When it comes to taking measurements, modern power conversion devices using switching techniques require special handling, including the need for differential probes. This is because, unlike their analog predecessors, they don’t employ transformers to reduce the line voltage. Instead, they use the rectified line voltage as the DC bus source (Figure 1). This topology has interesting implications with respect to grounding and differential signals. In the configuration of Figure 1, the circuit full-wave rectifies the AC line. For a 120rms AC line, the peak-to-peak voltage is 340 VAC. The full-wave-rectified voltage across the capacitor is 170 volts DC. A 240 volt line would double those numbers. This is used as the DC source for the switched-mode voltage regulator. The power switches are configured in a half-bridge topology, with upper and lower switches alternately connected to the output. The voltage regulator (not shown) generates pulse width modulated (PWM) signals which regulate the output voltage by driving the gate- to-source voltage of the MOSFETs. Why you need differential probes Looking at Figure 1, there are some things to note. First, no point in the circuit is referenced to ground. The input line has a hot and a neutral wire. The neutral is ground
Figure 1: Shown is a functional block diagram of a switched-mode power converter for full-wave rectification of the line voltage to generate a DC bus voltage. Image source: Teledyne LeCroy
This presents another problem for a ground-referenced oscilloscope measurement. The solution to this measurement problem is to use a differential probe. Given the voltages encountered – up to 680 volts – it will have to be a high-voltage differential probe (Figure 2). A differential probe measures the difference between the inputs. High-voltage differential probes include attenuators and overload protection on each input. Typical attenuation values are in the range
referenced at its source and may be several volts off ground before reaching the powered device. The voltages in the power converter are essentially floating. Attempting to make a voltage measurement with an oscilloscope using a common passive probe requires connecting the oscilloscope ground somewhere. Connecting a ground lead to any point in this circuit could cause problems. The second thing to note is that the upper MOSFET voltages are riding on the lower MOSFET’s drain voltage. This is switching between zero volts and the DC bus voltage.
High-voltage differential oscilloscope probes: why you need them
Figure 2: Shown is the functional block diagram of a high-voltage differential probe which does not require a ground connection as it measures the voltage difference between the + and – probe inputs. Image source: Art Pini
Written by Art Pini
we get technical
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