Electronic – Solid state device for switching (muxing) a bipolar high current signal

multiplexersolid-state-relay

I'm looking into solid state relays for an application where I need to switch between two different transducers to allow two different modes of operation for an echo sounder. Basically a MUX that allows selecting either transducer #1 or transducer #2.

Short overview of how an echo sounder works: the echo sounder generates a short pulse (1-10 ms, 60 Vrms) which is emitted through the transducer. Then afterwards the echo sounder goes into listening mode receiving the returned echo which reflects of objects and the bottom as it travels through the water column.

Developing a MUX has some challenging requirements for this kind of signal:

  • transmit requires a relatively high voltage signal to pass through (max 60 Vrms sinusoid of 1-10 ms duration), whereas when the echo sounder is listening it will receive a signal which has a low amplitude (< 10 mV).

  • The signal should pass through without distortion (close to linear transfer curve).

  • The received signal should not be colored by noise (high S/N ratio, > ~90 dB).

  • Signal is bipolar.

Is this even possible with SSRs? A better device? With solid state relays I'm wondering if a device capable of relatively high current will not perform well for low signals during receiving (non-linearity or too noisy). Of course a mechanical relay would solve all these but might not last as long if switching frequently and reliability is important (i.e. years).

Drawing shows TX and RX circuitry switched during transmit and receive (listen) phase. Operator can switch the mux to use transducer 1 or 2 when echo sounder is off.

Assumptions: Both transmit and received signals have no DC component. Transmit is a pure tone of e.g. 35 kHz. The received signals will be more wideband, but will be bandpass filtered in the receiver. The switching between transducer 1 and 2 can be restricted to only occur when system is turned off. Transducers are ceramic. Impedance of transducer at transmit frequency is on the order of 1.5 kOhm. Assume max 40 mA inst. current during transmit pulse. Assume there is no ringing in existing circuit as transmit pulse has smooth envelope (ramp up/down). The transmit circuitry is only connected for as long as the pulse is transmitted, afterwards the receiver is connected and listens until the next transmit ping.

Best Answer

Using some sort of FET-based solid state relay with isolation sounds like a great fit for this application. OMRON has a portfolio of FET SSRs (list) that probably will give you a reasonable output voltage and output on-state resistance. The G3VM series probably has the best options, with 100V output versions. Try a G3VM-202J1 at 8 ohms on resistance, 200V; single channel G3VM-201J1? Or G3VM-101ER 0.2 ohm on resistance, 100V?

From IXYS, the LCC120 could be another candidate at 20 ohms and 250V. From Vishay, suggested by another thread, the VO1400AEFTR, but it's only rated to 60V.

Additionally, if your signal duty cycle is low enough, you'll be less concerned with the continuous on-current limitation/rating of the FET SSR because of the lower energy dissipated by the device.

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