The relationship between the nominal voltage and current of the power adapter.power adapter price

　　I believe that power supply designers are not unfamiliar with power adapters, but when we go deeper, do power supply engineers know enough about power adapters? In fact, the discussion about power adapters in major forums is almost the most basic. This article will explain the relationship between the nominal voltage and current of the power adapter for novices.

　　It is not difficult to explain in terms of terms. The nominal voltage of the power adapter usually refers to the open-circuit output voltage, that is, the voltage value without any load and no current output. Therefore, it can also be considered as the upper limit of the output voltage of the power supply. For the case where an active voltage regulator unit or a voltage reference element is used inside the power supply, generally speaking, a DC voltmeter with high internal resistance can be used to directly measure the nominal voltage (more accurately, the method of electromotive force bridge should be used, which belongs to the general physics of colleges and universities). Course experiment, not to repeat), even if the mains voltage fluctuates to a certain extent, its output is a stable and constant value.

　　But this only refers to the formal transformer, and for the cheap small transformers on the market, such as those used for walkmans, it is basically a traditional magnetic core transformer plus four rectifier diode bridge rectifiers plus A large filter capacitor is enough. In this case, the value measured by an ordinary DC voltmeter will be greater than the nominal voltage. The reason is that the output of the bridge rectifier is a pulsating DC, which is simply a positive half cycle of a sinusoidal voltage signal. The connected time chain will become flat after being filtered by a large capacitor, but the ripple coefficient is still large (the ripple coefficient is the ratio of the amplitude of the voltage signal fluctuation to the average voltage, the smaller the voltage, the closer the voltage is to DC), The so-called nominal voltage refers to the time integral of this voltage divided by the integral time. A simple understanding is the average value of time. If it is measured with an ordinary DC voltmeter, the measured value is very close to the maximum value of the voltage signal, so it cannot be measured. allow. If the utility power fluctuates, the output of this type of power supply will also change.

　　The real no-load voltage of an ordinary power adapter is not necessarily exactly the same as the nominal voltage, because the characteristics of the electronic components cannot be completely consistent, so a certain error is allowed, and the civil situation is controlled at about 0.1% to 5% according to the needs of the application. The smaller the error, the higher the requirements for the consistency of electronic components, the higher the cost in industrial production, and of course the more expensive the price.

　　The second is the nominal current value of the power supply. Any power supply has a certain internal resistance, so when the power supply outputs current, a voltage drop will be generated internally, which is equal to the output current multiplied by the internal resistance of the power supply. It causes two things, one is to generate heat, which is equal to the square of the output current multiplied by the internal resistance, so the power supply will heat up, and the other is that the output voltage becomes the nominal voltage minus the internal voltage drop, causing the output voltage to drop.

　　The usual design generally limits a current value after considering the heat dissipation problem. When the output current reaches this value, the output voltage is reduced to 95% of the nominal voltage, or other proportions. Each manufacturer can set it according to the different needs of the load products. Higher or lower ratio, this current value is the nominal current. For example, the nominal current of a 72W ibm16V power adapter is 4.5A. If the load resistance is too low, causing the output current to exceed the nominal current, two things will generally happen. One is that the individual components burn out due to the heat exceeding the heat dissipation capacity, causing the power supply to be damaged. The other is that the heat dissipation design has a margin, which is only reflected as The output voltage is further reduced, and if it is reduced too much, the load may not function properly.

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