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  1. #1
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    Capacitors in input voltage circuit before and after reverse polarity protection diod

    Hi,

    Why are capacitors connected right before and after the reverse polarity protection schottky diode in input voltage circuit? What is the use of capacitors in this case? And if i supply 20volts input, how much voltage should be outcome before and after the reverse polarity diode ??

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    Re: Capacitors in input voltage circuit before and after reverse polarity protection

    Hi,

    Show a circuit and give test conditions.

    Klaus
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    Re: Capacitors in input voltage circuit before and after reverse polarity protection

    Because the diode series L, R will "kill" the high frequency
    performance of any upstream capacitors, you place the
    low value high frequency caps inside the reverse blocking
    diode close-in to the chip(s). Otherwise the current loop
    is too large for HF decoupling to be effective.

    I wouldn't bother estimating the Schottky Vf drop with no
    part info given. I've seen Schottkies from 0.3V (Si) up to
    almost 4V (SiC), and Schottkies have an inferior high current
    conductivity modulation (i.e. none) so they have a much
    higher resistance per die size.



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  4. #4
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    Voltage transient in BLDC motor application

    Hi,
    I applied dc input of 12v to my bldc motor since its normal voltage range is 12v. And when i measured the power supply circuit after reverse polarity diode with oscilloscope, i saw voltage transient(spike) when after switched on motor run. Why is it happening? Is it because of back emf from bldc motor? Is it normal if it happens so? Or should i change any of my pcb circuit component?

    Thanks
    Last edited by FvM; 27th September 2019 at 13:57. Reason: Merged with closely related previous thread



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    Re: Capacitors in input voltage circuit before and after reverse polarity protection

    Post #3 is answering your question. You need a sufficient bypass capacitor on the motor side of the diode. Otherwise there's a certain risk to damage the BLDC circuit.



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    Re: Capacitors in input voltage circuit before and after reverse polarity protection

    Hi fvm,
    For which question you are saying to add bypass capacitor??



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    Re: Capacitors in input voltage circuit before and after reverse polarity protection

    The latest question about spikes. You need a bypass capacitor to get rid of it.



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    Re: Capacitors in input voltage circuit before and after reverse polarity protection

    Click image for larger version. 

Name:	volt transients.jpg 
Views:	5 
Size:	489.3 KB 
ID:	155733

    Hi FVm,
    please look at the attached image. This is how my input dc supply is looking when i tested with oscilloscope. The voltage spike is coming when i only run the motor for example like from 2000rpm to 3000rpm, 3000rpm to 4000rpm and like this so on.., . But when i run the motor completely from its 0 to 12000 its max speed, no voltage spikes are appearing and motor is rotating to 12000 from o rpm. Is this the normal behaviour? or we need to add bypass capacitor at motor back emf flow direction?

    - - - Updated - - -



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    Re: Capacitors in input voltage circuit before and after reverse polarity protection

    you have not explained your questions very well and you are asking several questions inside this thread - your graph above may be the result of noise in your measuring set-up - it is impossible to tell - it is likely though, that when the motor reaches 3000 rpm from 2000 rpm the current reduces to maintain the acquired new speed - if you have too little capacitance on the DC bus the voltage will go up - just the same as if you stop it from 4000 rpm with a fast but soft stop.



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  10. #10
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    Re: Capacitors in input voltage circuit before and after reverse polarity protection

    Is it asking too much to get a time reference and correct orientation for a sketched waveform?

    Thanks anyway for qualifying the "voltage transients" a bit. This looks quite clearly like a kind of back emf, circulated energy stored in the motor inductance. Obviously the BLDC isn't designed to be supplied by an unfiltered rectifier. It's assumed that the power source can sink reactive current.

    You didn't tell what your BLDC application is. Consider that energy may be recuperated when breaking the load. If the energy source is a battery, you surely want to charge the battery.



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