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Best way to use a current sense transformer?

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bogdanserban

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I'm building a current sense circuit that uses a current transformer and triggers the SD pin of the SG3525 controller at a set current value.
The chip terminates the outputs at a minimum of 0.7V at the SD pin (which is normally pulled to ground). But it can also do pulse limiting if the overcurrent period is short.
I calculated the CT to produce 0.7V at 10A using the typical rectifier and resistor circuit. My question is how can this circuit be improved? Some used an SCR, triggered the gate with the CT voltage and pulled a reference voltage to the SD pin, other used more complicated circuits with transistors, zeners and stuff.

Screenshot_20190803_211043.png
 

Hi,

Improved in which regard?
Cost, part count, voltage precision, speed, drift....

Klaus
 

Hi,

Improved in which regard?
Cost, part count, voltage precision, speed, drift....

Klaus
I'd say stability, meaning I don't want false triggers and certainly I don't want it not to trigger at all. I guess that for overcurrent/shortcircuit protection it should be good enough, but what about a constant current scenario?
 

SD triggers the soft start, in so far it does no pulse limiting, rather hickup current limiting. SCR would even create a latching overcurrent shutdown.

Obvious initial question: what's the intended function? Please clarify.
 

Schottky diodes are a better choice as lower Von drop and faster to turn on and off - you set the trip volts with the 150 ohm resistor, this value seems high, for a 100:1 CT 10A gives 100mA which is also a little high, 200:1 gives 50mA ( better ) 0.7v would then require a 14 ohm R.

As to triggering, put a 1k ( 1k - 3k3 ) to the pin on the soft start pin, you can then use a transistor to pull this pin to gnd when 0.7V exceeded, giving cycle by cycle current limit and gradually pulling down the SS cap as well
 

The intended function is to offer isolated current feedback without using an optocoupler and sense resistor.
The CT that I have is designed for 5A/5mA. So for a resistor value of 140 ohms I get 0.7V.
 

Hi,

Now you repeated what we already knew.

But we don't know:
* frequency, input waveform, output waveform, expected reaction time, max ripple ...
* what's your application
* whether you want peak detection or averaging

Don't get me wrong, I don't expect that you have fully developed the complete application. But for me the informations are rather vague.

***

You say you want stability. Now it can be regulation loop stability, or long time stability, or temperature stability..

Klaus
 

There's two applications: a switchmode PSU with overcurrent protection, where the currennt feedback is used only as protection (from overcurrent and shortcircuit). The reaction time should be fast enough to protect the switching devices, so it's peak detection here.
Second, an application where the CT serves to monitor the output current and, through feedback, help keep a constant current at the output. Could be a lab PSU with constant current mode or a welding inverter.
Frequency is somewhere between 50kHz and 100kHz.
 

Stable constant current regulation needs respective frequency compensation. SD input isn't specifically designed for it although it can be probably achieved with respective circuit.

Precise pulse current limiting can be better achieved with current mode controller.
 

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