Current mode control in buck switch power,such as UC3842/3/4

[huojinsi]

uc3842 buck

Hi all:
I am studying how to operate regarding current mode control in buck switch power. I have some questions that cann't be understanded.

Question 1. From some papers, i know the inductor current is independent of duty cycle,so i cann't understand "how to vary about duty cycle? what is the result of duty cycle varying?"
Question 2. in current mode,i know the inductor average current is constant by slope compensation. But i cann't understand the inductor peak current is constant or not, what relation is the inductor peak current with duty cycle?

Pls help me explain.Thanks in advance!

 

[VVV]

buck current mode control

If you are operating in continuous current mode, then the duty-cycle only adjusts the output voltage. The peak current (and implicitly the average current) are adjusted depending on the output of the error amplifier. See pictures. In the first case, the error amplifier output is lower, so the transistor peak current is lower.
Note the duty-cycle is the same, as it should be. In reality, there will be a small variation, due to increased voltage drops across circuit elements.

The error amplifier output increases or decreases as a result of the output voltage variations, but since the EA has a large gain, the output voltage changes very little, although the error amplifier output has large variations. These variations actually control the inductor current, as we have seen.

If the load begins to draw less current, then the output voltage increases a little, so the error amplifier output decreases (a lot) , forcing the peak inductor current lower, until the average equals the load current (imagine going from the picture 2 condition back to picture 1).

 

[huojinsi]

uc3842 schematic

Thks VVV!
Will u talk about ur understanding for the inner current loop ?

 

[VVV]

current-mode error amplifier gain

There isn't much to talk about. The UC3842 only uses peak current mode, so the inner current loop is built with just a comparator and the gain of the loop is rather low.

Basically, you should see the inner loop like a black box with the input being the error amplifier output, Vea and the output of the black box being simply the power supply output current. So, the output of the error amplifier controls the output current, like I showed in the previous post: Vea increases, it allows the output current to increase. The output current multiplied by the load resistance gives the output voltage. This is then fed to the voltage loop, the "outer" loop, which is really the error amplifier.

So now what is the gain of the current (inner) loop? First, let's assume that the inductor current ripple is very small, such that Ipk≈Iout.
Now as we have seen, the comparator will limit the peak inductor current to a certain value, simply by comparing the Vea to the voltage across the sense resistor, Rs. In reality, it compares Vea/3 to the voltage across Rs.
Generally, the sensed current is not the load current, but the primary current, thus, you sense a current that is N times lower, where N is the turns ratio of the transformer. Therefore, you can write:

Ipkprimary=Ipk/N

The voltage across Rs is Ipkprimary*Rs

So the comparator ensures Ipkprimary*Rs is always equal to Vea/3

Then, Vea/3=Ipkprimary*Rs=Ipk/N*Rs≈Iout/N*Rs

So this is the gain of the inner loop Gil=Iout/Vea (output/ input, and it has the dimension of a conductance)

Gil=N/(3*Rs) That is not a high gain, especially if N is low, such in the case of DC/DC converters.

With that, the output voltage is Vout=Rload*Iout≈Vea*N/(3*Rs)

As you can see, you do not have much control over it, since the turns ratio is usually fixed based on the input/ output voltages and maximum allowable duty-cycle, while Rs is chosen such that the voltage across it is about 0.7V at full load. That pretty much fixes your inner loop gain.

Thie is an idealized situation. We assumed that Ipk≈Iout, but in reality, the waveform will look much like my picture, wo there is some error there, since the peak is higher than the actual Iout. Also. I mentioned that ususally you sense the current on the primary side. That means that you will also see the magnetizing current of the transformer (another ramp) added on top of the actual reflected current. That will make the peak "look" higher than it actually is.

All this boils down to one thing: the current you are controlling is NOT the output current (that you wish you controlled), but a rather inaccurate image of it. The lower the output current, the greater the error.

Having said all that, current mode control is great and peak current mode control works just fine, being used in zillions of products.

(I hope I did not make any mistakes in my little dissertation here; you should double check that).

 

[huojinsi]

uc3842 application

Thks VVV much
ur understanding and derivation is very clear! I have known much from u! Now,i finally clear about the inductor acting as a voltage controlled current source.

So this is the gain of the inner loop Gil=Iout/Vea (output/ input, and it has the dimension of a conductance)

How to define the gain of outer voltage loop? is it Gain=Vout/Vea? Pls check for me!

About current mode control, i have some papers downloaded from TI website ,but i feel the content of those papers is a little deep. Do u have some most basic materials regarding current mode?

 

[IanP]

uc3842 power supply schematic

Try this paper: "Current Mode Control"
https://www.venable.biz/tp-05.pdf
Regards,
IanP

BTW, VVV has done great job !!!

 

[VVV]

uc3842 application note

The gain you mention is actually the power stage gain, that is from the error amplifier output to the power supply output.

Gps=Vout/Vea

The overall loop gain is

Go=Gps*Gea,

where Gea is the error amplifier gain. You shape this Gea to obtain an overall loop gain that satisfies the requirements (stability and transient response).

The TI website is one of the best for power supply information. There are many appnotes and the power supply design seminars from Unitrode.

Thanks, IanP.

 

[huojinsi]

uc3842 application buck schematic

Thks VVV,IanP. I have knew lots from ur guide!

 

[lj053]

buck current control

thanks VVV, Ianp.