High current boost circuit solution based on DC/DC

" title="DC/">DC/" title="DC/">DC/" title="DC/">DC/" title="DC/">DC/" title="DC/">DC/DC cannot meet the current demand. At present, the commonly used method is to invert the 12V battery voltage to AC 220V, and then generate DC 18.5V and other multiple outputs from AC 220V. Although it can meet the current demand, the power efficiency will be greatly reduced after two conversions, only about 60%. Such conversion efficiency is unacceptable for car battery power supply.

To address this problem, this paper proposes a method for designing a high-current output boost DC/DC module based on a two-phase step-up DC/DC controller LT3782.

1 LT3782 Introduction

LT3782 is a two-phase step-up DC/DC controller produced by Linear Technology Corporation of the United States. It is a 28-pin SSOP package chip with a programmable switching frequency between 150 and 500kHz. Due to the two-phase BOOST topology, the requirements for the output field effect tube leakage current and the Schottky diode current are reduced by half, that is, the two outputs have a phase difference of 180°, and the two outputs suppress the output ripple current from each other. The output ripple is 1/3 of the single-phase BOOST conversion circuit. The power supply efficiency is high and the heat dissipation requirements are small. Figure 1 is the pin diagram of LT3782. Pin 29 is the heat dissipation pin at the bottom of the chip. Pin 27 is connected to the input power supply; pin 4 is grounded; pin 11 is used to set the switching frequency; pins 20 and 23BGATE are used to drive the gate of the field effect tube; pins 8, 9, 12 and 13SENSE are used to feedback the output current of the field effect tube; pin 16 is the output voltage feedback pin, the voltage of this pin is 2.44V, and the output voltage value can be set through the feedback resistor. Pin 17 is a low voltage shutdown pin. When the voltage of this pin is greater than 2.45V, the device starts to work. When the voltage of this pin is less than 0.3V, the device enters low voltage shutdown mode. Pin 14 is a soft start pin. When power is applied, the output voltage gradually changes from 0V to the set output voltage value. The typical startup time can be calculated by the following formula:

Where: C is the capacitance value connecting pin 14 to ground, in μF; t is the typical startup time.

2 Circuit Implementation

2.1 Overall design of switching power supply

The circuit is implemented as shown in Figure 2. The 12V car battery voltage powers the LT3782 through the plug JP1 and R5. The two-phase oscillation output generated by the LT3782 drives the N-channel field effect transistors Q1 and Q2. The field effect transistor outputs are rectified by Schottky diodes D1 and D2 respectively, and then filtered and output by capacitor C7.

2.2 Switching power supply parameter setting

In Figure 2, resistor R1 is used to set the switching frequency of LT3782, which is programmable between 150 and 500kHz. Here, the switching frequency is 250kHz, and the resistor value R1 is 75kHz, referring to Figure 3.

Referring to the LT3782 data sheet, the peak current of the N-channel field effect tube can be deduced to be about 15A through the input/output voltage relationship and duty cycle. The power supply current limit can be set by setting the resistance of resistors R8 and R10 to avoid excessive power supply current and burn out the subsequent circuits. The threshold voltage of the SENSE pin of the LT3782 is 60mV, so the resistance of resistors R8 and R10 is 0.004Ω.

The output voltage is set by setting the ratio of resistors R13, R7 and R11. The FB pin voltage of LT3782 is 2.44V reference voltage, so the output voltage can be calculated by the following formula:

Vout=2.44[1+（R13+R7）/R11]

The over-discharge protection of the car battery is achieved by setting the minimum operating voltage of the power supply. The power supply can only work normally when the voltage of the RUN pin of LT3782 is higher than 2.45V. Through the voltage division of resistors R6 and R9, the power supply can only work normally when the car battery voltage is greater than 10V, thus avoiding over-discharge of the battery.

3 Circuit Test

In order to verify the performance of the switching power supply, the method shown in Figure 4 is used for verification. Because the input current of the power supply is large, the input DC power supply uses Agilent's 6574A2J07. Its output current can reach up to 42A, and the output voltage is adjustable in the range of 0 to 50V. The output load uses Kenwood's PEL1022201, through which the output voltage and current can be intuitively seen. After verification, the circuit fully meets the needs of the user unit.

4 Conclusion

Based on the two-phase step-up DC/DC controller LT3782 produced by Linear Technology Corporation of the United States, a high-current output step-up DC/DC module is designed. When powered by a 12V car battery, the module can provide multiple outputs such as 24V, 18.5V, etc. with a current of up to 7A as needed. Due to the use of new two-phase DC/DC technology, the power efficiency reaches more than 90%. Compared with the method of converting the power supply to AC 220V and then converting it into the required voltage, the efficiency is significantly improved, which is in line with the current development direction of building a conservation-oriented society and is more practical.