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Power factor correction (PFC) circuit basics
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PFC Circuit Basics: The CrCM Boost Converter
[MUSIC PLAYING] This is part 2 of 6 of the Power Factor Correction Circuit Basic Seminar. In this section, we will be discussing the critical conduction mode boost converter. This converter will be broken into two parts-- an operation discussion and then a discussion on performance consideration.
The boost converter is the workhorse of the PFC. It can regulate from a very low input voltage. Since the input voltage always passes through zero, this is a distinct advantage. The boost converter also has a lower [? DIBT ?] on its input due to the presence of the inductor.
This can make the EMI signature of the PFC a bit softer than some of the other topologies. The boost converter has three modes of operation that we will touch on-- CCM, which is continuous conduction mode, CrCM, which is critical conduction mode, and DCM, which is discontinuous conduction mode.
For now, let's focus on critical conduction mode since that's what are shown on in the wave forms on the slide. However, I think it's important to be aware that both CCM and CrCM have the same Vout over Vin ratio. Take a look at the wave forms that are shown.
At the top here is the inductor current ramping from zero to some peak value back down to zero again. This signature where it starts and stops at 0 amps is what distinguishes a CCM from a CrCM system. Basically anytime the current comes to zero, just touches zero and goes back up, that is a critical conduction mode system.
Here's the MOSFET drain voltage where that voltage rises up to the output voltage corresponding to when the diode turns on. And then it rings back down to some minimum. We'll talk more about this minimum later. Here's the corresponding MOSFET-- 8 voltages, the current that flows through the MOSFET, and the current that goes through the diode.
OK, let's spend a little bit of time talking about the critical conduction mode PFC. One of the things that distinguishes this controller is its ultra-simplified control methodology. Basically, what it does is it operates with constant on time. And it turns out that if the on-time of the pulse is always constant over a full line cycle, the average of the inductor current also becomes sinusoidal like the input voltage.
Take a look at the equation shown here. Basically this shows what the average input current looks like as a function of input voltage, the inductor value, and the on-time. If everything in this equation is constant except for the input voltage, the average inductor current will follow the input voltage in wave shape perfectly.
This converter operates on the boundary between discontinuous conduction mode and continuous conduction mode. We talk about what continuous conduction mode was. It's basically where the inductor current is always positive. Discontinuous conduction mode occurs, though, when the inductor current goes to zero. And it stays there for a while instead of immediately rising as is the case in critical conduction mode.
One of the consequences of this control methodology is a large switching frequency variation. Take a look at the plot shown here in the middle. You can see that for critical conduction mode at different input voltages, you can have a huge variation in the switching frequency over the course of a line cycle. Here, the x-axis is showing a half line cycle of time variation. And you can imagine the input voltage going from zero to its peak and back down to zero there.
In this plot, we have a variation in the frequency from just below 100 kilohertz to well above 500 kilohertz at the zero crossings. There's some idealism built into this. But, in general, this is one of the challenges associated with this topology. We'll talk about some ways to address this in a few minutes. In addition, some of the advantages that this topology brings are zero current switching for the boost of up diode.
This means that essentially you have no reverse recovery because you're always turning on the switch after the current through the diode has gone to zero. In addition, if you take a look at the plot shown here on the right, up on top is a half cycle of the input voltage as it varies from zero to its peak and back down to zero.
Down here at the bottom, we're looking at several wave forms. One of them includes the cycle by cycle inductor current. The other one, the square wave-looking thing is the drain voltage of the MOSFET. Here in light blue shows the value of the input voltage for this particular condition. And then the dashed lines show what the output voltage is in one half the output voltage.
And this whole operating point down here at the bottom is relative to the point shown in red. Another interesting point to highlight here is in this particular example of these wave forms, the input voltage in between the output voltage and one half the output voltage. Under this condition, the converter gets something called valley switching. And what that means is the current in the inductor is going to drop to zero.
And that zero will correspond to when the voltage on the drain of the MOSFET reaches its minimum. Now, if the input voltage is below one half the output voltage, something different happens. And in this case, what happens is the drain of the MOSFET actually resonates all the way to zero before the switch turns on so you get zero voltage switching. And under these conditions, an improvement in the efficiency.
So now let's talk about how we can address that wide switching cycle variation that is inherent to a critical conduction mode system. There are two cases I'm going to go through. One of them occurs when the input voltage is below one half the output voltage. That's the case shown here. You can see that on the plot on the right.
The other case will be when the input voltage is above one half the output voltage. And I'll show that on the next slide. There are three fundamental places that we can make the decision to turn on. The first one is the zeroth valley. This occurs when the trained source voltage first touches zero.
There is no dependency on inductor current. So no inductor current sensing is necessary. The operation of this mode is identical to first valley when we're above one half the output voltage. And we'll display that on the next slide.
This particular mode does tend to introduce some distortion into the system. And that comes from the fact that now the inductor current does not start from zero. If the inductor current starts from zero, then the peak current in the inductor is proportional to the input current.
But once the inductor current goes negative due to this negative ringing of the inductor current-- and if we turn on when the inductor current is still negative, the on time-- the constant on-time that we've applied to this system is robbed of some of its current generation capability. And this fact introduces some distortion into the system.
This could be rectified by switching at what I call the first valley. In this particular case in the switch is turned on when the drain the source voltage reaches zero and the inductor current has resonated back up to 0 amps, at this point in time, the switch is turned on. And the current in the inductor ramps up to the peak that is proportional to the on time.
The third mode of operation is the second valley. And really, this could be any subsequent valley that you're switching on. And the valley is referred to as the drain of the MOSFET when it reaches zero. In this particular case because we're below one half the output voltage, the drain of the MOSFET will always resonate down to zero.
And you can see in the plot here what we're doing is the inductor current resonates down to zero. The drain hits zero. And then it resonates back up. And then it comes back down. The second time it comes back down, we turn on the switch. And at this point in time because we're just touching zero, the current will be starting from zero.
Now, this very effectively can address the high switching frequency problem of the system. But it does introduce some distortion into the system. And that distortion can be addressed by compensating what the on-time is. You can look into the UCC 28056 for details on how this is done.
描述
2020年 2月 10日
This video series explores the basics of power factor correction (PFC) circuits. In this video, we discuss performance considerations to take into account while using CrCM.
