Power factor correction (PFC) forces the input current to follow the input voltage (V_{IN})_{ }so that any electrical load appears like a resistor. This action requires sensing the input voltage and modulating a current reference based on that sensing. The current loop will force the input current to follow the reference. This is called average current-mode control, as shown in Figure 1.

**Figure 1: PFC average current-mode control**

You can find lots of commercial PFC controllers with low total harmonic distortion (THD) using this average current control algorithm in the market. However, these PFC controllers need a dedicated pin to sense V_{IN} and a precision analog multiplier for current-reference modulation.

Another PFC control algorithm that does not need V_{IN} sensing but can still provide average current-mode control has become very popular recently. TI’s UCC28180 belongs to this family. Because it lacks a V_{IN} sense pin and the precision analog multiplier, it comes in a smaller package, enabling lower system cost, and is very easy to use.

But when we introduce the UCC28180 to designers, many times their first response is, “What? Without V_{IN} sensing? How does that work?” In this post, I will try to answer this question.

Figure 2 shows the control algorithm used in the UCC28180. A low-bandwidth voltage loop regulates the output voltage. The input current is measured as V_{Iin} and compared with a saw-wave V_{ramp}. The amplitude of V_{ramp} is proportional to the voltage-loop output. Because PFC uses the boost topology, the input-voltage information is already there, but hidden. The control algorithm shown in Figure 2 employs the hidden information.

**Figure 2: PFC without V _{IN} sensing**

The PWM output signal always starts low at the beginning of the switching cycle, triggered by the internal clock, as shown in Figure 3. The PWM output stays low until V_{ramp} rises linearly to intersect the V_{Iin} voltage. The V_{ramp}/V_{Iin} intersection determines switch turning off time t_{OFF}.

**Figure 3: PWM generation**

From Figure 3:

Here T is the switching period. For boost converter operating in continuous conduction mode (CCM):

Combining Equations 1 and 2 gives you Equation 3:

The voltage output, V_{OUT}, is a constant in steady state. Since the PFC voltage loop is very slow, V_{ramp} is also a constant in steady state. Thus, the input current is solely proportional to V_{IN}. If V_{IN} is sinusoidal, the input current must be sinusoidal, achieving good PFC. It looks like magic, doesn’t it?

Learn more about TI’s PFC solutions.

Additional resources:

- Get started designing with the UCC28180:
- Get to know the UCC28051, UCC28063, UCC28019A and UCC28070
- Read more of Bosheng's blogs on PFC:
- It is not just a PFC controller, it is also a power meter
- How to reduce PFC harmonics and improve THD using harmonic injection: Part 1, Part 2
- How To: Improve power factor and THD using DFF control

Mr Bosheng, thank you very much. I appreciate your approach and your mathematical explanation.

Stephan.