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    5. Non-MLCC Replacement Case: Replacing Polymer Capacitors with MLCC

    Ceramic CapacitorExamples of Problem Solving
    Non-MLCC Replacement Case: Replacing Polymer Capacitors with MLCC

    1. Introduction

    Over 100uF MLCC

    Recently, MLCCs have made progress in increasing their capacitance, and it is now possible to offer over 100uF MLCCs, such as 220uF or 330uF. (Click here for product search of the over 100uF) On the other hand, in servers and base station equipment, the power supplies of CPUs and memory circuits handle large currents, so capacitors of large capacity are required.
    Polymer capacitors are widely used as output capacitors (decoupling capacitors) in the output section of DC-DC converters for smoothing purposes. By replacing these polymer capacitors with large-capacity MLCCs, there are advantages such as downsizing equipment, increasing reliability, and achieving high noise reduction effects.　This content provides an example of the benefits of replacing polymer capacitors with high-capacity MLCCs for the output capacitors used in DC-DC converter test boards.

    Image of replacing polymer capacitors with MLCC

    2. Comparison of Polymer Capacitors and MLCCs

    Compared to polymer capacitors, MLCCs have the following advantages:

    1. Significant reduction in Ripple/Spike noise (refer to Figure 1)
      • MLCCs have lower ESR and ESL compared to polymer capacitors, resulting in a greater reduction effect on output noise.
    2. High reliability and Long service life (See Figure 2)
      • Due to their small ESR compared to polymer capacitors, MLCCs generate less heat from Ripple current.
        Additionally, they have a longer service life compared to polymer capacitors.
    3. Enables downsizing of equipment
      • Due to their smaller size compared to polymer capacitors, MLCCs enable downsizing of equipment.
    (a) Impedance, ESR vs. Frequency
    (b) S21 curve
    • *Polymer Ta: Polymer Tantalum Capacitor
      SWF: Switching Frequency

    S21 is lower than polymer capacitor.
    → Ripple and Spike noise can be further reduced.

    Figure 1 Impedance curve and Insertion loss

    (a) Polymer Capacitor / 1411 size / 100uF
    (b) MLCC / 1206 size / 100uF

    [unit: inch]

    The slope of temperature rise of MLCC is looser than that of polymer capacitor.
    → Long life, Reliability

    Figure 2 Temprature rize curve

    3. Polymer capacitor Replacement evaluation for DC-DC converter

    Circuit with Replacement Evaluation

    The following is the test board and circuit diagram of the DC-DC converter used for replacement evaluation. The polymer capacitors C1 and C2 on the output side in this diagram are the replacement targets.

    figure 3 (a) DC-DC converter circuit
    Figur 3 (b) DC-DC converter test board

    DC-DC Converter Specifications

    • C1, C2: Polymer Capacitor 330uF / 4V / 2917 size
    • Switching Frequency: 400kHz
    • Input voltage: 14V, Output voltage: 1.5V
    • Output current: 30A

    [unit: inch]

    Replacement proposal

    The proposed replacement of polymer capacitor with MLCC is shown.
    After that, the phase compensation circuit constants were adjusted to match the power supply characteristics. (See Figure 4)

    Output Capacitor C1, C2: Polymer Capacitor 330uF / 4V / 2917 size → MLCC 220uF / 4V / 1206 size

    [unit: inch]

    • Since the impedance of the high frequency range is low, it is possible to lower capacity. (See Figure 1)
    • Occupied area can be reduced by 83%!
    C4: 220pF → 22pF Figure 4 Adjustment of Phase compensation circuit

    Evaluation item and Results

    We observed and compared (1) Ripple and Spike noise, (2) Load transient*1, (3) Stability*2, and (4) Power conversion efficiency.

    (1) Ripple and Spike noise

    Figure 5 Ripple / Spike noise

    Ripple is improved by 24%, Spike noise by 16%!

    (2) Load transient

    Figure 6 Load transient

    The Load Transient is equal to or less than its initial state.

    (3) Stability

    Diagram of Gain/Phase vs. Frequency
    Measured item Initial Replacement Criteria
    Phase margin (deg) 60.8 51.9 ≥45
    Gain margin (dB) −8.84 −11.3 ≤−10
    Cross over freq. (kHz) 53.1 72.4 ≤80

    SWF/5=400kHz/5

    Figure 7 Stability

    The Phase margin, Gain margin, and Cross over frequency after replacement meet the stability criteria!

    (4) Efficiency

    Figure 8 Efficiency

    The Efficiency does not change before or after replacement, no problem.

    4. Summary

    In this content, we introduced a case study of replacing output capacitors on a DC-DC converter test board. By replacing the output capacitor from a polymer capacitor to an MLCC with low ESR and low ESL characteristics, it was confirmed that Ripple and Spike noise could be reduced. Additionally, Load transient and efficiency remained equivalent, and stability criteria were satisfied. The occupied area could also be reduced by 83%. This change will also improve the reliability of the capacitors.
    For high-capacity capacitors in the output section of DC-DC converters, we recommend using MLCCs due to their small size, high reliability, and high noise reduction effects. (Click here for product search of the over 100uF)

    • *1Load transient:
      Observe the magnitude of voltage fluctuations caused by changes in the load current.
      In the case of a sudden current increase, the DC-DC converter cannot respond instantaneously. In the meantime, a deficient charge "⊿Q" is generated. At this time, the output capacitor temporarily discharges the charge to catch up with the current increase. The output voltage momentarily drops when the output capacitor discharges. In this evaluation, we are observing this voltage drop ⊿V.
      ⊿痴＝⊿蚕/颁
      If the load current suddenly decreases, the output voltage will rise momentarily in reverse.
    Figure 9 Description of "Load transient"
    • *2Stability:
      In a feed back circuit such as a DC-DC converter, stability is checked by observing the gain and phase characteristics of the feed back loop (blue dashed line in the figure). Changing the output capacitor changes the Gain and Phase characteristics. At this time, if the Phase is delayed and the Gain increases, the power supply circuit will oscillate depending on the condition. If the replacement results in an unstable condition, adjust the phase compensation circuit constant to ensure stability. As evaluation items, there are Phase margin, Gain margin, and Cross over frequency. (See Figure 10)

    Feed back loop:
    Changing the output capacitor changes the Gain and Phase of this loop. Observe the Gain and Phase curves to see if they meet the criteria. See Figure 10 (b), Table 1.

    Phase compensation circuit:
    Adjust these constants so that the feed back loop Gain and Phase satisfy the stability criteria (Table 1).

    Figure 10 (a) DCDC converter circuit

    Figure 10 (b) Gain/Phase vs. frequency (Bode diagram)
    Table 1 The criteria of Stability
    Measured item Criteria
    Phase margin (deg) ≥45
    Gain margin (dB) ≤−10
    Cross over freq. (kHz) SWF/5

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    5. Non-MLCC Replacement Case: Replacing Polymer Capacitors with MLCC
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