AC/DC Converter for 3kVA Power Supply

3 Principle of single-cycle controlled Boost PFC converter ^[2]

Figure 2 shows the principle block diagram of the single-cycle controlled Boost PFC converter. According to the definition of power factor, the following equation must be satisfied to achieve power factor correction:

(1)

Where Re is the input impedance of the converter _.

_If Re can be equivalent to a pure resistor within any switching cycle , the power factor is . The relationship between the input voltage and output voltage of the Boost converter within a switching cycle can be expressed as:

Figure 2 Schematic diagram of single-cycle controlled Boost PFC converter

In the figure: Voltage feedback error amplifier parameters

(2)

Order: (3)

Where Rs _is the equivalent sampling resistor

Combining equations (1) to (3) yields:

(4)

_When the output filter capacitor _Co is large enough, Uo can be regarded as a constant. Within a switching cycle, Uin can be regarded _as constant. From formula (4), it can be seen that Iin always follows _{Uin ,} so that the input impedance of the converter is equivalent to a resistor, achieving power factor correction. The control objectives are as follows :

(5)

[page]4 Parameter design ^[3][4]

Design requirements:

AC input voltage: U _in =165~275V

Rated output power: P _o =3000VA

Output voltage: U _o =380V

Switching frequency: _fs = 50kHz

Power factor: λ ≥0.99

4.1 Inductor Design

In the power factor correction converter, the design of the inductor

The quality of inductance design is directly related to the performance of the converter. The inductance is determined by the following formula:

(6)

Where U _in(pk)min is the peak value of the minimum input voltage, which is 233V here, and D is the maximum duty cycle that occurs when the input voltage is the lowest, which is 0.417. fs is the switching frequency, which is 50kHz. ΔIl is the peak - _to -peak value of the ripple current, which is 20% of the maximum peak-to-peak value of the inductor current. After calculation, _L is 0.36mH. The parasitic capacitance between turns of the PFC inductor will cause the drain current to oscillate when the switch tube is turned on, so its parasitic capacitance between turns should be minimized. For this reason, it is best to use a single-layer winding in the winding of the inductor and try to avoid using a double-layer winding.

4.2 Output Capacitor

The DC side output capacitor has two functions: 1. Filter out the DC voltage ripple caused by the high-frequency switching action of the device; 2. When the load changes, the DC voltage fluctuation is maintained within the limited range within the delay time of the inertia link of the rectifier. When the ripple frequency caused by the switching action is relatively high, only a smaller capacitor is needed to meet the first requirement. The second requirement is related to factors such as the size of the load, the output ripple voltage and the holding time. It is generally taken as =15~50ms. The capacitance value of the capacitor is determined by the following formula

(7)

Here U _omin is the minimum output voltage, which is generally 300 V. The calculated capacitance is 3300 μF, and four aluminum electrolytic capacitors with a withstand voltage of 500 V and a capacitance of 820 μF are connected in parallel.

In addition, the power tube uses IXFK80N50P, with a withstand voltage of 500V and a maximum forward on-state current of 80A. The freewheeling diode uses RURG7560 fast recovery diode, with a withstand voltage of 600V, a forward rated current of 75A, and a reverse recovery time of <70ns. The fast recovery diode is selected to reduce the conduction and radiation interference caused by reverse recovery, and at the same time reduce the loss. If the RC network is connected in parallel to the freewheeling diode, a better effect can also be achieved.

IR Company has launched the IR1150 series products dedicated to AC/DC power factor correction circuits using the single-cycle control technology shown in Figure 2. The core of its single-cycle control circuit is integrated inside the IR1150. The converter is based on the IR1150. Its typical working principle diagram is shown in Figure 3. Compared with the traditional average current mode PFC circuit, the circuit structure is simple and the number of external components required is small. For example, in a 1kVA server switching power supply, 40% of resistors and capacitors can be saved, and 50% of the PFC controller circuit board area can be saved. In low-power applications where the power density problem is more prominent, such as high-power notebooks and LCD TV adapters, if the CCM mode IR1150 controller is used, the peak current can be reduced. Therefore, the single-cycle control Boost PFC is suitable for the front stage of the switching DC power supply.

Figure 3 Boost PFC circuit diagram based on IR1150

[page]5 Experimental results and analysis
The single-cycle control Boost PFC experimental circuit given in Figure 3 was tested according to the above parameters, and the experimental waveform shown in Figure 4 was obtained by using a TDS420 oscilloscope. Figure 4 (a) is a waveform diagram of the drain-source voltage and drive voltage of the MOSFET with an effective input voltage of 200V, in which the channel 1 probe has a 20-fold attenuation, so the voltage amplitude of each grid is 20 times the voltage number of each grid shown in the figure. Figures 4 (b), (c), and (d) are waveform diagrams of the input voltage and current of the Boost PFC converter. It can be seen from the figure that the input current Iin waveform is approximately a sine wave, but because the grid voltage waveform still has a certain degree of distortion at the peak, the current waveform also has a certain degree of distortion at the peak. In addition, when the voltage passes through zero, the current also has a certain degree of zero-crossing distortion. Figure 4 (b) shows the input voltage and current waveforms when the input voltage is 165V and the output power is 2701.8VA. Figure 4 (c) shows the input voltage and current waveforms when the input voltage is 220V and the output power is 2983.0VA. Figure 4 (d) shows the input voltage and current waveforms when the input voltage is 255V and the output power is 2979.2VA. Table 1 gives a set of experimental data with an input voltage of 200V. Throughout the experiment, the output voltage of the converter is always stable at 380V. It can be seen that the single-cycle control can quickly achieve the control target for input voltage disturbances and load changes.

(a) MOSFET drain-source voltage (400V/grid) and drive voltage (10V/grid) waveform (b) Input voltage (200V/grid) and current (50A/grid) waveform

(c) Input voltage (200V/grid) and current (20A/grid) waveforms (d) Input voltage (200V/grid) and current (20A/grid) waveforms Figure

4 Input voltage and current waveforms

Table 1 Experimental data when input voltage is 200V