Mains power supply 5 volts 6 amps with high specific parameters. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Power Supplies

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The article brought to the attention of readers describes a pulse converter for powering electronic devices with a voltage of 5 V from an AC mains. The converter does not contain scarce and expensive elements, it is easy to manufacture and adjust.

The power supply is equipped with protection against output voltage surges and overcurrent with automatic return to operating mode after its elimination.

Main technical parameters

On fig. 1 shows a diagram of the device. The control unit implements the pulse-width principle of output voltage stabilization. On the elements DD1.1, DD1.2, a master oscillator is made, operating at a frequency of about 100 kHz with a duty cycle close to two. Pulses with a duration of about 5 μs through the capacitor C11 are fed to the input of the element DD1.3, and then amplified by the current by the elements DD1.4-DD1.6 connected in parallel. To stabilize the output voltage of the power supply, the pulse duration is reduced during regulation. Transistor VT1 "shortens" the pulses. Opening each period of operation of the generator, it forcibly sets a low level at the input of element DD1.3. This state is maintained until the end of the next period by a discharged capacitor C11.

(click to enlarge)

On transistors VT2, VT3, a powerful current amplifier is made, which provides forced switching of the switching transistor VT4. Voltage diagrams on the main elements of the power source during its startup are shown in fig. 2. When the transistor VT4 is open, the current flowing through it and the winding I of the transformer T1 increases linearly (Fig. 2,6). The pulse voltage from the current sensor R11 through the resistor R7 is supplied to the base of the transistor VT1. To prevent false opening of the transistor, current surges are smoothed out by capacitor C12. For the first few periods after starting, the instantaneous voltage at the base of transistor VT1 remains less than the opening voltage U6e open * 0,7 V (Fig. 2, c). As soon as the instantaneous voltage during the next period reaches the threshold of 0,7 V, the transistor VT1 will open, which, in turn, will lead to the closing of the switching transistor VT4. Thus, the current in the winding I, and hence in the load, cannot exceed a certain value predetermined by the resistance of the resistor R11. This ensures that the power supply is protected from overcurrent.

The phasing of the windings of the transformer T1 is set such that during the open state of the transistor VT4, the diodes VD7 and VD9 are closed by reverse voltage. When the switching transistor closes, the voltage on all windings changes sign and increases until these diodes open. Then the energy accumulated during the pulse in the magnetic field of the transformer T1 is directed to charge the capacitors of the output filter C15-C17 and the capacitor C9. Note that since the phasing of the windings II and III coincides, the voltage across the capacitor C9 in the output voltage stabilization mode is also stabilized regardless of the value of the input voltage of the power source.

The power supply control element is a DA2 KR142EN19A microcircuit. When the voltage at control pin 1 of the microcircuit reaches 2,5 V, a current begins to flow through it and through the emitting diode of the optocoupler, which increases with increasing output voltage. The phototransistor of the optocoupler opens, and the current flowing through the resistors R5, R7 and R11 creates a voltage drop across them, which also increases with the output voltage. The instantaneous voltage at the base of transistor VT1, equal to the sum of the voltage drop across resistor R7 and current sensor R11, cannot exceed 0,7 V. Therefore, with an increase in the current of the phototransistor of the optocoupler, the constant voltage across resistor R7 increases and the amplitude of the pulse component across resistor R11 decreases, which, in turn, occurs only due to a decrease in the duration of the open state of the switching transistor VT4. If the pulse duration decreases, then the “portion” of energy pumped over each period by the transformer T1 to the load is also reduced.

Thus, if the output voltage of the power supply is less than the nominal value, for example, during its startup, the pulse duration and energy transferred to the output are maximum. When the output voltage reaches the nominal level, a feedback signal will appear, as a result of which the pulse duration will decrease to a value at which the output voltage stabilizes. If for some reason the output voltage increases, for example, when the load current suddenly decreases, the feedback signal also increases, and the pulse duration decreases down to zero and the output voltage of the power supply returns to the nominal value.

On the DA1 chip, the converter start-up node is made. Its purpose is to block the operation of the control unit if the supply voltage is less than 7,3 V. This circumstance is due to the fact that the switch - the IRFBE20 field effect transistor - does not fully open when the gate voltage is less than 7 V.

The launch node works as follows. When the power supply is turned on, the capacitor C9 starts charging through the resistor R8. While the voltage across the capacitor is a few volts, the output (pin 3) of the DA1 chip is kept low and the operation of the control unit is blocked. At this moment, the DA1 chip at pin 1 consumes a current of 0,2 mA and the voltage drop across the resistor R1 is about 3 V. After about 0,15 ... 0,25 s, the voltage across the capacitor will reach 10 V, at which the voltage at pin 1 DA1 chip is equal to the threshold value (7,3 V). A high level appears at its output, allowing the operation of the master oscillator and the control unit. The converter starts up. At this time, the control unit is powered by the energy stored in the capacitor C9. The voltage at the output of the converter will begin to increase, which means that it will also increase on winding II during the pause. When it becomes greater than the voltage across the capacitor C9, the diode VD7 will open and the capacitor will continue to be recharged every period from the auxiliary winding II.

Here, however, one should pay attention to an important feature of the power supply. The charging current of the capacitor through the resistor R8, depending on the input voltage of the power source, is 1...1.5 mA, and the consumption of the control unit during operation is 10...12 mA. This means that during startup, the capacitor C9 is discharged. If its voltage drops to the threshold level of the DA1 microcircuit, the control unit will turn off, and since it consumes no more than 0,3 mA in the off state, the voltage across capacitor C9 will increase until it is turned on again. This happens either during overload or with a large capacitive load, when the output voltage does not have time to increase to the nominal value during the starting time of 20 ... 30 ms. In this case, it is necessary to increase the capacitance of the capacitor C9. By the way, this feature of the operation of the control unit allows the power source to be in overload mode for an indefinitely long time, since in this case it operates in a pulsating mode, and the operating time (start-up) is 8 ... 10 times less than the idle time. The switching elements do not even heat up!

Another feature of the power supply is the protection of the load from overvoltage, which occurs, for example, if any element in the feedback circuit fails. In operating mode, the voltage across the capacitor C9 is approximately 10 V and the Zener diode VO1 is closed. In the event of an open circuit in the feedback circuit, the output voltage rises above the nominal value. But along with it, the voltage across the capacitor C9 increases and at a value of about 13 V, the zener diode VD1 opens. The process lasts 50 ... 500 ms, during which the current through the zener diode gradually increases, repeatedly exceeding its maximum value. At the same time, the crystal of the element heats up and melts - the zener diode practically turns into a jumper with resistance from units to several tens of ohms. The voltage across the capacitor C9 is reduced to values ​​that are insufficient to turn on the control unit. The output voltage, having received an increment of 1,3 ... 1,8 times depending on the load current, decreases to zero.

An additional filter is made on the L2C19 elements, which reduces the amplitude of the output voltage ripples.

To reduce the penetration of high-frequency interference into the network, a C1-C3L1C4-C7 filter is installed at the input, which also smoothes the pulsed current consumed during operation at a frequency of 100 Hz.

Thermistor RK1 (TP-10) has a relatively high resistance in the cold state, which limits the inrush current of the converter when turned on and protects the rectifier diodes. During operation, the thermistor heats up, its resistance decreases several times and practically does not affect the efficiency of the power supply.

When the transistor VT4 is closed, a voltage pulse appears on the I winding of the transformer T1 (in Fig. 2, d it is shown by a dotted line in the first three periods of the voltage UcVT4). whose amplitude is determined by the leakage inductance. To reduce it, a VD8R9C14 circuit is installed in the converter. It eliminates the risk of breakdown of the switching transistor and reduces the requirements for the maximum voltage on its drain, which increases the reliability of the converter as a whole.

The power supply is made mainly on standard domestic and imported elements, with the exception of winding products. Inductors L1 and L2 are wound on K10x6x4,5 rings made of permalloy MP 140. The magnetic cores are first insulated with one layer of varnished cloth. Each winding is wound with a PETV wire 0,35 turn to turn in two layers on its half of the ring, and there must be a gap of at least 1 mm between the windings of the inductor L1. The windings of the inductor L1 contain 26 turns each, and the inductor L2 contains seven turns, but each has eight conductors. The wound chokes are impregnated with BF-2 glue and dried at a temperature of about 60°C.

The transformer is the main and most important part of the power supply. The quality of its manufacture depends on the reliability and stability of the converter, its dynamic characteristics and operation in idle and overload modes. The transformer is made on a K17x10x6,5 ring made of permalloy MP140. Before winding, the magnetic core is insulated with two layers of varnished cloth. The wire is laid tightly, but without tension. Each layer of the winding is coated with BF-2 glue, and then wrapped with varnished cloth.

Winding I is wound first. It contains 228 turns of PETV 0,2 ... 0,25 wire, wound round to round in two layers, between which one layer of varnished fabric is laid. The winding is insulated with two layers of varnished cloth. Winding III is wound next. It contains seven turns of PETV 0,5 wire in six conductors distributed evenly around the perimeter of the ring. One layer of varnished fabric is laid on top of it. And finally, winding II is wound last, containing 13 turns of PETV 0,15 ... 0,2 wire in two conductors, which is evenly laid around the perimeter of the ring with some interference to fit tightly to winding III. After that, the finished transformer is wrapped with two layers of varnished cloth, coated on the outside with BF-2 glue and dried at a temperature of 60 ° C.

In place of the VT4 transistor, you can use another one with a permissible drain voltage of at least 800 V and a maximum current of 3 ... 5 A, for example, BUZ80A, KP786A, and in place of the VD8 diode, any high-speed diode with a permissible reverse voltage of at least 800 V and current 1...3 A, for example, FR106.

The power supply is made on a board with dimensions of 95x50 mm and a thickness of 1,5 mm. There are six holes in the corners of the board and in the middle of the long sides, through which the board is screwed to the heat sink. On one side of the board, a VT4 transistor and a VD9 diode are soldered with flanges outward, and on the other, the remaining parts are installed. To reduce the size of the board, all elements, except for the capacitors C8, C9, the DD1 microcircuit, the resistor R9, the transformer and the optocoupler, are installed vertically so that their maximum height above the board does not exceed 20 mm.

The heat sink is connected to the common point of capacitors C1 and C2. In this case, it is better to connect the power supply to a three-prong grounded socket. These measures can significantly reduce the noise emitted by the converter.

The heat sink of the converter is a U-shaped bracket 95 mm long, 60 mm wide and 30 mm high, bent from aluminum sheet with a thickness of at least 2 mm. The converter is installed on the "bottom" of this "trough" with the metal flanges of the VT4 and VD9 elements down and attracted with M0,05 screws through the holes in the board. The flanges are pre-insulated with heat-conducting gaskets, for example, from Noma-con, Bergquist, or, in extreme cases, with mica XNUMX mm thick. Thus, structurally, the transducer is, as it were, in a metal casing that protects it from mechanical impact.

To increase reliability, it is desirable to cover the converter board with 2-3 layers of varnish to eliminate the possibility of breakdown at high ambient humidity.

If all the elements of the power source are in good condition, correctly manufactured and connected in accordance with the diagram, it is not difficult to establish. An oscilloscope is connected in parallel with the resistor R10. A laboratory power supply, for example, B9-5, with a maximum current of no more than 45 ... 15 mA is connected to the capacitor C17 in the appropriate polarity, and the voltage begins to slowly increase, starting from zero. At a voltage of 9,5 ... 10,5 V, a logical unit voltage is set at the output of the DA1 microcircuit, the master oscillator turns on and rectangular pulses with a frequency of approximately 100 kHz and a duty cycle of about 2 should appear on the oscilloscope screen (Fig. 2, a). Further, the voltage should not be increased, because at a value of about 13 V, the zener diode VD1 may open. The current consumed by the control unit must not exceed the specified maximum. If we now reduce the supply voltage, at 7,2 ... 7,6 V, the generation will disappear. This means that the converter control unit is working properly.

Next, a load with a resistance of 4 ... 5 Ohms and a power of 10 ... 15 W is connected to the output of the converter, and voltage is supplied to the input from the second laboratory power supply B5-49, and with the control unit running, the input voltage begins to increase. First, set it at a level of 7 ... 10 V and check with an oscilloscope that the windings of the transformer T1 are connected correctly. In addition, they control the shape of the voltage at the drain of the transistor VT4 (Fig. 2,d), and check the voltage at the output of the converter with a voltmeter. With an input voltage of 150 ... 170 V, the output voltage reaches 5 V and stabilizes. After that, the power supply of the control unit is turned off and continues to work on one input. A further increase in the input voltage should lead to a decrease in the width of the control pulse (Fig. 2, a), which should also be controlled on the resistor R10. Further, at an input voltage of 200 V, the load current is increased (but not more than 7 A) and its value is fixed, at which the output voltage of the converter begins to decrease. If this cannot be done at a current of up to 7 A, the resistance of the resistor R11 is increased. As a result of adjustment, its rating should be set so that at a load current of 6,5 ... 7 A and the minimum allowable input voltage, the output voltage of the converter begins to decrease. This completes the adjustment of the power supply.

If the quality of the winding of the transformer T1 is poor, the voltage “surges” on the transistor \L "4 increase, which can cause unstable operation of the power supply and even breakdown of the switching transistor.

If you need a source with a different output voltage, you must do the following: change the resistance of resistors R13, R14, given that the threshold voltage of the DA2 chip is 2,5 V; change in direct proportion to the number of turns and inversely proportional to the cross section of the conductors of the winding III; select the VD9 diode and capacitors C15-C17, C19 for the appropriate voltage; install resistor R16 with resistance (in ohms) calculated according to the formula R16=100(UBblx-4).

When setting up and working with the converter, remember that its elements are under high voltage, life-threatening. Be attentive and careful!

Author: A. Mironov, Lyubertsy, Moscow Region; Publication: cxem.net

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