Design of Half Bridge DC / DC Converter with Multi-channel Independent Power Supply

Design of half bridge DC / DC converter with multi-channel independent power supply

A half bridge DC / DC converter with high power and multi-channel independent power supply is introduced. Active power factor correction technology is adopted to realize the high power factor of the system. The DC / DC main circuit adopts a half bridge converter controlled by high-speed dual PWM chip uc3825, and IR2304 with fast dynamic response, strong driving ability and high working frequency is used as the driving chip of MOSFET. Keywords: half bridge converter; UC3825ï¼› Power factor correction; IR2304ï¼› multiple; independent

0 Introduction     With the development of power electronics technology, power supply technology is widely used in various industries. The requirements for power supply are also different. This paper introduces a switching power supply with large power, multi-channel output (20 channels and above) and independent of each other.     The design adopts AC / DC / AC / DC conversion scheme. The DC voltage after primary rectification passes through the active power factor correction link to improve the power factor of the system, then is inverted by the half bridge conversion circuit, isolated and reduced by the high-frequency transformer, and finally rectified to output the DC voltage. The main links of the system are active power factor correction circuit, DC / DC circuit, power factor correction circuit, PWM control circuit and protection circuit. The power factor correction circuit is composed of uc3854a / B control chip to improve the power factor. The new chip uc3825 is used as the control chip to replace SG3525. Not only the peripheral circuit is simple, but also has the function of tolerance overvoltage and current limiting. The new IR2304 is also used as the driving chip, which has fast dynamic response and its own dead zone to prevent the direct connection of the upper and lower tubes of the half bridge.

1 active power factor correction circuit     In order to improve the power factor of the system, diode rectifier can not be used in the rectifier link, and uc3854a / B control chip is used to form a power factor correction circuit. Uc3854a / B is a new high power factor corrector integrated control circuit chip of unitrod company. It is an improvement on the basis of UC3854. It is characterized by average current control, power factor close to 1, high bandwidth and limiting power grid current distortion ≤ 3%. Fig. 1 is an active power factor correction circuit controlled by uc3854a / b.

Active power factor correction circuit controlled by uc3854a / uc3854b

The circuit consists of two parts. Uc3854a / B and peripheral components constitute the control part to control the input current and output voltage at the network side. The power part consists of L2, CS, s and other components to form a boost boost circuit. Skm75gbl23d module of ximenkang company is selected for switch s, and its working frequency is 35 kHz. Boost inductance L2 is 2mh / 20A. C5 adopts two 450V / 470 µ f electrolytic capacitors in parallel. In order to improve the efficiency of the circuit when the power is small, the designed PFC circuit does not carry out power factor correction under light load, and the power factor correction circuit is automatically put into use when the load is large. This part of the control is realized by the comparator part in Fig. 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 to make its output terminal low level, D5 is turned on, and the low level of ENA (enable terminal) to block uc3854a / b. When the load is large, ENA is at high level to make uc3854a / b work. D6 is connected to SS (soft start end). When the load is light, D6 is turned on to make Ss low level; When the load increase requires uc3854a / B to work, the potential at SS terminal rises slowly from zero, and the duty cycle of control output pulse increases slowly to realize soft start.

2 main circuit and control circuit 2.1 main circuit     Flyback power supply is generally used in circuits below 100W, and the designed maximum power of this power supply is 300W, which is obviously not suitable. In the high-frequency switching power supply with high power, the common main conversion circuits are push-pull circuit, half bridge circuit, full bridge circuit and so on. The push-pull circuit has few switching devices and high output power, but the switching tube bears high voltage (twice the power supply voltage), and the transformer has 6 taps, so the structure is complex; The voltage borne by the full bridge circuit switch is not high and the output power is large, but there are many switching devices (4) and the driving circuit is complex; Half bridge circuit switches bear low voltage, few switching devices and simple driving. According to the comprehensive comparison of the electrical performance and cost of various topology schemes, the half bridge DC / DC converter is selected as the main circuit. Figure 2 is the main circuit topology.

In Fig. 2, S1, S2, C1, C2 and main transformer T1 constitute a half bridge DC / DC conversion circuit. The MOSFET adopts 11nc380. The operating frequency of the circuit is 80 kHz. The transformer adopts E55 ferrite core without adding air gap. The "Three-stage" winding method is adopted to reduce the leakage inductance. R1 and R2 are used to ensure uniform capacitor voltage division. R3, C3, R4 and C4 are absorption circuits at both ends of MOS tube. C5 is a DC isolation capacitor, which is used to block the DC component proportional to the unbalanced volt second value, and balance the unequal volt second value of the switch every time. C5 adopts high-quality CBB non inductive capacitor. CT is the current transformer, which is used for sampling during current control. D3 and D4 adopt fast recovery diodes and output out2 to supply power to the control chip after L1, C6 and C7 smoothing filtering. RS and R6 are the sampling resistors of feedback voltage. The output out3 of the main transformer is high-frequency low-voltage AC. As shown in Figure 2, the feedback voltage and the output voltage are the same winding, so that the stability of the output voltage can be guaranteed to the greatest extent when the load changes. The latter stage can be connected with one or more multi-channel output transformers, and then rectified through the rectifier circuit, which can not only ensure that each output is independent, but also obtain any voltage. Therefore, it can meet the requirements of multi-channel different voltage power supply and high precision for DSP. 2.2 control circuit     The control circuit of the system adopts high-speed dual PWM controller uc3825. Its internal circuit is mainly composed of high-frequency oscillator, PWM comparator, current limiting comparator, overcurrent comparator, reference voltage source, fault latch, soft start circuit, undervoltage locking, PWM latch, output driver, etc. It has more advantages than SG3525:     1) The oscillation circuit is improved, the accuracy of oscillation frequency is improved, and the dead time control is more accurate;     2) It has current limiting control function, and the threshold current has 5% tolerance;     3) Low starting current (100mA);     4) Uc3825 off comparator is a high-speed overcurrent comparator. It has a threshold value of 1.2V to ensure that the soft start capacitor is fully discharged before the chip is restarted. When the threshold value is exceeded, the output is in a low-level state to prevent short circuit caused by simultaneous conduction of upper and lower bridge arms. Fig. 3 control circuit of main circuit.

R808 and r809 of the front stage form a starting circuit with the voltage stabilizing tube. After triggering uc3825 to start working, it is self powered by the feedback output out1. PWM modulation wave is generated by R1 and CT oscillation. RT and CT are generally selected according to formula (1) and formula (2).     RT=3V/(10mA)*(1-Dmax)      (1)

CT=(1.6*Dmax)/(Rt*f)               (2) Where: F = 80kHz, is the frequency taken.     Pin 1 (INV), pin 2 (E / a) and pin 3 (HI) constitute an error amplifier for voltage feedback. Pin 9 (Ilim) limits the current, pin 8 (SS) is soft start, and pin 11 (0uta) and pin 14 (0utb) are output drive signals. As can be seen from Figure 3, uc3825 has complete functions and simple peripheral circuit, 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 pulse transformer, which has the advantages of small volume and low price. However, when directly driven, the leading edge and trailing edge of the pulse are not steep enough, which affects the switching speed of MOSFET. Here, IR2304 chip is adopted. It is a multifunctional 600V high-end and low-end driver IC newly launched by IR company. It has the following advantages.     1) The chip has small size (DIP8) and high integration (it 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 1000 ns, matching delay time 50 ns.     3) It has strong driving ability, can drive 600V main circuit system, has 61 MA / 130mA output driving ability, and the grid driving input voltage is up to 10 20V.     4) The operating frequency is high and can support high-frequency switches of 100 kHz or below.     5) Input and output in-phase design provides high-end and low-end independent control drive output, which can be controlled by two independent CMOS or lstfl inputs compatible with 3.3V, 5V and 15V input logic, which brings great flexibility to the design.     6) Low power design, rugged and high anti noise efficiency. IR2304 adopts high-voltage integrated circuit technology. The integrated design not only reduces the cost and simplifies the circuit, but also reduces the design risk and saves the space of the circuit board. Compared with other discrete, pulse transformer and optocoupler solutions, IR2304 can save the number and space of components and improve reliability.     7) With power undervoltage protection and Shutdown Logic, IR2304 has two non inverted input and cross conduction protection functions, integrating the protection function specially set for half bridge MOSFET or IGBT circuit of driving motor. When the power supply voltage drops below 4.7V, the undervoltage locking (uvl0) function will immediately turn off both outputs to prevent through current and device failure. When the power supply voltage is greater than 5V, the output will be released (the comprehensive lag is generally 0.3V). Overvoltage (HVIC) and anti blocking CMOS technology make IR2304 very rugged. In addition, IR2304 is also equipped with a large pulse current buffer stage to minimize cross conduction; At the same time, the sohmill trigger input design with pull-down function can effectively isolate noise and prevent accidental opening of devices.     Figure 4 shows the wiring diagram of IR2304,

It can be seen that IR2304 has the advantages of simple connection and few peripheral components. VCC is self powered by out2 in the main circuit, Lin and Hin are respectively connected to the two output terminals of uc3825, VD should adopt fast recovery diode, C1 is filter capacitor, C2 is bootstrap capacitor, it is best to adopt tantalum capacitor with good performance, R1 and R2 current limiting resistors. 2.4 protection circuit design     For DC / DC power products, the protection circuit of the system is required to work in case of abnormal conditions (such as overcurrent and overload), so that the converter can stop 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 sent to uc3825 pin 9 (iljim) after conversion. When the current exceeds the predetermined value, uc3825 automatically blocks the output pulse for protection. 3 experimental results and waveforms     Fig. 6 Fig. 9 are partial experimental waveforms of the prototype.

4 Conclusion     The engineering prototype has passed the performance test. The system uses uc3854a / B control chip to form a power factor correction circuit. It does not work under light load, which is conducive to improve the efficiency. The circuit is automatically put into operation under heavy load, which improves the power factor at high power. The main circuit control adopts a new chip uc3825, which has the function of tolerance overvoltage and current limit, simple peripheral circuit and good stability. It also adopts a new IR2304 with fast dynamic response, strong driving ability and high working frequency as the driving chip, which has the functions of power undervoltage protection and shutdown logic. Compared with the previous power supply, it only increases the cost, However, the stability of the system is greatly improved, the accuracy of frequency is improved, and the output voltage is more stable. When the load changes from the lightest to 300W, the change of output voltage is less than 1%. The transformer output can adopt multiple taps on the main transformer or connect to the secondary transformer to obtain independent output of different voltages.

Design of Half Bridge DC / DC Converter with Multi-channel Independent Power Supply 1

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