Author: Reno Rossetti

    Step down (step-down) and step-up (step-up) converters are the main products of DC-DC non isolated power conversion industry. They serve very different applications and purposes, so you rarely think of comparison. It may look like comparing apples and oranges. However, performance parameters such as accuracy, efficiency, cost, size and noise are one thing in common. In fact, the two configurations can be compared. For this comparison, we will use Simplis ™ Simulation. For simplicity, we will discuss the ideal operation in the steady state, where the closed-loop correction is negligible and the device can be fully represented as a simplified powertrain driven by the selected duty cycle (e.g. 50%).

    In Figure 1, the buck converter power supply system is represented by a 12V peak to peak square wave generator with 50% duty cycle and 330khz frequency. The LC filter consists of a 15 µ h inductor and a 30 µ f capacitor supplying 1 Ω load.

    Figure 1 Simplified buck converter circuit

    The circuit is simulated with Simplis, and the main waveform is shown in Figure 2. The blue waveform is the delta capacitor current with + / - 300mA peak to peak centered on zero. The red waveform is a 6V centered output voltage with a perfect sinusoidal voltage ripple of + / - 4MV amplitude. The green waveform is the inductance current of 6a, and the ripple is + / - 300mA. Naturally, the ripple current of inductor and capacitor is the same, because the load current is constant.

    Figure 2 Buck converter waveform

    In Fig. 3, the boost converter power supply system is represented by a classic MOSFET diode inductor combination. The MOSFET switches with a duty cycle of 50%, and the input voltage source V3 is set to generate an output voltage of 6V. The output voltage, inductor and capacitor values are the same as those of the step-down converter described above. In both cases, the same LC passive device supplies 6V to 1 Ω.

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     Figure 3 Powertrain boost converter simplification

     However, the presence of rectifier diodes here significantly changes the shape of the electric wave. During the on time of MOSFET Q1 (50% of the cycle), the current flowing to the output is interrupted, and the load current must be completely supplied by the capacitor. This is fundamentally different from the case of step-down converter, in which the capacitor must only compensate the ripple current. The blue curve in Figure 4 shows that the capacitor current is now + / - 6A (relative to + / - 300mA), and there is also a large negative current spike due to the diode bias reversal from + 1V to - 6V. Due to the huge current ripple, the voltage ripple (red curve) is + / - 150mV (relative to + / - 4MV). Finally, the inductor current is centered at 12a, which is twice that of the buck converter, so as to provide 6A current to the load and another 6A current to the capacitor during MOSFET shutdown.



    Figure 4 Boost converter waveform

     Under the same load, the same LC filter must work harder in boost mode than in Buck mode. In the case of boost, the capacitor switches 6A instead of 300mA peak, and the inductor carries 12a instead of average 6A. This means that in order to obtain good performance in practical application, the passive components in the boost converter housing must be larger than the buck converter, resulting in higher BOM cost and PCB size. Higher voltage ripple means lower output voltage accuracy, because the accuracy range provided by the customer is consumed by the voltage ripple. Lower accuracy ultimately means lower efficiency, because the regulator output must be set higher to ensure the minimum voltage required for the normal operation of the load. Due to capacitor ESR and inductor ESL, higher capacitor current ripple and higher inductor current mean higher ohmic loss and lower efficiency. Finally, the switching 6A current in the capacitor will lead to worse EMI performance.

     In conclusion, the performance comparison between buck converter and boost converter shows the advantages of Buck Converter in BOM cost, PCB size, efficiency, accuracy and efficiency Inherent advantages in EMI. On the other hand, if your voltage needs to be boosted, please say goodbye to the buck and welcome to the boost converter, which will become the only game in town.

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