Switching power supply 220/15 volts 70 watts on a KA2S0880 chip. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Power Supplies

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Bypassing the standard outdated PWM modulators, let's start, perhaps, with more advanced power supply circuits that use power switch switching at zero inductor current, or in foreign terms - off-line switch. Such circuits differ from conventional ones in their very high efficiency, low noise level, and, when choosing the appropriate element base, in their simplicity of design and ease of configuration.

Figure 1 shows a 70W power supply circuit for powering a stereo amplifier within 2x20W. The power converter is built on the KA2S0880 chip, which includes all the necessary components for building the primary part of the power supply. It should be noted that Fairchild Corporation, having developed this microcircuit, did a great job - the microcircuit is very stable in operation and has all the necessary protections. The power supply unit assembled on the basis of this microcircuit has real protection against overload and short circuit, load protection in case of emergency voltage going beyond the permissible limits, and the possibility of introducing a sleep mode. An obvious disadvantage of this circuit is that the unit does not turn on at full load. First you need to turn it on separately, then load it.

Features:

   Supply voltage: 200…240V Output voltage: No load. . . . . . . . . . . . . . . . . ±16,5V at full load. . . . . . . . . . . . . . ±15…±15,5V Maximum long-term output power, also limited by the microcircuit. . . . . . . 70W operating frequency. . . . . . . . . . . . . . . . . 20kHz device efficiency. . . . . . . . . . . . . . . . . 90…93%

The power supply is designed for a symmetrical load, in which the current consumption on the plus and minus sides is equal - low-frequency amplifiers. An uneven load causes overstrain on one of the shoulders and the block can go into protection. When selecting parts, do not forget about the requirements for their parameters and the design of the device. Rectifier diodes must have a reverse voltage of at least 200 Volts, capacitors C11 and C12 are deliberately selected for a voltage of 50 Volts, i.e. large-sized ones - the fact is that they will heat up, at frequencies of about 20-30 kHz they have a minimum impedance at which voltage surges are effectively suppressed, and, as a result, they heat up. Pay attention to the appearance of the components, especially the microcircuit and rectifier diodes - a scratched, nondescript, ugly case indicates either poor-quality manufacturing of the part, or “low-quality” production. Do not use capacitors of the K73-17 series, they often fail. The chip can be produced either by Fairchild or Samsung (SEC)

Circuits that contain transformers are very critical to the phasing of their windings. When phasing the windings, it is required to make sure that the beginnings and ends of the windings are connected to their points in the circuit. If the phasing is incorrect, the windings will run out of phase, which will disrupt the circuit and may damage components. The beginnings of the windings in the diagram are marked with a dot at one of the winding terminals. It's like speakers - the outputs are marked with pluses. It is best for you and me to wind the windings as in Figure 2 - either as option 1 or as option 2, but without mixing these options .

This will make it easier for us to figure out which output will be the beginning and which the end. An example of winding phasing is in Figure 3, the dots show the beginnings of the windings.

The transformer is wound on a Ш12Х12 core made of M2000 ferrite, with a gap in the magnetic core of 0,2 mm. The primary winding is 36 turns, divided into two equal parts. One part is wound into the first layer, the second into the last. Between them there are secondary windings: output - 7+7 turns in two wires each, microcircuit power winding - 7 turns. All windings are wound with wire with a diameter of 0,6 mm. We make a gap using paper, glue it to the ends of the ferrite, put everything together with the coil and glue the magnetic circuit with superglue.

A unit assembled without installation errors starts working immediately and without glitches. However, in order to protect ourselves from possible errors, we will turn on the device for the first time step by step.

Instead of a fuse, we turn on a regular 220V 100W lamp. It will prevent possible damage to the microcircuit. Let's unsolder the zener diodes from the thyristors. To the output of the power supply between “+” and “-” we connect a load - a 30-40 Ohm nichrome spiral with a power of at least 100 W. We will use it only to check the power supply. Such spirals are sold in stores for repairing electric heaters, either the spiral separately or in a glass tube. We only need part of the spiral. We measure the required resistance with a tester and connect it to the output of the power supply. Do not forget that the spiral is connected between the “+” and “-” of the source, and we will measure the voltage from the common wire (GND). Connect the tester to the “+” output of the power supply and plug the unit into a power outlet. After a second, the output voltage should reach +16,5 volts. We wait 5 seconds, turn off the unit and watch the heating of the parts. If there are suspiciously heated elements, do not ignore them!!! Do not forget that you have just assembled a MAINS power supply, which has a “hidden” but powerful destructive force :) If the output voltage is more than 16 volts, for example, 20, 30 volts, then the feedback circuit is not working. This could be either due to errors in the circuit or faulty parts. Will need to check. If the voltage is less than 16 volts and the microcircuit gets very hot in 5 seconds, it means that our secondary windings are incorrectly phased in relation to the primary.

It may turn out that when the unit is connected to the network, there is nothing at the output :( In this case, we will check the voltage on the network capacitor - about 300 volts, the voltage on the third leg of the microcircuit relative to the primary common wire (pin 2). It should jump within 12-15 volts - this microcircuit is trying to start, but something is preventing it.Let's check its feed circuit - the auxiliary winding and its rectifier, winding phasing.If everything is correct, the microcircuit may have gone into protection due to a short circuit in the load, faulty rectifier diodes, overload Turn off the unit and wait for the discharge of the mains capacitor below 30 volts and try to turn it on again with the connected coil not 30-40 ohms, but 50-60.It is also possible that the diodes D 4 and D 5 cannot operate at high frequencies, that is, they are not suitable for this circuit. In this case, the transformer whistles, breaks down, poor thing :( If this doesn’t work out, then let’s remember how many turns we wound and how :). If the voltage at the third pin of the microcircuit goes far beyond 20 volts, for example, 30, 40 volts, then we have too many turns wound on the auxiliary winding, or this winding is again incorrectly phased in relation to the primary.

The next stage is to check the operation of the unit without load. This is checking the feedback circuit for stabilization. It is carried out by an optocoupler. The required output voltage is set by the zener diode D 6, however, it will be one and a half volts higher than the zener diode :) If we measure exactly the required voltage on the spiral, i.e. 15-16 volts, then turn off the load. The voltage should not change, well, a volt and a half doesn’t bother us. We will be ready to immediately disconnect the unit from the outlet if the voltage rises sharply without load, otherwise the rectifier diodes, capacitors and optocoupler can be killed.

Next, we check the load protection when the output voltage is exceeded. The protection is triggered in emergency mode, without attempting to restart the unit. There is protection on both the positive and negative arms, and they work independently, but the effect is common :) The principle of operation is that a short circuit is created at the output, due to which the microcircuit goes into protection. Thyristors have good performance, and in the event of an accident, power is removed from the load in just a couple of milliseconds. If suddenly in the future this circuit works, then you need to check the power supply from the very beginning using the same method. To check, we will forcibly increase the output voltage by several volts. To do this, we will connect another one in series with the zener diode for several volts - 4,7 or 5,1 or 6,2V. We short-circuit it with a jumper and turn on the unit. We measure the output voltage - it is normal. We open the jumper, the transformer should “tick” and the unit should turn off. We wait for the mains capacitor to discharge, put the jumper back on and turn it on. The output voltages should be normal.

If the block worked out all the tests without glitches, then we hang up a load of 15 ohms on it and leave it for half an hour. After this, the device is recognized as fit for service to the fatherland. :)

PCB mounting

The printed circuit board is developed separately for the specific design of the transformer frame and its pinout.

When designing a printed circuit board, the following points must be taken into account:

Do not place interconnected parts far from each other. Pulse currents flow along the tracks, emitting interference into the surrounding space, and the longer the track, the more interference it produces. Maintain sufficient distance between the tracks of the network part. If the voltage between adjacent tracks is 200-300 volts, the distance between them should be at least 4-5mm. Also maintain the distance between the tracks and the parts of the network and secondary parts. The only component we can't do anything with is the optocoupler. It has a distance between the legs of about a centimeter, all other distances between the network and the secondary part must be at least 1 cm. On the secondary side, the trace from the optocoupler should be connected as close as possible to diode D4. To ensure that the trace can handle high currents, it is often filled with solder. But you can’t do this with every track. If possible, let it be wider rather than thicker, otherwise there will be a parasitic connection between the thick traces, which can produce noise at the output and do many other dirty tricks. Capacitors C15, C 16 should be connected closer to the diodes, and not to the electrolytes C11, C 12. VERY IMPORTANT!!!! See figure 4.

The path goes from diode D1 to ceramic capacitor C1, from it to electrolyte C2, from it to coil L1 - that’s correct.
Figure 5 - so wrong.

A track that has multiple elements hanging from it should BYPASS each of them, not go past them. In pulse technology, millimeters of distance are often very important. For example: Figure 6.

If the connection point of the ceramic capacitor C1 is moved 5mm further from the diode D1, the stabilization will deteriorate by half a volt, and the efficiency will drop by 1%.

And here are photos of the assembled prototype:

Publication: radiokot.ru

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