SLYT838 January 2023 UCD3138

2 Peak current-mode control for CCM PFC

Peak current-mode control [4] is widely used in DC/DC converters, but it is not suitable for PFC because PFC needs to control the average current, not the peak current. Controlling the inductor peak current results in poor THD and a low-power factor.

Through the use of a special PWM generator, as shown in Figure 2-1, peak current-mode control becomes possible for PFC. In Figure 2-1, the sensed switching current I_Q is compared with a saw wave. The saw wave peak voltage (V_RAMP) starts at the beginning of each switching period, and its magnitude linearly drops to 0 V at the end of the switching period. The boost switch (Q) turns on at the beginning of the switching period. Q turns off when I_Q exceeds the saw wave.

This kind of PWM generator already exists in almost all digital power controllers, such as TI’s C2000™ real-time microcontrollers and the UCD3138. These digital controllers have a peak current-mode control module with programmable slope compensation. Programming the compensation with a slope of V_RAMP/T generates the intended saw wave.

Figure 2-1 PWM waveform generation for the proposed method in CCM.

To achieve a unity power factor, Equation 1 calculates the peak value of the saw wave V_RAMP as:

Equation 1.

V_{R A M P} = G_{v} * V_{o u t} + \frac{T_{o n} * V_{o u t} * R}{2 * L}

where G_v is the voltage loop output, V_out is the PFC output voltage, L is the inductance of the boost inductor, R is the current-sense resistor at the current transformer output, and T_on is the PFC PWM on-time.

Since the PWM on-time is almost the same in two consecutive switching cycles, you can use the T_on information from the previous switching cycle to calculate the V_RAMP value for this switching cycle.

Take a look at how to achieve a unity power factor with this control method. From Figure 3, during T_on time, the input voltage applies to the inductor, causing the inductor current to rise from I₁ to I₂. Employing Equation 2:

Equation 2.

I_{2} - I_{1} = \frac{V_{i n} * T_{o n}}{L}

where V_in is the PFC input voltage. Equation 3 calculates the average inductor current in each switching cycle as:

Equation 3.

I_{a v g} = \frac{{(I}_{1} + I_{2})}{2}

Substituting Equation 2 into Equation 3 results in Equation 4:

Equation 4.

I_{a v g} = I_{2} - \frac{V_{i n} * T_{o n}}{2 * L}

From Figure 2-1, Equation 5 is:

Equation 5.

\frac{I_{2} * R}{V_{R A M P}} = \frac{T_{o f f}}{T}

Equation 6 applies to PFC operating at CCM in steady state:

Equation 6.

\frac{T_{o f f}}{T} = \frac{V_{i n}}{V_{o u t}}

Substituting Equation 6 into Equation 5 and solving for I₂ results in Equation 7:

Equation 7.

I_{2} = V_{R A M P} * \frac{V_{i n}}{{R * V}_{o u t}}

Substituting Equation 1 and Equation 7 into Equation 4 results in Equation 8:

Equation 8.

I_{a v g} = \frac{G_{v}}{R} * V_{i n} + \frac{V_{i n} * T_{o n}}{2 * L} - \frac{V_{i n} * T_{o n}}{2 * L} = \frac{G_{v}}{R} * V_{i n}

In Equation 8, G_v is the PFC voltage loop output. It is constant in steady state; therefore, I_avg is proportional to V_in and follows the shape of V_in. If V_in is a sine wave, I_avg will also be a sine wave. Controlling the inductor peak current achieves a unity power factor.

Compared to traditional average current-mode control, this method eliminates the power losses caused by the current shunt resistor. And compared to the current transformer sensing method, which requires a precise sampling position, this method does not need to sample the current. Instead, an analog comparator determines the PWM off instant, eliminating the sample offset issue.

To save system costs, some designers prefer combo control, where a single controller controls both PFC and the DC/DC controller. You can place the combo controller on either the primary or secondary side of the AC/DC power supply; each has its advantages and disadvantages. If the combo controller is chosen to be put on the primary side, the DC/DC output voltage and current information need to be sent to primary side across the isolation boundary, and the communication between the controller and host also needs to across the isolation boundary. If the combo controller is chosen to be put on the secondary side, because the conventional average current-mode control method requires input AC voltage information, the input voltage must be sensed and used to modulate the current-loop reference. Sensing the input voltage across the isolation boundary is a challenge.

In the new control method, Equation 1 includes only V_out, not V_in. Because there is no need to sense V_in, you can eliminate the V_in sensing circuit. This control method needs only the current transformer output and V_out information. Because the current transformer provides isolation, a low-cost optocoupler can sense V_out and send it to the secondary side. You can then put the PFC controller on the secondary side of the AC/DC power supply and combine it with the DC/DC controller, which is also on the secondary side, to create a combo controller, which will significantly reduce system costs.