the shielding effect of the chassis, improve the leakage of the gap of the chassis, and perform good grounding treatment. The external anti-jamming capabilities, such as surges and lightning strikes, should be optimized for AC input and DC output ports. Generally, the combined lightning waveform of 1.2 / 50μs open circuit voltage and 8 / 20μs short circuit current can be solved by the combination of zinc oxide varistor and gas discharge tube due to the small energy
To reduce the internal interference of the switching power supply, achieve its own electromagnetic compatibility, and improve the stability and reliability of the switching power supply, we should start from the following aspects:
Pay attention to the correct distinction between the PCB layout of the digital circuit and the analog circuit, and the correct decoupling of the power supply of the digital circuit and the analog circuit;
Pay attention to the single-point grounding of digital circuits and analog circuits, single-point grounding of high-current circuits and small currents, especially current and voltage sampling circuits to reduce common resistance interference and reduce the impact of ground loops;
When wiring, pay attention to the spacing and signal properties between adjacent lines to avoid crosstalk; reduce the impedance of the ground wire; reduce the area surrounded by the high-voltage high-current line, especially the primary side of the transformer, the switch tube, and the power filter capacitor circuit;
Reduce the area surrounded by the output rectifier circuit, freewheeling diode circuit and DC filter circuit; reduce the leakage inductance of the transformer and the distributed capacitance of the filter inductance; use a filter capacitor with a high resonance frequency.
The power test program launched by TEK can conduct a pre-conformance test on current harmonics according to the EN61000-3-2 standard. For details, please refer to: http: //
Q3: Are there any special requirements for starting the switching power supply at low temperature (eg below -20 ℃)?
A3: The key is the temperature range selected by the device. Such as capacitors, MOSFETs, diodes, etc.
Q4: How to accurately test the ripple and noise of the switching power supply? Is it necessary to test the Ripple & noise in a special laboratory, because other equipment in the experiment has a greater impact on it? What about setting?
A4: Of course it is ideal if you have a special laboratory for ripple measurement. The issues that should be noted when this condition is not met are:
The oscilloscope should have a good ground.
If your measurement standard has bandwidth limitation requirements, you should turn on the 20MHz bandwidth limitation in TDS430A,
AC coupling using oscilloscope
Use BNC cable and measure with TDS430A's 50 ohm input impedance file (in this case, you may need a 50 ohm high power load, BNC adapter or make a test fixture)
In order to improve the measurement accuracy, the probe of the oscilloscope should not be used. The ground wire of the oscilloscope probe will introduce relatively large noise.
Q5: Is it possible to use an oscilloscope for power factor measurement in AC / DC switching power supplies? How to measure?
A5: In fact, measuring the power factor with an oscilloscope is measuring the phase difference between voltage and current, that is, cosφ. At the same time, the Tektronix TDS5000 power test system also automatically measures the relevant parameters of the PFC (such as: THD, True Power, Apparent Power, Power Factor, etc.) ).
Q6: Using the FFT function of the Tektronix oscilloscope, you can see the frequency and amplitude of the radiation of the switching power supply, but is the concept of the amplitude value the same as the value of the certification center? If not, how to convert? And I also found that if Choose different V / DIV when looking at the waveform. Is there a different amplitude in the FFT state? Is it normal? --- I use the model TDS1012.
A6: The amplitude measured by the FFT function of the oscilloscope can only be used as a qualitative analysis, not as a quantitative analysis, so it only has reference value. If you want to analyze the spectrum amplitude, you can choose the Blackman-Harris window, which will be good. Some; when converting V / div, it will definitely affect the amplitude of the FFT, because this is limited by the resolution of the oscilloscope's own ADC, so in order to improve the measurement accuracy, we generally choose to fill the waveform as much as possible to fill the entire screen (But never exceed the screen), that is, select a smaller V / div gear.
Q7: When designing a soft-switching PWM converter (such as a PWM half-bridge switching converter), how to observe the MOSFET Vt / It trace with an oscilloscope?
A7: First of all, your oscilloscope must have a time delay correction function between channels, so that the basic accuracy can only be guaranteed when the correlation data is calculated. You use high voltage differential voltage probes and current probes to measure. In the power test program launched by TEK, the entire working process of the MOSFET can be dynamically observed.
Q8: Excuse me, the choice of output capacitance and output inductance should be determined according to the power supply requirements of the load. Should the values ​​of L and C be applied according to the determined formula on the datasheet? If the value calculated according to the formula has a problem in practical application , So what should we replace?
A8: The calculation formulas of the output choke and output filter capacitor of different topologies are different. You should choose the appropriate calculation formula according to the circuit structure you choose. The size of the output capacitor is mainly determined by the suppression of the output ripple voltage to a few millivolts, which requires the calculation of ESR, which can then be selected according to the DATASHEET provided by the manufacturer. However, when selecting the capacitor, the change of the load, the range of the current change, the output inductance, etc. will all change the characteristics of the capacitor.
Q9: The thorny problem often encountered in the design of switching power supplies is efficiency. The efficiency of the whole machine depends largely on the loss of the switch tube. After our circuits and devices are selected, the measurement of the switch waveform of the switch tube is very important, and the working state of the switch can be judged and improved according to its data. So how to correctly operate and pay attention to those problems when using oscilloscope for this test?
A9: There are two major themes in switching power supplies: improving efficiency and improving reliability. The efficiency must measure the loss, the loss is mainly concentrated on the switch tube and the magnetic element. To this end, we should measure the turn-on loss, cut-off loss, and conduction loss with an oscilloscope. Similarly, we can measure the core loss and dynamic inductance of transformers and inductors.
Q10: In actual work, when a sudden glitch signal is encountered, how can I capture and test it?
A10: For example, when we conduct clock testing, we often encounter occasional glitch signals, which will cause malfunctions in our circuits, so capturing this signal becomes the key to the test. Since we cannot determine whether the glitch is positive or not in advance Negative, so we must first use the digital fluorescence function of the TDS5000 oscilloscope, that is, fast waveform capture mode combined with infinite afterglow to view glitches, and then use the oscilloscope's advanced trigger function-pulse width trigger according to signal characteristics, such as: less than normal clock pulse width trigger.
Q11: Is there a more general method of calculating transformer parameters for flyback switching power supplies? In the flyback switching power supply, a transformer algorithm is used, and it needs to be adjusted many times.
A11: Although the design of the transformer is calculated through theory, it still needs multiple experiments to adjust because of the differences in the magnetic core and winding method. Generally, the primary inductance is calculated, the core material and frame size are selected according to the output power, and then the single-ended design transformer such as the core cross-sectional area and other parameters is determined with the rejection manual to reset the flux of the core.
Q12: How is the power factor measured? How is the conversion efficiency of a certain module measured?
A12: Power factor: In a DC circuit, voltage times current is active power. However, in AC circuits, the voltage times the current is the apparent power, and part of the power that can work (ie, active power) will be less than the apparent power. The ratio of active power to apparent power is called power factor, which is expressed in COSΦ. In fact, the simplest measurement method is to measure the phase difference between voltage and current. The result is the power factor. The measurement of these problems you mentioned, TEK's power test system can easily complete automatic measurement.
Q13: At the high frequency end, how to judge the influence of the impedance of the oscilloscope probe itself on the signal?
A13: The probes of the oscilloscope have specific indicators. You can refer to the equivalent impedance-frequency diagram of the probe to determine the equivalent impedance of the probe at your frequency point.
Q14: Is there any way to use an oscilloscope to measure the working condition of a high-frequency transformer or inductive magnetic core? For example, the magnetic flux density Bw of the magnetic core?
A14: There is a function in the power test program launched by TEK. The analysis of the BH curve can reflect the working state of the magnetic core. It can also measure the dynamic inductance value and obtain the core loss.
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