Design of half-bridge converter with multiple independent power supplies

　　With the development of power electronics technology, power supply technology is widely used in various industries. The power requirements also vary. This article introduces a switching power supply with large power, multiple outputs (20 channels and above) and independent of each other.

　The design adopts AC/DC/AC/DC conversion scheme. The primary rectified DC voltage goes through the active power factor correction link to improve the power factor of the system. After being inverted by the half-bridge conversion circuit, it is isolated and stepped down by the high-frequency transformer, and finally the DC voltage is rectified and output. The main parts of the system are active power factor correction circuit, DC/DC circuit, power factor correction circuit, PWM control circuit and protection circuit. The UC3854A/B control chip is used to form a power factor correction circuit to improve the power factor. The new chip UC3825 is used as the control chip to replace the SG3525. It not only has a simple peripheral circuit, but also has a tolerance overvoltage and current limiting function. The new IR2304 is also used as The driver chip has fast dynamic response and a built-in dead zone to prevent the upper and lower tubes of the half-bridge from flowing through.

1 Active power factor correction circuit

　　In order to improve the power factor of the system, diode rectification cannot be used in the rectification link, and the UC3854A/B control chip is used to form a power factor correction circuit. UC3854A/BUnitrode is a new high power factor corrector integrated control circuit chip, which is an improvement on UC3854. It is characterized by the use of average current control, power factor close to 1, high bandwidth, and limiting grid current distortion to ≤3%. Figure 1 is an active power factor correction circuit controlled by UC3854A/B. Figure 1 Active power factor correction circuit controlled by UC3854A/B

                     　

　　The circuit consists of two parts. UC3854A/B and peripheral components constitute the control part to control the grid-side input current and output voltage. The power part consists of L2, Cs, S and other components to form a Boost circuit. The switch S is selected from Ximenkang's SKM75GBl23D module, and its operating frequency is selected at 35 kHz. The boost inductor L2 is 2mH/20A. C5 uses two 450V/470μF electrolytic capacitors in parallel. In order to improve the efficiency of the circuit when the power is small, the designed PFC circuit does not perform power factor correction when the load is light, and the power factor correction circuit is automatically put into use when the load is large. This part of the control is implemented by the comparator part in Figure 1. R10 and R11 are load detection resistors. When the load is light, the signals detected on R10 and R11 are input to the comparator, causing its output end to be low level, D5 is turned on, and ENA (enable end) is given a low level to block UC3854A/B. When the load is large, ENA is high level to allow UC3854A/B to work. D6 is connected to SS (soft start terminal). When the load is light, D6 is turned on, making SS low level; when the load increases and requires UC3854A/B to work, the SS terminal potential rises slowly from zero to control the output pulse duty. The ratio slowly increases to achieve soft start.

2 Main circuit and control circuit

2.1 Main circuit

　　Flyback power supplies are generally used in circuits below 100w, but this power supply is designed to have a maximum power of 300w, which is obviously not suitable. In high-power high-frequency switching power supplies, commonly used main conversion circuits include push-pull circuits, half-bridge circuits, full-bridge circuits, etc. Among them, the push-pull circuit uses fewer switching devices and has a high output power, but the switching tube withstands a high voltage (twice the power supply voltage), and the transformer has 6 taps, making the structure complex; the full-bridge circuit switching tube does not withstand a high voltage, The output power is large, but it requires many switching devices (4) and the driving circuit is complex; the voltage of the switch tube of the half-bridge circuit is low, the switching devices are few, and the driving is simple. Based on a comprehensive comparison of the electrical performance and cost of various topology solutions, this power supply uses a half-bridge DC/DC converter as the main circuit. Figure 2 is the main circuit topology diagram.

　　In Figure 2, S1, S2, C1, C2 and main transformer T1 constitute a half-bridge DC/DC conversion circuit. The MOSFET is 11NC380. The operating frequency of the circuit is 80 kHz. The transformer uses E55 ferrite core and does not require an air gap. The "three-stage" winding method is used during winding to reduce leakage inductance. R1 and R2 are used to ensure uniform voltage division of the capacitor, and R3, C3, R4, and C4 are the absorption circuits at both ends of the MOS tube. C5 is a DC blocking capacitor, used to block the DC component that is proportional to the unbalanced volt-second value, and balance the unequal volt-second value of the switching tube each time. C5 uses high-quality CBB non-inductive capacitors. Ct is a current transformer, used for sampling during current control. D3 and D4 use fast recovery diodes. After smoothing and filtering by L1, C6 and C7, the output OUT2 is used to power the control chip. Rs and R6 are the sampling resistors of the feedback voltage. The output OUT3 of the main transformer is high-frequency low-voltage alternating current. As shown in Figure 2, the feedback voltage and the output voltage have the same winding, which can maximize the stability of the output voltage when the load changes. The subsequent stage can be connected to one or more multi-output transformers, and then rectified through a rectifier circuit. This ensures that each output is independent and can obtain any voltage. Therefore, it can meet the requirements of DSP and other applications that require multiple power supplies with different voltages and high precision.

2.2 Control circuit

　　The system's control circuit uses a high-speed dual-channel PWM controller UC3825. Its internal circuit mainly consists of a high-frequency oscillator, PWM comparator, current limit comparator, over-current comparator, reference voltage source, fault latch, and soft-start circuit. , undervoltage lockout, PWM latch, output driver, etc. It has more advantages than SG3525:

　　1) The oscillation circuit is improved, the accuracy of the oscillation frequency is improved, and it has more precise dead zone control;
　　2) It has a current limiting control function, and the threshold current has a 5% tolerance;
　　3) Low starting current (100MA);
　　4) The UC3825 shutdown comparator is a high-speed overcurrent comparator. It has a threshold value of 1.2v to ensure that the soft-start capacitor is completely discharged before the chip restarts. When the threshold value is exceeded, the output is in a low level state to prevent The upper and lower bridge arms are turned on at the same time, causing a short circuit. Figure 3 is the control circuit of the main circuit.　

　　The R808 and R809 of the front stage and the voltage regulator tube form a startup circuit. After triggering the UC3825 to start working, it is self-powered by the feedback output OUT1. The modulation wave of PWM is generated by R1 and CT oscillation. RT and CT are generally selected according to equations (1) and (2).

　　RT=3V/{（10mA）*(1-Dmax)} (1)

　　CT=(1.6*Dmax)/(Rt*f) (2)

　　In the formula: f=80kHz, which is the frequency taken.

　　Pin 1 (INV), pin 2 (E/A) and pin 3 (HI) form an error amplifier for voltage feedback. Pin 9 (ILIM) is current limiting, pin 8 (SS) is soft start, and pin 11 (0UTA) and pin 14 (0UTB) are the output drive signals. As can be seen from Figure 3, UC3825 has relatively complete functions and simple peripheral circuits, which can effectively reduce PCB wiring and peripheral components and improve the reliability of the system.

　　2.3 Drive circuit

　　MOSFET can be driven by a pulse transformer, which has the advantages of small size and low price. However, when driven directly, the leading and trailing edges of the pulse are not steep enough, which affects the switching speed of the MOSFET. Here, the IR2304 chip is used, which is a newly launched multi-functional 600v high-end and low-end driver integrated circuit by IR Company. It has the following advantages.

　　1) The chip is small in size (DIP8) and highly integrated (can drive the upper and lower switching devices of the same bridge arm at the same time).
　　2) Fast dynamic response, on-off delay time 220/220 ns (typical value), internal dead time 1000ns, matching delay time 50ns.
　　3) Strong driving ability, can drive 600v main circuit system, has 61 mA/130mA output driving ability, and the gate driving input voltage is as wide as 10~20V.
　　4) High operating frequency and can support high-frequency switching of 100 kHz or below.
　　5) Input and output in-phase design, providing high-end and low-end independent control drive output, which can be controlled through two independent CMOS or LSTFL inputs compatible with 3.3v, 5v and 15v input logic, bringing great flexibility to the design .
　　6) Low power consumption design, sturdy and durable with high anti-noise performance. IR2304 uses high-voltage integrated circuit technology. The integrated design not only reduces costs and simplifies circuits, but also reduces design risks and saves circuit board space. Compared with other discrete, pulse transformer and optocoupler solutions, IR2304 can save more components and less space. space and improve reliability.
　　7) Equipped with power undervoltage protection and shutdown logic, IR2304 has two non-inverting inputs and cross-conduction protection functions, integrating protection functions designed for half-bridge MOSFET or IGBT circuits that drive motors. When the power supply voltage drops below 4.7v, the undervoltage lockout (UVL0) function will immediately turn off the two outputs to prevent shoot-through current and device failure. When the power supply voltage is greater than 5v, the output will be released (the comprehensive hysteresis is generally 0.3v). Overvoltage (HVIC) and anti-latchup CMOS technology make the IR2304 very rugged. In addition, the IR2304 is also equipped with a large pulse current buffer stage to minimize cross-conduction; it also adopts a Schmitt (Sohmill) trigger input design with a pull-down function to effectively isolate noise and prevent the device from accidentally turning on.

　　Figure 4 shows the connection diagram of IR2304

　　It can be seen that IR2304 has the advantages of simple wiring and few peripheral components. Among them, VCC is self-powered by OUT2 in the main circuit, LIN and HIN are connected to the two output terminals of UC3825 respectively. VD should use a fast recovery diode, C1 is a filter capacitor, and C2 is a bootstrap capacitor. It is best to use a tantalum capacitor with good performance, R1 and R2 are current limiting resistors.

　　2.4 Protection circuit design

　　For DC/DC power supply products, it is required that when an abnormal situation occurs (such as overcurrent, overload), the system's protection circuit works so that the converter stops working in time. The protection circuit design of UC3825 is also relatively simple, as shown in Figure 5　

　　The sampling current obtained through the current transformer is converted and sent to pin 9 (ILJIM) of UC3825. When the current exceeds the predetermined value, UC3825 automatically blocks the output pulse for protection.

3 Experimental results and waveforms

　　Figures 6 to 9 show some experimental waveforms of the prototype.

4 Conclusion

　　The engineering prototype produced has passed performance testing. The system uses the UC3854A/B control chip to form a power factor correction circuit. It does not work when the load is light, which is beneficial to improving efficiency. When the load is heavy, the circuit automatically puts into work, improving the power factor at high power. The main circuit control uses a new chip UC3825, which has a tolerance overvoltage and current limiting function, and the peripheral circuit is simple and has good stability. It also uses a new IR2304 with fast dynamic response, strong driving ability and high working frequency as the driver chip. , with power supply under-voltage protection and shutdown logic functions. Compared with the previous power supply, it only adds a limited cost, but the stability of the system is greatly improved, the frequency accuracy is improved, and the output voltage is more stable. When the light changes to 300W, the output voltage changes <1%. The transformer output can either use multiple taps on the main transformer or connect to a secondary transformer to obtain independent outputs of different voltages.