How to Use Oscilloscope to Effectively Assist Switching Power Supply Design

Q1: The ripple of the output voltage of a switching power supply is an important indicator. How to correctly use an oscilloscope to measure this indicator?

A1: Ripple is defined as a noise signal containing periodic and random components attached to the DC level, called PARD (Periodic And Random Deviation) in English. It is defined as the peak-to-peak value of the noise. Things to note when measuring ripple:

The ground wire of the oscilloscope probe will bring a lot of ripples, so you should unplug the ground wire and use the probe ground wire directly for measurement. Of course, the best measurement method is to use a 50 ohm terminal resistor and connect it directly to the oscilloscope with a BNC cable. It should be noted that the power consumption of the 50 ohm resistor should be considered, and a high-power resistor may be required.

Relevant standard requirements, such as whether to separate periodic power frequency ripple and switching ripple, high-frequency noise, etc. Another example is whether the measurement frequency should be limited to below 20MHz.

Q2: Switching power supplies always have electromagnetic radiation, and are more likely to be interfered with by other electrical equipment. How can we achieve the goal of not being interfered with by other electrical equipment and effectively radiating the electromagnetic field?

A2: Since 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 the electromagnetic compatibility of the whole machine, there are mainly common impedance coupling, 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 object. Common impedance coupling is mainly because the interference source and the interfered object have common impedance electrically, through which the interference signal enters the interfered object. Line coupling is mainly the mutual coupling caused by parallel wiring of the conductors or PCB lines that generate interference voltage and interference current. Electric field coupling is mainly due to the coupling of the induced electric field generated by the existence of potential difference to the interfered object. Magnetic field coupling is mainly the coupling of the low-frequency magnetic field generated near the high-current pulse power line to the interfered object. Electromagnetic wave coupling is mainly due to the high-frequency electromagnetic waves generated by the pulsating voltage or current, which radiate outward through space and produce coupling to the corresponding interfered object. In fact, each coupling method cannot be strictly distinguished, but the emphasis is different.

From the three elements of electromagnetic compatibility, to solve the electromagnetic compatibility of the 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 interfered object. When solving the electromagnetic compatibility inside the switching power supply, the above three methods can be used in combination.

The premise is cost-effectiveness and ease of implementation. The external interference generated by the switching power supply, such as power line harmonic current, power line conducted interference, electromagnetic field radiation interference, etc., can only be solved by reducing the interference source. On the one hand, the design of the input and output filter circuits can be enhanced, the performance of the active power factor correction (APFC) circuit can be improved, the voltage and current change rate of the switch tube and the rectifier freewheeling diode can be reduced, and various soft switching circuit topologies and control methods can be adopted. On the other hand, the shielding effect of the casing should be strengthened, the gap leakage of the casing should be improved, and good grounding treatment should be performed. As for the external anti-interference ability, such as surge and lightning strike, the lightning protection capability of the AC input and DC output ports should be optimized. Usually, for the combined lightning waveform of 1.2/50μs open-circuit voltage and 8/20μs short-circuit current, due to the small energy, a combination of zinc oxide varistors and gas discharge tubes can be used to solve it.

To reduce the internal interference of the switching power supply, realize 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 digital circuit and analog circuit PCB wiring, and the correct decoupling of digital circuit and analog circuit power supplies;
• Pay attention to the single-point grounding of digital circuits and analog circuits, and the single-point grounding of large current circuits and small current circuits, especially current and voltage sampling circuits, to reduce common impedance interference and reduce the impact of ground loops;

• When wiring, pay attention to the spacing between adjacent lines and the nature of the signal to avoid crosstalk; reduce ground line impedance; reduce the area surrounded by high-voltage and high-current lines, especially the primary side of the transformer and the switch tube, power supply filter capacitor circuit;

• Reduce the area enclosed by the output rectifier circuit, the freewheeling diode circuit and the DC filter circuit; reduce the leakage inductance of the transformer and the distributed capacitance of the filter inductor; use filter capacitors with high resonant frequency, etc.

The power test solution launched by TEK can perform pre-compliance testing on current harmonics according to the EN61000-3-2 standard. For details, please refer to: http://www.kingcable.com.cn/kingcableweb/tektronix/glcl/b7.htm

Q3: Are there any special requirements for starting the switching power supply at low temperatures (e.g. below -20°C)?

A3: The key is the temperature range of the device selection, such as capacitors, MOSFETs, diodes, etc.

Q4: How to accurately test the ripple and noise of a switching power supply? Do I need to test ripple and noise in a dedicated laboratory, because other equipment in the experiment has a greater impact on it. How should I set it in TDS430?

A4: Of course, it would be ideal if you have a dedicated laboratory for ripple measurement. If you do not have this condition, you should pay attention to the following issues:

• The oscilloscope should have a good ground.
• If your measurement standard requires bandwidth limitation, you should enable the 20MHz bandwidth limit in TDS430A.
• Using AC Coupling of an Oscilloscope
• Use a BNC cable and measure with the 50 ohm input impedance of the TDS430A (you may need a 50 ohm high power load, a BNC adapter, or a test fixture)
• To improve measurement accuracy, the oscilloscope probe should not be used, as the ground wire of the oscilloscope probe will introduce relatively large noise.

Q5: Can an oscilloscope be used to measure the power factor in an AC/DC switching power supply? How to measure it?

A5: In fact, using an oscilloscope to measure the power factor is to measure 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 PFC (such as: THD, True Power, Apparent Power, Power Factor, etc.).

Q6: The FFT function of the Tektronix oscilloscope can show the frequency and amplitude of the radiation of the switching power supply, but are the amplitude values ​​here the same as the values ​​of the certification center? If not, how to convert? And I also found that if I select different V/DIV when viewing the waveform, there will be different amplitudes in the FFT state? Is this normal? ---The model I use is TDS1012.

A6: The amplitude measured by the FFT function of the oscilloscope can only be used for qualitative analysis, not quantitative analysis, so it is only of reference value. If you want to analyze the spectrum amplitude, you can choose the Blackman-Harris window, which will have a better effect. When converting V/div, it will definitely affect the FFT amplitude, because this is limited by the resolution of the ADC of the oscilloscope itself. Therefore, in order to improve the measurement accuracy, we generally choose to make the waveform occupy the entire screen as much as possible (but never exceed the screen), that is, choose a smaller V/div position.

Q7: When designing a soft-switching PWM converter (such as a PWM half-bridge switching converter), how can one use an oscilloscope to observe the MOSFET Vt/It trajectory?

A7: First of all, your oscilloscope should have the function of delay correction between channels, so that the basic accuracy can be guaranteed when performing relevant calculations. You can use high-voltage differential voltage probe and current probe for measurement. The power test solution launched by TEK can dynamically observe the entire working process of MOSFET. You can refer to the following website: http://www.kingcable.com.cn/kingcableweb/tektronix/glcl/intro.htm

Q8: The selection of output capacitors and output inductors should be determined according to the power supply requirements of the load. Should the values ​​of L and C be applied according to the formulas specified in the datasheet? If the values ​​calculated according to the formulas have problems in actual applications, what should we base our decisions on when to replace them?

A8: The calculation formulas for output chokes and output filter capacitors 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 how many millivolts the output ripple voltage should be suppressed to. This requires calculating the ESR, and then you can choose according to the DATASHEET provided by the manufacturer. However, when selecting capacitors, you must also consider the changes in load, the current range, the output inductance, etc., which will change the capacitor characteristics.

Q9: The most difficult problem encountered in the design of switching power supplies is the efficiency problem. The efficiency of the whole machine depends largely on the loss of the switch tube. After our circuit and device are selected, the switching waveform measurement of the switch tube is very important, and its data can be used to judge and improve the working state of the switch. So how should we operate correctly and pay attention to what problems when using an oscilloscope to perform this test?

A9: There are two major themes in switching power supplies: improving efficiency and improving reliability. Efficiency requires measuring losses, which are mainly concentrated on the switch tube and magnetic components. For this reason, we should use an oscilloscope to measure the turn-on loss, cut-off loss, and conduction loss. Similarly, for transformers and inductors, we can measure their core loss and dynamic inductance. TEK's TDSPWR2 can do this. You can refer to the following website: http://www.kingcable.com.cn/kingcableweb/tektronix/glcl/intro.htm

Q10: In actual work, when encountering sudden glitch signals, how do you capture and test them?

A10: For example, when we are doing clock testing, we often encounter occasional glitch signals, which will cause our circuit to malfunction. Therefore, capturing the signal becomes the key to the test. Since we cannot determine whether the glitch is positive or negative in advance, we must first use the digital phosphor function of the TDS5000 oscilloscope, that is, the fast waveform capture mode combined with infinite afterglow to view the glitch characteristics, and then use the oscilloscope's advanced trigger function - pulse width trigger according to the signal characteristics, such as: triggering when the pulse width is less than the normal clock pulse width.

Q11: Is there a more universal method for calculating transformer parameters in flyback switching power supplies? Using a transformer algorithm in a flyback switching power supply always requires many adjustments.

A11: Although the design of the transformer is calculated theoretically, it still requires multiple tests and adjustments due to differences in the magnetic core and winding method. Generally, the primary inductance is calculated first, and the magnetic core material and frame size are selected according to the output power. Then, some parameters such as the core cross-sectional area are determined according to the manual. The purpose of single-ended transformer design is to reset the magnetic flux of the magnetic core.

Q12: How is the power factor measured? How is the conversion efficiency of a module measured?

A12: Power factor: In a DC circuit, voltage multiplied by current is active power. But in an AC circuit, voltage multiplied by current is apparent power, and the part of power that can do work (i.e. active power) will be less than the apparent power. The ratio of active power to apparent power is called power factor, which is represented by COSΦ. In fact, the simplest way to measure it is to measure the phase difference between voltage and current, and the result is power factor. TEK's power test system can easily complete the automatic measurement of the problems you mentioned. Please refer to: http://www.kingcable.com.cn/kingcableweb/tektronix/glcl/intro.htm

Q13: At the high frequency end, how do you determine the impact 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 graph 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 inductor core? For example, the working magnetic flux density Bw of the core?

A14: The power test solution launched by TEK has a function, BH curve analysis, which can reflect the working state of the magnetic core, and it can also measure the dynamic inductance value and obtain the magnetic core loss. For detailed introduction, please refer to: http://www.kingcable.com.cn/kingcableweb/tektronix/glcl/a2.htm