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[moved] Calculate the Input/Output Capacitor Value on DCDC converter

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stoui333

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Hi everyone,

I have a general question on how to calculate the input and out capacitor values for a DCDC converter for filtering/smoothing.

My example:

DCDC Converter: Synqor NQ60W60QTC25NRS
Input Voltage: 36v
Output Voltage: 12v
Load: 5A

Is there any mathematical formulas to find what capacitor values to use?
Is there any rule of thumb?

Also, i have read that for the specific purpose (filtering/smoothing) is better to use a ceramic capacitor instead of an electrolytic, is that right?

Thanks in advance for your answers!
 


Hi,

and of course: frequency..

Klaus
 

Minimum capacitance formulas for DCDC

C= (Vout(peak) x DutyCycle% x Switching Period) / (Vripple x Rload)

or

C= (Duty Cycle x Imax ) / (Vripple x switching frequency)

The voltage rating of capacitor must be smallar than 2 times of input voltage.

since so far both worked for me.

For rectifier capacitor you can use this: C=(0.7 x Imax)/(Vripple x frequency)
(don't ask why 0.7)

ceramic capacitor instead of an electrolytic

As I know so far ceramics have much more smallar capacitances than electrolytics/alluminums...
 

For input and output filter, C value is only the beginning.
You need to keep an eye on input filter ESR for assembly
"wallplug" efficiency (every electron is dragged across the
ESR, twice). Output filter ESR and ESL both add to load-
step deflection beyond what the control loop would ideally
do, and can magnify responses like overshoot (as the loop
will be "wound up" further if the filter C is not "stiff").

I'd recommend you start with a good converter SPICE model
and equally good models of your capacitors, parasitics-
inclusive (and insist on using components for which this
info, either credible SPICE models or the curves to fit your
own, are provided by the mfr; don't be shy about exercising
the applications engineers at vendors, this is what they live
for).
 

From the data sheet:

"Output capacitance: An output capacitor of at least
100uF with an ESR of ~50mOhms is recommended to damp Note that all control signals are referenced to Vsense-, which
the resonance of the output filter. More capacitance can be in this arrangement is at -Vout potential.
added to improve dynamic response to load variation; there
is no upper limit on the value of the output capacitance.
Very large output capacitors may slow the converter's start
up time due to the current required to charge them being
limited by current limit or Isetpoint."

and:

Input filtering: These modules should be connected to a Vin+ +Vin +Vout Vin-,Vout+
low-impedance source. A highly inductive source can affect Cin1 HVNQ Vsense+ Cout
the stability of the module. An input capacitor must be placed On/Off DC-DC Vset
adjacent to the input pins of the module to minimize input Converter Rvset
Vsense-
ripple voltage and ensure stability. See SynQor's application -Vin -Vout
note titled Input System Stability for guidance on selecting Vout-
appropriate input filter elements.

https://www.synqor.com/documents/appnotes/appnt_System_Instability.pdf
 

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