LM7481-Q1: Power sources priority and redundancy - circuit design

Hi Niv,

Welcome to E2E!

You surely need features like OVP/UVP. Please refer to this document below which explains how to use priority power MUXing. 

Priority Power MUX using Ideal Diodes in Automotive Zonal Modules

I would recommend you use LM7481-Q1 for this application. The parallel FETs indeed will take longer to turn-on but to tackle that, we have a solution by driving the blocking FET with CAP pin. This way the turn-on time will be minimum.

Regards,

Shiven Dhir

Hi Shiven,
Thank you for your response.

I have already reviewed the document you attached, but it doesn't provide an exact solution to my requirements.

I understand that I need the OVP function to ensure that the power path from the battery will be OFF when the power path from the power supply is ON and when the power path from the power supply disconnects, the OV pin of U2 will be pulled down, and U2 will exit OVP mode so that the power path from the battery turns ON.

However, I don't need the UVP function; I want to use this pin for the Enable/Disable function to turn ON and OFF the output voltage to the load.

When both U1 and U2 receive an Enable signal, will the prioritization of the power path depend solely on the OVP function?

Additionally, from what I understand from you, will the LM7481-Q1 combined with transistor Q5 and by driving the blocking FET with CAP pin provide me higher gate drive strength than the LM74722-Q1?

Hi Niv,

Your understanding is correct on OVP. Regarding UVP, yes you can use it as EN signal also.

When both EN signals are high, prioritization will happen solely on OVP.

Regarding the gate drive strength, yes LM7481-Q1 combination will have stronger drive as LM74722-Q1 has 50uA.

Here, point of interest is the gate drive of blocking FET as this FET only influence the time of "no power"

Regards,

Shiven Dhir

Hi Shiven, thanks for the response.

Based on your answer, I started the schematic drawing and the component selection. I’m attaching a PDF file of the initial schematic.

I chose to use Infineon Power MOSFETs, part number IQD020N10NM5CGATMA1, for all the MOSFETs.

As you can see in the drawing, only one pair of back-to-back MOSFETs is present in the power path from the power supply, where around 5A will flow.
In contrast, in the power path from the battery, where around 250A will flow, I had to connect 5 pairs of MOSFETs in parallel.

I would appreciate your feedback on the schematic so far.
Additionally, to generate a higher current for charging the GATE of the MOSFETs in parallel, I want to implement the addition of the NPN transistor. I would appreciate help in selecting the appropriate transistor and the resistor and diode connected to it.

I don’t think I fully understood the potential at the CAP pin or the current it will push to perform the calculations needed for choosing the transistor and resistor.

Thanks a lot,
Niv

4130.Schematic Prints.pdf

Hi Niv,

We have a Pspice model of this controller available to TI.com which you can run in Pspice for TI. With the model you can design the resistor value which will decide the turn-on of the MOSFET.

Regarding the rating: 

1. The voltage rating of Q5 transistor should be > [VAUX(MAX) + 15V]
2. The value of R5 resistance is depends on how much current you want to inject into the HGATE,
   • With R5 = 0 ohms, the current injected would be around 2.7 mA
   • If you want to reduce the current injection into HGATE to slow down the turn on of the HFET, you can increase the value of R5. For example, if you want to limit the max current injection into the HGATE to 500 uA, then R5 = [(VAUX(max)+15V)/500 uA]
3. Rating of diode D2 should be > VPRIM(max)

Regards,

Shiven Dhir

Hi Shiven, thanks for the response.

1. The voltage rating of Q5 transistor should be > [VAUX(MAX) + 15V]
   When you say voltage rating of Q5, do you mean collector-emitter breakdown voltage / collector-base breakdown voltage / emitter-base breakdown voltage?
   
   
2. The value of R5 resistance is depends on how much current you want to inject into the HGATE
   How do I calculate how much current I need to inject into HGATE?
   What will be the Gate Drive Voltage that the MOSFET will receive when it is connected to the emitter of Q5?
   With R5 = 0 ohms, the current injected would be around 2.7 mA
   In the LM74810-Q1 datasheet, I(CAP) is specified as 3.8mA. Could it be that the picture you attached is related to another device?
   
   
3. Rating of diode D2 should be > VPRIM(max)
   Did you mean D3 (or Dx in my schematic drawing)?
   When you say voltage rating of D3, do you mean to the diode reverse voltage (V_R)?
   What type of diode should it be?

Updating:
VPRIM(max)=53V
VAUX(max)=50.4V

I would be very grateful if you could review everything and provide more detailed responses.

Thank you very much!

Hi Shiven, thanks for the response.

1. It is the VCEO (collector-emitter)
   OK, Thank you!
   
   
2. Current to be injected into HGATE depends on system requirements. Higher the current, faster the switching of HGATE. HGATE will be driven to around 10V VGS.
   I understand what you are saying but let me ask this more correctly
   - how do I calculate the MOSFET turn-ON time and turn-OFF time?
   - What parameters do I need to know for the calculation? (I_gate / Vgs / Qg / Ciss), what equation should I use?
   I want the on and off times of the MOSFETs in both power paths to be as synchronized as possible so that the load does not feel the change between the paths and the transition between the paths will be as smooth as possible.
   
   
3. I mean Dx in your schematic. Rating that I referred to is V_R. It can be a general-purpose switching diode.
   OK, Thank you!