Designing low-power SMPS on the LNK501 chip using the VDS program. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Power supply

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The LNK501 chip is very convenient for building switching power supplies up to 5 watts. But the PIXIs Designer program recommended by the manufacturer has limitations that do not allow full use of the capabilities of this microcircuit. The author of the proposed article successfully overcame these limitations - he calculated the pulse transformer using another program - VIPer Design Software, designed for VIPer microcircuits.

Low-power flyback converters (FFCs) are widely used in mobile phone chargers, power supplies for audio players, digital cameras, modems, and various computer peripherals. Modern elements make it possible to make them miniature, circuit-technically simple and cheap.

How to make a switching power supply on the LNK501 chip with a stabilized output voltage of 3 or 5,5 V is described in the article [1]. But when I needed a similar source with a voltage of 12 V and an output current of up to 0,2 A, quite unexpectedly (after all, the power of such a device does not exceed 2,4 W, which is quite consistent with the microcircuit used), a negative result was obtained. It turned out that the specialized PIXIs Designer program mentioned in the article [1], which is designed to calculate a pulse transformer in an OHP, cannot cope with such a task. The program assumes the use with this microcircuit of transformers on ferrite magnetic cores of only three types - EE13, EE16, EE19. When designing the required converter, even in the largest of the listed magnetic cores, ferrite enters saturation (the calculated magnetic induction reaches a value of 0,45 T, which significantly exceeds the allowable 0,38 T). Since the contents of the cells in the program are blocked from editing, the user will not be able to enter the parameters of his own transformer into the calculations. This means that the program does not allow the user to calculate power supplies for standard voltages of 9 and 12 V, which significantly narrows the scope of LNK501 microcircuits.

Fig. 1

Therefore, to design the required SMPS, it was decided to use the VDS - VIPer Design Software program. The magazine "Radio" has already described the use of this program for the design of OCP on ICs of the TOPSwitch-ll series [2]. The experience with LNK501 was also successful. The schematic diagram of the calculated device is shown in fig. 1. It differs from the prototype (Fig. 1 in [1]) by the ratings of some elements and the presence of the HL1 on indicator, which, together with the current-limiting resistor R4, provides a minimum load.

Fig. 2

On fig. Figure 2 shows the load characteristic of the unit at an input voltage of 220 V. A rated resistance load draws 0,17 A at an output voltage of 12 V. In fig. 3 shows the dependence of the output voltage on the input voltage at a nominal load resistance.

Fig. 3

The principle of operation of the OCP on the LNK501 chip is described in detail in the article [1]. The task of designing an SMPS is reduced to the calculation of a pulse transformer. The sequence of such a calculation in the VDS program: set the input voltage interval 176 ... 264 V; select the VIPer53A SHI controller in the DIP8 package, the value of the reflected voltage is 50 V (as in [1]), the switching frequency is 42 kHz; output voltage and current - 12 V and 0,2 A, respectively.

For a pulse transformer, a magnetic core made of ferrite M2000NM1 of size B22 was used, between the halves of which a washer made of non-magnetic material 0,1 mm thick was inserted (the equivalent total non-magnetic gap is 0,2 mm). The primary winding contains 87 turns of PEV-2 wire with a diameter of 0,21 mm. Its measured inductance is 2,62 mH. In the program, a close foreign analogue was chosen - the RM8 magnetic circuit made of ferrite N27. After the forced installation in the program of the measured inductance of the primary winding and the number of turns in it, a quite acceptable result was obtained - at a maximum current of 228 mA, the magnetic induction does not exceed 0,109 T.

In accordance with the passport data of the LNK501 microcircuit, the internal current limitation occurs at the level of 0,24 ... 0,27 A. For reliable and stable operation of the power supply, it is desirable not to increase the current through the microcircuit above 0,24 A at the maximum load current.

Fig. 4

For any combination of input voltage and load current, the transformer operates in intermittent current mode, as recommended by the developers of the LNK501 chip. The results of the calculation using the VDS program showed that with the parameters of the primary winding of the transformer set forcibly, the secondary should contain 22 turns of PEV-2 wire with a diameter of 0,6 mm. There is no coupling winding in the device transformer, so the corresponding program information is not used. The windings are isolated from each other by several layers of varnished fabric. All elements of the power supply (except SA1 and FU1) are mounted on a printed circuit board (Fig. 4) made of one-sided foil-coated fiberglass 1,5 mm thick. The appearance of the board with details is shown in the photo (Fig. 5). During assembly, the halves of the transformer magnetic circuit are tightly attached to the board with an M1 screw through the central hole, the screw is isolated from the magnetic circuit with a getinax washer and a piece of PVC tube put on the screw, the nut on the board is fixed with a drop of nitro paint. There are no acoustic noises in the transformer even without any impregnation. Due to the small external electromagnetic stray fields of the armored magnetic circuit, the short-circuited screen made of copper tape, recommended in [XNUMX], was not used.

Fig. 5

At the adjustment stage, the resistance of the resistor R2 was reduced from the original 20 kΩ to 18 kΩ in order to set the output voltage to 12V at a load current of 0,2 A. Heating of any elements was not noted. This made it possible to install the chip on the board through the DIP8 adapter panel.

Thus, using the VDS program, it is possible to quickly and efficiently complete a low-power OHP project on a suitable magnetic circuit and the LNK501 chip that the radio amateur has. The oscillograms of currents and voltages obtained by the program are close to real ones.

Literature

1. Pletnev E. Small-sized switching power supply based on the LNK501 chip. - Radio, 2006, No. 5, p. 32-34.
2. Kosenko S. Designing flyback SMPS on TOPSwitch-ll using the VDS program. - Radio, 2006, No. 3, p. 30-32.

Author: S. Kosenko, Voronezh; Publication: radioradar.net

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