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The multiphase buck converter is a circuit topology where basic buck converter circuits are placed in parallel between the input and load. Each of the n "phases" is turned on at equally spaced intervals over the switching period. This circuit is typically used with the synchronous buck topology.
*In such a circuit, only the inductor, transistor and diode "sets" are in parallel, while the input and output capacitor and terminals are common. Each "set" is called a "phase", and the sum of all the "phase" currents is equal to the load current.
*The switches are fired at different time intervals and not simultaneously. The switching cycle is divided into n intervals, and the switchs are fired in their respective intervals. This increases the apparent frequency of the DC-DC converter, without altering the actual frequency.
*An increase in apparent frequency has the advantage that ripple frequency is increased without an increase in switching losses, as the actual frequency stays the same. The increase in ripple frequency means small output capacitor. Also, having n phases reduces the input ripple current, because the load current is Handled by n phases switching seperately. This also means that a smaller input capacitor is needed.
This type of converter can respond to load changes as quickly as if it switched n times faster, without the increase in switching losses that would cause. Thus, it can respond to rapidly changing loads, such as modern microprocessors.
There is also a significant decrease in switching ripple. Not only is there the decrease due to the increased effective frequency,[8] but any time that n times the duty cycle is an integer, the switching ripple goes to 0; the rate at which the inductor current is increasing in the phases which are switched on exactly matches the rate at which it is decreasing in the phases which are switched off.
Another advantage is that the load current is split among the n phases of the multiphase converter. This load splitting allows the heat losses on each of the switches to be spread across a larger area.
This circuit topology is used in computer motherboards to convert the 12 VDC power supply to a lower voltage (around 1 V), suitable for the CPU. Modern CPU power requirements can exceed 200 W,[9] can change very rapidly, and have very tight ripple requirements, less than 10 mV. Typical motherboard power supplies use 3 or 4 phases.
One major challenge inherent in the multiphase converter is ensuring the load current is balanced evenly across the n phases. This current balancing can be performed in a number of ways. Current can be measured "losslessly" by sensing the voltage across the inductor or the lower switch (when it is turned on). This technique is considered lossless because it relies on resistive losses inherent in the buck converter topology. Another technique is to insert a small resistor in the circuit and measure the voltage across it. This approach is more accurate and adjustable, but incurs several costs—space, efficiency and money.
Finally, the current can be measured at the input. Voltage can be measured losslessly, across the upper switch, or using a power resistor, to approximate the current being drawn. This approach is technically more challenging, since switching noise cannot be easily filtered out. However, it is less expensive than emplacing a sense resistor for each phase.
*Marked comments are my own.
Wikipedia: https://en.m.wikipedia.org/wiki/Buck_converter#Multiphase_buck
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