SG3525 as peak current controller

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Tahmid

Tahmid

Member
Joined 2008
Hi jegandren,
You cannot directly do current-mode control with SG3525 as it is a voltage mode controller. But current controlling is possible with a current sense device and a comparator which will shut down the SG3525 in over current condition.
Thanks.
 
C

ChocoHolic

Member
Joined 2003
I like the 'nastiness' of that paper.
It shows, that you had a good time working on this :cheerful:

Yes, I agree. It makes sense to feedback the current signal to the error amp, because your current sensing does not feedback DC information. Designs that want realize a cycle by cycle pulse cut via the shut down input, must contain the DC information of the transformer currents. So it would not work with current transformer sensing (from my theoretical pov..., not tried..)
Hm, regarding your general feedback structure:
Is there a reason for your quite special way of superimposing the current feedback signal to the reference signal for the error amp? My straight forward thinking would lead to a set up, where I would sum both signals at the error amp....
 
Z

zilog

Member
Joined 2005
gnuffel.googlepages.com
ChocoHolic said:
Yes, I agree. It makes sense to feedback the current signal to the error amp, because your current sensing does not feedback DC information. Designs that want realize a cycle by cycle pulse cut via the shut down input, must contain the DC information of the transformer currents. So it would not work with current transformer sensing (from my theoretical pov..., not tried..)
Hm, regarding your general feedback structure:
Is there a reason for your quite special way of superimposing the current feedback signal to the reference signal for the error amp? My straight forward thinking would lead to a set up, where I would sum both signals at the error amp....
Click to expand...


The error amplifier arrangement is in my mind a product of the average current mode control scheme, where you apply lowpass filtering to (setpoint voltage - measured current) and decide on this value how much pulse-width you like.

Regarding the DC-component of the input current, I am quite convinced that you will retain this since either current transformer has time to reset to zero while the other conducts.
 
Eva

Eva

Member
Joined 2003
With some external help, SG3525A can do both average or peak current control, or both (average with cycle by cycle peak limiting). Average control requires an external op-amp (or two because the one included is not quite good). Peak limiting requires a comparator with its output tied to the shutdown input. I used both methods in my 15V 125A power supply.
 
C

ChocoHolic

Member
Joined 2003
Eva said:
.....Peak limiting requires a comparator with its output tied to the shutdown input. I used both methods in my 15V 125A power supply.
Click to expand...

That's exactly what I mean.
It should be possible to use the shut down pin of the SG3525 directly for cycle by cycle peak current control. The shut down does directly cut the pulse, that's nice.
But it also does cause a slow discharge of the soft start cap, which might cause issues if you run this control loop acting every cycle. Or not? I was considering an external current source (or in simplest way just a resistor to Vref) at the soft start pin, by this it should be possible to override the soft start discharge function and use the shut down input like a regular cycle by cycle control input.
 
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