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TIDUF99 November 2024

3.5 Input Capacitance

Select input capacitors carefully to both reduce the size and satisfy the big ripple current capability (see the How to select input capacitors for a buck converter analog applications journal).

To get a satisfied MPPT effect, such as 99.5% of maximum power tracking, the input ripple voltage value needs to be small, for many panels, when the V_panel is within 97.5%–102.5% of the V_mpp, the output power of the panel is above 99.5% of maximum power. For most panels, the MPP voltage is higher than 30V. So, 0.75V is taken as the maximum input ripple voltage (ΔV_in).

Figure 3-9 Input Current Waveform

The AC current flowing through the input capacitors results in input voltage ripple. Even the majority of the ripple current goes through MLCC, thanks to the low equivalent series resistance (ESR), ripple voltage results from this can be ignored. The rest of the ripple current goes through the electrolytic capacitor if the system has one, although the electrolytic capacitor has a much bigger ESR, the AC current is small, the overall impact for input voltage ripple is minor.

Use Equation 4 to estimate the required effective capacitance that meets the ripple voltage requirement. At 50% duty cycle, the input capacitance C_in is biggest.

Equation 4.

C_{I N} \geq \frac{D \times (1 - D) \times I_{O}}{∆ V_{i n} \times f_{s w}}

Where I_o is 18A and f_sw is 300kHz, C_in needs to be bigger than 20μF. Considering the DC bias effect of the MLCC as the voltage increases, the actual value taken needs to be larger depending on the practical situation.

Additionally, the input capacitors also need to meet the thermal stress caused by the ripple current, the bigger the footprint, the lower the temperature rise. Use Equation 5 to calculate the root mean square (RMS) current of the input ripple current.

Equation 5.

I_{i n_r m s} = I_{O} \times \sqrt{D \times (1 - D) + \frac{1}{12} \times {(\frac{V_{O}}{L \times f_{s w} \times I_{O}})}^{2} \times {(1 - D)}^{2} \times D}

Duty cycle has a significant impact on the input RMS ripple current. Figure 3-10 is a plot of Input RMS current to Load Current Ratio versus Duty Cycle, from which the largest ripple current RMS can be observed. The largest ripple current occurs when the duty cycle is 0.5. The maximum value of I_{in_rms} is 9.5A. To reduce the temperature rise of the MLCC, the 1210 footprint is chosen. Meanwhile, it is better to parallel multiple capacitors with small capacity than just use one with a bigger capacity.

TIDA-010949 Input RMS, Load Current Ratio
vs Duty Cycle

Figure 3-10 Input RMS, Load Current Ratio vs Duty Cycle

Place additional small MLCCs with low equivalent series inductance (ESL) and low ESR as close as possible to the input side of the FETs, especially using GaN devices with high di/dt and dv/dt slope. These MLCCs can greatly alleviate overshoot of the switching node waveform without sacrificing efficiency.

Bulk capacitors like aluminum electrolytic capacitors can also be added to satisfy the transient response if the response speed of the system is important. Because of the high ESR of electrolytic capacitors, the ripple current can be approximated by dividing the input ripple voltage by the ESR. Also, the waveform is triangular, so the RMS value can be estimated with Equation 6.

Equation 6.

I_{b u l k_r m s} = \frac{1}{2 \sqrt{3}} \times \frac{∆ V_{i n}}{E S R}

Take care when selecting a bulk capacitor due to the low tolerance for RMS current.