TPS23880: How to choose MOSFET ?

Hi Pierre,

Please make sure the TPS23880's ILIM foldback curve and inrush foldback curve are within the SOA curve when you choose MOSFET. Thanks.

Best regards,

Penny

Hi Penny,

Thank you for your reply even if your suggestion didn't help me.

With the curves, the maximum current is around 1.25 A for Vds wich goes to 0 V. 

To inject PoE, the transistor has to be closed so the gate is controlled. As we know, Vds could not be 0V because of the Rds(on). So I have to know Rds(on) for 1.25 A to find the SOA. I dind't find the Rds (on) curve function to the current on the CSD19538Q3A. How could I find it ?

Thank you for your help

Pierre

Hi Pierre,

I don't know if I understand your question. I think there's only one SOA for each MOSFET. For example, CSD19538 has below SOA. 

Then you refer to the foldback curve for TPS23881 and make sure the foldback curve is within the SOA range. Thanks.

Best regards,

Penny

Hi Penny,

I forgot to tell you that i'm working with the TPS23880 evaluation module documentation.

As you can see on the schematic : 

Transistors are used to close the loop between VPWR and GND to injecte the power inside the RJ45 port. I need to characterize the good MOSFET to insure that his SOA is enough. I worked on the CSD to understand correctly. 

To do that according to the figure 30 above and schematic :

Transistor OPENED : Ids = 0 A and Vds goes to VPWR 

Transistor CLOSED : Ids = 1.25 A and Vds goes to 0 V but it's not 0 V due to Rds (on)

The OPENED position of the MOSFET is not important in our case because there isn't current through the transistor. We will analyse the CLOSED position. 

Vds (MOSFET) = Rds (on) x Ids. How to find Rds(on) for Ids at 1.25 A ? This is given for 5 A only on fig 7 page 5 

According to the SOA curve, with the practical VDS value, I could know if 1.25 A is under or upper than the Rds linear curve (grey) to insure a good life condition of the MOSFET with a marge.

Thank you for your help

Hi Penny,

According to the page 31 of the datasheet of TPS23880 and datasheet of CSD19538Q3A, the CSD can only be used for the configuration 2xFBn =0, ALTFBn = 0 2xFBn =0, ALTFBn = 1

If FBn = 1, current through the transistor is 1.25 A at Vdrain = 0V so SOA curve is below the application 

As you can see on figure 9-3, the current is 1.25 A while vdrain is less than 1.5 V. It means that the Rds(on) should be around 1 Ohm but it's not, here it's around 60 mOhm.

So, Vd is not more than 0.1 V. 

In my opinion the CSD19538Q3A can be used only for IEE compliant PD but is not protected against partial and total short circuit. 

Regards,

Pierre