Ripple current vs Ripple Voltage in capacitor selection

Thread Starter

anishkgt

Joined Mar 21, 2017
529
Hey all,

What type of capacitor is best suited for input filtering. MLCC or Polymer capacitors ? This is for a buck converter ADP1850. Design specs are as follows

Vin: 12V
Vout: 8.1V
iOut(max): 83A
Freq: 220kHz
Duty Cycle: .675
Peak I: 10A
RMS I: 3.16A
L: 1.2uH
Capacitor peak RMS I: 11.7A
Cin, Min: 103uf


This will be used to charge a 3S2P, 8.1V 260F bank of capacitors. On page18 they talk about both ripple current and voltage
The input capacitor needs sufficient ripple current rating to handle the input ripple, as well as an ESR that is low enough to mitigate input voltage ripple.
and shows formula for both MLCC and Ploymer. For the applicaiton, calculated ripple current is about 11.7A. This MLCC is rated 16V and for my application Vout is 8.1V. As per the datasheet temp vs ripple current rating is shown in the range of 0-5A. Does it that mean that is the max rating ?

I am confused if i should go with the Polymer because of the high ripple current but would prefer the MLCC (two in parallel for this application) because of their low ESR. What does insulation resistance of 100 MΩ·μF mean for these capacitors ?
 

Delta prime

Joined Nov 15, 2019
425
MLCC or Polymer capacitors ?
Hello there, :) you seem to know what you're doing just think of this as a reinforcement.
Some of the main advantages of MLCCs are:
Versatility: Ceramic capacitors are available in a very wide range of capacitance values, spanning nine orders of magnitude. They can also be produced with high accuracy and low thermal sensitivity (class 1 devices) or with greater capacitance values but less accuracy and thermal stability (class 2).
Small size: MLCCs are surface-mount chip devices and so are compact compared to most other capacitor types, for a given capacitance and voltage.
Economy: There are no expensive materials required. As a result, ceramic capacitors can, in principle, be produced in large numbers at low cost.
Durability: Ceramic capacitors are made from durable materials, which are unlikely to wear out over time.
Safety: There is no risk of leakage, toxicity or combustion as there can be with liquid electrolytes.
Good electrical characteristics: Ceramics have low parasitic resistance (ESR) and inductance (ESL) – board layout is the largest contributor to the latter in most cases.
As a result, MLCCs are widely used in electronics circuits particularly for decoupling power supplies (this makes up about 70% of the market for MLCCs), filtering noise on input signals, suppressing noise generation and controlling the frequency response of circuits.
Polymer capacitors
Polymer capacitors are a type of electrolytic capacitor.
In traditional electrolytes, one electrode is made of metal and the other is a conductive solution (the electrolyte). Because of this structure, they are polarised, with the metal electrode being the anode (positive) terminal.
The dielectric is formed from a layer of oxide on the metal surface. This is very thin and has a high dielectric constant, meaning that electrolytic capacitors can achieve a high capacitance in a small volume.
In a polymer capacitor, the electrolyte is replaced with a conductive polymer that is deposited on the oxide surface to form the cathode.
Polymer capacitors address some of the issues with liquid electrolytes and also share several characteristics with MLCCs. In particular:
They are more compact than other electrolytic
They’re available in surface mount packages
The absence of liquid means there is no danger of leakage
The capacitance value is stable over time with changing voltage and temperature
They have a lower ESR than other electrolytes and hence high ripple current ratings
An incidental benefit of polymer caps is that they are not sensitive to the piezoelectric effects that can cause ceramic devices to either emit sound as a result of AC voltages across them or to pick up ambient sounds as changes in voltage.
 
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