Where to Place Bulk Caps

[rhnrgn]

Hello,

In a motor drive application PCB i have huge 8 Bulk Caps (330uF each) but couldnt decide where to place before or after DC-DC Swicthing converter?

DC motors and MCU are powered after DC-DC convertion.

1_If i place them before DC-DC convertion then start of motors excessive current will do damage to converter.
2_If i place them after converter, when first power up the system converter also consume huge power in order to fill the caps.

What do you think?

Thank you.

 

[barry]

1) Why will excessive current damage the converter? Is the converter not sized properly for its load? Generally, DC-DC converters will have some sort of internal overcurrent protection. If you are worried about starting current for the motor, then by all means put a cap on the input to the motor.

2) That's just a fact of life, and the laws of physics. I=C*dv/dt. You can limit the power-on current with a thermistor. Also, many converters have a soft-start function so that you can limit the inrush to the output caps.

What are you using for a converter?

 

[rhnrgn]

Dear Barry,

Thank you for your answer.

1_I dont have enough time to design system that is why if i did something wrong that will couse lots of time to me. I am just trying to think all possibilities =). Converter is 1 Amps load capacity. There is 16 DC motors each of them 5V 50ma while loaded. This is approximately 800ma. 100ma to digital circuit. Total 900ma each pcb will consume. As you know DC motors consume excessive power while they start motion this is around 1ms. Yes I worry about start current. I think i put half of caps before the converter and other half of them after converter. By the way i am using drv8837 for each DC motor.

2_ Recom R-78C-1.0 https://www.recom-international.com/pdf/Innoline/R-78Cxx-1.0.pdf (If you advice any other converter that will be welcomed =) nowadays i am also trying to find any other converter because this one is really expensive around 9 euros and i am going to use 28 converters in this project)

Question= I am also thinking about adding ferrite beads between motor and motor driver. I hope this will help me to soft start. What do you think? Can you suggest any inductors?

Thank You

 

[barry]

You'll notice that the spec for the Recom converter says it has continuous short-circuit protection, so I don't think you have to worry about blowing it up. Worst case is that you can't supply enough starting current to get the motor moving and it just sits there. What is the STARTING current for the motor?

A ferrite is not going help with soft start. You could do something like putting an RC on the gate of a MOSFET, so that will apply power to the motor slowly.

If you're looking for the most inexpensive solution to your DC-DC converter, you can build your own. There are lots of devices available from TI, Linear Tech, etc.

 

[rhnrgn]

I would add caps to input and output and use inrush current limiters 1A (ICL) or use 100mA with several in // for lower value.

1. Add Polyfuses where needed for SC protection where needed.
2. Adding series esistance to ESR of motor coil, Driver ESr and Power Supply reduces max. torque.
3. While caps with low ESR improve surge handling and improve torque.
4. So ICL allows slow charge, Caps allow quick discharge and Polyfuse allows short cct protection.
5. They can be ganged in parallel like resistors.

ICL's
https://www.digikey.com/product-sea...0&page=1&quantity=10&ptm=0&fid=0&pageSize=250

 

[chuckey]

The MCU should start before the motors are powered, else after a supply glitch, the MCU outputs could be in a random state and drive the motors unexpectedly until the MCU sorts its self out.
I would put a .1 MF disc soldered onto both motor terminals to the motor case, which should be earthed. The sparking from motor brushes are easily radiated from their supply lines and could get into the MCU.
Frank