PFC+PWM circuit design based on ICE1CS02

The PFC stage outputs a stable DC voltage. When the PWM stage MOS tube is turned on, the DS voltages of both tubes are zero, and the transformer input current increases linearly due to the secondary energy storage inductor, as shown in the current waveform in Figure 3.
When turned off, the primary potential of the main transformer is reversed, and the energy is returned to the input end through diodes D6 and D7, and the magnetic reset is completed, as shown in the voltage waveform in Figure 3.

The basic working principle of the circuit is verified by simulation experiments.
3 Main transformer design
Transformer design is the focus of the entire circuit design. Now let's discuss the design of the PWM level forward transformer for a 500 W circuit (the design requirements are based on simulation parameters as an example):
According to the relationship between the output power and the core size, the effective area of ​​the core is roughly estimated, and the effective area of ​​the selected core model should be larger than the theoretical calculated value. The EE42 core is selected, and its effective area Ae is 2.33 cm2.
The circuit operating frequency is constant. Considering that the saturation magnetic induction intensity Bs will decrease at high temperatures, and in order to reduce the core loss during high-frequency operation, the maximum working magnetic induction is generally selected to be 2000~2500 Gs.
UP=UD-△U1=393-3=390 V
UP is the voltage amplitude of the primary winding of the transformer; UD is the PFC level output DC voltage; △U1 is the sum of the conduction voltage drops of the primary winding and the MOS tube, which can be ignored in the calculation. Similarly, the voltage amplitude Us of the secondary winding of the transformer is:

D is the duty cycle. Since the transformer in the double-arm forward circuit needs magnetic reset, and according to the principle of equal volt-second time, the maximum D cannot be greater than 0.5, and 0.35~0.4 is taken here.
The number of turns of the primary side of the main transformer:

EON is the volt-second quantity of the transformer when the power tube is turned on; △B is the flux increment, and 0.15~0.2 T is taken here. TON is the conduction time.
The number of turns of the secondary side of the main transformer:

According to the effective value of the current and the experience of wire selection, and considering the skin effect of the wire during high-frequency operation, when the current is large, multiple strands are wound in parallel, and the diameter of each strand shall not be greater than 2 times the penetration depth. The wire diameter and number of strands of the enameled wire can be adjusted appropriately so that each layer of the wire package can be just wound. If the calculated number of turns of the primary and secondary sides is not an integer, the smaller number of turns can be selected to round up, and then the number of turns of other windings can be calculated based on the turns ratio. The primary number of turns is selected as 33 turns and the secondary number of turns is selected as 5 turns.
According to the formula Ku=Ae/Q, the window coefficient Ku is about 0.3~0.35.
The number of turns is adjusted in the process of calculating the secondary rounding. The maximum magnetic induction should be checked by the formula Np=(Vin×Ton)/(△B×Ae), and the maximum magnetic induction is within 3000 Gs.

4 Measured waveform analysis
The following waveforms are obtained under the conditions of input voltage 220 VAC and output power 500W unless otherwise specified.
4.1 PFC level waveform analysis
The main function of the PFC circuit is to adjust the input current waveform to make it sinusoidal and synchronize the phase with the voltage waveform by sampling the input voltage waveform.
As can be seen from Figure 4, after the power factor correction circuit, the input current is sinusoidal, and the phase is consistent with the voltage waveform. After testing, the power factor reached 0.99.

4.2 PWM level waveform analysis
For the PWM level dual-tube forward circuit, its input voltage is the PFC level output voltage, which is basically stable at 400 VDC. Due to the transformer leakage inductance and some parasitic parameters, there are some slight differences between the actual test waveform and the simulated waveform. The experimental waveforms are shown in Figures 5 and 6.

During the magnetic reset process, the maximum voltage of the upper and lower power tubes does not exceed the sum of the DC input voltage and the forward voltage drop of diodes D6 and D7. After the magnetic reset is completed, the primary voltage of the high-frequency transformer is clamped at zero. At this time, the voltages of M2 and M3 are clamped at half of the input voltage. This stage is maintained until the next conduction of the MOS tube. In Figure 5, the DS voltage drops to a platform, which is half of the input voltage, and also marks the completion of the magnetic reset.

5 Conclusion
The working principle of the PFC+PWM circuit is analyzed. Through the test results of the 500 W principle prototype, it can be seen that the efficiency of the prototype can reach 88% when fully loaded; the power factor is not less than 0.99; the load adjustment rate is not more than 0.5%; the output 24 V DC voltage has a high accuracy. It is verified that the circuit has the advantages of simple driving circuit, high reliability, small size, and high efficiency, and achieves the purpose of DC/DC conversion under the premise of improving the power factor.