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TPSI2072-Q1: Capacitive isolated Solid state relay to drive a 24V DC relay from 2 different MCU's.

tilak kc
Prodigy 20 points
Part Number: TPSI2072-Q1
Other Parts Discussed in Thread: CSD17483F4, TPSI3050, CSD18543Q3A

Hi ,

I want to design an output logic drive circuit for 24V DC relays.

We are driving the external 24V DC relay using 2 FET logic circuits as shown below - The 2 FETS are driven by 2 different MCU's. When both the FET's turn on-> then the relay will be energised. We are using optocouplers to provide isolation between MCU pin and FET.

Kindly suggest a similar circuit connection to drive the relay from 2 MCU's using solid state relay from TI which has capacitive isolation and functional safety capable. For ex - TPSI2072-Q1. 
When we send logic high from both the MCU's then only the relay should energise.

Thanks in advance,    Tilak

  • 0
    TI__Prodigy 486 points

    Hi Tilak,

    Thank you for the question.

    The TPSI2072-Q1 is an isolated switch, meaning the device has internal FETs with a 50 mA maximum recommended operating current. The scheme I would suggest would look something like this:

     

    Each of the FETs in our device has an Ron of 65 Ohms, making it 130 Ohms in total. At 24V, 50mA would flow through the FETs. This solution eliminates the need for external FETs, but it comes very close to the maximum recommended operating conditions. A solution to this is to add some more resistance in series.

     

     What is the minimum current needed to energize the relay?

     

    Sam Skranak

    Product Marketing & Applications | Solid-state relays

  • 0 in reply to Samuel Skranak
    Prodigy 20 points

    Hi,

    The relay rated voltage is  24V DC (Max full operating voltage - 19.2VDC) with coil resistance of 350ohm.

    Minimum current required will be around 55mA.

  • 0 in reply to tilak kc
    Prodigy 20 points

    Also please suggest an FET with voltage drain to source of 30V (similar to csd17483f4) and

    VGS(TH) of greater then 2V.

    Hi,

    The relay rated voltage is  24V DC (Max full operating voltage - 19.2VDC) with coil resistance of 350ohm.

    Minimum current required will be around 55mA.

  • 0 in reply to tilak kc
    TI__Prodigy 486 points

    Hi Tilak,

     

    Since the minimum current is 55 mA, TPSI2072-Q1 is not suitable. However, we have high reliability isolated switch drivers that can drive the FETs from your original schematic (CSD17483F4). The device I recommend is TPSI3050, here is the product page: https://www.ti.com/product/TPSI3050

     

    A block diagram with TPSI3050 would look like this:

     

    How does this solution look to you?

     

    Thanks,

    Sam Skranak

    Product Marketing & Applications | Solid-state relays

  • 0 in reply to Samuel Skranak
    Prodigy 20 points

    Sir,
    Can i use csd18543q3a as FET in above circuit - it has 2V VGS(TH) .
    whereas in case of csd17483f4 - VGS(TH) is 0.85V which is very less and unsafe turn on can occur.

  • 0 in reply to tilak kc
    TI__Prodigy 486 points

    Hi Tilak, 

    Yes, CSD18543Q3A is suitable!

    Thanks, 

    Sam