More questions on power factor correction

[simbaliya]

Today I have read some material about CCM and DCM of PFC pre-regulator. I think for DCM I've already understood the machanism there, which I will illustrate in the following. But for CCM, I still can not figure out how does it work.

So firstly I'll give my own understanding of working machanism of boost PFC preregulator in DCM. In DCM(assume the circuit works at critical conduction mode by using zero current detector), since the average inductor current is calculated with this formular IL,avg= delta IL / 2 =(Vi * Ton) / 2L, to make the avg line current to be proportional to Vi, we just need Ton to be constant. In addition, to obtain regulated DC output, since Vo = Vi / (1-D), we need to properly modulate duty cycle to make Vi / (1-D) constant. But since Ton has to be constant, PWM method is not working in this case. We can only modulate the switching frequency therefore to modulate the duty cycle D to get constant output voltage.
If the above is correct, how can I get a pulse sequence with constant on time but variable frequency?

Then for CCM, I 've read so many materials yet still can not find the formular to calculate the avg line current. Most of the book seems like automatically admit that IL,avg is proportional to Vi, obviously it's not convincable without any formular(like the one in DCM). So I hope you guys can tell me the formular and the source of it so I can read it.

Thank you!:smile:

 

[FvM]

Most of the book seems like automatically admit that IL,avg is proportional to Vi, obviously it's not convincable without any formular(like the one in DCM).

It's not automatic, it has to be enforced by the controller, using an input voltage measurement and a multiplier.

 

[simbaliya]

It's not automatic, it has to be enforced by the controller, using an input voltage measurement and a multiplier.

Can I understand it as in critical conduction mode, the line curent is automaticlly proportional to Vi because the formular itself is natural. But in CCM we need control strategy to make it so. Is that correct?

 

[FvM]

Yes, that's the basic situation. You'll notice the difference when looking at the circuit implementation.

 

[simbaliya]

Yes, that's the basic situation. You'll notice the difference when looking at the circuit implementation.

But for DCM I still have some doubts. If we regulate the switching frequency to get constant Vo, however to stay in critical conduction mode we use zero current detector to turn on the switch once the line current reaches zero, that seems to violate the regulated sw frequency.

 

[mtwieg]

Then for CCM, I 've read so many materials yet still can not find the formular to calculate the avg line current. Most of the book seems like automatically admit that IL,avg is proportional to Vi, obviously it's not convincable without any formular(like the one in DCM). So I hope you guys can tell me the formular and the source of it so I can read it.

Thank you!:smile:

There are a few types of CCM, mainly peak current mode control, and average current mode control. They are both (usually) constant frequency controls. Peak current mode control compares the instantaneous input current to the reference current (output of the multiplier) and uses the output of that comparator to control the PWM. Average current mode control takes the difference between the instantaneous input current and reference current with an error amplifier, filters that, then compares that filtered error signal with a ramp signal, and the result of that comparison is the PWM waveform.

For peak CMC, the peak current is only defined by the feedback network, not the average current. The actual average current should track it closely though. With average CMC, the actual average current will be much more closely regulated, assuming the control loop has sufficient bandwidth.

edit: wait, when you say CCM, do you mean critical conduction mode or continuous current mode? Usually people refer to critical conduction mode as transition mode (TM) to avoid the confusion...

 

[simbaliya]

edit: wait, when you say CCM, do you mean critical conduction mode or continuous current mode? Usually people refer to critical conduction mode as transition mode (TM) to avoid the confusion...

Ya, CCM is for continuous conduction mode. Thank you for ur patient explanation. But can you oso answer my doubt on #5?

 

[mtwieg]

Ya, CCM is for continuous conduction mode. Thank you for ur patient explanation. But can you oso answer my doubt on #5?

What #5? Are you talking about the first thread you made...?

 

[simbaliya]

What #5? Are you talking about the first thread you made...?

I was referring to this question

But for DCM I still have some doubts. If we regulate the switching frequency to get constant Vo, however to stay in critical conduction mode we use zero current detector to turn on the switch once the line current reaches zero, that seems to violate the regulated sw frequency.

 

[mtwieg]

I was referring to this question

Critical conduction mode and Discontinuous mode are two completely separate things. With crtitical conduction mode you don't operate with a fixed frequency. DCM control can be fixed or variable frequency.

 

[Orson Cart]

you can't vary the frequency with a standard booster to regulate the output volts, CM is a common choice for boosters these days as the choke has the lowest volume (related to Ipk^2L) and Ipk^2 is much lower for CCM, hence the peak mosfet current is lower too and the filtering requirements (for Diff mode) are the least also, DCM (fixed freq) or boundary mode (variable freq) is useful for lower powers where you can get components to handle the peak currents relatively easily - Regards, Orson Cart.