Use optically isolated probes for testing high-power DC power supplies
Challenges in testing high-speed power supplies
Modern power supply designs often require the precise measurement of fast switching signals such as gate-to-source voltage (Vgs) and drain current (Id). Standard differential probes often struggle in this regard because they lack sufficient common-mode rejection and have limited bandwidth , leading to erroneous or distorted readings and complicating validation.
Real-world test setup
A leading power supply manufacturer in Guangdong faced precisely this challenge when developing a high-performance DC power supply based on SiC MOSFETs. The engineers observed irregular oscillations in the Vgs waveform of the top transistor when using conventional differential probes.
To solve this problem, the test team used an optically isolated probe solution in combination with a high-resolution oscilloscope and a flexible current probe . This setup provided reliable measurements and allowed engineers to clearly observe and understand the switching processes without misinterpretations due to poor isolation or limited bandwidth.
Instruments used for the test solution
Optically isolated probe tip (MOIP series) - Offers strong isolation and a high common-mode rejection ratio (CMRR) even at gigahertz frequencies.
High-resolution oscilloscope (MHO3 series) - Offers a large memory and high sampling rates for precise waveform capture.
Flexible Rogowski current measuring head (RCP series) - Enables accurate current measurements in compact PCB environments.
The combination of these high-performance instruments ensures signal integrity in both voltage and current measurements in high-frequency circuits.
On-site testing facility
The on-site test environment shows a complete measurement setup consisting of a high-resolution digital oscilloscope (MHO3 series - MHO3-5004), an optically isolated probe (MOIP series - MOIP1000P), a flexible Rogowski current probe (RCP series - RCP600XS) and the device under test (DUT).
The connection diagram shows how the MOIP1000P optical isolation probe is connected to the upper switch's Vgs signal via an MMCX coaxial cable. Simultaneously, the RCP600XS Rogowski coil is used to measure the upper switch's drain current (Id) by routing the conductor through the probe loop directly to the device pins.
The oscilloscope's waveform display shows both voltage and current measurements simultaneously. Channel 4 (green curve) displays the waveform of the upper switch Vgs, while channel 2 (blue curve) displays the corresponding current waveform of the upper switch Id, allowing for an accurate correlation between switching behavior and current flow.
Kundenerfahrung & Feedback
Vor dem Einsatz der optisch isolierten Messtastertechnologie verließ sich das Ingenieurteam bei der Signalmessung auf herkömmliche differentielle Messtaster. Während der Tests traten während der Ein- und Ausschaltvorgänge merkliche Oszillationen in der Wellenform des oberen Schalters auf. Da das Team zuvor nur wenig Erfahrung mit optischen Isolationslösungen hatte, nahm es zunächst an, dass das Problem von der Schaltung selbst herrührte.
Infolgedessen wurde viel Zeit damit verbracht, das Schaltungslayout zu ändern, die Parameter abzustimmen und alternative Konfigurationen zu testen - doch das Problem der Oszillation blieb bestehen. Als die optische Isolationssonde als Versuchslösung eingeführt wurde, war die Verbesserung sofort sichtbar. Die erfassten Wellenformen stimmten genau mit den theoretischen Modellen und den Simulationsergebnissen überein, so dass die Ingenieure das tatsächliche Verhalten der Schaltung schnell erkennen und eine kritische F&E-Herausforderung lösen konnten.
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