High power current sense resistor with low values are rare?

[Flyback]

Hello,
We are doing a 680W boost converter (vin = 18V-28V, VOUT = 34V)
we need a high power current sense resistor of 3W or more.
All I can find is the CSS2725 series by Stackpole, but it only comes in 250uR, 500uR or 1mH.

Is there a range of 3W+ current sense resistors with values in between these?

CSS2725
https://www.seielect.com/catalog/sei-css_cssh.pdf

 

[ronsimpson]

I went to digikey.com
Searched for RESISTOR.
Clicked on Chip Resistor Surface mount.
Selected 3 watt resistors and Resistance range (0.1m to 1m) ohm
Now I can see 8 Manufacturers. Values of: 0.3, 0.5 and 1mohm.

I think you need a current sense transformer.
>very little current loss.
>no high current resistor.

 

[Flyback]

Thanks RonSimpson...you confirm my fears that a range of close values at sub 1 milliohm levels just doesn't happen.......I wish to see a family of resistors of 500uR, 560uR , 620uR, 750uR, 820uR, 910uR,...AT 3W plus, ( or even 2W+)....but they just don't occur? , anywhere?

You're right, I really need the current sense transformer, but my boss insists on using the LT3791-1 controller and with that part, with buck and boost modes, its impossible to implement a current sense transformer, due to the different direction source currents across buck and boost modes.

LT3791-1 datasheet
http://cds.linear.com/docs/en/datasheet/37911fa.pdf

 

[KeepItSimpleStupid]

Why not an Allegro hall effect sensor.

Besides, to be accurate at all, you need a 4 terminal resistor.

 

[ronsimpson]

you can parallel resistors to get different values.