1 kilowatt power supply for ULF. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Power Supplies

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In amateur radio magazines, switching power supply circuits with a power of more than 500 W are not common. Therefore, a switching stabilized power supply was developed with the following parameters:

A schematic diagram of a switching power supply (UPS) is shown in fig. one.

Rice. 1 (click to enlarge)

The circuit is based on the DAI TL494CN chip of the family of controllers with pulse-width modulation. This microcircuit is used in the UPS of computers and has proven itself very well. Consider its operation in the converter circuit in more detail. The TL494CN includes an error amplifier, a built-in variable oscillator, a dead time adjustment comparator, a control trigger, a 5V precision voltage reference (REF), and an output stage control circuit. The error amplifier outputs a common mode voltage in the range of 0,3...2 V. The dead time adjustment comparator has a constant offset that limits the minimum dead time to about 5% of the output pulse width. Independent output drivers on transistors provide the ability to operate the output stage in a common-emitter circuit. The current of the output transistors of the microcircuit is up to 200 mA. TL494CN is operational at a supply voltage of 7 ... 40 V. In fig. 2 shows the switching circuit of the microcircuit and the structural layout of its internal circuits.

Fig. 2

When power is applied, the sawtooth voltage generator 2 and the reference voltage source 5 are started. The sawtooth voltage from the output of generator 2 (Fig. 3a) is supplied to the inverting inputs of comparators 3 and 4. The non-inverting input of comparator 4 receives voltage from error amplifier 1. Since the output There is still no voltage from the power supply at this moment, the feedback signal from the divider R2R4 to the non-inverting input of the error amplifier is zero. The inverting input of this amplifier is supplied with a positive voltage from the divider R5R7, to which the reference voltage Uop from the ION output is already connected. The output voltage of error amplifier 1 is initially zero, but as the voltage in the feedback circuit from the divider R2R4 increases, it increases.

The voltage at the output of the error amplifier also increases. Therefore, the output voltage of comparator 4 has the form of a sequence of pulses increasing in width (Fig. 3,6). The non-inverting input of comparator 3, which provides a pause, is connected to pin 4 of the microcircuit. This pin is supplied with voltage from an external RC circuit C2R3 connected to the reference voltage bus Uorr. When the reference voltage appears, it is applied to this circuit.

As capacitor C2 is charged, the current through it and resistor R3 decreases: the voltage Uop on resistor R3 has the form of a falling exponential (Fig. 3, c) The output voltage of comparator 3 is a sequence of pulses decreasing in width (Fig. 3, d) From the diagram output voltages of comparators 3 and 4 (Fig. 3,6, d) it can be seen that they are mutually opposite. The output voltages of comparators 3 and 4 are input to the “2OR” logic element. Therefore, the pulse width at the output of the logic element is determined by the widest input pulse.

The output voltage of the "2OR" element is shown in Fig. 3d, from which it follows that at the initial moment of time the width of the output pulses of comparator 3 exceeds the width of the output pulses of comparator 4, therefore switching comparator 4 does not affect the width of the output pulse of the “2OR” element. In the time interval (t0; t1) (Fig. 3a), the decisive role is played by the output voltage of comparator 3. In Fig. 3, f, g shows the output pulses on the collectors of transistors VT1, VT2. The width of these pulses gradually increases in the interval (t0; t1). At time t1, the output pulse of comparator 3 is compared with the output pulse of comparator 4. In this case, control of the “2OR” logical element is transferred from comparator 3 to comparator 4, since its output pulses begin to exceed the width of the output pulses of comparator 3. Thus, in the period of time (t0; t1) output pulses on the collectors of transistors VT1, VT2 smoothly increase and ensure a smooth start of the voltage converter.

Fig. 3

Before each UPS is turned on, capacitor C2 (Fig. 2), which ensures a smooth start, must be discharged. It's time to turn to the general diagram of Fig. 1 voltage converter. The function of the soft start capacitor in it is performed by capacitor C3. When the power is removed, the capacitor quickly discharges through resistor R1, the base-collector junction of transistor VT1 and diode VD1. Transistors VT1, VT2 perform the function of trigger protection. When an unlocking voltage is applied to the base of transistor VT2, it opens. At the same time, transistor VT1 opens, shunting capacitor C3 and thus blocking the operation of the voltage converter. The voltage from the collector of transistor VT1 through the circuit R4VD2 keeps transistor VT2 open. Trigger protection is turned off only after the supply voltage is removed. Powerful field-effect transistors with a fairly large gate-source capacitance are used as power switches. Therefore, to control these transistors, two blocks of switches are used on transistors VT3, VT5, VT7 and VT4, VT6, VT8.

Consider the work of one of them. When a high voltage is present at pin 8 of the DAI chip (the transistor inside the chip is closed), field-effect transistors VT3 and VT7 open. The latter shunts the gate capacitance of the transistor VT9, instantly discharging it. Transistor VT5 is closed. As soon as a low voltage is established at pin 8 of the microcircuit, transistors VT3 and VT7 will close, and VT5 will open and an unlocking voltage will be applied to the gate of transistor VT9. Resistor R18 prevents the failure of transistors VT5, VT7 if one of them is closed and the other is not fully open.

Voltage oscillograms at the gates of transistors VT9, VT10 are shown in Fig. 3,3, i. The gate circuits of transistors VT9, VT10 include resistors R20, R21, which together with the gate capacitances form a low-pass filter that reduces the level of harmonics when the keys are opened. Circuits R22, R23, C8, C9, VD5-VD8 also serve to reduce harmonics during converter operation. The primary winding of transformer T1 is connected to the drain circuits of transistors VT9, VT10. To stabilize the converter voltage, the feedback voltage is removed from winding III of the transformer. Through a divider on resistors R7, R8 it goes to the DA1 chip. Resistor R10 can be used to regulate the output voltage of the UPS within small limits. Elements R6, C4 determine the operating frequency of the internal sawtooth voltage generator of the DA1 microcircuit (with the ratings indicated in the diagram, this frequency is close to 50 kHz).

By changing the resistance of the resistor R6 and the capacitance of the capacitor C4, it is possible, if necessary, to change the frequency of the voltage converter. The power part of the circuit is fed through the mains filter C10, Cl1, L1, rectifier VD4 and capacitors C12, C13. Resistor R24 ​​discharges the filter capacitor in the off converter. Chip DA1 and keys on transistors VT3-VT8 are powered by a stabilized power supply on the elements T2, VD3, C5-C7 and stabilizer DA2. Resistor R25 serves to reduce the inrush current through the filter capacitors when the UPS is connected to the network. The converter output voltage rectifier is made according to the bridge circuit on VD12-VD15 diodes.

The smooth start of the voltage converter allows the use of filter capacitors of fairly large capacity in the secondary circuits, which is necessary when powering a power amplifier. Chokes L2, L3, together with filter capacitors, smooth out the ripples in the UPS output voltage. The protection of the voltage-to-flow converter is made using transistors VT11, VT12. As the current through resistors R27-R30 increases, transistors VT11, VT12 open and the LEDs in optocouplers Ul.l, U1.2 light up. The transistors of the optocouplers open and supply an unlocking voltage to the base of the transistor VT2, which causes the trigger protection to operate. Capacitor C1 prevents the protection against random impulse noise from triggering.

Construction and details

Structurally, the UPS is made on a single-sided printed circuit board (Fig. 4a, b).

Rice. 4 a, b (click to enlarge)

All circuit elements are located on the board, except SA1, FU1 and T2. Also on a separate small board are resistors R22, R23 and capacitors C8, C9. They are connected by wires to the main board at the points indicated by the letters a, b, c. Resistors R22, R23 get very hot during operation, so the board with them should be positioned so that the resistors do not heat up the rest of the circuit elements. Diodes VD12-VD15 are mounted on a separate needle radiator 10x12 cm and connected to the main board with a wire with a diameter of at least 1 mm. On one side of the printed circuit board there is a radiator (Fig. 4,6) 170 cm long and 10 cm high.

It is advisable to use a needle radiator, but in a pinch, any other will do. Board elements DA2, VD4, VT9, VT10 are attached to this radiator through insulating gaskets. A fan is installed on the opposite side of the radiator so that the air flow from it blows well over the radiator. You can use a fan from a computer power supply. Power is supplied to it through a resistor with a resistance of 320 Ohms and a power of 7,5 W from the +50 V output of the converter. You can use a PEV type resistor and attach it anywhere in the body. It is also possible to wind an additional winding in transformer T1 to power the fan (Fig. 1). To do this, you will need to wind two turns of wire with a diameter of 0,4 mm and connect the fan according to Fig. 5.

Fig. 5

The transformer T1 of the converter is wound on four 2000NM ferrite rings folded together with dimensions K45x28x12. Winding data of the transformer are given in the table.

The windings I and II of the transformer are separated from the rest of the windings by two or three layers of varnished cloth. Transformer T2 is used ready-made with an alternating voltage of 16 V. Coil L1 consists of 2x20 turns wound on a ferrite ring made of 2000NM ferrite with dimensions KZ1x18x7 in two wires with a diameter of 1 mm. Coils L2, L3 are wound on pieces of ferrite with a diameter of 8 ... 10 mm and a length of about 25 mm with a wire with a diameter of 1,2 mm in one layer along the entire length of the ferrite. In the converter circuit, it is desirable to use imported electrolytic capacitors with a mark of 105 °. In extreme cases, it is permissible to use other capacitors that are suitable in size. Capacitor C12 is made up of three capacitors with a capacity of 220 uFx400 V.

Non-electrolytic capacitors of any type, for example K73-17. As a resistor R25, three resistors of the SCK105 type or similar, connected in parallel, used in computer power supplies, are used. Resistors R22, R23 type C5-5-10W, R27-R30 - C5-16V-5W. The remaining resistors are of any type, for example MLT. Trimmer resistor R9 type SPZ-19AV or other small-sized. It is desirable to use high-frequency diodes as indicated in the diagram (KD212 and KD2999), since imported diodes, which are now widely used, do not always work well at high frequencies, especially above 50 kHz.

Diode bridges can be used in any suitable size: VD3 - with a rectified current of at least 500 mA; VD4 - with a rectified current of at least 8 A and a voltage of at least 400 V. BSS88 transistors can be replaced with other similar field-effect transistors with an insulated gate and n-channel (drain-source voltage more than 50 V, drain current 0,15 ... 0,5, 123 A). These can be BSS108, BS2, 1336SK2, etc. transistors. Instead of powerful field-effect transistors 956SK2, transistors of types 787SK50, IRFPE494 are suitable. The TL494CN chip can be replaced by the TL25LN chip, which will allow the voltage converter to be used at ambient temperatures down to -494 ° C, since the TL0CN is only operable at temperatures above 7500 ° C. Also, instead of it, you can use the analog KA101V. Optocoupler AOT101BS can be replaced by AOT2501AC, PS2-2. KR142EN8E or 7815 can be used as a DA7815 chip. If the 502 chip is used in an insulated case, an insulating gasket is not required when installing it on a radiator. Transistors KT503E, KT502E can be replaced by KT503G, KT510G, and diodes KD503A - with almost any pulse diodes, for example, KD522, KDXNUMX, etc.

Setting

Before turning on the converter for the first time in the network, it is necessary to remove the mains voltage from the power circuits and apply power only to the T2 transformer. First of all, make sure that the supply voltage is +15 V from the DA2 output. Then, using an oscilloscope, they make sure that there are pulses at the gates of field-effect transistors VT9, VT10 and that they correspond to the oscillograms of Fig. 3, and. When the capacitor C9 is short-circuited, the pulses should disappear, and zero voltage should be set on the gates VT10, VT9. Further, setting the slider of the resistor RXNUMX to the middle position, the supply voltage is applied to the rest of the circuit.

Using a voltmeter, control the voltage at pin 1 of DA1, setting the value to 2,5 V by selecting the resistance of resistor R7. The trimmer resistor R9 can slightly change the output voltage of the converter, however, it is necessary to control the pulses at the gates of field-effect transistors VT9, VT10 so that their duration does not approach the extreme limits (too short or too long), but is in the middle position. Otherwise, with an increase in load or a change in the supply voltage, the stabilization of the output voltage will deteriorate.

In order not to overload the voltage converter and not burn out powerful field-effect transistors, it is better to set the current protection as follows. Instead of resistors R27-R30, resistors with a resistance of 1 ohm and a power of 2 watts are temporarily soldered. A load and an ammeter are connected to the output of the converter. The load current is set to 1,3 ... 1,4 A and by selecting the resistances of the resistors R32, R33, the current protection is activated. Then the resistors R27-R30 are soldered in place. This completes the setting of the voltage converter. If a different voltage is required to power the amplifier or some other load, then the output voltage of the converter can be changed by changing the number of turns of the windings IV and V of the transformer T1. It should be borne in mind that one turn of the secondary winding accounts for about 7 V.

Based on materials from Radioamator magazine; Publication: cxem.net

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