Doubts about LDO power supply power and output current.

SHU_2017

Doubts about LDO power supply power and output current. [Copy link]

 

Background: I designed a vehicle controller to control the movement of three motors. I have encountered the problem of insufficient power of the power module. Since it is my first time to do this, these problems have troubled me for several days, so I would like to ask you for advice. Input: The power module on the PCB board (using a low-dropout linear regulator LDO) is a step-down module that reduces voltage from 12V to 5V. The schematic diagram is below; Output: The load needs to connect 3 motors. The working voltage of these three motors is 12V, which is directly introduced from the input terminal; the maximum working current is 1A, 4A and 3.5A respectively; there is a working condition where the working current is at the maximum at the same time, and the driving circuit schematic is as follows; Question: 1. I think the current at the motor flows directly to the external power supply. When the power module on the PCB board considers its current output, it will not include the current at the motor, but only consider the current of the MCU and other ICs on the board. I don’t know if there is any problem with my idea. Please correct me; 2. Is the design power of the power module on the PCB board the output current of the IC multiplied by the output voltage? If not, how should it be calculated? (I call the maximum power that the power module can work normally as the design power) 3. If the working output current of the LDO chip I use is 1A, and the load current on the PCB board needs 2A to work, how to modify the power module design to make the PCB work properly? (I only thought of connecting a push-pull amplifier circuit behind the LDO chip) 4. When the PCB board is input with 12V voltage, the output current is 2.5A; when the input is 15V, the output is 3.5A; I cannot understand this phenomenon; I think that when the power is constant, when the high voltage is input, the current should decrease instead of increase? Is it because the voltage increases, the output current at the motor increases, so the current increases? Then why can't the current at the motor reach 3.5A at 12V? What limits the motor current?

maychang

"1. I think the current at the motor flows directly to the external power supply. The power module on the PCB board does not include the current at the motor when considering its current output. It only considers the current of the MCU and other ICs on the board. I don't know if there is anything wrong with my idea." No problem.

maychang

"2. Is the design power of the power module on the PCB board the IC's output current multiplied by the output voltage? If not, how should it be calculated?" No. It is the 12V input voltage multiplied by the current consumed by all ICs on the board. It is not the power module output voltage multiplied by the current consumed by all ICs on the board.

maychang

"3. If the working output current of the LDO chip I use is 1A, and the load current on the PCB needs 2A to work, how can I modify the power module design to make the PCB work properly?" Use at least three LDO chips you use to power each IC on the circuit board, or replace it with a LDO chip that can output a larger current. However, even if you use three LDO chips you use to power, it is better to use a switching power supply chip instead. The switching power supply chip powers each IC on the circuit board, generates less heat, and is smaller in size.

maychang

"4. When 12V voltage is input on the PCB board, the output current is 2.5A; when 15V is input, the output current is 3.5A. I can't understand this phenomenon. I think when the power is constant, when high voltage is input, the current should decrease instead of increase? Is it because the voltage increases, the output current at the motor increases, so the current increases?" The power consumption of the motor is not constant power. If it is constant power, the current consumed by the motor should decrease, but the motor does not consume power at constant power.

maychang

"Why can't the current at the motor reach 3.5A at 12V? What limits the motor current?" This is determined by your motor. Motors come in different sizes, and this should be common sense. The rated current of the motor when the rated voltage is input and the rated mechanical load is how much, which should be marked on your motor nameplate or in the manual.

SHU_2017

maychang published on 2018-12-5 07:55 "Why can't the current at the motor reach 3.5A when it is 12V? What limits the motor current?" This is determined by your motor. The motor has...

For example, when the motor is unloaded, the current is 0.3A; when the motor is loaded, it is powered by a PCB board, 12V voltage input, the current is only 2.5A, it cannot carry the load, and the motor is blocked; when it is directly powered by a 12V DC power supply, the current reaches 3.5A and can drive the load. My question is why the current cannot reach 3.5A when it is powered by a PCB board, what factors limit it? If it is a power module on the PCB board, but the 12V supply voltage at the motor is directly drawn from the 12V input voltage, why is it affected by the power module?

maychang

SHU_2017 Published on 2018-12-5 08:42 For example, when the motor is unloaded, the current is 0.3A; when the motor is loaded, it is powered by a PCB board, with a 12V voltage input, and the current is only 2.5A, which cannot carry the load, and the power supply...

There are two possibilities: First, your 12V power supply capacity is insufficient, and the voltage at the 12V power supply end drops to less than 12V when the motor is heavily loaded. Second, the wire is too long, and the voltage drop on the wire is too large when the motor is heavily loaded. The voltage at both ends of the 12V power supply can maintain 12V, but the voltage at both ends of the motor is not enough to reach 12V. Directly powered by a 12V DC power supply, one possibility is that your 12V DC power supply capacity is sufficient and the motor can work normally, another possibility is that when powered by a 12V DC power supply, the wire is shorter and the voltage drop is smaller.

maychang

SHU_2017 Published on 2018-12-5 08:42 For example, when the motor is unloaded, the current is 0.3A; when the motor is loaded, it is powered by a PCB board, with a 12V voltage input, and the current is only 2.5A, which cannot carry the load...

"My question is why the current cannot reach 3.5A when powered by a PCB board, and what factors limit it?" The possibility that the power module on the PCB board affects the operation of the motor is very small.

SHU_2017

maychang posted on 2018-12-5 08:47 There are two possibilities: One is that your 12V power supply capacity is insufficient, and the voltage at the 12V power supply end drops to less than 12V when the motor load is heavy. The other is that the conductor...

I use a 12V DC power supply to power the PCB, and then connect the positive and negative poles of the motor to the output port of the PCB board; the 12V DC power supply directly powers the motor. The source of the power supply is a 12V DC power supply, and the difference in the length of the wires of the two connection methods is less than 10cm, so this impact should not be that great.

maychang

SHU_2017 posted on 2018-12-5 08:54 I use a 12V DC power supply to power the PCB, and then connect the positive and negative poles of the motor to the output port of the PCB board; the 12V DC power supply directly powers the motor. The source of the power supply...

The difference of 10cm in the length of the wires of the two connection methods will not have as great an impact as you described, and will not reduce the motor current from 3.5A to 2.5A.

maychang

SHU_2017 Published on 2018-12-5 08:54 I use a 12V DC power supply to power the PCB, and then connect the positive and negative poles of the motor to the output port of the PCB board; the 12V DC power supply directly powers the motor. The source of the power supply...

"I use a 12V DC power supply to power the PCB, and then connect the positive and negative poles of the motor to the output port of the PCB board; the 12V DC power supply directly powers the motor. The source of the power supply is the 12V DC power supply," It depends on how your 12V DC power supply is connected to the circuit board, and then how it is connected to the motor. When the DC power supply is connected to the circuit board, you may use a plug and socket, and when the circuit board is connected to the motor, you may also use a plug and socket. It is recommended to use two multimeters, after calibration (make sure the readings of the two multimeters are consistent), to measure the output end of the DC power supply and both ends of the motor respectively. If there is a difference in the readings of the two multimeters, and the voltage across the DC power supply is higher than the voltage across the motor, then the contact of the plug and socket is not good enough, there is contact resistance, or the wires on the circuit board are too thin.

SHU_2017

maychang posted on 2018-12-5 09:40 "I use a 12V DC power supply to power the PCB, and then connect the positive and negative poles of the motor to the output port of the PCB board; the 12V DC power supply directly powers the motor. The source of the power supply...

OK, thank you for your patient answer

SHU_2017

The problem was solved. Finally, it was found that the wire at the output end was too thin. Thank you for your help.

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