Calculating the source decoupling resistors for paralleling the MOSFETs

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Kiran21
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1. The current to charge the gate capacitors of the MOSFET's  is decided based on the  turn ON time for the MOSFET's and under peak power dissipation by the gate driver.

2. Based on the gate driver output limiting resistors are determined. 

3. later the  the resistor is distributed on the gate and source line. 

  the total gate resistor is chosen in the same way  as listed above ? in the application note " Batt Switch 48V", a reference pic is also attached.

I have doubt on why the resistors are not equally distributed on the gate and the source like 27 ohm each ? any reasons why 47 ohm is used on the source terminal. 

It would be better if the calculations are shared.

 

Regards, 

Kiran

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Abhilash_P
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Hi,

    Thank you for posting on the Infineon community.

Yes, your understanding is correct. The gate resistance is selected based on the criteria's' that you have mentioned.

But since the MOSFETs' are paralleled and used in common source configuration, a source resistance must be used as decoupling resistors. Adding external source resistors (R9 to R13 in the schematic) provides the negative feedback needed for stable operation. The gate-source voltage applied is given as,
VGS = VG * ID * R9.   for Q1. Similarly for rest of the MOSFETs'. 

 

Regards,
Abhilash P

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Abhilash_P
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Hi,

    Thank you for posting on the Infineon community.

Yes, your understanding is correct. The gate resistance is selected based on the criteria's' that you have mentioned.

But since the MOSFETs' are paralleled and used in common source configuration, a source resistance must be used as decoupling resistors. Adding external source resistors (R9 to R13 in the schematic) provides the negative feedback needed for stable operation. The gate-source voltage applied is given as,
VGS = VG * ID * R9.   for Q1. Similarly for rest of the MOSFETs'. 

 

Regards,
Abhilash P

Hi Abhilash, 

Can you please help me by  elaborating on  about the  source resistance  used as decoupling resistors and its negative feedback action and also on how source potential is ID*R9.  

Regards,

Kiran

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Abhilash_P
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Hi,

   The resistance placed in series with the source path helps in better current sharing in parallel MOSFETs'.

Higher the resistors, better the current sharing. We need to also consider tradeoff  between the current sharing and the efficiency. It is basically used as a negative feedback to the Rdson resistance spread of all the parallel MOSFETs'. 

 

Regards,
Abhilash P

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Kiran21
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Hi, 

Source resistors only have role only during the gate capacitors being charged by gate driver right?  During steady state i.e., MOSFET operated in ohmic region, how negative feedback is created by source resistor. 

 

Regards, 

Kiran 

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Abhilash_P
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Hi,

   As explained in my previous response, this resistor is used for proper current sharing across parallel mosfets' since a single gate driver is used to drive multiple MOSFETs' and also since it is used in a common source configuration. 

 

Regards,
Abhilash P