In the present article describes the switching Converter to power electronic devices voltage of 5 V from the AC. The Converter does not contain scarce and expensive, easy to manufacture and building.
The power supply has protection from shots of the output voltage and overload protection current with automatic return to operation mode after its removal.
Main technical parameters
- Input voltage, V.....150 240...
- Input voltage frequency, Hz...................50...60
- Conversion frequency, kHz.........................100
- Output voltage, V.........5
- The amplitude of the output ripple voltage, mV, not more.....................50
- The load current, And................0...6
- Ambient temperature, °C..............-10...+50
- The static instability of the output voltage when the input voltage, load current and ambient temperature in the full interval, %, of the nominal value, not exceeding.........3
- Dimensions, mm.............60x95x30
In Fig. 1 shows a diagram of the device. The management node implements pulse width the principle of stabilizing the output voltage. On the elements of DD1.1, DD1.2 holds a master oscillator operating at a frequency of about 100 kHz with a duty cycle close to two. Pulses with a duration of about 5 microseconds through a capacitor C11 to the input element DD1.3, and then amplified current elements connected in parallel DD1.4-DD1.6. To stabilize the output voltage of the power source, the pulse duration during regulation decreases. "Shortens" the pulses of the transistor VT1. Opening each period of operation of the generator, it enforces the input element DD1.3 low level. This state is held until the end of the next period is discharged by the capacitor C11.
Transistors VT2, VT3 made a powerful current amplifier that provides rapid switching of the switching transistor VT4. Chart of the voltage on the main elements of the power source during the run 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). Pulse voltage from the current sensor through a resistor R11 R7 is supplied to the base of the transistor VT1. To exclude false opening of the transistor, the emission current of the smoothing capacitor C12. After the first run several periods of the instantaneous voltage on the base of the transistor VT1 is less than the voltage of opening U6э revelation * 0.7 V (Fig. 2, in). As soon as the instantaneous voltage during another period reaches 0.7 V, the transistor VT1 appears that, in turn, will lead to the closing of the switching transistor VT4. Thus, the current in the winding I, and hence in the load may not exceed a certain value, a predetermined resistance of the resistor R11. This prevents the power supply from overcurrent.
The phasing of the windings of the transformer T1 is set such that, at the time of the transistor VT4 diode VD7 and VD9 closed reverse voltage. When the switching transistor is closed, the voltage across all the windings changes sign and increases until such time as these diodes will not open. Then the energy stored during the time of the pulse in the magnetic field of the transformer T1 is sent to the charging of the output filter capacitors C15-C17 and capacitor C9. Note that, since the phasing of the windings II and III are the same, the voltage across the capacitor C9 in the stabilization mode the output voltage is also stable regardless of the value of the input voltage of the power source.
Control element power supply - chip DA2 CREA. When the voltage on the control pin 1 of the chip reaches 2.5 V, through it and through the optocouplers emitting diode current begins to flow, increasing with increasing output voltage. The phototransistor of the optocoupler is opened, and the current flowing through the resistors R5, R7 and R11, creates at which the voltage drop, also increasing with increasing output voltage. The instantaneous voltage on the base of the transistor VT1, is equal to the sum of the voltage drop across the resistor R7 and the current sensor R11 may not exceed 0.7 V. Therefore, when the current of the phototransistor of the optocoupler increases the DC voltage across the resistor R7 and decreases the amplitude of the pulse component of the resistor R11, which, in turn, is only due to the reduction of the duration of the open state of the switching transistor VT4. If the pulse duration is reduced, it is reduced and a portion of the energy pumped each period by the transformer T1 to the load.
Thus, if the output voltage of the power source is less than the nominal value, for example, at the time of its launch, the pulse duration and the energy transferred to the output, max. When the output voltage reaches the nominal level, you will receive the feedback signal, whereby the pulse duration is reduced to the value at which the output voltage is stabilized. If for any reason the output voltage increases, for example, the sudden decrease of the load current, the feedback signal also increases, and the pulse width is reduced down to zero and the output voltage of the power supply returns to the nominal value.
On the chip DA1 made a start node of the Converter. Its purpose - to block the work of the management node, if the supply voltage is less than the 7.3 V. This fact is due to the fact that the switch MOSFETs IRFBE20 is not fully opened when the voltage at the gate less than 7 V.
The triggering unit operates as follows. When you turn on the power source capacitor C9 begins to charge through resistor R8. Until the voltage on the capacitor unit is volts, the output (pin 3) of the chip DA1 is held low and the work of the management node is locked. At this point, DA1 chip on pin 1 consumes a current of 0.2 mA and the voltage drop across the resistor R1 is about 3 V. after Approximately 0,15 0,25...with the voltage on the capacitor reaches 10 V, at which the voltage at pin 1 of the chip DA1 equal to the threshold value (7.3 In). At its output goes high level, allowing the work of the master oscillator and a management node. Start Converter. At this time, the control unit is powered by energy stored in a capacitor C9. The output voltage of the Converter will start to increase, which means that it will increase and in the winding II during pause. When it becomes more voltage on the capacitor C9, the diode VD7 opens and the capacitor will continue each period to charge auxiliary winding II.
Here, however, you should pay attention to an important feature of the power source. 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 - 10... 12 mA. This means that during startup, the capacitor C9 is discharged. If the voltage drops to the threshold level circuits DA1, the control unit switches off, and because in the off state it consumes no more than 0.3 mA, the voltage across the capacitor C9 will increase to re-enable. This happens either when overloaded or when a large capacitive load when the output voltage lags behind the starting time 20...30 MS to increase to the nominal value. In this case, it is necessary to increase the capacitance of the capacitor C9. Incidentally, the above feature of the operation of the control unit enables the power source to be in the overload mode indefinitely, because it works in pulsed mode, and the operation time (run) 8... 10 times less time idle state. The switching elements while not even hot!
Another feature of the power source to protect the load from over-voltage, which occurs, for example, in case of failure of any element in the feedback circuit. In operation the voltage on the capacitor C9 - about 10 V and the Zener diode VO1 is closed. In case of a break in the feedback circuit, the output voltage increases above a nominal value. But increasing the voltage on the capacitor C9 and if the value is about 13 In the Zener diode VD1 open. The process lasts for 50...500 MS, during which the current through the Zener diode increases gradually, greatly exceeding its maximum value. However, the crystal element is heated and melted the Zener diode becomes almost a jumper with resistance from a few to a few tens of ohms. The voltage across the capacitor C9 is reduced to values insufficient to enable the management node. Output the same voltage, receiving, depending on the current load increment 1.3 1.8 times..., decreases to zero.
The elements L2C19 made an additional filter that reduces the amplitude of the output ripple voltage.
To reduce the penetration of high-frequency interference in the network, the input filter is installed C1-C3L1C4-C7, which also smooths consumed during operation of the pulse current with a frequency of 100 Hz.
The thermistor RK1 (TR-10) has a relatively high resistance in the cold state, which limits the starting current of the inverter when turned on, and protects the rectifier diodes. During operation, the thermistor heats up, its resistance decreases several times in the efficiency of the power source virtually no effect.
When closing the transistor VT4 on the winding I of the transformer T1 occurs a voltage pulse (in Fig. 2,g it is shown by dashed lines in the first three periods of the voltage UcVT4). the amplitude of which is determined by the leakage inductance. To decrease, installed in the inverter circuit VD8R9C14. It eliminates the danger of breakdown of the switching transistor and reduces the requirements on the maximum voltage at its drain, which increases the reliability of the transducer.
The power source is made on a standard domestic and imported elements, with the exception of coilers. Inductors L1 and L2 are wound on the rings CHH,5 of permalloy MP 140. The first magnetic isolate one layer of varnished cloth. Each winding wound wire PETV 0,35 round in two layers in their half of the ring, and between the windings of the inductor L1 must be a gap of at least 1 mm Winding of the inductor L1 contain 26 turns, and the inductor L2 seven turns, but in eight conductors each. Wound chokes impregnated with BF-2 and dried at a temperature of about 60°C.
Transformer - the main and the most important detail of the power source. The quality of its production is dependent on the reliability and stability of operation of the Converter, its dynamic characteristics and operation at idle and overload. The transformer is made on the ring CHH,5 of permalloy MP. Before winding the magnetic isolate two layers of varnished cloth. The wire is placed tightly, but without interference. Each layer of the winding glue BF-2, and then wrapped with a varnished cloth.
The first winding is wound I. It contains 228 turns of wire PETV 0,2...0,25 wound coil to a coil in two layers, between which are laid one layer of varnished cloth. Isolate winding two layers of varnished cloth. Following the winding is wound III. It contains seven turns of wire PETV 0.5 in six conductors, distributed evenly around the perimeter of the ring. On top of it lay a single layer of varnished cloth. And finally, the last wound coil II, containing 13 turns of wire PETV 0,15 0,2...in two conductors, which are evenly placed around the perimeter of the ring with some tension for a snug fit to the winding III. After that, the finished transformer is wrapped with two layers of varnished cloth, coat the outside with BF-2 and dried at 60°C.
In place of the transistor VT4 can be applied with other allowable voltage at the drain of not less than 800 V and a maximum current of 3...5 A, for example, BUZ80A, CPA, and in place of the diode VD8 any speed diode with a permissible reverse voltage of 800 V and a current of 1...3 A, for example, FR106.
The power source is made on the circuit Board sizes 95x50 mm and 1,5 mm thick In the corners of the PCB and the midpoints of the long sides there are six openings which cost screwed to the heat sink. On the one side of PCB soldered the transistor VT4 and diode VD9 flanges to the outside, and on the other - installed the rest of the details. To reduce the size of the Board all the elements, except for the capacitors C8, C9, DD1 chip, resistor R9, transformer and optocoupler, mounted vertically to the maximum height above the Board did not exceed 20 mm.
The heat sink is connected to the common point of the capacitors C1 and C2. In this case, the power source it is better to connect a three-prong outlet with a protective grounding connection. These measures can significantly reduce the radiated transmitter interference.
The heat sink Converter The u - shaped bracket length 95, a width of 60 and a height of 30 mm, bent from sheet aluminum with a minimum thickness of 2 mm. Transducer mounted on the bottom of the "trough" of the metal flanges of the elements VT4 and VD9 down and draw screws MOH through the holes in the Board. Flanges pre-insulate thermally conductive strips, for example, firms "Noma-con", "bergkvist, or in the extreme case of mica thickness of 0.05 mm. Thus, structurally, the Converter finds himself in a metal casing that protects it from mechanical damage.
To improve the reliability charge Converter is preferably covered with 2-3 layers of lacquer to eliminate the possibility of breakdown at high humidity environment.
If all elements of the power source is serviceable, properly constructed and connected in accordance with the scheme, in establishing it is not complicated. In parallel with the resistor R10 is connected to the oscilloscope. To the capacitor C9 to connect to the polarity of the laboratory power supply, for example, B5-45, with a set maximum current of no more than 15... 17 mA and slowly increase the voltage, starting from zero. When the voltage of 9.5 10.5...at circuit output DA1 is set to the voltage of the logic unit, the master oscillator is enabled and on the oscilloscope screen should appear rectangular pulses with a frequency of about 100 kHz and a duty cycle of about 2 (Fig. 2,a). Further voltage increase should not be, because when the value of about 13 To may open a Zener diode VD1. The current consumed by the management node must not exceed the stated maximum. If we now reduce the voltage, of 7.2-7.6 In generation will disappear. This means that the management node of the Converter is working properly.
Further to the Converter output connect the load resistance 4...5 Ohms and a power of 10...15 W, and the input set voltage from the second laboratory of the power source B5-49 and when the management node, start to increase the input voltage. First install it on level 7... 10 V and an oscilloscope to check the correctness of connection of the windings of the transformer T1. In addition, controlling the shape of the voltage at the drain of transistor VT4 (Fig. 2,g), and a voltmeter check the voltage at the Converter output. When the input voltage is 150...170 In the output voltage reaches 5 V and stabilized. Thereafter, the power source control unit off and continue to work on a single input. Further increase of the input voltage should lead to a reduction in the width of a control pulse (Fig. 2,a), which should also be monitored in the resistor R10. Further, when the input voltage 200 To increase the load current (but not more than 7 (A) and record the value at which the output voltage begins to decrease. If at currents up to 7 And do not increase the resistance of the resistor R11. As a result of adjusting its value must be set so that, when the load current of 6.5...7 A and the minimum allowable input voltage Converter output voltage begins to decrease. This adjustment of the power source ends.
For winding of the transformer T1 increases "emissions" voltage on the transistor \L"4, which may cause unstable operation of the power source and even breakdown of the switching transistor.
If you need a source with a different output voltage, do the following: change the resistance of the resistors R13, R14, given that the threshold voltage of the chip DA2 equal to 2.5; change is directly proportional to the number of turns and inversely proportional to the cross section of the conductors of the winding III; pick up the diode VD9 and capacitors C15-C17, C19 to the appropriate voltage set resistor R16 to the resistance (in ohms) is calculated by the formula R16=100(UBblx-4).
During the establishment and operation of the inverter remember that its elements are under high stress, life-threatening. Be careful and cautious!
Author: A. Mironov, Lyubertsy, Moscow region; Publication: www.cxem.net