Exploring Non-Optical SSR Isolation

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With optically isolated solid state relays (SSR) so common, it’s easy to assume that all of these products will use that coupling architecture. However, any isolation mechanism could theoretically be incorporated into an SSR. Although they are not common, a few non-optical types are commercially available.

Continue with an optical isolation review or jump to:

First, let’s quickly review the optical isolation structure. The Digi-key site currently (as of this posting) does not include a filter for the isolation type, so it’s important to recognize the optical feature in a drawing.

Here is the main category for solid state relays: [click here https://www.digikey.com/products/en/relays/solid-state-relays/183 ]. Most random picks will produce an optically isolated device. Note the photodiode and coupling symbols that are common in all of those. The output side may vary, but all optically isolated SSRs will look similar in this regard.

Part number CLA311CT-ND is one example (IXYS number LCB716STR). [click here https://www.digikey.com/short/zbmpmr ].

Image source: [click here https://www.ixysic.com/home/pdfs.nsf/0/C189E884A3BC9DA98525749E004A25EC/$file/LCB716.pdf

IXYSSSR

Part number 306-1308-ND is another example of optical coupling (Coto Technology number C347S). [click here https://www.digikey.com/short/zbmz7f ].

Image source: [click here https://www.cotorelay.com/wp-content/uploads/2014/10/c247s_c347s_mosfet_relay_datasheet.pdf ].

COTOSSR

Capacitor Coupled Isolation

An alternative is capacitor coupled isolation. Within the Panasonic AQY2 series, some part numbers with an AQY2C prefix use this configuration: [click here https://www.digikey.com/short/zbmrjt ]. This works by generating an oscillating signal which is capacitively coupled across the isolation barrier, and which is then used to charge the gates of the output MOSFETs.

The datasheet shows the internal structure and a comparison to other SSR types within the series:

PanCCInternal

PanCCCompare

Source of images: [click here Catalogs/Datasheets Download - Panasonic ]

Transformer Isolation

A third isolation technique within SSRs is through transformer inductance. Texas Instruments has a reference design for this type. Note the circuit diagram and partially populated bill of materials.

Reference Design: [click here https://www.ti.com/lit/ug/tidub92b/tidub92b.pdf?ts=1594758350599&ref_url=https%253A%252F%252Fwww.google.com%252F ]. Be sure to read the “Important Notice for TI Reference Designs” at the end of the article, as there are rules regarding the use of this material.

TITransformer SSR
TITransformer SSR705×311 26.9 KB

Partial Bill of Materials:

  • CSD19537Q3
  • SN74LVC1G19
  • SN74AUP1G74
  • SN74AUP3G14
  • LMC555

Hybrid SSR

There may also be references to “hybrid” SSRs. In general, these designs are only partially solid state, but they may provide isolation (depending on design), and some of the problems encountered with mechanical relays are averted.

Examples can be found in the OMRON G9H series: [click here https://www.digikey.com/short/zbz2pc ].

OmronG9HLayout

Image source: [click here http://www.ia.omron.com/data_pdf/cat/g9h_ds_e_6_2_csm175.pdf ].

Similar designs use the basic principle of a reed relay within the overall SSR topology. These are “hybrid” in the sense that they include isolation, but are not truly solid state given the presence of moving contacts.

Summary

Each design has its pros and cons, or to be more precise, applications where it is best suited. Check the datasheets and other manufacturer documents to find the highlighted features and specs unique to each product.

For example, here is a summary of the Panasonic AQY2C product mentioned earlier:

Main advantages:

  • lower input current (0.2mA max) required to turn on, vs. > 2mA for most others
  • higher operating temperature capability of +105 °C (most others are +85 °C)
  • smaller size (requires less space than opto interface)
  • no wear-out mechanism (LED types degrade over time)

Disadvantages:

  • low isolation voltage (only about 200V, compared to >1KV for most others)
  • max load voltage currently only 100V (less for some versions), whereas some opto types can be > 1000V

For more detailed information on general SSR functions, the following article is useful: [click here https://www.digikey.com/en/articles/how-to-safely-and-efficiently-switch-current-or-voltage-using-ssrs ].