Positioning of current sesne resistor after current sense transformer?

[grizedale]

When doing primary current sensing in an offline half-bridge converter, where do you put the sense resistor? (i.e. the resistor at the secondary side of the current sense transformer)


Do you put it on the current transformer side of the rectifier bridge, or the PWM controller side of rectifier bridge.?


Page 17 (bottom ) of the following puts it at the controller side of the rectifier, ................

**broken link removed**


...but other references put it on the other side (coil-side) of the rectifier.

-some even split the resistor into two and have one each side of the rectifier.

So , my question would be...;where should the resistor go and why?

 

[FvM]

I'm under the impression, that CT operation has been sufficiently discussed in your present thread https://www.edaboard.com/threads/222904/ As long as you don't undisclose the "other references", I don't know what to comment here.

Supplementing what's been previously said. Using a rectifier bridge on the secondary has of course the prerequisite, that the CT's specification allows for the respective voltage integral at the secondary, and that the error introduced by the magnetization current is still acceptable. For a 10 % accurate current limiting application, is much more suitable than for a 0.1% current measurement.

Best CT performance will be achieved when effectively shorting the output, so a low impedance shunt with succeeding amplifier and active rectifier can be suggested in some cases (other than current limiting). But surely not a shunt followed by a bridge rectifier.

 

[tsvetelin_velkov]

I think the schematic on page 17 is correct. Why do you want to put it before the bridge?

Before or after, I think there is no big difference, as usually the current transformer is a current to voltage transforming circuit - you will have the current sensed and of course integrated as voltage on Cf. You need to select the value of Cf carefully so that your feedback is not too much delayed, but also any short burst must be filtered out.

 

[grizedale]

putting the resistor before the bridge is the way Marty Brown does it on page 128 of the Power Supply Cookbook.

-However, the example he is giving there is a peak current mode Half Bridge smps, with no balancing winding, and no series capacitor in the primary....so i am wondering what to make of this text.....

 

[Orson Cart]

putting the resistor across the CT (i.e. before the bridge) will lower the CT current seen in the sense resistor after the bridge - and so make the measurement non linear and inaccurate in the later resistor.