[SOLVED] flyback capacitor regulator for a flying capacitor topology

[danilorj]

Hey guys,

do you know anyway of regulating the voltage in the c_out capacitor of the figure sent attached? I mean, any control method. The C_in capacitor is regulated by the switch of the flyback topology, but the c_out capacitor does not regulate only by ratio transformer. It must have a different control of the switch of the flyback or it might have to introduce an other switch in the secondary side of transformer in order to regulate this capacitor, I don't know I'm just saying. Any ideas I would appreciate.

 

[D.A.(Tony)Stewart]

They use a capacitance transformer , which is usually a porcelain bushing with a high breakdown and small capacitance tapped to a low voltage with large capacitance at low voltage.

----||---v--||------
In . . . Out . . Gnd


ABB used to make them, maybe still do.

 

[danilorj]

But it can only be solved with a different transformer?

 

[mtwieg]

So you're just asking how to regulate a flyback transformer? There are tons of possibilities, you just need to measure the output voltage, compare it to a reference, and use some sort of control law (like a PI controller) to control the duty cycle of the switch.

 

[danilorj]

What switch? That is what I am asking. The flyback switch is already used to regulate the input capacitor as shown in the figure above. I was thinking about introducing an other switch in the secondary of the transformer, but how?

 

[Orson Cart]

Hello there, what switching freq for the 1st switch? or is it one shot or only a few shots?
If 20kHz (for e.g.) or higher you will need a standard control circuit to reduce the PWM to zero as you reach the target voltage, applying the right PWM for a given load to keep the o/p volts in the range you desire, usually a feedback optocoupler is used across the isolation boundary - if the Vout is very high then special opto's are required.
Else at no load even a very small PWM will rapidly charge your Cout to very high values...!

 

[D.A.(Tony)Stewart]

THey make IGBT's rated for kV these days. THe only reason for a flying cap design is to spread the voltage beyond the limits of a single unit.

I was thinking of high power grid converters and >20kV that could be exposed to much higher stray transients, when I suggested Cap. Xfmr.

Just remembered that I have used cascaded of 15x cheap resistors in a ladder ( each R rated for 1kV) to drive opto-isolators with mirror feedback for CTR tracking ( which are normally very innacurate) for -15kV resonant sine step-up DC regulators regulated to 1% for laser printer drums.

 

[danilorj]

The problem is that I need to regulate this Cout capacitor at the value of 1000 V, but with this flyback converter or the push-pull converter I can't make it. I am using a comparison between the reference value and the value of the voltage of Cin capacitor and I can regulate the voltage I want of 440 V using a PI controller. The transformer ratio of 440:1000 does not work for regulating the Cout capacitor

 

[Orson Cart]

You need to look at the Cout cap & compare its volts to a fixed ref and use the error signal to drive an opto to control the PWM down to zero if necessary - this is a standard feedback control.
You cannot just look at Vin, and assume the turns ratio to get Vout.

 

[danilorj]

So with only one switch and an optocoupler I can control both Cin and Cout capacitors?

 

[danilorj]

I guess this is what you mean.



And this is what I did to generate the PWM signals to drive the flyback switch. (I put a DC source in the place of a Cin capacitor and I want to regulate only the Cout capacitor using a flyback switch).




The problem it that it is not working! Look at the value of the voltage in the display below in the second picture, it is going down, it should be regulated in 1000 V.

 

[Orson Cart]

It works in the real world, better to try and develop a schematic for the desired power circuit and control and then model that. If you want a real world design I think you may want input from a power electronics engineer, e.g. pwrtrnx.com