SC8701 is a synchronous 4 switch buck-boost controller, support 2.7~36V wide voltage input, and 2~36V output, support input current limiting, output current limiting, over temperature protection functions, output efficiency up to 96%, using QFN4X4 package, has the advantages of small size, high efficiency.
This design is to design a 9~30V input, 12V/10A output DC power module, at the same time with anti-reverse connection, input/output overcurrent protection, undervoltage protection, short circuit protection and other functions.
This is the power supply part of SC8701. The whole module is divided into power supply part, single-chip control part and detection part. For the sake of confidentiality, only the power supply part can be provided here. Anti-reverse connection circuit is a popular circuit, composed of a Q5NMOS tube, which controls the working state of the whole circuit by controlling the conduction of the MOS tube. It can be clearly seen from the circuit that when the input power is connected correctly, the gate voltage of the MOS tube will have a voltage of about 10V due to the effect of R3 and R15 voltage resistors. The MOS tube can be conduction in saturated state, the current can pass R6 resistance to level after, to the ground, and at last, by SG MOS tube to the input power of the cathode, the formation of a circuit, when the input, MOS tube grid voltage, in the cutoff state, therefore, the current will not be able to form a loop, the whole current of course won't work.
For SC8701 power module circuit, basically all the chip manual will have a recommended circuit, and we will have to do is actually the recommended reference circuit, and then according to the design requirements, through the various parameters of the chip manual to calculate the parameters of the device, you can get what we want for a functional outcome. In this design, the most important things about this circuit are the selection of inductor and the selection of driving MOS tube. The input/output current limiting protection is set by setting R26 and R29 (there is a calculation formula in the manual), and the output voltage is set by R23 and R28 resistors. The inductance is mainly related to the output current and the output ripple, and the driving MOS tube is related to the output current and input voltage.
The inductor can be selected by L=[Vo* (1-D)]/ R * F *Io, where Vo is the output voltage, D is the duty cycle, which is equal to the output voltage/input voltage, R is the ripple coefficient, F is the switching frequency, and Io is the output current. For this chip, a recommended value of 3.3UH was directly used because the chip manual had recommended values. There is another important parameter for the inductor, that is the saturation current. Because the output current is 10A, the saturation current of the current should be about twice the output current. Therefore, the inductor adopts type 1770 inductor, and the saturation current is 17A.
For driving MOS, ID current, RDS resistance and VDS are mainly considered. Output current is 10. A, current ID will have to be far greater than this value, the RDS is the MOS tube within a tolerance of about 10 a such a big current, the internal resistance must be as small as possible, because a big resistance, will increase the pressure drop, a output ability is not enough, one is due to the large pressure drop will be converted into heat energy, therefore, MOS tube will be fever is very serious, Easy to cause the failure of MOS tube. Since the input voltage should be between 9 and 30V, the VDS should also be above 30V at least. Therefore, it is necessary to select the MOS tube with high current and low internal resistance, and the VDS should also meet the requirements. Finally, IRLR8726 was selected, and its internal resistance was only 5.8Mω under the condition of saturated conduction.
The most important part of the whole power supply is the inductor and driving MOS tube, and the rest of the peripheral devices only need to correspond to the chip manual according to the design requirements, and the parameter values can be easily obtained.
After the design of the principle, is the layout of the PCB design, and debugging.
PCB design according to the manufacturer's suggestion, the use of 4 layers of board wiring.
When designing PCB, we should pay attention to the line width of the power supply. For such a large current, the line width must be very thick. Otherwise, when the current is pulled to 10A, the voltage will drop very low. Chip design, a peripheral device to try to close to the chip, the line is the shortest, MOS pipe line, for the driver requires extra bold, because the pins of the drive current is compared commonly big, has a high voltage, do have a purpose is to ensure that MOS tube after the electricity, the voltage in a platform to achieve the shortest time, protect the MOS tube.
Because the power of this power supply is relatively high, all power cables are routed through the upper and lower power supply layers. In addition, Windows are opened to increase heat dissipation. PCB board design, mainly need to rely on their own peacetime drawing, more experience, so here is not much to say.
After the PCB drawing play back plate test, found that there is a problem, when I load output of 12 v / 2 a, power supply can be normal output, but when the output load current is 3 a and above, after the output is 0, that is to say, chip did not work, then use a multimeter pens and to poke chips enable pin, but returned to normal output, To this phenomenon, at first I was very confused, I suspect the chip pins virtual welding, but there are still the same after the new welding phenomenon, and the level of test meter and couldn't change pin, because there is no output shows that enable pin may be for the high level, after stamp and explained again into a low level, through the analysis, determine the pin level is not stable, A 0.1uF capacitor was welded to the conversion pin. The problem was finally solved. After looking at the PCB diagram, it was found that the wire was connected across two power supply planes, which caused some interference.