Calculator to Design a High Power 3kW PFC (Power Factor Correction) Circuit

In this post I will explain how to use this active PFC calculator for calculating high current, high PFC designs. In our example we will be using 230 VAC input and 3 kW output and Vout equal to around 390 V, and we will try to see how each resistor and capacitor gets calculated in a PFC circuit using the specialized IC UCC28180. So let us start understanding right away:

Purpose Of This PFC

We use this PFC because when we take power straight from the AC mains then the current becomes very ugly shape, it becomes sharp pulses and it puts big stress on the wiring, transformer, and the electricity company side.

When we add this boost PFC stage then we force the input current to follow the same smooth sine wave shape like the mains voltage, so this keeps the power factor very high near 1.0 and the THD becomes low and the AC line becomes happy.

Now the mains will not heat, your breakers will not complain and your whole 3 kW UPS will draw clean current from the grid, so this is why we put this boost PFC stage before our main DC bus.

The Active PFC Calculator

What the Calculator Does

This calculator helps us because this PFC design always depends directly on many numbers like AC input voltage, DC output voltage, power, switching frequency, and selected ripple percent, so normally we would sit and do each formula one by one, but this is slow and confusing.

That means this calculator takes all our values in one go and then it shows the inductor size, duty cycle, average current, peak current, and even the required inductance in mH, and this helps us to pick the correct MOSFET, correct diode, correct choke, correct Cbus, and correct current limit.

When we use this tool then we do not guess anything, everything becomes direct and fast and so now anybody can design a proper boost PFC just by entering the numbers and pressing the calculate button.

Circuit Diagram

How it Works

Input Voltage Related Values

We first take the 230 VAC RMS and we convert it into peak value, so we do 230 * 1.414 and we get around 325 V peak. We do this because UCC28180 senses the peak shape indirectly through the scaling network at VSENSE. So we keep this value in mind for all coming calculations.

VSENSE Divider Calculation

Now let us talk about VSENSE pin. This pin sees a scaled down portion of the boosted 390 V output so that the controller can regulate the bus.

The internal reference at this pin is around 2.5 V, so we need to create a divider that turns 390 V into 2.5 V.

Now we choose Rtop as 1 mega ohm because we want to reduce current wastage, so we do simple divider math:

2.5 V = 390 * Rbottom / (Rtop + Rbottom)

Now we solve for Rbottom and we get around 6.6 kilo ohm, so we select 6.8 kilo ohm because that is a standard value.

So the divider becomes 1 mega ohm at top and 6.8 kilo ohm at bottom and this gives around 2.5 V at VSENSE when the boost output is 390 V.

VCOMP Compensation Network

Now we go to VCOMP pin, which gets an RC network for stability, and we normally use something like 22 kilo ohm in series with 100 nano farad and 1 micro farad to ground.

This combination filters the error signal coming from the VSENSE divider, and we choose these values because they give a correct phase boost and smooth loop response at this power level. off course we can fine tune but we keep these standard values as a safe starting point for a 3 kW boost PFC.

FREQ Pin Capacitor Calculation

Now we select the switching frequency and we choose 65 kHz because it is a good compromise for 3 kW. So UCC28180 datasheet gives a formula for FREQ timing capacitor and we apply the formula and we get around 1.2 nano farad.

So we select a standard 1.2 nano farad or 1.0 nano farad depending on availability which sets the switching frequency for the MOSFET gate.

ISENSE Resistor Calculation

Now we calculate the current sense resistor RSENSE. We have 3 kW power and 390 V output so average output current is around 3000 / 390 = 7.7 A.

But boost inductor ripple and peak shaping makes peak current much higher, assume around 30 to 35 A peak.

The ISENSE limit threshold inside UCC28180 is around 0.5 V so we do 0.5 V / 35 A = 0.014 ohm meaning around 14 milli ohm.

So we select a resistor like 10 milli ohm to 15 milli ohm with 3 W to 5 W rating or we use a metal strip resistor rated for high pulse.

ICOMP Network Calculation

Now let us talk about ICOMP pin.

This pin needs an RC filter to shape the current loop and the usual starting values are 2.2 kilo ohm in series with 4.7 nano farad and a 100 nano farad to ground.

These values stabilize the average current control loop which we can fine tune later, but these values work well for a 3 kW design. These components give us correct bandwidth for the inner current regulation loop.

Input Capacitor Calculation

Now we calculate the small input capacitor that sits before the boost inductor, and that capacitor is not for energy storage but for EMI filtering, so we keep around 1 micro farad to 2.2 micro farad rated at 400 V or more.

We do not use a big capacitor here because PFC does not allow smoothing of the input current and only add minimum capacitance to reduce HF noise.

Output Capacitor Calculation

Now we calculate the output capacitor at 390 V, here we want small ripple so we select around 470 micro farad to 680 micro farad at 450 V using two or three capacitors in parallel.

The required capacitance is based on the hold up time and ripple voltage, assume around 5 percent ripple.

We keep ESR very low so we select good quality electrolytic capacitors and also add a 1 micro farad film capacitor across the bus to kill HF spikes.

Putting All Values Together

• Now we see that VSENSE uses 1 mega ohm and 6.8 kilo ohm.
• FREQ uses around 1.0 to 1.2 nano farad.
• ISENSE uses around 10 milli ohm.
• ICOMP uses 2.2 kilo ohm and 4.7 nano farad and 100 nano farad.
• VCOMP uses 22 kilo ohm and 100 nano farad and 1 micro farad.
• Input capacitor is around 1 micro farad and output capacitor is around 470 to 680 micro farad. This collection forms the working RC network for the PFC controller.

Final Explanation Ending

So now we reach the end and we see that this whole UCC28180 PFC design is not complicated once we break it into small parts. We took 230 VAC input and 390 V output for 3 kW and we saw how each resistor and capacitor has one clear role.

VSENSE divider tells the chip what the real bus voltage is, ICOMP and VCOMP RC parts fix the current loop and voltage loop, FREQ capacitor sets the switching frequency, and ISENSE resistor decides the peak current limit.

When we choose these values correctly then the controller automatically keeps the input current in the same shape as the AC sine wave. So now we understand that this high power PFC circuit is only simple rules and simple formulas, nothing too complex....