Regarding the selection of power supply filter capacitors

[Benjam]

Ask a question, the DC bus voltage is 48V, so what capacitance should I choose as the filter capacitor?

[gcewing]

More information is needed to answer that question. How much current does the supply need to be able to deliver? What kind of regulator are you using and what is its output voltage?

[Benjam]

Thank you for your reply，I want design one PMSM Motor driver. so the Motor datasheet info as follows:

Code: [Select]

Max Speed:2900RPM
Rated Power:307Watts
Continuous Torque:1.65N-m
Peak Torque:5.37N-m
Continuous Current:8.90amps
Peak Current:29.4amps
Torque Sensitivity0.191N-m/A
Back EMF Constant:11.6V/kRPM
Ambient Temp.:25.0deg. C


I am a novice and I am studying hard. The problem I have now is that I want to design a DCBus circuit. I have read many books and have mentioned in the application that it is necessary to design a filter capacitor. So I’m stuck here. I don’t know what capacitance to choose, because I want the PCB design to be small enough, so I can’t use too big a capacitor.

The bus voltage of the motor is 48V.

The extra information is that the MCU I use is the STM32F4 series combined with DRV8301, then combined with TPS54360B, and LM2940IMP-5.0/NOPB, and then combined with TPS73533DRVR. My intention is to drive the MOSFET through DRV8301 to control U\V\W. , And then TPS54360B reduces 48V to 24V, LM2940IMP reduces 24V to 5V, and TPS73533DRVR reduces 5V to 3.3V. This is what I probably want to do.

I see some design schematics, either directly use a 1000uF capacitor, or use 8 100uF parallel (similar to ODrive), I don't know why they chose this way, I believe their choice must have a reason. So I want to figure out what they are based on? I am stupid. The Internet is full of various materials. I am confused about what I learn. I hope that my friends on EEVBLOG can help me so that my learning path will not become so difficult.

The above information is some information that I can sort out at present. If any information is still lacking, I will try my best to add it. Thank you!

[Terry Bites]

Draw a diagram, if you only have a phone photo and post it. Parallel caps can help to reduce the ESR and keep the hight of your pcb down.  Will a 30A surge cause a dip in your bus voltage? You need to know the minumum input voltage for your buck converter. Will it still work when your motor starts. Where does your 48V come from?

[Benjam]

Hi Terry Thank you for your reply,

This power supply is currently selected by me, but some of its instructions, I don’t understand too much, such as "maximum output current is 10A, so this item will not fit for your device with over 10A current.", its link is:



https://www.amazon.com/elxjar-Adjustable-Converter-110V-220V-Transformer/dp/B07SY7HFJX/ref=sr_1_14?crid=1XGXUKTG807LO&dchild=1&keywords=48v+power+supply&qid=1632500353&sprefix=48v+power%2Caps% 2C448&sr=8-14


I don't know how to choose. Does it mean that the maximum current is 10A, which does not meet the 30A you said?

I am drawing a schematic diagram now, and I will upload it in about 20 minutes.

[Benjam]

Hi Terry,

I found this:

https://www.meanwell.com/webapp/product/search.aspx?prod=UHP-1000

You help me take a look, thank you

[Benjam]

This is a simple schematic that I just drew. I don't know if it is correct or if there is a potential risk.

[Benjam]

This one seems to be more suitable, it has a 250% peak power capability feature.

https://www.meanwell.com/webapp/product/search.aspx?prod=HRP-600N

[Benjam]

I don't know why he has so many capacitors in parallel here.
What is the calculation based on?

[ledtester]

I don't know why he has so many capacitors in parallel here.
What is the calculation based on?

An eevblog episode on this topic:

https://www.eevblog.com/2015/05/09/eevblog-742-why-electrolytic-capacitors-are-connected-in-parallel/

and a EE StackExchange discussion:

https://electronics.stackexchange.com/questions/239764/why-multiple-capacitors-in-parallel

[Terry Bites]

Soft start will relieve you of a lot of PSU sag and energy storage concerns. Mechanical energy stored in the system has to go somwhere when the motor stops. So the Capaciotrs need to mange this as well as smoothing out the rail. https://media.monolithicpower.com/document/AN132_r1.1.pdf?_ga=2.197651951.1720216975.1627312172-71868909.1613146038

The DRV8301 is a PWM controller, so implementing soft start and stall detection for your motor is just a matter of MCU coding.

See https://www.ti.com/lit/pdf/slva552 on dealing with over current and other fault conditions.

[gcewing]

The power supplies being considered already provide a smoothed and regulated output and seem to be designed for driving motors etc., so I'm not sure what purpose the capacitors are meant to serve. Without knowing that, it's impossible to say how big they need to be.

If you're intending the capacitors to supply the startup current surge for the motor, you're going to need some really big ones. From I = C dV/dt, a current of 20A will suck all the charge out of a 1000uF capacitor charged to 48V in 1000uF * 48V / 20A = 2.4 milliseconds.

I don't know how long the motor will take to start up, that depends on what it's driving, but let's say it's a quarter of a second. Also you won't want the capacitors discharged all the way to 0V in that time. Maybe a sag of 12V is acceptable. For that you would need something like 400,000uF.

[Terry Bites]

see this guide to calculating bus capacitance needs https://www.google.co.uk/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&cad=rja&uact=8&ved=2ahUKEwil8JuHoJrzAhUFoVwKHRdqBZwQFnoECAIQAQ&url=https%3A%2F%2Fwww.cde.com%2Fresources%2Fdownloads%2FSelecting-and-applying-DC-Link-bus-capacitors-for-inverter-applications-09-30-20a.pdf&usg=AOvVaw1--qQP9YlrXanLrjGA8VMe

[Benjam]

Thanks to all those who responded and helped me. Your answers have given me a great help. I need to continue to study hard. I will try to read and understand the content of the link address you posted. If there are other things that I don’t understand, I will Continue to consult, thanks to the EEVBLOG forum, which provides an opportunity to exchange and learn

[Jwillis]

There are a few main reasons for the caps . A capacitor is connected across the motor terminals. Capacitor reduces the spikes in the motor current and reduces the magnetic interference. Capacitor smooth the rotation of the motor during the frequent changes in motor load. The capacitor absorbs the back emf of the motor .

Your motor should have a start up energy either on the motor it self or in the datasheet . This is usually in joules . If that isn't available you can work that out by measuring the inrush current or calculating the inrush current by measuring the rest resistance of the coils and the voltage applied . Calculate the watts   1 Watt = 1 Joule second


 Multiply 0.5 times the square of the voltage.  Divide the start-up energy requirement, in joules. This is the capacitance required in Farads .