Power supply for portable equipment from the USB port. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Power Supplies
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AC adapters are often used to power portable equipment at home. This is not always convenient, since there is not always a free power outlet at the place of use. And if you need to have two or three different power sources? One solution is to make a universal power supply, if possible, and use, for example, a computer USB port as an external source.
As you know, the standard USB connector provides power for external devices with a voltage of 5 V at a current of about 500 mA. Unfortunately, most portable equipment requires 9 or 12 V to operate. The advent of specialized voltage converter microcircuits greatly simplifies the task of creating power supplies with the necessary parameters.
On fig. Figure 8.1 shows a diagram of a power supply option that allows you to get 9 or 12 V power from a 5 V USB port.
Fig. 8.1
The basis of the circuit is a specialized microcircuit MC34063 (the Russian analogue is K1156EU5). The MC34063 chip is a DC / DC converter control circuit. It contains a thermally compensated voltage reference (RTF), a comparator, a variable duty cycle oscillator, a current limiting circuit, an output stage, and a high current switch.
This chip is specially designed for use in boost, buck and inverting converters with a minimum number of elements.
The voltage received by the boost converter is determined by two resistors - R2 and R3.
The selection of resistors is carried out on the basis that the input of the comparator (pin 5) should have a voltage of 1,25 V. The calculation of the resistor values for the circuit in fig. 8.1 can be produced according to a simple formula:
By setting the output voltage and the value of the resistor R3, you can easily calculate the value of the resistor R2.
The figure of the printed circuit board of the converter is shown in fig. 8.2, the layout of the elements is in fig. 8.3.
Fig. 8.2
Fig. 8.3
Since the output voltage is set by a resistor divider, you can improve the circuit by adding a switch to it, allowing you to receive different voltages as needed. A version of the converter for two output voltages (9 and 12 V) is shown in fig. 8.4.
Fig. 8.4
The figure of the printed circuit board of the converter is shown in fig. 8.5, the layout of the elements is in fig. 8.6.
Fig. 8.5
Fig. 8.6
Resistors used in the device - any, with a power of 0,125 to 0,5 W, type MLT or C2-29, non-polar capacitors - type KD, KM, K10-17, etc. Electrolytic capacitors - type K50-29, K50-35 or similar. The inductance of the inductor L1 is 120-180 μH, with a power dissipation of at least 200 mW. As a choke L2, an integral inductance of the EC24 type or similar is used. The inductance of this inductor is from 10 to 33 uH.
Authors: Bashirov S.R., Bashirov A.S.
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