Hi !

I am trying to buid a DC-DC converter to step down the voltage from 16-17V to 13.7V to charge an AGM lead-acid battery. But I want this converter to have an adjustable current limit. I'm interested in 1-1.5A. And the switching frequency between 400-800KHz so I can use smaler LC.

I tested this schematics and is working really well... until the current reach the limit. Then the ripple/noise/oscilations - I don't know what it is - rise suddenly from 50 mVpp to 1Vpp. I used an ACS724 current sensor on the negative side, and I got same results. I used a PNP transistor with base-emitter on the current sensing resistor, and colector connecte through 100k in the FB pin - failed again ! Everything I do, it keeps oscilating with 1Vpp.

I apreciate any sugestion on this schematic or if you have any ideas, any other switchig IC, preferably with variable current limiting function included... I can't believe I'm the first to try a DC-DC converter with adjustable current limiting. There must be a schematic somewhere on the internet...

 

The Battery won't care about "Ripple" or "Noise".
This is made obvious by the minimal amount
of Filtering that was deemed necessary by whoever designed this Schematic.

First, add a 1000uF Capacitor at "VOP-2" to Ground.

If You really really don't like Noise,
add some Hysteresis to the Current-Sensing portion of the Feedback-Circuitry.
Even if this doesn't reduce the absolute Peak-Current,
it will still reduce the Average-Current, and therefore prevent Battery-Overheating,
which is the whole purpose of Current-Limiting.
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But, can I add hysterezis to an amplifier... ?

 

I don't understand your question.
Do You know what Hysteresis is ?
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Not realy... I used hysterezis with comparators, but not with amplifiers.

 

The Op-Amp in the Current-Limiting-Circuit can be changed-out for
a Dual-Op-Amp with similar specifications,
this will give You an extra Op-Amp to work with.

The additional Op-Amp can be set up to have a very small amount of Positive-Feedback,
and be positioned between the currently existing Op-Amp, and the Diode feeding the FB-Input.

It will then act much slower, more like an "On/Off" switch,
instead of chopping-up the Output at some random high-frequency.

Install the large Capacitor first and see if that makes things more acceptable to You.
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The noise and oscillation are because as soon as the over-current protection operates, the over current output switches back on. It becomes a feedback oscillator.

 

Wow ! I put 100 nF (C8 and C12) on every opamp input and the oscilations are gone. It has ony the ripple that was present before entering the limiting.