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INNO3678C I sense pin resistor value selection

Posted by: plangtorr on 12/02/2021

Hi, I'm designing a SMPS Circuit with INNO3678C and I have a few questions about resistor selection around I sense pin.

I refered 'DER-726 - 65 W Isolated Flyback Adapter Using InnoSwitch3-EP PowiGaN and MinE-CAP' to design my circuit.

 

1) As attched 1_3, it uses 0 ohm resistor for R13 but I don't know what's difference with short circuit.

 

2) And there's GND between R13 and R16, so it seems that there would be no signal to I sense pin.

How does it work? It would be great if you give me some technical explanation and criteria.

 

3) Plus, what happens if I change R13 0ohm to 10ohm?

Actually, I use R13 as 10ohm and R16 as 0.003ohm to regulate secondary load current under 9A.

I read some criteria to choose R13 value but i have no idea about choosing R16 value.

It would be truly great if you give me some technical explanation and criteria.

 

4) Last, I want to know where GND should be.

In 1_3, GND locates between R13 and R16 so there's potential difference between GND and GNDpin.

But in 2_2, GND locates between R16 and C9 so GND shorted with GNDpin.

What's difference? For your information, I got 2_2 circuit from INNOSWITCH3-ep Family datasheet.

If you give me some technical explanation and criteria, it would be super great.

 

I'm sorry for many questions. Have nice day.

Files

Attachment Size
1_3.PNG 128.89 KB
2_2.PNG 72.74 KB

Comments

Submitted by PI-Chloe on 12/09/2021

Hi Plangtorr 

Good Day! 

1) As attched 1_3, it uses 0 ohm resistor for R13 but I don't know what's difference with short circuit.

    >>> The R13 is used as a provision if there is a need to filter the  signal to the IS pin.  

 

2) And there's GND between R13 and R16, so it seems that there would be no signal to I sense pin.

How does it work? It would be great if you give me some technical explanation and criteria.

   >>>> Current will flow to R13, the voltage develop across the R13 will be sense by the IS pin and GND pin. The IS pin and GND pin will provide information to the IC for the output current information. 

 

3) Plus, what happens if I change R13 0ohm to 10ohm?

Actually, I use R13 as 10ohm and R16 as 0.003ohm to regulate secondary load current under 9A.

I read some criteria to choose R13 value but i have no idea about choosing R16 value.

It would be truly great if you give me some technical explanation and criteria.

   >>>> R13 to 10ohms is Ok. It will just filter the signal to IS pin. Filtering is done by the RC circuit R13 and C15. 

  >>>> R16 is the output current sense resistor. It determine the level in which the INNO IC will undergo protection due to excessive output current. You can refer to AN-72 application note of INNO3. 

4) Last, I want to know where GND should be.

In 1_3, GND locates between R13 and R16 so there's potential difference between GND and GNDpin.

But in 2_2, GND locates between R16 and C9 so GND shorted with GNDpin.

What's difference? For your information, I got 2_2 circuit from INNOSWITCH3-ep Family datasheet.

If you give me some technical explanation and criteria, it would be super great.

   >>> 1_3 and 2_2  configuration are the same 

             1_3 System side is the GND    while 2_2 system side is J4 

             1_3 will go the current sense resistor R16 while 2_2 will to the current sense resistor R14 

              1_3 will be connected to IC GND Pin via R16  while 2_2 will be connected t to the IC GND Pin via R14.  

             R16 (1_3) and R14 (2_2) functions as a current sense resistor. The IC will determine the voltage across the current sense resistor. the GND pin and IS pin will provide the current sense resistor voltage information to the IC.

Thanks 

Regards 

PI_Chloe 

 

 

 

 

 

 

Submitted by plangtorr on 12/09/2021

Thx for you answer

Then how do I select R and C for filter?

We use 0.003 ohm for R is, and 10 ohm 1nF for filter and I think those are not enough to work as filter.

As I think, if noise goes through I sense pin, it would regulate current at wrong timing.

So I read datasheet and design guide, and it says 10 ~ 100 nF Cap would be good.

But I can't find out the way to select R for filter.

Do you have any recommanding value?

Or, if noise doesn't matter anyway, let me know.

 

Thanks again, Have a nice day.

 

Submitted by PI-Chloe on 12/13/2021

Hi Plangtorr 

Good Day! 

Noise is dependent on PCB layout. 

If the lay out is good, no need to add filter. 

In DER726, R13 is added just a place holder if there is a need to add filtering. 

If there is a noise generated on the IS and GND pin, voltage across the IS and GND pin should be measure then identify the duration of the noise spike then compute for the RC in order to snub the noise.

Again if the board is working properly without the filter, It is ok not to place any filter. 

Thanks 

Regards 

PI_CHLOE