Design of the hottest low voltage high current swi

Abstract: with the development of computer and communication technology, low voltage and high current switching power supply has become an important research topic. The design process of a switching power supply with output voltage of 3.3V and output current of 20a is introduced

key words: active clamping; Synchronous rectification; Forward converter

1 introduction

switching power supply is a power supply that uses modern power electronic technology to control different test modes in the process of switching crystal test. The operator can control the time ratio of on and off by setting parameters in the human-machine interface and maintain a stable output voltage. Since the 1990s, switching power supply has successively entered the field of various electronic and electrical equipment. Switching power supply has been widely used in computers, program-controlled exchanges, communications, electronic testing equipment power supply, control equipment power supply, etc. With the power supply comprehensively promoting the development of intelligent manufacturing standard system construction technology, the price of 4.35v lithium cobalt oxide as cathode material with low voltage and high current has risen to 260000 yuan/ton. Switching power supply has attracted more and more attention because of its high technical content and wide application. In switching power supply, forward and flyback have the advantages of simple circuit topology and electrical isolation of input and output. They are widely used in small and medium power conversion occasions. Compared with the flyback type, the copper loss of the forward converter transformer is lower. At the same time, the ripple voltage and current attenuation of the secondary side of the forward circuit is more obvious than that of the flyback type. Therefore, it is generally considered that the forward converter is suitable for low voltage, high current and high power occasions

2 basic technology

2.1 active clamping technology

forward DC/DC converter has the inherent disadvantage that the high-frequency transformer must be magnetically reset during the power transistor cut-off period. In case of transformer core saturation, special magnetic reset circuit must be used. There are three commonly used reset methods, namely, the traditional additional winding method, RCD clamping method, and active clamping if normal method. The three methods have their own advantages and disadvantages: the advantages of the magnetic reset winding positive excitation converter are that the technology is mature and reliable, and the magnetization energy can be fed back to the DC circuit without damage. However, the additional magnetic reset winding complicates the structure of the transformer, and the shutdown voltage spike caused by the leakage inductance of the transformer needs to be suppressed by the RC buffer circuit. The duty cycle D is 0.5, and the voltage stress borne by the power switch is proportional to the input power supply voltage. The advantage of RCD clamp forward converter is that the magnetic reset circuit is simple, the duty cycle D can be greater than 0.5, and the power switch tube bears low voltage stress, but most of the magnetization energy is consumed in the clamp resistance, so it is generally suitable for power conversion occasions with low conversion efficiency and low price. Active clamping technology is the most efficient of the three technologies. Its circuit diagram is shown in Figure 1 and its working principle is shown in Figure 2. Before the DT period, the switch S1 is turned on, and the exciting current IM is negative, that is, from Cr flows through S1 to tr. in the DT stage, the driving pulse UGS of the switch s turns it on, and ugs1=0 turns S1 off. Under the action of VIN, the exciting current changes from negative to positive, and the primary power is transmitted to the secondary side through the transformer to charge the output inductance L; In the period of (1-D) t, ugs=0, s is off, ugs1 to make S1 conductive, Im charges CR through the anti parallel diode of S1, under the action of the resonant circuit composed of Cr and tr leakage inductance, Im changes from positive to negative, and the transformer is reverse excited. From the above analysis, it can be seen that the transformer core of the active clamp forward converter works in the bidirectional symmetrical magnetization state, which improves the utilization rate of the core. The steady-state voltage of the clamp capacitor is automatically adjusted with the switch duty cycle, so the duty cycle can be greater than 50%; When VO is constant, the stress of main switch and auxiliary switch changes little with VIN; Therefore, within the allowable range of duty cycle and switching stress, it can adapt to the situation of large input voltage variation range. The disadvantage is that a tube is added, which makes the circuit complex

Figure 1 active clamp synchronous rectification forward circuit diagram

Figure 2 active clamp circuit working principle diagram

2.2 synchronous rectification technology

in low-voltage and high current power converters, if traditional ordinary diodes or Schottky diodes are used for rectification, due to their large forward voltage drop (low-voltage silicon diodes have a forward voltage drop of about 0.7V, Schottky diodes have a forward voltage drop of about 0.45V, and new low-voltage Schottky diodes can reach 0.32v), Rectifier loss has become the main loss of the converter, which can not meet the needs of high efficiency and small size of low-voltage and high current switching power supply

the volt ampere characteristic of MOSFET when it is turned on is a linear resistance, called on state resistance RDS. The on state resistance of new low-voltage MOSFET devices is very small, such as IRL3102 (20V, 61A), irl2203s (30V, 116a), IRL3803S (30V, 100a) on state resistance is 0.013, 0.007 and 0.006 respectively. When they pass through 20A current, the on state voltage drop is less than 0.3V. In addition, power MOSFET has short switching time and high input impedance, which makes MOSFET the preferred rectifier for low voltage and high current power converters. Power MOSFET is a voltage type control device. When it is used as a rectifier element, it requires the control voltage to be synchronized with the phase of the voltage to be rectified in order to complete the rectification function, so it is called synchronous rectification circuit. Figure 1 shows a typical step-down synchronous switching converter circuit (when there is no SR in the circuit, it is an ordinary step-down switching converter circuit)

3 circuit design

the designed power parameters are as follows: the input voltage is 50 (110%) V, the output voltage is 3.3V, the current is 20a, and the working frequency is 100kHz

the adopted main circuit topology is shown in Figure 1. As the active clamping adopts flyback clamping circuit, its clamping capacitor voltage is:

vc=vin

the selected control IC chip is uc3844, and its maximum duty cycle is 50%, so the maximum voltage on the capacitor is VIN, and the capacitance withstand voltage is more than 60V. As long as it is large enough, the circuit can work normally. The clamping capacitor selected in this circuit is 47 f/100v

the driving of active clamp tube S1 must be separated from the ground on the primary side of the transformer, and the driving signal of S1 must be reversed from the driving signal of switch tube S. the driving of two tubes can be realized by using UCC3580, but this chip is not common, so uc3844 and IR2110 are selected here. The control signal from uc3844 is used as the low-end input of IR2110, and its inverse signal is used as the high-end input of IR2110. The high-end drive of IR2110 is isolated by internal bootstrap circuit. In this way, we have achieved the goal of driving two switches

in the output rectifier circuit, when the freewheeling diode (i.e. the reverse diode of SR) is turned on by the forward voltage, the SR should be driven on in time to reduce the voltage drop and loss. However, in order to prevent Sr and Sr1 from conducting at the same time, causing a short-circuit accident, there must be a dead time. At this time, diode D is still used to conduct. The switching instantaneous of SR should closely cooperate with the on-off instantaneous of freewheeling diode, so the switching speed is very high. In addition, IRL3102 is selected from the perspective of cost

the design of transformer is similar to that of general forward converter transformer, but the drive of synchronous rectifier should be considered. The driving opening voltage of the selected synchronous rectifier is about 4V, the output voltage of the circuit is 3.3V, and the output end is equivalent to a step-down circuit, with a maximum duty cycle of 0.5, so the secondary side voltage of the transformer is at least 6.6v. Because the voltage resistance of the silicon oxide layer between the gate and source of MOSFET is limited, it will be permanently damaged once it is broken down, so in fact, the maximum value of the gate source voltage is between 20 ~ 30V. If the voltage exceeds 20V, a regulator tube should be connected to the gate

4 experimental results and waveform analysis

the UDS waveforms of switch S1 and s are shown in Figure 3. Refa is the voltage drop waveform of S tube, 50v/div, refb is the voltage drop waveform of S1 tube, 50v/div. At this time, the circuit works at vin=60v, the switching stress of S1 and S is about 120V, and d=0.5. Figure 4 shows the output voltage of the transformer, that is, the driving signal of synchronous rectifier Sr1 and Sr. the positive part is the driving signal of Sr and the negative part is the driving signal of Sr1. The experimental waveform is basically consistent with the analyzed waveform, but there is a small spike in the voltage at the moment of switching, which is caused by the stray parameters of the circuit. The working efficiency of the circuit is measured to be about 90%, which basically meets the design requirements

Fig. 3 UDS waveform of switch tubes s and S1

Fig. 4 driving waveform of synchronous rectifier tube

5 Conclusion

3.3v/20a switching power supply design shows that active inverter plus synchronous rectifier circuit can achieve high efficiency when used in low-voltage and high current forward circuit design without PFC circuit