Switching power supply on the LX1552 PWM controller. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Switching power supply on the LX1552 PWM controller. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Power Supplies

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The proposed switching power supply (SMPS) is used to power the master oscillator and four independent, galvanically isolated drivers of a powerful power supply unit with a bridge converter. The SMPS is made according to a flyback architecture and has output voltage stabilization and overcurrent protection.

Switching power supply on the LX1552 PWM controller
(click to enlarge)

Purpose and possible replacement of components.

Thermistor RK1 (with a negative temperature coefficient of resistance) is necessary to reduce the inrush current that occurs at the moment the SMPS is turned on and is due to the charge of the electrolytic capacitor G13. The circuit used SCK-472 with an initial resistance of 47 ohms and a maximum operating current of 2 A. It can be changed to similar thermistors B57237-S 220-M (2,8 A, 22 ohms), B57236-S 250-M (2,5 A, 25 ohms), B57236-S 800-M (1,6 A, 80 ohms) or SCK-252R0 (2 A, 25 ohms); Varistor RU1 B72220-S321-K101 from Epcos with a response voltage of 320 V protects the input circuits of the device from overvoltages. It can be exchanged for varistors TVR20471, TVR20621, B72214-S301-K101, B72214-S321-K101, B72220-S301-K101 or B72220-S381-K101; fuse FU1 serves to protect the supply network from short circuit current in case of failure of the SMPS components.

Recommended fuses - VP1-2V, H520RT-2A/250V or. H630RT-2A/250V; the mains rectifier is assembled on a VD1 diode assembly (KBL408, KBL407, RS407 or RS510) and a capacitive rectifier C13, C15. Capacitor C15 with a polyester dielectric shunts the electrolytic capacitor C13 at high frequency; ceramic capacitor C1 filters the reference voltage V REF; elements C2, R4 determine the frequency of pulse generation. By setting the largest value of the duty cycle D and the conversion frequency F (in hertz), using empirical formulas, you can calculate the resistance R4 (if 0D0,95) and the capacitance C2:

C3-R3 is the correction circuit of the error signal amplifier, and R1-R2 is the voltage divider applied to the inverting input of the error signal amplifier; capacitors C4, C5 (Y-class) and C6 (X2-class with polyester dielectric type B81133-C1224-M or. ) together with chokes L81131 and L1474 form an electromagnetic compatibility filter that blocks the propagation of ripples from the SMPS to the supply network. Inductors L81141 and L1334 (81133 mH each) are PLA1474AN32923R2474R1B manufactured by Murata Manufacturing Co. According to the documentation, these chokes have a rated voltage of 2 V and a current of 1 A; capacitors C2, C1,5, C10 and C1522 - ceramic, noise suppression; controller DA0 monitors voltage fluctuations on capacitors C2 and C300 and, by means of pulse-width regulation, returns the voltage applied to them to its original value.

As a result, the constant voltages at the output of the SMPS are also stabilized to some extent, and the winding II of the TV1 transformer plays the role of a group stabilization winding. The device uses a specialized LX15521M controller in a DIP-8 package. The maximum direct output current of the final stage DA1 is 200 mA, the pulse current is 1 A; resistor R6 provides the initial start-up of the master oscillator DA1 (start-up current is approximately 250 μA).

The resistance R6 can be calculated using the formula

(Uc min=90 V - minimum mains voltage, ls=250 µA - start current). For a margin, it is better to take a resistor of a slightly lower resistance; elements VD4 (SF12, can be changed to BYD77D, BYD1100, BYV27-200, SBYV27-200, ES1 B) C9, C14 form an auxiliary rectifier of impulse voltage from winding II TV1, which supplies DA1 in steady state. On the conclusions of VD4 (as well as VD7 ... VD11) you need to put on ferrite beads that replace the damping RC chains; resistor R5, connected in series with the gate of the MOS transistor VT1, reduces the high-frequency parasitic oscillatory process during switching, the protective diode VD2 (1,5KE18CA, P6KE18CA, SMBJ16CA or SMBJ15CA) limits the gate-source voltage of VT1 at the moment of charging its parasitic gate-source capacitances and gate-drain, and the resistor R10 discharges the gate-source capacitance VT1 in the pauses of the trigger voltage pulses from the output DA1.

The current protection circuit is made on C12, R7, R9 and R11.

The non-inductive resistor R11 acts as a shunt, on which a voltage drops proportional to the current through the drain-source VT1. The trimming resistor R9 sets the required sensitivity of the protection circuit. The L-shaped filter C12-R7 eliminates short peaks that occur at the beginning of the pulses, due to the parasitic parameters of the key.

The key MOSFET VT1 is 2SK3550-01R from Fuji Electric (2SK3341-01, 2SK3549-01, STW11NK100Z or STW12NK90Z are also suitable). The transistor has a maximum drain-to-source reverse voltage of 900 V and the highest direct drain current of 10 A (pulsed - 40 A).

The drain-source voltage drop in the open state is 1,08 V. The transistor is installed on the cooler HS113-50 (HS151-50) f. Kinstein Co or similar with thermal paste fit. To protect VT1 from breakdown, a damping chain C16-R8-VD3-VD5 is installed. Resistor R8 - non-inductive, carbon. Protective diode VD3 - 1,5KE250A, it can be replaced with 1.5KE200A, 1.5KE220A or 1.5KE300A, and VD5 type HER508 - with HFA06TB120 or HFA06PB120. Diode VD6 - opposed (HER508, UF3010 or UF5408).

Pulse transformer TV1 has an ETD34 W-shaped magnetic circuit with a round core made of 3F3 material. A non-magnetic gap of 0,8 mm is required in the core. The primary winding I TV1 contains 35 turns of PEV-2, PETV or PETV-2 and is wound in three wires (0,38 mm each), winding II - 6 turns of a single wire 0,27 mm. Windings III, VI should be, if possible, the same. They are wound in three wires (0,32 mm) with 6 turns each. Winding VII contains 5 turns and is also wound in three wires (0,38 mm).

First, about half of the turns of the primary winding are laid on the dielectric frame, three layers of insulation from the Mylar tape are laid, after which the secondary windings are placed, the interwinding insulation is laid again, and then the winding of the primary winding ends.

There must also be insulation between the secondary windings.

After placing all the windings, several layers of fluoroplastic tape are wound and the transformer is assembled. Now, over the windings around all three cores, a shielding short-circuited coil of copper tape is circled, the edges of which are soldered to each other and electrically connected to the cathode of the mains rectifier.

Ultrafast diodes VD7. VD11 brand SF54 rectify the pulses that occur on the windings III, VII TV1. These diodes can be changed to BYW29E-150, BYW80-200 or MUR820.

Ceramic capacitors C17...C21 shunt high frequency electrolytic capacitors C22...C26. Resistors R12 ... R16 discharge capacitors C17 ... C26 after the source is turned off and, in addition, serve as a load of the SMPS.

Fixed resistors up to 2 W used in the power supply can be MLT, OMLT, S2-23 or P1-4 brands. Ceramic capacitors C1 .C3, C9, C12, C17 C21 -K10-17, K10-62, K10-73 or similar.

Setting and adjusting

First of all, the engine of the tuning resistor R9 is set to the extreme right position according to the diagram. After checking the installation and phasing of the TV1 windings, the source is connected to the network through an incandescent lamp (220 V 60 W). It protects the SMPS from failure in the event of installation errors or defective parts. If everything is in order, the lamp does not glow, and constant voltages are present at the outputs of the SMPS. Now, instead of an incandescent lamp, an AC ammeter with a measurement limit of 1.2 A is connected in series with the SMPS, and load equivalents are connected to the outputs of the device.

The current consumed by the SMPS should not exceed 0,7 A.

Using an oscilloscope, make sure that the VT1 gate receives rectangular pulses with a repetition rate of approximately 120 kHz. To accurately set the frequency, you can select the resistance R4 and capacitance C2 within a small range. Then the output voltages of the SMPS are checked and, if necessary, adjusted by selecting the resistance R2.

The final stage is the adjustment of the current protection using the trimming resistor R9, as well as checking the heating of the SMPS components in the long-term mode.

Author: E.Moskatov, Taganrog, Rostov region.

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