22 CAPACITIVE DIGITAL ISOLATORS www.we-online.com/en Issue 3 2024 Power Electronics Europe www.power-mag.com voltage of ±30 V max. with a variation period of one second. The current consumption has been minimized to < 85 mA for the transmitter and < 25 mA for the receiver with a voltage supply of 15 V. Both the transmitter and the receiver are electrically isolated, the transmitter between the measurement data acquisition and the signal transmission path and the receiver between the signal transmission path and the data output. Special DC/DC power modules and digital isolators with galvanic isolation and particularly low parasitic coupling capacitance were used to achieve this isolation in the circuitry. The signal is transmitted between the transmitter and receiver via a two-wire cable. Depending on the electromagnetic environmental influences, the distance can be several hundred meters. Transmit circuit Figure 2 shows the block diagram of the transmitter. The circuit is divided into six blocks: 1. probe: transducer with voltage divider and amplifier for measuring positive and negative polarity. 2. level shifter: level shifter for the voltage-frequency converter. 3. voltage-frequency converter: digital output signal, frequency dependent on the input voltage. 4. digital isolator: galvanic isolation between measuring potential and interface. 5. interface buffer: Low-impedance line driver with balanced output. 6. power supply: DC/DC converter, galvanically isolated converters for the probe head section. To ensure functional reliability, measures for transient protection and filters are provided both on the probe side and at the driver output; low-pass filters are also provided before and after the DC/DC power modules to effectively attenuate RF coupling. Receiver circuit Figure 3 shows the block circuit of the receiver. The circuit is divided into five blocks: 1. input buffer: signal pick-up, signal conditioning with balanced input. The link indicator shows whether there is a detectable connection to the transmitter. 2. digital isolator: galvanic isolation between input signal and secondary signal processing / output interface. Additional galvanically isolated voltage for the inputside buffer. 3. frequency-to-voltage converter: Generates an output voltage from a digital signal. The voltage level depends on the frequency of the input signal. 4. interface buffer with polarity indicator: Level converter for the output signal. The output signal has a positive polarity, the polarity indicator shows the polarity of the input signal. 5. power supply: DC/DC converter for the secondary-side supply. Numerous EMC measures are also provided in the receiver section. The signal input from the twisted pair cable is equipped with transient protection and a common mode filter to effectively attenuate interference from the cable. The power supplies around the DC/DC converters are equipped with low-pass filters on both the input and output sides to significantly reduce electromagnetic interference from outside and inside the circuit caused by the switching operations of the DC/DC converters. This ensures a high signal-to-noise ratio and high reliability. Capacitive digital isolators The digital isolator from Würth Elektronik [1,2] consists of an oscillator and a modulator on the primary side. A demodulator and a signal buffer are located on the seco ndary side. The components on the primary side are electrically isolated from the components on the secondary side by a capacitive structure with an isolation barrier made of Figure 3: Block diagram of the receiver for potentialfree voltage measurement. Figure 4: Basic structure of a digital isolator from Würth Elektronik.
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