SG 3525 with PI compensator connection

[biswaIITH]

hello frnds....i am using SG3525 for closed loop control in my project... I gave feedback to SG3525 without any compensation ..RESULT: initially i got stable pulses from SG3525 ,but when i am giving power to main converter ckt,there is fluctuation in the pulses and suddenly they got vanished....

I think the feedback needs some compensation to avoid the above mentioned problem...I am planning to implement a PI compensator with in-built error amplifier (pin 1 ,pin 2)of SG 3525 and compensation pin(9)...

I am attaching my schematic below..IView attachment SG3525 with PI compensator connection.pdf need ur expert suggestion to rectify or improve my ckt

 

[red_alert]

By putting that series capacitor, you only have a negative feedback at high frequencies thus for DC/low frequency it is an open loop (prone to oscillations).

Try the simple compensation network from the datasheet (a resistor between pin 9 and pin 1 and a capacitor from pin 9 to ground).

 

[biswaIITH]

sry that problem was a minor one which is due to a floating load terminal...i am facing a new problem now...as soon as i am increasing the supply ac voltage to the converter from 0v....the output duty ratio is getting saturated at maximum (i.e.48.6% approx) which is causing substantially large input current....i have seen the duty ratio(approx) in simulation..it is arnd 20%..the high input current prevents me from increasing further...

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For my application ,i need to apply 110Vrms supply...My question is whether SG3525 or any PWM ic for that matter operates satisfactorily when we increase the input voltage to the system(i mean the converter pulses to which are given from SG3525)

 

[red_alert]

Seems like the output voltage feedback is not working. Do you have any compensation network? Are you using the right pins (IN- for feedback and IN+ for reference)? Can you post the schematic you're actually using?

 

[biswaIITH]

YES i have connected all pins correctly...i am not using compensation...why do u think the feedback is not working???

 

[red_alert]

You've just said the duty cycle has rised up to the maximum after the power on thus it seems like there's no negative feedback to keep it around that theoretically 20%.

What happens with the output voltage, when the duty cycle reaches that maximum value?

I gave feedback to SG3525 without any compensation ..RESULT: initially i got stable pulses from SG3525 ,but when i am giving power to main converter ckt,there is fluctuation in the pulses and suddenly they got vanished..

Is the SG3525 power supply (12V) still stable when you got those fluctuations? Have you used a decoupling capacitor across its Vcc and GND pins? And try to put at least a simple resistor from IN- to compensation pin.

 

[biswaIITH]

no there are no more fluctuations as i have mentioned in the previous post...now let me tell you the way i think the whole closed loop is working in my application....

When i am turning the supply on,the feedback sees a zero voltage at the output .The feedback is connected to the negative pin of the error amplifier and the positive pin of the error amp is connected to 5v reference..so it is trying to achieve that 5v by applying a high duty ratio..but it will not be able to achieve that because for that the input should be close to 110vrms(i have set it accordingly i.e. when the input is 110vrms, the load voltage will be 30v and the feedback will be 5v and the duty ratio will be arnd 18-20%)...But, as i am varying the input voltage slowly when i am reaching say 10 vrms,it is drawing a huge current because of that maximum duty ratio(as the duty ratio gets latched on to that maximum value)

 

[red_alert]

Looks like a normal behaviour. As long as your maximum duty cycle is 49% (by using only one SG3525 output) the IC it's keeping it at maximum value to reach that output.

So you basically want to design a buck converter (110V > 30V). The base rule is the input voltage shouldn't be lower than the output voltage (30V). So when you're applying a 10V input voltage, the IC is rising up the duty cycle to the maximum but that's the best it can do (the output couldn't rise more than 10V to reach your intended 30V).

Be aware that the reverse MOSFET diode is forward conducting when the input voltage is lower than the output voltage.

If you want to use a wide input range (manually adjusted 0 - 110V) you should consider a buck-boost topology.

If you apply the full input voltage (110V) the circuit should actually work (the IC is keeping the duty cycle at maximum till the output reaches 30V then it start to decrease the duty cycle due to negative feedback thus keeping that 30V output steady).

 

[biswaIITH]

that is what i thought of doing(i.e. applying directly 110 v rms) but i was worried about the protection of the circuit..

shall i use a fuse of certain current rating and apply directly 110 vrms????

 

[red_alert]

You might as well manually adjusting the input voltage from 0 to 110V but keep in mind that the duty cycle would stay at maximum until you reach 30V. After that, the duty cycle should begin to decrease as to keep the output at 30V.

 

[biswaIITH]

But the problem is input current is also increasing as i am manually increasing the input voltage...At 10-15vrms , it is reaching around 1 A....How to limit the input current????

 

[red_alert]

Are you using a typical buck converter topology? What's your MOSFET rated current?

At start up, the output capacitors are discharged so the current through MOSFET is bigger (if the capacitor is large, too).

When you keep the input voltage at 10-15V, what's the output voltage? That bigger current is steady, if you keep the input voltage unchanged? What's your load?

 

[biswaIITH]

yes it is a buck-boost followed by a buck topology(both operating in discontinuous mode)....load is around 60ohm..rated current through mosfet is 2.5 amp...yes that current is steady when i am keeping the input voltage constant...

 

[red_alert]

If the input voltage is 15 V and load is 60 ohm, how come you have a steady current of 1 A?? Something happened on the way to heaven..

That 1 A is just too big to represent the MOSFET switching losses. It might be the MOSFETs running in linear region. It's better to analyse a complete schematic, anyway. Could you post it?

 

[red_alert]

I assumed the output voltage is 15 V, too (the output is following the input). What's the output voltage, anyway?

You said it's a buck-boost followed by a buck converter. Which stage are you driving with this SG3525? Have you individually tested the buck-boost and the buck converters?

 

[biswaIITH]

yes the output voltage is almost following the input voltage...i havnt seen the exact value..i wil see it today..i am driving both the stages by SG3525 with two TLP350s.....No i have not tested them individually...

 

[biswaIITH]

i am planning to start the supply with 110Vrms...But ,for that i need to employ certain protective equipments...planning to use NTC...But i have a small confusion...In my schematic which i have attached below ,u will get to see 2 points(pointA nd point B)The peak (transient)current at point A is slightly more than 1 amp..But at point B (just before the dc-dc converter)the peak transient current is 7-8 amps as the dc-dc converter is operating in discontinuous mode...

so my question is
what should be the maximum current rating of my NTC???
I think if i use an NTC of (3A) at point A ,that will serve the purpose...

 

[red_alert]

Are you sure you don't want a PTC instead?

A NTC is mainly used for inrush current limitation - it's resistance is decreasing as the current rises (and the body of the NTC is heated).

A PTC has an opposite behaviour thus limiting the current in abnormal situations (and it's acting like a shunt at start up).

 

[biswaIITH]

I think i will try with NTC...But where shall i use it???

 

[red_alert]

In series with the power supply, usually. But, once again, a NTC won't help you in overcurrent situations.

Beside, the voltage drop across it might resemble the slow increasing input voltage condition you're trying to avoid.