Newest integrated PFC/PWM combination solution for efficient PC power supply

Power supply engineers are always looking for simple design methods that can implement a series of circuit protection functions while making the power supply meet increasingly stringent efficiency specifications. This article will explore a highly integrated semiconductor solution that combines a boost power factor correction converter with a two-switch forward pulse width control converter , which requires very few external components and can have multiple circuit protection and compensation functions and meet IEC-1000-3-2 specifications.

PFC+PWM control

FAN480X is composed of two average current mode controllers: power factor correction (PFC) and pulse width control (PWM). The PFC stage uses a switch charging multiplier technology to obtain a higher power factor and lower total harmonic distortion (THD); while PWM can choose to use current mode control or voltage mode control. PFC regulation is rising edge modulation, while PWM uses falling edge modulation, because the use of different trigger modulation can reduce the ripple voltage on the PFC output capacitor . In addition, FAN480X adds a programmable two-stage PFC output function, which can improve the system efficiency at low voltage input and light load.

FAN480X has a variety of protection functions, including soft start of PWM and PFC, PFC overvoltage/undervoltage, cycle-by-cycle current limiting, PFC input undervoltage, etc., to ensure that the power supply and subsequent devices are not damaged. Users can use the equations described in this article to select the required key components. Figure 1 is the application circuit diagram of FAN480X ATX, where the output power is 300W (10W is standby power), the AC input voltage range is 75VAC~264VAC, and the PFC circuit provides a 380V output voltage as the input of the subsequent double-tube forward converter. The switching frequency of both parts is 65kHz.

Figure 1. Schematic diagram of a two-switch forward converter for a PFC/PWM integrated solution

The PFC part of FAN480X works in continuous current mode, which can help reduce the rate of change of boost inductor current and is suitable for applications with higher power. The gain regulator can provide the power supply with higher power factor and lower total harmonic distortion. It is the core of the PFC stage and can respond to the current loop for different input voltages, frequencies, effective voltages and PFC output voltages, as shown in equation (1). The function of the gain regulator is to generate a control signal to the PFC stage to control its duty cycle so that the output voltage remains stable; the inverse of the square of VRMS can provide constant power for high and low voltages. Figures 2 and 3 show the working principle and application circuit of the gain regulator of FAN480x respectively.

Figure 2. Gain adjuster operating principle.

Figure 3. FAN480x gain adjuster application circuit

PFC current loop compensation

FAN480X has two control loops in the PFC part, one is the current control loop and the other is the voltage control loop. The current control loop controls the current based on the reference signal generated by IAC. The voltage control loop stabilizes the output voltage and maintains the balance of total harmonic distortion. Figure 4 is a simplified current loop diagram, in which the PWM module part includes a comparator, a trigger, and a MOSFET drive output. The voltage-controlled voltage source combines the input voltage source, a rectifier, a MOSFET, and a boost diode.

Figure 4: Current loop diagram

The current control loop is compensated for the poles generated by the L1R5 product in the higher frequency section, so there is no need to consider the inductance characteristics when analyzing the voltage control loop. The system transfer function of the current loop can be obtained by small signal analysis, as shown in formula (2), where VRAMP is 2.55V.

Figure 5: Bode plot of the frequency response of the current loop

Figure 5 is a Bode plot of the frequency response of the current loop, where GPWM_Boost is the open-loop frequency response of the system current loop; GPWM_Boost_fc is the frequency response of the power error amplifier compensation; and GClose is the closed-loop gain. The system current closed-loop bandwidth is determined by the crossover frequency fC (crossover frequency) when the closed-loop gain is 1. Using equation (3), the required compensation gain when the system current closed-loop gain is 1 can be calculated.

The current loop compensation network includes an original pole representing that the system has no steady-state error, a zero point that can increase the bandwidth and phase margin of the system closed loop, and a pole that can reduce the interference of the system closed loop at high frequencies. The crossover frequency should be set at 1/6~1/10 of the switching frequency, and adjusting the zero and pole to appropriate values ​​can stabilize the system and obtain better transient response, so it is recommended to place the zero at one-tenth of the bandwidth of the crossover frequency and the pole at ten times the bandwidth of the crossover frequency.

Figure 6: Bode plot of the loop gain of the current loop of a 300W power supply

PFC voltage loop compensation

Figure 7 is a schematic diagram of a voltage loop control. The principle is to control the voltage control loop through a current source and charge and discharge the output capacitor. It is assumed here that the current loop control generates a current sine wave, charges and discharges C17, and C17 provides a DC current to the load resistor . The voltage error amplifier controls the amplitude of the current, and the voltage loop includes the current loop. In other words, the voltage-controlled current source integrates the input voltage source, rectifier, inductor and diode to generate a current sine wave with an amplitude proportional to the output of the voltage error amplifier.

Figure 7: Schematic diagram of voltage loop control

To avoid increasing the amplitude of the third harmonic in the current waveform and reduce the total harmonic distortion, the voltage loop bandwidth should be set between 10 and 30 Hz. A lower bandwidth will minimize the second harmonic and total harmonic distortion that appear in the current waveform. The main reason for using a low bandwidth voltage loop is the difference in phase between the input voltage and the ripple on the PFC output voltage; it is a natural reaction that the PFC load determines the phase difference. If the ripple voltage is not attenuated, the ripple voltage will enter the gain regulator proportionally and form a distortion of the current waveform. The capacitor roll-off feature of C16 is usually used to reduce the amplitude of the second harmonic, but too low a voltage loop bandwidth will cause transient response problems, so it is acceptable to allow some second harmonics. This method helps to balance the needs of total harmonic distortion and transient response. Assuming the crossover frequency is 30 Hz and the zero frequency is 3 Hz, we will set a pole at the crossover frequency. The second harmonic ripple size on the output capacitor C17 can be obtained using the following formula.

In the above formula, fline is the line frequency, ZC17 is the capacitive reactance of the large capacitor under the second harmonic, VC17_SH is the ripple voltage of the second harmonic, △VEA is the output range of the voltage error amplifier, and VVEA-H and VVEA-L are the maximum and minimum values ​​of the voltage error amplifier, respectively. According to the voltage loop control schematic diagram of Figure 7, the gain of the voltage error amplifier and the proportional relationship of the resistor can be expressed by equations (8) and (9).

Here, α is the ratio of total harmonic distortion, △VEA is the output range of the voltage error amplifier, and GVD is the gain of the voltage divider network. The gain of the voltage error amplifier at the second harmonic is expressed by equations (10) and (11).

GEA_SH and ZEA_SH are the error amplifier gain and the capacitive reactance at the second harmonic frequency, and GmV is the transconductance of the voltage error amplifier. To ensure sufficient roll-off characteristics of the voltage closed-loop gain at the second harmonic frequency, the C16 capacitor can be determined by equation (12).

Figure 8: Frequency response Bode plot of the voltage loop (omitted)

Figure 8 is the frequency response Bode plot of the voltage loop. The same method as the current loop is used to find R12 and C15. Equation (13) defines the gain of the boost part at the crossover frequency (GVL_Boost_fVC). Equation (14) converts Equation (13) into log form. The closed-loop gain ensures that the curve drops and intersects the horizontal axis. The values ​​of R12 and C15 can be determined by Equations (15) and (16).

Figure 9: Bode plot of a 300W power supply voltage closed loop

Two control modes of PWM

FAN480x provides two PWM control modes, namely voltage mode and current mode. Voltage mode can provide a more stable system, but the system response speed is slower than current mode. Current mode can provide a faster response speed, but it is easily affected by noise. The operation of voltage mode is to generate the PWM duty cycle by comparing the FBPWM voltage with the internal triangle voltage waveform of the RAMP pin, as shown in Figure 10; while the duty cycle of current mode is generated by comparing the FBPWM voltage with the signal on the sense resistor under the PWM MOSFET, as shown in Figure 11. Figure 12 is the compensation circuit of the output voltage. The small signal analysis of compensation can be calculated by equation (17), and the compensation method is similar to the PFC stage. Figures 13 and 14 are the PF value and efficiency charts of the 300W power supply.

Figure 10. PWM - Voltage Mode

Figure 11. PWM - Current Mode

Figure 12: Output voltage compensation circuit

Figure 13. PF and THD performance vs. Vrms line voltage

Figure 14. Efficiency and THD performance vs. output power

in conclusion

FAN480x is an average current mode control method that combines power factor correction and pulse width control, which can provide a higher power factor and lower total harmonic distortion for the power supply; PFC-level current loop compensation can make the input current follow the input voltage, and voltage loop compensation can provide a stable output voltage. FAN480x provides a simple design method and multiple protection functions, and requires fewer external components to enable the power supply to meet increasingly stringent energy efficiency standards and IEC-1000-3-2 specifications.