# How to choose a DC-DC Converter for a motor with inrush current? (Theory)

I am wondering how to choose a DC-DC converter for a motor.

This is a theoretical question for understanding.

For example, if I have a converter that can output 5V with a maximum current of 1A. Assume the input voltage was 24V and the converter can have a max input voltage of 40V

If the motor, requires 5V and has a rated current of 0.2A, then assuming for a thumb rule, the inrush current was 10x the rated current so 2A). This converter would not be suitable, would it?

Hence how do you select a DC-DC converter for driving motors with the inrush current?

I am currently doing a project that involves a lot of components (can't show the circuit, unfortunately) and so multiple motors, etc but I was unsure on how to choose the DC-DC converter to supply the motors.

• If you can in this situation you drive the motor with something that limits the inrush current, preferably an amplifier that is constant-current up to maximum voltage. This will limit the torque (permanent-magnet DC motor torque is roughly proportional to current), but for a lot of applications that'll be OK. – TimWescott May 27 '19 at 17:17
• Whether the motor needs help depends on the mechanism it's attached to. If the mechanism is dominated by static friction then the most torque needed will be breaking that friction. On the other end of the spectrum, if the "mechanism" is a propeller, then the maximum torque requirement will happen at top speed. There is no one answer here. – TimWescott May 27 '19 at 17:27
• It also depends on what the particular DC-DC converter does when it's overloaded. Many limit, so you'd just get 1A into your motor, which apart from lower starting torque is just fine. Some fold-back, so you'd get a very low current into your motor, because the converter assumes it's a fault, which may never start the motor. – Neil_UK May 27 '19 at 18:29
• DC-DC converters don't make great supplies for DC motors without acceleration control like VFM's with V/f control. With the FET losses dependant on Ron/Rload the motor load appears to be 10% of the rated load or 10% x V/I = DCR typically +/-20% So your FET Ron now needs to be << 1% of the motor rated impedance V/I. For an H bridge that means the sum of two FET's Ron. So for 5V/0.2A, = 25 Ohms your FETs need to be <= 100 mOhm and if you accelerate in the opposite direction at full speed then surge current doubles for a lot longer time down to 1x at stop speed.. – Tony Stewart Sunnyskyguy EE75 May 27 '19 at 18:56
• Problem with inrush currents and switching power supplies is the DUTY CYCLE. You can't get 300% duty cycle for 10 milliSeconds to kick-start the motor. – analogsystemsrf May 27 '19 at 23:39