Powerful switching power supply, 220/2x50 volts 800 watts. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Power Supplies

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Switching power supplies are widely used in modern electronic equipment. The attention of readers is invited to a switching power supply with a power of 800 W. It differs from those described earlier by the use of field-effect transistors and a transformer with a primary winding with an average output in the converter. The first provides higher efficiency and reduced high-frequency interference, and the second - half the current through the key transistors and eliminates the need for an isolation transformer in their gate circuits.

The disadvantage of such a circuit solution is the high voltage on the halves of the primary winding, which requires the use of transistors with the appropriate allowable voltage. True, unlike a bridge converter, in this case, two transistors are enough instead of four, which slightly simplifies the design and increases the efficiency of the device. In the proposed UPS, a push-pull converter with a transformer is used, the primary winding of which has an average output. It has high efficiency, low ripple and weakly radiates interference into the surrounding space. The author uses it to power a two-channel powered version of the UMZCH.

UPS input voltage - 180...240 V, rated output voltage (with input 220 V) - 2x50 V, maximum load power - 800 W, converter operating frequency - 90 kHz. The schematic diagram of the UPS is shown in fig. 4.47. As you can see, this is a converter with external excitation without output voltage stabilization. At the input of the device, a high-frequency filter C1, L1, C2 is included, which prevents interference from entering the network. After passing it, the mains voltage is rectified by the diode bridge VD1 ... VD4, the ripples are smoothed out by the capacitor C3. Rectified DC voltage (about 310 V) is used to power the high-frequency converter.

(click to enlarge)

The converter control device is made on microcircuits DD1...DD3. It is powered by a separate stabilized source, consisting of a step-down transformer T1, a rectifier VD5 and a voltage regulator on transistors VT1, VT2 and a zener diode VD6. On the elements DD1.1, DD1.2, a master oscillator is assembled that generates pulses with a repetition rate of about 360 kHz. This is followed by a frequency divider by 4, made on the triggers of the DD2 chip. With the help of elements DD3.1, DD3.2 additional pauses are created between pulses. A pause is nothing more than a logical 0 level at the outputs of these elements, which appears when there is a logical 1 level at the outputs of the element DD1.2 and triggers DD2.1 and DD2.2. The low level voltage at the output of DD3.1 (DD3.2) blocks DD1.3 (DD1.4) in the "closed" state (at the output - logic level 1). The duration of the pause is equal to 1/3 of the duration of the voltage pulse at pins 1 DD3.1 and 13 DD3.2, which is quite enough to close the key transistor. From the outputs of the elements DD1.3 and DD1.4, the finally generated pulses are fed to transistor switches (VT5, VT6), which, through resistors R10, R11, control the gates of powerful field-effect transistors VT9, VT10 (see Fig. 4.48).

Pulses from the direct and inverse outputs of the DD2.2 trigger are fed to the inputs of a device made on transistors VT3, VT4, VT7, VT8. Opening in turn, VT3 and VT7, VT4 and VT8 create conditions for the rapid discharge of the input capacitances of the key transistors VT9, VT10, i.e. their fast closing. Resistors of relatively high resistance R9 and R10 are included in the gate circuit of transistors VT10 and VT11. Together with the capacitance of the gates, they form low-pass filters that reduce the level of harmonics when the keys open.

For the same purpose, elements VD9 ... VD12, R16, R17, C12, C13 were introduced. The primary winding of the transformer T9 is included in the drain circuits of transistors VT10, VT2. The output voltage rectifiers are made according to the bridge circuit on VD13...VD20 diodes, which somewhat reduces the efficiency of the device, but significantly (more than five times) reduces the ripple level at the UPS output. It is important to note that the shape of the oscillations, almost rectangular at maximum load, smoothly turns into close to sinusoidal when the power is reduced to 10 ... 20 W, which has a positive effect on the noise level of the UMZCH fed from this unit at low volume. The rectified voltage of the winding IV of the transformer T2 is used to power the fans.

The device uses capacitors K73-17 (C1, C2, C4), K50-17 (C3), MBM (C12, C13), K73-16 (C14 ... C21, C24, C25), K50-35 (C5. ..C7), KM (the rest). Instead of those indicated in the diagram, it is permissible to use microcircuits of the K176, K564 series. Diodes D246 (VD1 ... VD4) are replaceable with any others rated for a forward current of at least 5 A and a reverse voltage of at least 350 V (KD202K, KD202M, KD202R, KD206B, D247B), or a diode rectifier bridge with the same parameters, diodes KD2997A (VD13 ... VD20) - on KD2997B, KD2999B, zener diode D8Yu (VD6) - on D814V. As VT1, you can use any transistors of the KT817, KT819 series, as VT2 ... VT4 and VT5, VT6 - respectively, any of the KT315, KT503, KT3102 and KT361, KT502, KT3107 series, in place of VT9, VT10 - KP707V1, KP707E1 . Transistors KT3102ZH (VT7, VT8) are not recommended to be replaced.

Transformer T1 - TS-10-1 or any other with a secondary voltage of 11 ... 13 V at a load current of at least 150 mA. The coil L1 of the power filter is wound on a ferrite (M2000NM1) ring of size K31x18,5x7 with PEV-1-1.0 wire (2x25 turns), transformer T2 is wound on three ferrite rings glued together of the same brand, but size K45x28x12. Winding I contains 2x42 turns of wire PEV-2-1,0 (wound in two wires), windings II and III - 7 turns each (in five wires PEV-2-0,8), winding IV - 2 turns PEV-2- 0,8. Three layers of insulation made of PTFE tape are laid between the windings.

The magnetic circuits of the chokes L2, L3 are ferrite (1500NMZ) rods with a diameter of 6 and a length of 25 mm (trimmers from B48 armor cores). The windings contain 12 turns of PEV-1-1,5 wire. Transistors VT9, VT10 are installed on heat sinks with fans used to cool Pentium microprocessors (similar nodes from 486 processors are also suitable). Diodes VD13...VD20 are mounted on heat sinks with a surface area of ​​about 200 cm2.

When installing the UPS, you should strive to ensure that all connections are as short as possible, and in the power section use a wire of the largest possible cross-section. It is desirable to enclose the UPS in a metal shield and connect it to the 0 V terminal of the source output, as shown in fig. 4.49.

The common wire of the power unit must not be connected to the screen. Since the UPS is not equipped with a short circuit and overload protection device, 10 A fuses must be included in the power circuit. The described UPS practically does not need to be adjusted. It is only important to correctly phase the halves of the primary winding of the transformer T2. If the parts are in good condition and there are no errors in the installation, the unit starts working immediately after being connected to the network. If necessary, the frequency of the converter is adjusted by selecting the resistor R3. To increase the reliability of the UPS, it is desirable to operate it with a UMZCH, which provides through blowing by a fan.

Author: Semyan A.P.

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