HV optically isolated scope probe captures bridge-gate signals
The HVFO probe extends Teledyne LeCroy’s capabilities in the area of power electronics measurements; optimized for small-signal floating measurements on a HV bus in power electronics designs, it offers optical isolation between the probe tip and the oscilloscope input to reduce adverse loading of the device under test (DUT); and also reduces noise, distortion, ringing, overshoots, and transients on the measured signal. It surpasses the measurement capabilities and signal fidelity of both conventional HV differential probes and acquisition systems that rely on galvanic channel-to-channel and channel-to-ground high voltage isolation. It also avoids reliance on dangerous test setups that require floating the oscilloscope and probe, or investments in specialized isolated oscilloscope or data acquisition systems that result in other performance compromises.
To avoid the need for a separate power connection to the probe head while HV measurements are in progress, LeCroy has made the HVFO battery powered; up to 6 hours are available on a charge, and a single micro-USB alternately provides charging ans signal connection, to eliminate the possibility of an electrical connection being made when isolated measurements are in progress.
The HVFO architecture is, LeCroy says, simple, with a single laser and fibre optic cable providing optical isolation and modulated signal + data communication. Multiple tips achieve different operating voltage ranges, from ±1 to ±40V. The HVFO is small, to fit in tight spaces, with what LeCroy terms “just enough performance for real-world needs”.
Its primary target measurement scenario, the gate drive circuit in a power electronics design is a series RLC circuit with parasitic capacitances across the semiconductor device. An upper-side device has an applied gate voltage floating above ground. Any measurement probe with high tip capacitance in parallel with the parasitic capacitance from the gate to emitter (CGE) or gate to source (CGS), and/or high impedance and low loop inductance in series with the gate drive impedance, will at best undesirably load the gate drive signal and at worst cause circuit malfunction. Measurement interference from the low-side device switching can also interfere with proper upper-side gate voltage measurement.
The HVFO has low tip capacitance and high input impedance, and because the amplifier is optically-isolated, the tip only has to measure the small signal gate-drive voltage [and not the high voltage on which it is superimposed]. Therefore, there is approximately 1/100th of the loading compared to a conventional HV differential probe; and superior noise and rejection provided by the high common-mode rejection ratio (CMRR), low lead loop inductance, and low attenuation.
For floating control or sensor signal measurements, it is difficult to measure signals with conventional instrumentation, because a probe with low input impedance or high attenuation will load the DUT and adversely affect system performance or signal fidelity, result in too much noise in the measurement, or both. The HVFO’s low tip capacitance, which is charged only to the small-signal floating sensor voltage level and not the common-mode voltage, will provide better response for faster signals.
The HVFO will be priced less than $4000.
Teledyne LeCroy; www.teledynelecroy.com
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