High-voltage differential oscilloscope probes
series. All of these probes are well- matched to switched-mode power converter measurements requiring ground isolation. They are fully integrated into the Teledyne LeCroy oscilloscope operating system and are automatically sensed and scaled for accurate measurements.
of 50:1 to 2000:1. This gives high- voltage differential probes an input voltage range from 1500 to 7000 volts. The device being measured is modeled as a differential source consisting of two differential sources, a positive component (V P ) and a negative component (V N ), as well as a common-mode component (V COM ). The common- mode component is shared with both the + and – inputs. The + input sees V P + V COM while the – input sees V COM - V N . The probe, ideally, measures the difference between these input voltages or V P +V N , eliminating the V COM term. Real- life differential probes attenuate the common-mode voltage but do not eliminate it completely. The differential probe’s common- mode rejection ratio (CMMR), the ratio of the attenuated common mode signal to its unattenuated amplitude expressed in decibels (dB), indicates the effectiveness of
means that the 170 volt common- mode signals will be attenuated by better than 65dB. The attenuated common-mode signal would have an amplitude of about 95 millivolts (mV). Since the gate-to- source voltage is on the order of 4 to 12 volts, the common-mode interference will have very little effect on the measurement. For higher voltages such as those associated with 1500 volt DC solar photovoltaic (PV) inverter measurements, I’d recommend the HVD3206A . It has a maximum differential voltage rating of 2000 volts (DC plus peak AC) and has the same bandwidth and CMRR as the HVD3106A-NOACC probe. Finally, for large three-phase machines and their controllers, the HVD3605A with its maximum voltage input of 7000 volts (DC plus peak AC), is the high-voltage differential probe I’d recommend. The high voltage range is the result of the 200:1 or 2000:1 attenuator available in this probe. It has a CMRR of 85dB at 60Hz, 70dB at 10 kilohertz (kHz), and 64dB at 1MHz, and an offset range of 6000 volts. The HVD3000A series probes all have gain accuracies of 1% or better, are available with and without accessories, and with oscilloscope cable lead lengths of 2.25 and 6 meters (m) (Figure 3). Accessory kits vary with the model and include voltage-suitable clips or micro-grabbers.
High-voltage differential probes are useful in situations where there is no ground reference and where the signal to be measured is riding on top of another, high-voltage signal.
Conclusion High-voltage differential probes are useful in situations where there is no ground reference and where the signal to be measured is riding on top of another, high-voltage signal. For these situations, Teledyne LeCroy provides the HVD3000A
the differential probe. This figure of merit is frequency-dependent, generally falling with increasing frequency. How to use a differential probe Let’s look at using a high-voltage differential probe to measure the upper gate to source voltage in a 120 volt input, switched-mode power converter, similar to the one shown in Figure 1. The DC bus voltage will be about 170 volts. The gate-to-source of the upper MOSFET will ride on the switching signal of the lower MOSFET, a PWM signal switching between zero
and 170 volts. The gate-to-source voltage will be on the order of 4 to 12 volts. For this measurement, I recommend a Teledyne LeCroy HVD3106A-NOACC high-voltage differential probe. This 120MHz bandwidth probe has a voltage rating of 1000 volts RMS. Its differential voltage rating is 1500 volts (DC plus peak AC), well matched to even a power converter operating off of 240 volts AC. It has an offset range of 1500 volts, making it easy to vertically expand the measured waveform to see details. The probe has a CMRR of 85dB up to 60 Hertz (Hz) and 65dB at 1 megahertz (MHz). This
Figure 3: Some of the high-voltage differential probe configurations for the HVD3000A series, which are offered with or without accessories and oscilloscope cable lengths up to 6m. Image source: Teledyne LeCroy
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