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Solid state relay: Frequently asked questions

Solid state relay: Frequently asked questions

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What is the maximum operating junction temperature of opto solid state relay (SSR) (for example, PVT322AS, PVG612PBF)?

The maximum operating temperature of opto SSRs is 105°C.  It is limited by internal LED temperature.

What are the different connection configurations available for SSRs (for example, PVG612PBF, PVX6012 etc.)?

Following are the possible configurations depending on how you connect the internal switches on the output side.

  1. Single connection of one internal switch
  2. Parallel connection of the internal switches
  3. Series connection of the internal switches 

Figure 1 shows the one of the possible configuration for the PVX6012 (an IGBT based output switch SSR) that can use both AC and DC supply.
KA-15276_Image1.pngFigure 1 PVX6012 relay internal circuit

If you use DC supply as shown in Figure 1, then one switch and one diode on the opposite side will conduct making it work.
If you use AC supply, during one polarity, one switch and diode on the opposite side will conduct and during the other polarity, the other switch and diode on the opposite side will conduct.

In the PVG612PBF, which is MOSFET based output switch SSR, the following configuration can be used in AC or DC operation.
 

KA-15276_Image2.png

 Figure 2 PVG612PBF internal circuit with AC/DC (series connection)

As MOSFETs have inherent body diode, the circuit operation is same as discussed above. Figure 3 shows a configuration which works only in DC.

KA-15276_Image3.png
Figure 3 PVG612PBF internal circuit with DC (single connection)


AC supply cannot be applied in this configuration because, in one polarity the body diode starts conducting and the supply will be clipped to forward voltage drop of the body diode.

Figure 4 shows the configuration which works with parallel connection. In the following configuration, the load current can be increased but cannot be doubled due to thermal limitation.

 

KA-15276_Image4.pngFigure 4 PVG612PBF internal circuit with DC (parallel connection)

How to calculate ON resistance required to safely turn on the output switch?

To turn on the switch, the gate needs to be activated which is done by applying some current at the input terminal which gets converted into light by the LED.
The formula used to limit the input current within the datasheet specified limit is as following:
KA-15276_IMAGE5.png

 

The above equation expresses the maximum allowed resistance in input to turn on the switch.

 

KA-15276_IMAGE6.png

 

 

The above equation expresses the minimum  allowed resistance in input to turn on the switch safely.
 Where,

VIN = Input voltage applied,

VLED = Internal LED forward voltage drop.
 

Note: Equation (1) and (2) gives maximum (Rcmax) and minimum (Rcmin) resistance values to be applied on the input to turn on the switch. To design:

  • Rcmax , the VLED should be generally considered at 85°C
  • Rcmin, the VLED should be considered at coldest operating temperature which can be determined from LED voltage drop graph.
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