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I need to design a buck converter (controlled by pwm using feedback) for a 24V dc motor under no load. The buck converter output can be controlled to vary between 15v and 24v. I understand that in order to choose a correct inductor value to ensure CCM operation, I need to know the load current, and that the lower the load current, the greater the required inductor value as per this design formula:

enter image description here The nominal voltage for the motor is 24V and there is a rated current of 130 mA. The spec also states that for 24V motor models the maximum no load current is 50mA. So for my design what output current should I be using to choose the inductor value? 50mA is the maximum no load current meaning that it could technically be less than 50mA so I think it would operate in DCM if I designed for this value. And I don't understand how the output current would vary when supplying less than 24v to the motor (since the voltage supplied to the motor could be as low as 15v). My intuition is that the current draw would be greater, so I should be considering the worst case scenario, 24V, but maybe this is not true.

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  • \$\begingroup\$ Why do you need CCM operation - after all the load is only a motor and not some high spec'd voltage supply requiring low ripple etc.? Also, if you designed it to be a synchronous buck converter (flyback diode supplemented with a MOSFET), you needn't worry about the lack of control when entering DCM because the synchronizing MOSFET ensures that the average output voltage is largely the input voltage x D under all load conditions. \$\endgroup\$ – Andy aka Sep 30 '17 at 9:06
  • \$\begingroup\$ it's for a project and there is a ripple requirement. I was told it was easier to design for operation in ccm. i did not consider a synchronous design because they've only taught us about asynchronous design \$\endgroup\$ – Jamila Sep 30 '17 at 9:10
  • \$\begingroup\$ The control loop is easier in CCM except that you then need to avoid DCM unless the control loop can smoothly transition the mode change into and out of DCM. Low ripple voltage on a motor seems an excessive demand being placed that is usually unimportant for motors. \$\endgroup\$ – Andy aka Sep 30 '17 at 9:13
  • \$\begingroup\$ it is for a uni course. i think the ripple requirement is just so we learn how to design for that \$\endgroup\$ – Jamila Sep 30 '17 at 9:16
  • \$\begingroup\$ Thanks for your help on that by the way. Yes that was it! \$\endgroup\$ – Jamila Sep 30 '17 at 9:42
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I don't understand how the output current would vary when supplying less than 24v to the motor (since the voltage supplied to the motor could be as low as 15v). My intuition is that the current draw would be greater

At lower voltages the DC motor current will tend to be proportionally less unless you anticipate a stall situation.

If the motor spec says 50 mA is the maximum no load current at 24 volts then (ignoring mechanical loads on the armature that you may have), I'd assume it could be typically about 30 mA and, at 15 volts (providing you don't suffer a motor stall), the current will be about 20 mA.

So, if you want CCM operation then aim for a minimum load current of 15 mA to have some degree of margin.

However, a more detailed examination of the motor's data sheet might give typical figures that you can use OR just test the motor on a DC supply.

Alternatively, operate at high enough PWM frequencies so that ripple will be naturally low and just design it for DCM operation.

OR design a synchronous buck converter (more efficient) and let the inductor current flow negative on light loads because you will still have decent ripple using a synchronous buck converter and control will still be quite close to Vout = Vin x duty cycle.

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