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Analysis of Small Voltage Variations Under Large Signal Conditions

Parent Category: 2015 HFE

By Matthias Beer, Renate Mittermair, Dr. Markus Herdin

Measuring small voltage variations when high voltage components are present is a common challenge in electronic design and testing. Typical applications are switched-mode power supplies (SMPS) where the voltage across the switching transistor has to be measured during the switching period. In this case, the voltage difference between ON and OFF state of the transistor can easily reach a hundred volts or more. To measure small voltage variations under such conditions with the required accuracy, more than 8 bit of vertical resolution would be required on an oscilloscope. The same requirements would apply, for example, when analyzing an AM modulated signal with low modulation index.

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Figure 1 • Cross section of MOSFET (single VDMOS transistor).

Switched Mode Power Supply

Switched-mode power supplies, commonly abbreviated as SMPS, are electronic power supplies which transfer power from a source to a load while converting the voltage and current characteristics. SMPS incorporate a switching regulator, typically a MOSFET (metal oxide semiconductor field-effect transistor) or IBGT (insulated-gate bipolar transistor). The pass-transistor translates power efficiently as it switches continuously and spends very little time in the high dissipation transitions; ideally, the power supply dissipation is zero. The output voltage regulation of the SMPS is realized by varying the duty-cycle of the switching transistor. The switches are realized using semiconductor devices such as FET’s, IBGT’s or diodes.

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Figure 2 • Block diagram of an active full-bridge converter (1: DC input supply; 2: DC/AC converter; 3: transformer; 4: active rectifier; 5: filter; 6: load).

In general, the SMPS can be classified in isolated and non-isolated topologies. For the latter topology type, buck or boost converters are typical candidates [1], whereas for the first one, flyback (isolated buck-boost converter) or full-bridge converters are known [2]. A very efficient converter for high-power ranges of up to several kW would be an active full-bridge converter (Fig. 2). Here, power MOSFETs are often used to realize the rectifier as they are designed to handle significant power levels while having a high switching speed and a good efficiency at low voltages at the same time.

In switched-mode power supply applications, one key parameter to determine the conducting loss of the commonly used power MOSFET is RDS(ON). In ON state, the transistor shows a resistive behavior between the drain and source terminal which is the sum of resistive elements between them (Fig. 3).

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Figure 3 • RDS(ON) as sum of resistive elements between source and drain terminal of MOSFET in ON state.

To measure RDS(ON), special requirements appear, as small drain-to-source voltages have to be measured in ON state, typically a few hundreds of mV, while having high drain-to-source voltages in OFF state. In addition to the large dynamic range, fast rise- and fall-times in the order of ns have to be considered. This requirement leads to special demands on the measurement tool, typically an oscilloscope.

Accurate Measurement of RDS(ON)

To calculate RDS(ON) of a SMPS MOSFET, the drain current and the drain-to-source voltage of the rectifier need to be measured (Fig. 2, section 4). As the rectifier is typically operated in the 100 kHz range, no special requirements appear in this regard. However, measuring small drain-to-source voltages in ON state is challenging, typically a few hundreds of mV, due to high drain-to-source voltage levels in OFF state and peaks during state switching. Viewing such signals with both large and small voltage details, referred to as high dynamic range requirement, is challenging to meet for a standard oscilloscope with 8 bit resolution of the A/D converter. For example, referring to Equation 1, the resolution of an 8 bit ADC would be around 39 mV when measuring a 100V signal.

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Equation 1 • Calculation of vertical resolution of a n-bit ADC with reference voltage Vcc.

Additionally, poor probe compensation and incorrect probing techniques can lead to significant signal distortions, leading to incorrect measurement results even if the oscilloscope offers the necessary dynamic range.

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Figure 4 • R&S RTE/RTO K17 High Definition Mode.

Analyzing Small Signal Details under High Dynamic Range Conditions

With the R&S digital oscilloscope RTE or RTO and the high-definition (HD) mode, referred to as option K17, it is possible to measure the drain-to-source voltage for RDS(ON) under high dynamic range conditions. The high-definition mode applies a digital low-pass filtering on the measured signal which reduces noise and increases the signal-to-noise ratio inside the filter bandwidth. With this method, depending on the selected filter bandwidth, a resolution improvement of 256 times compared to a typical 8 bit resolution can be achieved. This enhancement corresponds to a vertical resolution of 16 bit (Fig. 4).

With this approach it is possible to see small signal details like the drain-to-source voltage in switched mode power supply applications, which would otherwise vanish in noise. Fig. 5 and Fig. 6 show such a measurement. Because the HD mode is not utilized in Fig. 5, the displayed noise is significantly higher compared to Fig. 6 were it is enabled. The improvement of the higher resolution can be seen on the measurement curve and in particular at the “zoomed” part of the signal (Fig. 6). Both are showing less noise and more signal details compared to the displayed result in Fig. 5 where the HD mode is not enabled.

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Figure 5 •Drain-to-Source voltage measurement of SMPS without utilizing HD mode.

A prerequisite to achieve accurate measurement results is the right probing technique. When measuring signals with high-frequency components, one has to make sure that the “loop” formed by the probing connections (signal pin and ground connection) is as short as possible. The spring-loaded tip of the R&S RT-ZP10 passive probe together with spring-type ground contacts allows a safe contact with minimal noise and interference coupling on the measured signal (Fig. 7). This allows a direct probing of the MOSFET pins and body.

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Figure 6 • Drain-to-Source voltage measurement of SMPS with HD mode enabled.

In addition, accurate probe compensation is very important for high-resolution measurements. A poorly compensated probe introduces measurement errors resulting in inaccurate readings, which can also influence differential measurements. For measurements where none of the MOSFET pins have ground potential, an active differential probe like the R&S RT-ZD10 can be used.

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Figure 7 • R&S RT-ZP10 passive probe with spring-type ground connection to reduce noise and interference coupling.

Avoiding Offset Problems When Calculating RDS(on)

Measuring different voltage levels under high dynamic range conditions requires additional steps to get an accurate result. To calculate RDS(ON) of a MOSFET, the offset accuracy of oscilloscopes is no longer sufficient to simply divide the measured drain-to-source voltage across the MOSFET by the drain current. Furthermore, when Rogowski probes are used to measure the current through the drain pin of the MOSFET, only the AC components are considered. Therefore, the resulting current measurement on the oscilloscope will have a DC offset. This problem can be addressed by taking advantage of the fact that the drain current shows a constant or nearly constant slope for a certain time interval while the MOSFET is in on-state. Concerning this matter, a differential method as shown in Fig. 8 can be applied to calculate RDS(ON).

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Figure 8 • Differential method to calculate RDS(ON) under high dynamic range conditions.

The procedure is to measure the slope of the drain-to-source voltage and derive ΔuDS first. Next, in order to get ΔiD, the slope of the drain current of the MOSFET has to be measured in the same time interval used for measuring ΔuDS. RDS(ON) can now be calculated based on ΔuDS and ΔiD.

Summary

To analyze signals with both large and small voltage details, special considerations need to be included. The discussed example of measuring RDS(ON) of a SMPS is a typical example for such an application and shows that special measurement capabilities are required to address these challenges. Today’s oscilloscopes like the R&S RTE and R&S RTO are featuring such measurement capabilities and allow the user to solve the measurement task in a fast and accurate way. With special features like the high-definition mode, option K17 on the R&S RTE and R&S RTO oscilloscopes, the signal analysis gets even more accurate and simplified as the noise of the displayed waveforms gets reduced and signal details are displayed with greater clarity.

About the Authors

Matthias Beer, Renate Mittermair, and Dr. Markus Herdin work in the Test & Measurement Division of Rohde & Schwarz GmbH & Co.KG, Munich, Germany.

References

[1] Barry Rowland, “Initial Evaluation of a DC/DC Switch Mode Power Supply”, Rohde & Schwarz Application Note 1TD04, 2013

[2]Christophe Basso, “Switch-Mode Power Supplies Spice Simulations and Practical Designs”, McGraw Hill Professional, 2008

[3]Bo Yang, “Topology Investigation for Front End DC/DC Power Conversion for Distributed Power System”, PhD thesis, Virginia Polytechnic Institute and State University, September 2003

[4]Davide Giacomini, “A novel high efficient approach to input bridges”, PCIM Europe, May 2008

[5]Ulrich Tietze, Christoph Schenk, “Electronic Circuits”, Springer, 2008

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