Power supply circuit can be divided into linear power supply mode and switching power supply mode according to its working principle.
Linear power supply mode This is the motherboard power supply mode many years ago. It is realized by changing the conduction degree of the transistor, which is equivalent to a variable resistor and connected in series in the power supply circuit. Because the variable resistor flows through the same current as the load, it consumes a lot of energy, which leads to the temperature rise and the voltage conversion efficiency is low. Especially in the power supply circuit that needs large current, linear power supply cannot be used. At present, this power supply mode has long been eliminated.
Switching power supply mode This is a widely used power supply mode. The PWM controller IC chip provides pulse width modulation and sends out pulse signals to turn on the field effect transistors MOSFET 1 and MOSFET2 in turn. Chokes L0 and L 1 are used as energy storage inductors, and together with the connected capacitors, an LC filter circuit is formed.
Its working principle is: when the voltage at both ends of the load (such as the voltage required by CPU) is to be reduced, the external power supply charges the inductor through the switching action of MOSFET to reach the required rated voltage. When the voltage across the load rises, the external power supply is disconnected through the switching effect of MOSFET, and the inductor releases the energy just charged, and then the inductor becomes the power supply to continue to supply power to the load. With the consumption of energy stored in the inductor, the voltage across the load begins to decrease gradually, so it is necessary to charge the external power supply through the switching effect of MOSFET. By analogy, in the process of continuous charging and discharging, it will become a stable voltage and will never increase or decrease the voltage at both ends of the load. This is the biggest advantage of switching power supply. In addition, because the MOSFET works in the switching state, its internal resistance when turned on and leakage current when turned off are very small, so its own power consumption is very small, which avoids the problem that the resistor part of the linear power supply in series in the circuit consumes a lot of energy. This is also the working principle of the so-called "single-phase power supply circuit".
General single-phase power supply can provide the maximum current of 25A, but the commonly used CPU has already exceeded this figure. The power of P4 processor can reach 70-80W, and the working current even reaches 50A. Single-phase power supply can not provide enough reliable power, so now the power supply circuit design of motherboard adopts two-phase or even multi-phase design. (Figure 2) is a schematic diagram of a two-phase power supply. It is easy to understand that two single-phase circuits are connected in parallel, so it can provide double current supply, which is more than enough to meet the needs of the current CPU in theory. But the above is only pure theory, and many practical factors need to be added, such as the performance of switching elements and the resistance of conductors, which are all factors that affect Vcore. In practical application, there are efficiency problems in the power supply, and the electric energy will not be converted into 100%. Generally speaking, the consumed electric energy will be converted into heat and emitted, so any common regulated power supply will always be the hottest part of electrical appliances. It should be noted that the higher the temperature, the lower its efficiency. In this way, if the conversion efficiency of the circuit is not very high, then the circuit using two-phase power supply may not meet the needs of CPU, so there are three-phase or even more power supply circuits. However, it also brings the complexity of motherboard wiring. If the wiring design is unreasonable at this time, it will affect the stability of high frequency work and a series of problems will appear. At present, many mainstream motherboard products in the market use three-phase power supply circuit, which can provide enough power for CPU. However, due to the lack of circuit design, the stability of the motherboard is limited to some extent in extreme cases. If we want to solve this problem, we must make greater efforts in circuit design and wiring, and the cost will rise.