High power factor power supply based on UCC28019

0 Introduction

The traditional AC/DC conversion uses diode full-bridge rectification, and the output end is directly connected to a large-capacity capacitor filter, resulting in a large amount of harmonics in the input current of the AC power supply. Harmonic current has serious harm to the power grid, not only causing distortion of the grid voltage, but also wasting a lot of electric energy. With the introduction of the concept of green power supply, power factor correction has been widely used. The so-called power factor correction refers to taking measures from the circuit to make the input current of the AC power supply sinusoidal and keep the same phase with the input voltage. UCC28019 is a power factor correction chip recently launched by TI. The chip uses the average current mode to correct the power factor, so that the distortion caused by the tracking error of the input current is less than 1%, and a power factor close to 1 is achieved. This article introduces the internal structure and working principle of UCC28019. On this basis, a high power factor power supply is designed.

l UCC28019 Introduction

UCC28019 is the latest active power factor correction (PFC) chip from TI of the United States. This chip uses the average current mode to correct the power factor and is suitable for power factor converters with a wide range of universal AC input and an output of 100W to 2kW. The chip has a fixed switching frequency (65kHz) and has many system protection functions such as peak current limiting, soft overcurrent protection, open-loop detection, input power-off protection, and output overvoltage/undervoltage protection.

1) UCC28019 pin function

The pin arrangement of UCC28019 is shown in Figure 1.

The functions of each pin are:

• GND pin – ground;
• ICOMP pin - current loop compensation, output of the transconductance current amplifier;
• ISENSE pin – inductor current detection;
• VINS foot - AC input voltage detection;
• VCOMP pin - voltage loop compensation, output of the transconductance voltage error amplifier;
• VSENSE pin - output voltage detection;
• VCC pin——power input terminal;
• GATE pin - gate drive output terminal;

2) Internal structure of UCC28019

The internal structure of UCC28019 is shown in Figure 2:

3) UCC28019 has the following protection functions

(1) Soft start (SS)

(2) VCC pin undervoltage lockout (UVLO)

(3) Input power-off protection (IBOP)

(4) Output overvoltage protection (OVP) / output undervoltage protection (UVD)

(5) Open loop protection/standby mode (OLP/Standby)

(6) Overcurrent protection

4) Gate drive

The gate drive output is designed according to the current optimization structure, which can directly drive the gate of large-capacity MOSFET at a higher switching speed. The internal clamping of the chip clamps the voltage on the MOSFET gate at 12.5V, and the external gate drive resistor RGATE limits the rise time of the parasitic inductance and parasitic capacitance of the gate drive circuit, and suppresses ringing, thereby reducing electromagnetic interference (EMI).

5) Current loop and voltage loop

The current loop is composed of the average current amplifier inside the chip, the PWM comparator, and the boost inductor and inductor current detection resistor outside the chip. The voltage loop is composed of the voltage error amplifier inside the chip, the nonlinear gain, and the output voltage detection resistor outside the chip.

2 How the system works

The circuit block diagram shown in Figure 3 simply describes the working principle of active power factor correction using UCC28019 as the control chip. The gate drive signal is modulated by the output signal of the current amplifier and the output signal of the voltage error amplifier through the pulse width comparator. When the system is in a quasi-steady state, there are:

Where: M1 is the gain of the current amplifier; M2 is the slope of the PWM wave; Rsense is the inductor current detection resistor; iLbst is the average inductor current; M(D) is the voltage conversion ratio of the boost converter; M1 and M2 are determined by the difference between the voltage error amplifier and the internal reference voltage of the chip, and both can control the amplitude of the input current, and the product of the two satisfies a certain relationship. When the system is in a quasi-steady state, the output voltage is a constant, and M1 and M2 are also constants, so there is a control loop that forces the inductor current to follow the input voltage waveform to maintain boost regulation. And because Uin is a sine wave, the average inductor current is also a sine wave.

3. Design of main circuit parameters

Figure 4 is a schematic diagram of a typical application circuit of UCC28019.

1) Selection of switching devices

The maximum peak current IDS_PEAK(max) of the switching device can be calculated by the following formula:

Select the corresponding power field effect transistor according to the maximum output voltage and maximum peak current.

2) Selection of input filter capacitor

When a 20% inductor current ripple IRIPPLE and a 6% high-frequency voltage ripple UIN_RIPPLE are allowed, the maximum value of the input filter capacitor CIN is determined by the input current ripple IRIPPLE and the input voltage ripple UIN_RIPPLE(max). The value of the input filter capacitor can be calculated using the following formula:

Select the corresponding capacitor based on the calculated capacitance value.

3) Selection of boost inductor

The boost inductor LBST is selected after determining the maximum value of the inductor peak current IL_PEAK(max):

The minimum value of the boost inductor is calculated based on the worst case (duty cycle is 0.5):

4) Selection of inductor current detection resistor

When the inductor current exceeds the maximum peak current by 25%, the voltage on the ISENSE pin reaches the minimum value of the soft overcurrent protection threshold, and RSENSE will trigger the soft overcurrent protection. RSENSE can be calculated by the following formula:

In addition, to protect the chip from the impact of the inrush current, a 220Ω resistor is connected in series at the ISENSE pin.

5) Selection of output capacitor

The output capacitor COUT is calculated by meeting the delay requirements of the converter. In one linear cycle, tHOLDUP = l/fLINE(min), the minimum value of the capacitor can be calculated by the following formula:

6) Selection of voltage feedback resistor

In order to reduce power consumption and minimize the voltage setting error, 1MΩ is used as the voltage divider resistor RFB1 at the top of the voltage feedback, and the bottom voltage divider resistor RFB2 is selected through the internal 5V reference voltage URR to meet the design indicators of the output voltage:

Select the corresponding electrolytic capacitor based on the calculated minimum output capacitance.

4 Design Examples and Experimental Results

Based on the analysis of the working principle and main parameter design of UCC28019, a high power factor power supply is designed. The power supply has an input of AC 220V, an output of DC 360V, and a power of 500W.

The voltage and current waveforms at the AC power input are shown in Figure 5:

The power factor of the power supply was measured by a clamp meter to be 0.991. The waveforms and data measured in the experiment show that after the system works normally, the harmonic content is basically eliminated, and the input current waveform is consistent with the input voltage waveform.

5 Conclusion

The high power factor power supply designed based on UCC28019 has the advantages of high power factor and low harmonic content. At the same time, the chip has the advantages of simple application, powerful protection function, strong driving ability, simple debugging, etc. It is a very excellent power factor correction chip.