AN90004

Probing considerations for fast switching applications
Rev. 1 — 17 October 2018

Application note

Document information
Information

ContentKeywords

GaN FET, fast switching measurement, oscilloscope probe, ground spring

Abstract

Accurate voltage waveform measurements of fast switching circuits require special care andattention in the use of the oscilloscope probe. This Application note details the use of a probeground spring to achieve the best results.

AN90004

Nexperia

Probing considerations for fast switching applications

1. Introduction
The latest GaN FET devices from Nexperia are capable of very fast switching of high voltages andcurrents. This requires special care to be taken in the measurement of the switching waveforms.
In order to accurately measure voltages on fast switching nodes, best measurement practices
should be followed. It should be noted that unless efforts are made to follow best practiceas closely as possible, then the voltages seen on the oscilloscope may be an artefact of themeasurement rather than a true representation of the real voltage on the node.
An example of a fast switching waveform is shown in Fig. 1 below. Note the difference between
measurements results with a) the oscilloscope probe ground connection made using a standard
probe ground wire and b) the same measurement with the ground connection made using a probeground spring.

Ground spring
VGround wire

3025201510
50-5-20Fig. 1.

AN90004

Application note

02040

8060time (μs)

Switching waveform; probe grounding method comparison

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Probing considerations for fast switching applications

2. Oscilloscope probe ground spring
The ground lead length of the oscilloscope probe can have a very detrimental effect on themeasurements taken. If you use the standard 10 cm loop supplied with the scope probe, themeasurements that you make will be prone to noise and pickup, as the loop acts as an aerial andpicks up noise in proportion to the loop area.
In order to make switching node measurements that are less prone to noise and pickup, thefollowing type of oscilloscope probe connections should be made:Fig. 2.

Probing a fast switching node

Note that the oscilloscope probe ground connection is made via a test probe ground spring, seeFig. 3. It is important that the ground wire is kept in parallel with the probe tip and the loop area iskept to an absolute minimum.

Fig. 3.

Probe ground spring

These oscilloscope probe accessories are available from electronic test equipment distributors andretailers - e.g. Mouser Electronics Part No: 940-PK1-5MM-118 (Mfr. Part No: PK1-5MM-118).

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Application note

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Probing considerations for fast switching applications

Fig. 4.

Oscilloscope probe with ground spring attached

It is possible to make your own probe ground spring using TCW25 tinned copper wire, see (Fig. 2).
Form the spring around the probe, remove and apply a little flux and solder to hold it in place. Trimthe ends and use appropriately.

AN90004

Application note

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Probing considerations for fast switching applications

3. Oscilloscope and probes
When measuring fast switching nodes always use a high-quality oscilloscope that does not
introduce unwanted parasitic capacitance, especially in the sensing loop.

Fig. 5.

Typical oscilloscope used in a Nexperia measurement lab

To make accurate measurements, the best quality scope probes should be used. These shouldhave high input impedance: preferably 100 MΩ or greater, and very low capacitance: less than
4 pF. Also, the greatest bandwidth scope probes should be used in line with the switching edges
that are being measured.
The impedance and capacitance of the scope probe can have an effect on the voltage that is being
measured. By using high input impedance and very low capacitance the effect of the probe on thenode being measured is minimised.
Always check that the oscilloscope probe you are using has the correct maximum voltage rating forthe signal being measured. Do not exceed the manufacturer's recommendations - voltages close toa probe’s maximum rating should be avoided.
Detailed information about probes and advice on measurement best practice is often available fromprobe manufacturers. As an example, Tektronix publish a comprehensive guide - ABCs of Probes
Primer, (http://info.tek.com/www-abcs-of-probes-primer.html).

AN90004

Application note

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Probing considerations for fast switching applications

4. Probe positioner
When measuring high voltages extreme caution should be taken and best practices adhered to.
With this in mind, the use of a probe positioner is highly recommended. The oscilloscope probes
can be adjusted to make contact with the required measurement points, leaving the test setup to beoperated “hands free”.
The following probe positioner is recommended: Type : MSA100 3D PROBE POSITIONER Farnell
part number 1552771.

Fig. 6.

Probe positioner

5. Revision history
Table 1. Revision history
Revision
DatenumberDescription

1.0

Initial version of the document

2018-10-17

AN90004

Application note

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Probing considerations for fast switching applications

6. Legal information
Definitions
Draft — The document is a draft version only. The content is still underinternal review and subject to formal approval, which may result inmodifications or additions. Nexperia does not give any representations or
warranties as to the accuracy or completeness of information included hereinand shall have no liability for the consequences of use of such information.

Disclaimers
Limited warranty and liability — Information in this document is believed
to be accurate and reliable. However, Nexperia does not give any
representations or warranties, expressed or implied, as to the accuracy
or completeness of such information and shall have no liability for theconsequences of use of such information. Nexperia takes no responsibility
for the content in this document if provided by an information source outsideof Nexperia.
In no event shall Nexperia be liable for any indirect, incidental, punitive,
special or consequential damages (including - without limitation - lostprofits, lost savings, business interruption, costs related to the removalor replacement of any products or rework charges) whether or not suchdamages are based on tort (including negligence), warranty, breach ofcontract or any other legal theory.
Notwithstanding any damages that customer might incur for any reason
whatsoever, Nexperia’s aggregate and cumulative liability towards customer
for the products described herein shall be limited in accordance with theTerms and conditions of commercial sale of Nexperia.
Right to make changes — Nexperia reserves the right to make changes
to information published in this document, including without limitation
specifications and product descriptions, at any time and without notice. This
document supersedes and replaces all information supplied prior to thepublication hereof.
Suitability for use — Nexperia products are not designed, authorized or
warranted to be suitable for use in life support, life-critical or safety-criticalsystems or equipment, nor in applications where failure or malfunctionof an Nexperia product can reasonably be expected to result in personal
injury, death or severe property or environmental damage. Nexperia and its
suppliers accept no liability for inclusion and/or use of Nexperia products insuch equipment or applications and therefore such inclusion and/or use is at
the customer’s own risk.
Applications — Applications that are described herein for any of theseproducts are for illustrative purposes only. Nexperia makes no representation
or warranty that such applications will be suitable for the specified use
without further testing or modification.
Customers are responsible for the design and operation of their applications
and products using Nexperia products, and Nexperia accepts no liability forany assistance with applications or customer product design. It is customer’ssole responsibility to determine whether the Nexperia product is suitableand fit for the customer’s applications and products planned, as well as
for the planned application and use of customer’s third party customer(s).
Customers should provide appropriate design and operating safeguards tominimize the risks associated with their applications and products.
Nexperia does not accept any liability related to any default, damage, costs
or problem which is based on any weakness or default in the customer’sapplications or products, or the application or use by customer’s third party
customer(s). Customer is responsible for doing all necessary testing for thecustomer’s applications and products using Nexperia products in order toavoid a default of the applications and the products or of the application oruse by customer’s third party customer(s). Nexperia does not accept any
liability in this respect.
Export control — This document as well as the item(s) described herein
may be subject to export control regulations. Export might require a priorauthorization from competent authorities.
Translations — A non-English (translated) version of a document is forreference only. The English version shall prevail in case of any discrepancy
between the translated and English versions.

Trademarks
Notice: All referenced brands, product names, service names andtrademarks are the property of their respective owners.

AN90004

Application note

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Probing considerations for fast switching applications

List of Tables
Table 1. Revision history......................................................6AN90004

Application note

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Probing considerations for fast switching applications

List of Figures
Fig. 1. Switching waveform; probe grounding method
comparison........................................................................... 2Fig. 2. Probing a fast switching node...................................3Fig. 3. Probe ground spring................................................. 3Fig. 4. Oscilloscope probe with ground spring attached.......4Fig. 5. Typical oscilloscope used in a Nexperia
measurement lab................................................................. 5Fig. 6. Probe positioner........................................................6

AN90004

Application note

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Rev. 1 — 17 October 2018

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Probing considerations for fast switching applications

Contents
1. Introduction................................................................... 2
2. Oscilloscope probe ground spring............................. 3
3. Oscilloscope and probes.............................................5
4. Probe positioner........................................................... 6
5. Revision history............................................................6
6. Legal information......................................................... 7© Nexperia B.V. 2018. All rights reserved

For more information, please visit: http://www.nexperia.com
For sales office addresses, please send an email to: salesaddresses@nexperia.com
Date of release: 17 October 2018

AN90004

Application note

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