Choosing the best operating frequency for your power supply is a complex trade-off process that includes size, efficiency, and cost. In general, low frequency designs tend to be the most efficient, but they are the largest and most costly. Although increasing the frequency can reduce the size and reduce the cost, it will increase the circuit loss. Next, we use a simple step-down power supply to describe these trade-offs.
We start with a filter component. These components occupy most of the power supply volume, while the size of the filter is inversely proportional to the operating frequency. On the other hand, each switching conversion is accompanied by energy loss; the higher the operating frequency, the higher the switching loss and the lower the efficiency. Second, higher frequency operation usually means that smaller component values ​​can be used. Therefore, higher frequency operation can bring significant cost savings.
Figure 1.1 shows the relationship between the frequency and volume of the buck power supply. At a frequency of 100 kHz, the inductor occupies most of the power supply volume (dark blue area). If we assume that the inductor volume is related to its energy, then its volume reduction will be proportional to the frequency. The above assumptions are not optimistic in this case because the core loss of the inductor at a certain frequency is greatly increased and the size is further reduced. If the design uses ceramic capacitors, the output capacitor volume (brown area) will decrease with frequency, ie the required capacitance will decrease. On the other hand, input capacitors are often chosen because of their ripple current rating. This rating does not change significantly with frequency, so its volume (yellow area) can often be kept constant. In addition, the semiconductor portion of the power supply does not change with frequency. Thus, passive devices can occupy most of the power supply volume due to low frequency switching. When we switch to a high operating frequency, the semiconductor (ie, the semiconductor volume, the light blue region) begins to occupy a large proportion of space.
Figure 1.1 The size of the power supply components is mainly occupied by semiconductors.
The graph shows that the semiconductor volume does not vary with frequency in nature, and this relationship may be oversimplified. There are two main types of semiconductor-related losses: conduction losses and switching losses. The conduction losses in the synchronous buck converter are inversely proportional to the die area of ​​the MOSFET. The larger the MOSFET area, the lower its resistance and conduction losses.
The switching loss is related to the speed of the MOSFET switch and how much input and output capacitance the MOSFET has. These are all related to the size of the device. Large volume devices have slower switching speeds and more capacitance. Figure 1.2 shows the relationship between two different operating frequencies (F). The conduction loss (Pcon) is independent of the operating frequency, while the switching losses (PswF1 and PswF2) are proportional to the operating frequency. Therefore, a higher operating frequency (PswF2) will result in higher switching losses. When the switching loss and conduction loss are equal, the total loss per operating frequency is the lowest. In addition, as the operating frequency increases, the total loss will be higher.
Figure 1.2 Increasing the operating frequency results in higher overall losses
However, at higher operating frequencies, the optimal die area is smaller, resulting in cost savings. In fact, at low frequencies, minimizing losses by adjusting the die area results in a very costly design. However, after moving to a higher operating frequency, we can optimize the die area to reduce losses, thereby reducing the semiconductor footprint of the power supply. The downside to this is that if we don't improve semiconductor technology, power efficiency will decrease.
As mentioned earlier, a higher operating frequency reduces the inductor volume; the required inner core plate is reduced. Higher frequencies also reduce the need for output capacitance. With ceramic capacitors, we can use lower capacitance values ​​or less. This helps to reduce the area of ​​the semiconductor die, which in turn reduces costs.
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