Design of low voltage and high current switching power supply

1 Introduction

A switching power supply is a power supply that uses modern power electronics technology to control the time ratio of the switching transistor to turn on and off to maintain a stable output voltage. Since the 1990s, switching power supplies have been introduced into various electronic and electrical equipment fields. Computers, programmable switches, communications, electronic testing equipment power supplies, control equipment power supplies, etc. have all widely used switching power supplies. With the development of power supply technology, low-voltage, high-current switching power supplies are increasingly valued because of their high technical content and wide application. Among switching power supplies, forward and flyback types have the advantages of simple circuit topology and input and output electrical isolation, and are widely used in small and medium power power conversion occasions. Compared with the flyback type, the forward converter transformer has lower copper loss. At the same time, the secondary ripple voltage and current attenuation of the forward circuit is more obvious than that of the flyback type. Therefore, it is generally believed that the forward converter is suitable for low voltage, high current, and high power occasions.

2 Basic Technology

2.1 Active clamping technology

The inherent disadvantage of the forward DC/DC converter is that the high-frequency transformer must be magnetically reset during the power transistor cutoff period. In order to prevent the transformer core from saturating, a special magnetic reset circuit must be used. There are three commonly used reset methods, namely the traditional additional winding method, the RCD clamping method, and the active clamping method. The three methods have their own advantages and disadvantages: The advantage of the magnetic reset winding method forward converter is that the technology is mature and reliable, and the magnetization energy can be fed back to the DC circuit without loss. However, the additional magnetic reset winding complicates the transformer structure, and the turn-off voltage spike caused by the transformer leakage inductance needs to be suppressed by the RC buffer circuit. The duty cycle D<0.5, and the voltage stress of the power switch tube is proportional to the input power supply voltage. The advantage of the RCD clamp forward converter is that the magnetic reset circuit is simple, the duty cycle D can be greater than 0.5, and the voltage stress of the power switch tube is low, but most of the magnetization energy is consumed in the clamping resistor, so it is generally suitable for power conversion occasions with low conversion efficiency and low price. Active clamping technology is the most efficient technology among 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 tube S1 is turned on, and the excitation current iM is negative, that is, it flows from Cr to Tr through S1. In the DT stage, the driving pulse ugs of the switch tube S turns it on, and at the same time ugs1=0, so that S1 is turned off. Under the action of Vin, the excitation current changes from negative to positive, and the primary power is transmitted to the secondary side through the transformer to charge the output inductor L; in the (1-D)T period, ugs=0, S is turned off, and the arrival of ugs1 makes S1 turned on, iM charges Cr through the anti-parallel diode of S1, and under the action of the resonant circuit composed of Cr and Tr leakage inductance, iM changes from positive to negative, and the transformer is reversely excited. From the above analysis, it can be seen that the core of the active clamp forward converter transformer works in a bidirectional symmetrical magnetization state, which improves the core utilization rate. 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 the main switch and auxiliary switch does not change much with Vin; therefore, within the allowable range of duty cycle and switch stress, it can adapt to the situation of a larger input voltage variation range. The disadvantage is that an additional tube is added, which makes the circuit complicated.

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, high-current power converters, if traditional ordinary diodes or Schottky diodes are used for rectification, due to their large forward conduction voltage drop (the forward voltage drop of low-voltage silicon diodes is about 0.7V, the forward voltage drop of Schottky diodes is about 0.45V, and the new low-voltage Schottky diodes can reach 0.32V), the rectification loss becomes the main loss of the converter, which cannot meet the needs of high efficiency and small size of low-voltage, high-current switching power supplies.

The volt-ampere characteristic of MOSFET when it is turned on is a linear resistance, called the 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) The on-state resistance is 0.013Ω, 0.007Ω and 0.006Ω respectively. When they pass 20A current, the on-state voltage drop is less than 0.3V. In addition, the power MOSFET has a short switching time and high input impedance. These characteristics make MOSFET the preferred rectifier device 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 to complete the rectification function, so it is called a synchronous rectifier circuit. Figure 1 is a typical step-down "synchronous" switching converter circuit (when there is no SR in the circuit, it is a "normal" step-down switching converter circuit).

3 Circuit Design

The designed power supply parameters are as follows: input voltage is 50 (1 ± 10%) V, output voltage is 3.3 V, current is 20 A, and operating frequency is 100 kHz.

The main circuit topology used is shown in Figure 1. Since the active clamp uses a FLYBACK type clamp circuit, its clamp capacitor voltage is:

Vc=Vin

The control IC chip selected is UC3844, and its maximum duty cycle is 50%, so the maximum voltage on the capacitor is Vin, and the capacitor withstand voltage is above 60V. As long as it is large enough, the circuit can work normally. The clamping capacitor selected in this circuit is 47μF/100V.

The drive of active clamp tube S1 must be isolated from the ground of the primary side of the transformer, and the drive signal of S1 must be in antiphase with the drive signal of switch tube S. The use of UCC3580 can realize the drive of two tubes, 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 inverted signal is used as the high-end input of IR2110. The high-end drive of IR2110 is isolated through the internal bootstrap circuit. In this way, we have achieved the purpose of driving two switch tubes.

In the output rectifier circuit, when the freewheeling diode (i.e. the anti-parallel diode of SR) is turned on by the forward voltage, SR should be driven to turn on in time to reduce the voltage drop and loss. However, in order to avoid SR and SR1 being turned on at the same time and causing a short circuit accident, there must be a "dead zone" time, and then the diode D is still turned on. The switching instant of SR must be closely coordinated with the on-off instant of the freewheeling diode, so the switching speed is very high. In addition, considering the comprehensive cost, IRL3102 is selected.

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

4 Experimental results and waveform analysis

The Uds waveform of the switch tubes S1 and S is shown in Figure 3. RefA is the voltage drop waveform of the S tube, 50V/div, and RefB is the voltage drop waveform of the S1 tube, 50V/div. The circuit is working at about Vin=60V at this time, the switch stress of S1 and S is about 120V, and D=0.5. Figure 4 is the output voltage of the transformer, that is, the drive signal of the synchronous rectifier tubes SR1 and SR. The positive part is the drive signal of SR, and the negative part is the drive signal of SR1. The waveform obtained by the experiment is basically consistent with the waveform analyzed, except that at the moment of switch conversion, the voltage has a small spike, 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.

Figure 3 UDS waveform of switch tubes S and S1

Figure 4 Synchronous rectifier drive waveform

5 Conclusion

The design of the 3.3V/20A switching power supply shows that the active inverter plus synchronous rectification circuit can achieve very high efficiency when used in the low-voltage and high-current forward circuit design without adding a PFC circuit.