Safety solutions for switching power supply measurements

Preface

Power supply is essential for almost every electronic product with external power supply, and switching power supply system (SMPS) has become the mainstream structure in digital computing, network and communication systems. The performance (or failure) of switching power supply may have a significant impact on an expensive large system.

To ensure the reliability, stability, compatibility and safety of the upcoming SMPS design, measurement is the only way. SMPS measurement is divided into three main parts: active device measurement, passive device measurement (mainly magnetic components) and power quality test. Some measurements may face floating voltage and high current; some measurements require a lot of mathematical analysis to get meaningful results. Power supply measurement can be complicated, especially in the switching power supply system measurement. Why is the safety technology so eye-catching? We should start with the current development trend of switching power supply (SMPS) technology and the challenges of switching power supply design.

The characteristics of the development trend of switching power supply technology are: higher and higher efficiency; higher and higher power density; instantaneous load; low voltage jump, high current; broadband power supply technology and compliance with EN6100003-4 A14 standard.

Switching power supply design faces the challenges of improving switching power supply efficiency, reducing switching losses, minimizing power losses in magnetic devices, and requiring faster control loop response. Switching power supply system reliability must be improved, massive data analysis must be performed, and broadband technology standards must be met; easy-to-use, reliable tools are needed to locate problems.

Switching power supply system

The dominant DC power architecture in most modern systems is the switching power system because it can effectively handle changing loads. The power "signal path" of a typical SMPS includes passive devices, active devices, and magnetic components. SMPS uses as few lossy components as possible (such as resistors and linear transistors), and mainly uses (ideally) lossless components: switching transistors, capacitors, and magnetic components.

SMPS devices have a control section that includes a pulse width regulator, a pulse frequency regulator, and a feedback loop. The control section may have its own power supply. Figure 1 is a simplified SMPS schematic showing the power conversion section, including active devices, passive devices, and magnetic components.

Figure 1 Simplified schematic diagram of a switching power supply

SMPS technology uses power semiconductor switching devices such as metal oxide field effect transistors (MOSFET) and insulated gate bipolar transistors (IGBT). These devices have short switching times and can withstand unstable voltage spikes. Equally important, they consume very little energy whether they are on or off, with high efficiency and low heat generation. The switching device largely determines the overall performance of the SMPS. The main measurements of the switching device include: switching loss, average power loss, safe operating area, etc.

Safety Measurements for Switching Power Systems

Safety measurements of industrial power supplies should include: measuring high voltage and high current, measuring three-phase power circuits, handling floating equipment or equipment with different grounding, verifying digital control circuits, verifying instantaneous power analysis, waveform analysis, phase angle and switching loss of power electronic devices, etc., which should comply with industry standards and statutory standards.

Why can't we use traditional oscilloscopes to measure

Traditional oscilloscopes powered by AC use "ground as the reference point for measurement", which means that the AC-powered oscilloscope must be connected to the ground wire, and the ground wire of the probe is connected to the reference point of all channels of the oscilloscope, and thus connected to the ground potential. However, the "differential floating ground measurement" of traditional AC-powered oscilloscopes is dangerous!

We measure Vc-d = (Va-b + V ground loop voltage) - V ground loop voltage (common mode).

Disconnect the neutral from the ground wire by cutting the ground wire of a standard three-prong AC outlet or by using an AC isolation transformer. Float the oscilloscope from the protective ground wire to reduce the effect of the ground loop. This method is not actually feasible because the neutral wire may be connected to the ground wire somewhere in the building's wiring. It is an unsafe measurement method that will cause personal injury and damage to instruments and circuits!

The reasons why traditional oscilloscope measurement techniques cannot be used are as follows:

Distributed capacitance and inductance drop bring an inductive load of more than 100pf to the test point, which may cause circuit damage!

Therefore, you cannot cut the ground wire of the oscilloscope to perform differential measurements! You cannot use an isolation transformer to perform differential measurements!

Distributed capacitance and inductance may also cause ringing that was not originally present! See Figure 2(a).

Figure 2a Parasitic inductance and capacitance cause oscillations that distort the signal and invalidate the measurement.

When the oscilloscope is not grounded, its electromagnetic compatibility characteristics cannot meet the design requirements, which may interfere with the circuit to be tested or be interfered by electromagnetic waves in space, affecting the measurement results!

Most oscilloscopes have the "signal common" terminal connected to a protective earth ground system, commonly referred to as "ground." The result of this is that all signals applied to and provided by the oscilloscope have a common connection point. This common connection point is usually the oscilloscope chassis, which is connected to ground by connecting the third conductor in the AC power supply power cord to a test point. This is an unsafe measurement practice. This practice raises the voltage of the instrument chassis (which is no longer grounded) to the same voltage as the test point to which the probe ground is connected. A user touching the instrument becomes the shortest path to ground. Figure 2(b) illustrates this dangerous situation.

Figure 2b. Floating measurement with dangerous voltages present on the oscilloscope chassis.

In Figure 2b, V1 is a "bias" voltage above true ground, while VMeas is the voltage to be measured. Depending on the unit under test (DUT), V1 may be hundreds of volts, while VMeas may be a fraction of a volt. Floating the chassis ground in this way poses a hazard to the user, the DUT, and the instrument.

Furthermore, it violates industrial health and safety regulations and the measurements obtained are poor. Moreover, an AC powered instrument will present a large parasitic capacitance when floating to ground. Therefore, the floating measurement will be corrupted by oscillations, i.e., ringing as shown in Figure 2(a).

Introduction of new floating measurement method

The so-called "floating" measurement means that neither of the two points of the measurement is at ground potential. This measurement is also often called differential measurement.

The voltage between the "signal common" and ground may rise to hundreds of volts. A "floating" ground reference oscilloscope disconnects the "signal common" from ground by invalidating the ground system or using an isolation transformer. To achieve this, the TPS2000 series oscilloscope with built-in isolation channel technology is required, allowing engineers and technicians to quickly, accurately and economically perform multi-channel isolated measurements.

Because the floating measurement technique exposes the accessible parts of the instrument, such as the case, cabinet, and connectors, to the potential of the probe ground connection point, it is dangerous not only because it increases the voltage present on the oscilloscope (the operator may be shocked), but also because it accumulates stress on the insulators of the oscilloscope's power transformer, which, although not causing an immediate failure, may cause dangerous failures (electric shock and fire) in the future, even if the oscilloscope is restored to normal ground operation. This has the potential to make not only the floating ground reference oscilloscope dangerous, but also the measurement method inaccurate, that is, the error in the potential is caused by directly connecting the total capacitance of the oscilloscope case to the circuit under test at the ground connection point. This is why the safety-first isolated channel (LsoIated Channel) technology is needed as a solution.

Isolation Channel Technology

In the broadband oscilloscope systems used today, the most common isolation method is the two-path method, which splits the input signal into two signals: low frequency and high frequency. This method requires expensive optocouplers and broadband linear transformers for each input channel.

Using innovative sound isolation technology, the two-way approach is eliminated and only one broadband signal path is used for each input channel from DC to the bandwidth of the oscilloscope. This technology can provide the first batch of oscilloscopes with four inputs (1isolated channels), low cost and battery power, which can provide the oscilloscope with eight hours of continuous operation. For engineers and technicians who need to make four-channel isolated measurements and want to obtain the performance and ease of use provided by low-cost and battery-powered oscilloscopes, choosing the TPS2000 series oscilloscope with built-in isolation channel technology is an ideal tool.

Figure 3 illustrates the concept of isolation channels.

Figure 3. Safety-first IsolatedChannel technology enables fast, accurate, and economical multi-channel isolation measurements.

The quad isolated channel input architecture provides true and complete channel-to-channel isolation for the “positive” input and “negative reference” leads (including the external trigger input).

The most stringent requirements are placed on floating measurements in power control circuits such as motor controllers, uninterruptible power supplies, and industrial equipment. In these applications, voltages and currents can be large enough to pose a threat to both the user and the test equipment.

To ensure measurement quality, isolated channel technology is the preferred solution, and this technology always puts safety first. If there is a large common-mode signal, effective channel-to-channel isolation can minimize the impact of parasitic effects. The smaller the capacity of the measurement system, the smaller its interaction with the environment. Fully isolated battery-powered instruments do not involve grounding issues themselves. Each probe has a "negative reference" wire isolated from the instrument chassis instead of using a fixed ground wire.

Furthermore, the “negative reference” leads of all input channels are isolated from each other. This is the best way to avoid the danger of short circuits. It also minimizes the signal-degrading impedance that can affect measurement quality in single-point grounded instruments.

The inputs of the TPS2000 Series oscilloscopes are always floating, whether running on battery power or connected to AC power via the AC adapter. Therefore, these oscilloscopes do not have the same limitations as traditional oscilloscopes, which focus on performance (bandwidth, versatility) at the expense of the ability to make floating measurements.

Power Quality Measurements

According to the composition of SMPS, its measurement can be divided into active device (switching element) measurement, source device (magnetic element) measurement, input AC power supply measurement and power quality measurement. Here we only introduce the power quality measurement.

Power quality does not only depend on the generator. It also depends on the design and manufacture of the power supply and the end-user load. The power quality characteristics of a power supply define the "health" of the power supply.

Real wires never provide a perfect sine wave, there is always some distortion and noise on the line. Switching power supplies impose a non-linear load on the power supply. Therefore, the voltage and current waveforms are not exactly the same. Current is drawn during a certain part of the input cycle, causing harmonics on the input current waveform. Determining the effects of these distortions is an important part of power engineering.

To determine the power consumption and distortion on the power supply line, power quality measurements should be made at the input stage, with voltage and current test points shown in Figure 4.

Figure 4 Schematic diagram of the power quality test points of the switching power supply. The power quality test must use the input VAC and IAC readings at the same time.

Power quality measurements include: active power; apparent power or reactive power; power factor; crest factor; current harmonics measurement according to EN61000-3-2 standard; total harmonic distortion (THD).

Use a digital oscilloscope with an operating software package such as TDSPWR3 to perform parallel power quality measurements.

A digital oscilloscope running a software package (such as TDSPWR3), such as the TDS5000B Series, is a powerful tool for replacing traditional power meters and harmonic analyzers for power quality measurements.

Using a digital oscilloscope has significant advantages over traditional tools. The instrument must be able to capture up to 50 harmonic components of the fundamental wave. Depending on the standards used in each region, the power supply frequency is usually 50Hz or 60Hz. In today's high-speed oscilloscopes, oversampling ensures that fast-changing events can be captured in detail (high resolution). In contrast, traditional power meters may ignore signal details due to their relatively long response time. Even at very high sampling resolution, the oscilloscope's record length is sufficient to capture an integer number of cycles.

Software tools can speed up the measurement process and minimize setup time. Most power quality measurements can be automated by full-featured power measurement software running on the oscilloscope, completing lengthy processes in seconds. Oscilloscopes reduce manual calculations, making them very versatile and efficient power meters.

Digital oscilloscope probes also help make safe, reliable power supply measurements. High-voltage differential probes for power applications are the tool of choice for observing floating voltage signals.

Current detection requires special considerations. There are several current probe configurations available: AC current probes are based on current transformer (CT) technology. CT probes are non-intrusive and cannot sense the DC component of the signal, which can cause inaccurate measurements. Shunts, which require the circuit to be cut, can create a voltage drop inside the probe, which can affect the accuracy of power measurements. AC/DC current probes are generally based on Hall effect sensor technology. This device senses AC/DC current in a non-intrusive manner, and is able to read both AC and DC components with one connection.

When a digital oscilloscope (such as the TDS5054B) is equipped with TDSPWR3 software, it becomes a true automatic power measurement platform. The software automatically sets up the oscilloscope and its initial measurement parameters. If necessary, these settings can be fine-tuned manually.

Figure 5 shows power quality and current harmonics readings obtained using a TDSSOOOB series oscilloscope and TDSPWR3 measurement and analysis software. The display shows a rich array of measurements, including active power, apparent power, crest factor, total harmonic distortion, power factor, and a bar graph display of current harmonics.

Conclusion

Professional technicians have to deal with the safety issues in SMPS high voltage and current measurement, that is, running in potentially dangerous floating measurement. In this regard, there are many optional technologies or products for floating measurement. Here, we only take the TPS 2000 broadband oscilloscope with built-in Isoated Channel I technology as an example of a solution, that is, isolation and floating measurement function + laboratory oscilloscope performance + field general + power-specific measurement and analysis, which equals high production efficiency, that is, the application of a new floating measurement method, which is characterized by versatility, accuracy or economy.