Design of Phase-Shifted Full-Bridge Converter Based on UCC3895

introduction

The phase-shifted full-bridge (FB) PWM converter is a widely used converter suitable for high power, low voltage and other occasions. The converter adopts PWM phase-shift control. Without adding other additional components, the circuit cost and complexity remain basically unchanged. The leakage inductance of the transformer and the junction capacitance of the power switch tube are used for resonance, so that the power tube can achieve zero voltage switching (ZVS), thereby reducing the switching loss. The efficiency of the converter can be greater than 80%, and the reduction of the switch voltage stress can further increase the switching frequency to 100 kHz ~ 500 kHz. Therefore, the converter adapts to the current development trend of high frequency and high efficiency of switching power supplies and has broad application prospects.

There are many ways to achieve phase-shift PWM control of full-bridge converters, such as: using discrete devices for logical combination, using dedicated integrated control chips, using DSP or CPLD digital implementation, etc. The first method is relatively complex and not conducive to industrial applications, and the third method is relatively expensive; and the use of dedicated integrated controllers is the method more commonly used by power supply developers and designers. The most commonly used phase-shift full-bridge integrated control chips today are mainly UC3879 and UC3875/6/7/8 series. As an improved version of UC3875, UC3879 has the same working principle and basic structure, but has been improved in some functions. UCC3895 is another high-performance PWM phase-shift controller produced by TI. It is an improved version of UC3879. In addition to the functions of UC38779, the biggest improvement is the addition of adaptive dead zone settings to adapt to different quasi-resonant soft switching requirements when the load changes. The PWM soft shutdown capability has been newly added. At the same time, because it uses the BCDMOS process, its power consumption is smaller and the operating frequency is higher, so it is more in line with the development requirements of high efficiency, high frequency and high reliability of power electronic devices.

This paper first introduces the electrical characteristics of UCC3895, the basic functions of the pins, and the implementation of voltage-type control or peak current control. Then, a phase-shifted full-bridge power supply with 220 V input, 24 V output, switching frequency of 20 kHz, and power of 500 W is designed using UCC3895, and the main waveforms of open-loop and closed-loop experiments are given.

1 UCC3895 Application Characteristics

UCC3895 has the following features: programmable output turn-on delay and adaptive delay setting; can be used in both current mode and voltage mode; can achieve output pulse duty cycle from 0% to 100% phase shift control; built-in 7 MHz bandwidth error comparison amplifier, maximum operating frequency 1 MHz, etc. Its internal structure block diagram is shown in Figure 1.

Pin ADS is a new control pin of the control chip, and its function is to set the ratio between the maximum value and the minimum value of the set output delay dead zone.

When the ADS pin is directly connected to the current sensing pin CS, the delay dead time is the smallest; when the ADS pin is directly grounded, the delay dead time is the largest.

The ADS pin can change the voltage on the DELAB and DELCD pins through the relationship listed in formula (1), thereby changing the output delay.

Where: VDEL is the voltage on the DELAB and DELCD pins; VCS is the voltage of the sampled current on the CS pin; VADS is the set voltage applied to the ADS pin.

Pin CS is the inverting input of the current detection comparator. When the circuit works in peak current mode, the pin signal can realize the current limiting function cycle by cycle. At the same time, when the circuit is overcurrent in any case, the chip immediately blocks the output and enters the soft start cycle to achieve effective protection function.

Pin RAMP, when UCC3895 works in voltage or average current control mode, when this pin is connected to the oscillator output pin CT, when this pin is connected to the current signal pin CS, UCC3895 works in peak current mode.

The operating frequency of the synchronous oscillator is determined by the timing capacitor CT and the timing resistor RT. The oscillation period can be approximated by equation (2):

The dead zone delay time of the two tubes on the same bridge arm can be determined by formula (3):

Where: RDEL is the resistance between pin DELAB and ground.

The parameter comparison of UCC3895 with traditional phase-shift control chips such as UC3875 and UC3879 is listed in Table 1.

From Table 1, we can see that the power consumption of UCC3895 is significantly reduced and the response speed is the fastest, but the driving ability is smaller than that of UC3875, so in practical applications, we must choose the chip reasonably according to the situation.

2 Main circuit and control circuit parameter design

The schematic diagram of the power system designed in this paper is shown in Figure 2.

The design of the transformer in the main circuit is the key to the circuit performance. According to reference [4], the area product method is used to design the high-frequency transformer. Assuming the output power of the transformer is Po, the efficiency of the transformer is η, the filling factor is Ku, the current density of the wire is J, the switch conduction time in a high-frequency cycle is tom, and the flux density of the transformer is △B, the following calculation formula can be obtained.

Because the high-frequency transformer is bidirectionally excited, △B=2 BmBm is the maximum working magnetic flux density of the magnetic core; assuming the switching frequency is f and the duty cycle is D, then ton=0.5D/fo Substituting the above relationship into formula (4) yields

Select parameters: Po = 500W; f = 20kHz; for the ferrite core of R2KB material, the maximum working magnetic induction intensity Bm = 1700Gs can be selected; filling factor Ku = 0.3; wire current density J = 3A/mm2; transformer conversion efficiency η = 0.98. It can be obtained that AcAw = 4.614 cm4, and by consulting the transformer core manual, it can be known that the EE80 core is selected.

The formula for calculating the number of turns on the primary side of the transformer is:

Where: Vimax is the maximum value of the input voltage.

The calculation formula for the number of turns on the secondary side of the transformer is:

Where: Vimin is the minimum value of the input voltage; Vomax is the maximum value of the output voltage.

Substituting the parameters into the calculation and rounding, the original number of secondary turns is: N1=114 turns, N2=14 turns.

Other main parameters in the main circuit are: MOSFET IXTH25N60 is used as the switching device; 1000 pF/630V is used as the parallel capacitor; and the resonant inductor is 30 μH.

The system adopts PI controlled voltage closed loop. Through the analysis of the mathematical model of the phase-shifted full-bridge converter and the simulation study of PSPICE14.0, the transfer function of the regulator is finally determined as:

3 Experimental Results

Based on the analysis of the main pin functions and basic characteristics of UCC3895, a prototype of a phase-shifted full-bridge converter with AC 220 V input and DC24 V output was made to verify the functions of the chip and the correctness of the designed parameters.

The control circuit uses UCC3895 controller and the driver uses IR2110. The control circuit principle is shown in Figure 3.

The main waveforms obtained from the experiment are shown in Figures 4 to 7.

In Figures 6 and 7, vgs1 and vgs2 are the drive signal waveforms of the super bridge arm power switch S1 and the lagging bridge arm S4, respectively; vDs1 and vDS4 are the voltage waveforms between the drain and source of S1 and S4, respectively. From the corresponding relationship between the drive signal and the drain-source voltage, it can be seen that the power switch achieves ZVS.

Figure 8 shows the corresponding relationship between the output voltage and current when the load changes from 0 to 3 A when the voltage loop is closed. It can be seen from Figure 8 that the selected regulator parameters meet the rapidity and stability of the system well.

4 Conclusion

Compared with the traditional phase-shifted controller, the new phase-shifted full-bridge controller UCC3895 has improved design, enhanced functions, and reduced power consumption while retaining basic functions, thereby further optimizing the efficiency and reliability of the entire converter system. This paper further verifies the feasibility and effectiveness of the chip for soft-switching phase-shifted full-bridge converter control by making an experimental prototype and conducting open-loop and closed-loop experimental studies.