Flyback switching power supply

Encyclopedia of radio electronics and electrical engineering / Power Supplies

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The article describes a switching power supply controlled by a specialized microcircuit. The device uses a standard pulse transformer from television units.

It would seem that power sources (PS), which are integral elements of absolutely all electronic devices, should be the least susceptible to rapid changes - after all, they have existed for more than half a century. But the development of modern circuit solutions does not bypass this most extensive area of ​​radio electronics.

Initially, traditional battery power supplies were replaced by network lamp power supplies with LC filters, and then by transistor and integrated linear regulators.

The struggle for efficiency and improvement of weight and size indicators contributed to the development and implementation of pulsed IP (SMPS). Along with half- and full-bridge SMPSs, flyback sources are widely used, since without special measures the danger of through current in bridge SMPSs (due to the supply of an opening voltage to one of the arms, when the other arm has not yet completely closed due to its inertial properties) led to the operation of switching elements in the short circuit mode and the failure of expensive high-voltage high-voltage transistors. These special measures greatly complicated the bridge SMPS, and therefore flyback SMPS became more widespread in household appliances, in which the switching transistor in the first cycle ensures the accumulation of electromagnetic energy in the windings and magnetic circuit of the storage transformer, and in the second - its transfer to the load. To be convinced of the relative complexity of such SMPS, just look at the circuits of the MP-403 power supply module for ZUSST, 4USTST TVs or the KRP-501 scanning and power cassette for 5USTST TVs.

And only the latest developments of Siemens specialists and domestic manufacturers, who created a microcircuit for controlling flyback SMPS TDA4605 (the domestic analogue of KR1033EU5 - the so-called PWM controller), greatly simplified the task of developing highly reliable and economical SMPS for radio amateurs. Although the handbook [1], containing information on PWM controllers, is not free from some errors, it should be noted that it is of great value for the designer and developer of the SMPS. The manual [2] describes the operation of the SMPS in 6th generation TV sets using the domestic KR1033EU5 chip, but there is no reference information (voltage values, signal waveforms) characterizing its operation. Unfortunately, none of the mentioned sources gives the winding parameters of the storage transformer. Nevertheless, using the available reference characteristics, for amateur radio purposes, it is always possible to adapt existing pulse transformers to create the necessary modern SMPS. The materials of the published article will help to solve this problem, they may also be useful for radio amateurs involved in the modernization and repair of domestic and imported video equipment.

The service functions performed by the microcircuit are very extensive:

• "soft" launch of the SMPS with a smooth increase in the duration of the starting pulses;
• stabilization of the output voltage in the operating mode by controlling the duration of switching pulses;
• "binding" the beginning of switching pulses to the moment of completion of the transfer of stored energy to the load;
• SMPS protection in idle and load short circuits;
• shutdown of the SMPS when the mains voltage fluctuates above the established limits;
• suppression of parasitic oscillatory processes initiated by a storage transformer;
• thermal protection (switching off the microcircuit at a case temperature above +125 °C).

The functional purpose of the pins of the microcircuit is given in Table. one.

Table 1

Pin number	Functional purpose
1	Control amplifier input
2	Signal input that simulates a change in current in the storage winding of a transformer
3	Mains voltage divider input for switching off switching pulses when it deviates from the permissible value
4	General conclusion
5	Switching pulse output
6	Chip supply voltage
7	Input for connecting an integrating capacitor that controls the duration of pulses during the "soft" start of the SMPS
8	Signal input for detecting the zero crossing of the pulse in the secondary winding

Main characteristics

Supply voltage	7,5 13 ...
Switching frequency of switching impulses, kHz, no more	165
SMPS power (determined by the type of magnetic circuit and the parameters of the switching transistor), W, not more than	250
Control amplifier input voltage, V	0,37 0,43 ...
Switching voltage change limits for recharging the gate-source capacitance of the switching MOSFET, V	-0,3 ... + 6
Limits of change of switching current, A	-1,5 ... + 1,5

The control amplifier is the main element of the microcircuit. Receiving a signal from the additional winding of the transformer and comparing it with the internal reference voltage, it generates switching pulses of various durations, which is determined by the values ​​of the current in the load and the rectified mains voltage. The duration of the pulses is changed in such a way as to maintain a constant voltage at the output of the SMPS.

The main element of the SMPS is a storage pulse transformer, which, in principle, can be anything. A wide range of output voltage regulation provided by the microcircuit, as well as a large set of output windings of the transformer, facilitate the task of creating an IP with the necessary parameters. It is advisable, for example, to consider the use of the TPI-8-1 pulse transformer, described earlier in the pages of the Radio magazine [3].

The scheme of the SMPS, created according to the materials [1,2, 1] and adapted to the use of the specified transformer, is shown in fig. 4 (unused transformer windings are not shown, pins 10 and XNUMX were absent initially).

The device contains an interference suppression filter that prevents high-frequency interference from penetrating into the supply network (L1, C1-C3); current-limiting resistor that limits the inrush current when the SMPS is turned on (R1); bridge rectifier mains voltage (VD1); voltage divider in the feedback circuit of the control amplifier of the microcircuit, which forms the level of stabilization of the output voltage of the SMPS (R2, R6, R7, VD2); filter in the power supply circuit of the SMPS, which reduces the level of input voltage ripple (C4); voltage divider to control changes in the mains voltage and turn off the SMPS in case of unacceptable fluctuations (R3, R4); sawtooth voltage shaper to simulate current changes in the storage windings of a pulse transformer (R5, C5); pulse shaper in the feedback signal circuit (VD3, C6); integrating capacitor in the control circuit for the "soft" start of the SMPS (C7); filter capacitor in the power circuit of the microcircuit (C8); current-limiting resistor in the start-up mode of the microcircuit before it enters the operating mode (R8); voltage rectifier that feeds the microcircuit from the communication winding (II) of the transformer in operating mode (VD4); pulse supply circuit for controlling the switching transistor (R9-R11, VD5); circuit for limiting peak voltage surges at the drain of the transistor (VD6, R12, C10); damping circuit to eliminate parasitic vibrations (C11, R13); noise suppression filter in the circuit for determining the beginning of the cycle of generating a switching pulse (transition of the output voltage pulse through zero) and the feedback circuit of the control amplifier (R14, C9, R15, C12); rectifier and output voltage filter (VD7, C13); current-limiting resistor in the output voltage circuit (R16).

The results of testing the device with different output windings and ratings of the elements used, shown in the diagram, to obtain an output voltage of 12 V at a load current of 1,25 A are shown in Table. 2.

Table 2

Winding III	Voltage, V
7-11	12,5
16-20	8
3-5	2,2
8-12	10
6-12	55

To select the output winding, use the table. 3, containing the parameters of copper winding wires, which are most often used in pulse transformers. Winding III, designed for a voltage of 24 V for "normal" use, contains 16 turns of three PEVTL-0,35 conductors connected in parallel. Their total cross section is about 0,3 mm²and equivalent to a conductor with a diameter of 0,62 mm. For current density 4,25 A/mm², corresponding to an increase in the temperature of the transformer by 30 ° C, the permissible current in the winding is 1,28 A, which fully satisfies the requirements (using the calculator, it is easy to continue the nomenclature of conductors in the direction of increasing and decreasing the diameter). If you use the windings V and VI (terminals 14, 18 and 16, 20, respectively [3]), connecting them in parallel, you can get a current of up to 3,5 A at the output of the SMPS.

Table 3

Insulated wire diameter, mm	Copper diameter, mm	Allowable current, A
0,41	0,35	0,41
0,44	0,38	0,48
0,47	0,41	0,56
0,5	0,44	0,65
0,53	0,47	0,74
0,55	0,49	0,8
0,58	0,51	0,87
0,6	0,53	0,94
0,62	0,55	1,01
0,64	0,57	1,08
0,66	0,59	1,16
0,69	0,62	1,28
0,72	0,64	1,37
0,75	0,67	1,5
0,77	0,69	1,59
0,8	0,72	1,73

As in the MP-403 power supply module, the storage winding is the I winding (pins 1, 19). Particular attention should be paid to the correct connection (phasing) of the leads (usually in the diagrams, the beginning of the winding is always indicated by a dot). The pin numbers of the additional winding for communication and power supply of the microcircuit are shown in Fig. 1.

It should be borne in mind that the operating current in the coupling winding depends on the total load power and does not necessarily reach a maximum value of 1,5 A. When evaluating the operating voltage of the windings, it must be remembered: the proportional relationship between the number of turns and the voltage is observed only for the secondary windings and does not apply to primary winding, since they operate in different half-cycles (cycles) of the pulse voltage, and the ratio between their operating voltages will depend on the duty cycle of the switching pulses.

The load equivalent during the adjustment is three PEVT-25 resistors connected in parallel with a resistance of 30 ohms each. Before applying the mains voltage, it is necessary to include a 1 A ammeter in the open circuit between points A and B (Fig. 0,5). load current or voltage on the secondary windings), but also as a reliable indicator of the on state of a silently operating SMPS. This will prevent accidental electric shock when making adjustments.

It is also useful to check the serviceability of the switching transistor by assembling the simplest measuring circuit according to the circuit in Fig. 2 (the pinout of the field-effect transistors KP707V2, KP812B1 and their foreign counterparts IRFBC30, IRFBC40, BUZ90A, 2SK1221, etc. is also shown there). Increasing the transistor gate voltage in steps of 0,1 V, make sure that, starting from the threshold voltage (1 ... 5 V, depending on the type and parameters of the transistor), the current in the drain circuit smoothly increases and reaches 500 μA after about In after opening. It is better to use power supplies with current protection preset at 0,5 mA. This will prevent damage to transistors even with connection errors due to their unknown pinout.

After carrying out these preparatory measures, the tuning resistor R7 should be set to the middle position and connected to the SMPS network. During the adjustment, it is better to place the device on the desktop with the elements down: then the printed circuit board will protect against injury during a possible explosion of oxide capacitors as a result of excess voltage due to incorrect connection of the windings.

If the voltage in the secondary windings is insufficient for the SMPS to enter the operating mode, characteristic clicks of the transformer will be heard along with a high tone ("click"), due to the periodic switching on of the starting mode as the voltage across the capacitor C8 increases to the threshold value. In the process of establishing an SMPS, first of all, it is necessary to check the influence of the position of the movable contact of the tuning resistor R7 on the parameters of the output pulses.

You should be very careful when choosing the parameters of the elements of the sawtooth voltage formation circuit (R5, C5), which determines the maximum duration of the open state of the switching transistor. The voltage on the capacitor C5 in the microcircuit is compared with the voltage at the input of the control amplifier, and the switching pulse stops when they match. If these elements are chosen incorrectly, at the moment the SMPS is disconnected from the network, the decrease in voltage at the output of the mains power filter will be compensated by an increase in the duration of the switching pulses and an excess of the allowable value of the drain current of the transistor, which will damage it.

In the process of establishing, to connect the SMPS to the network, reliable switching elements (toggle switches, switches, and not mains plugs and sockets) should be used, since the resulting contact bounce can cause the switching transistor to fail.

After the adjustment is completed, the device must confidently enter the operating mode, as evidenced by the silent operation of the SMPS and the readings of the control ammeter in the range of 100 ... 350 mA, depending on the load. If this does not happen, then the device has faulty parts or errors in installation.

After the first few tens of seconds of operation, the SMPS should be disconnected from the network and control the thermal conditions of the transistor, transformer, diodes, then repeat the same after several tens of minutes of operation. If there is no overheating, it is necessary to adjust the output voltage and check the shape of the signals in accordance with Fig.3.

An analysis of the operation of the device showed that when using a ready-made pulse transformer, it is better to leave the storage winding unchanged, and select the communication winding for a voltage of 8 ... containing six turns (winding with pin numbers 9 - 8).

It may turn out that the selected transformer does not provide the required parameters of the SMPS, as a result of which the replacement of the secondary windings will be required. Rigid technology for the manufacture of pulse transformers (distribution of windings in a strictly specified order, maintaining gaps between the edge of the winding and the outer side of the frame, choosing the diameter of the wires depending on the operating current, distributing an incomplete "discharged" layer over the entire width of the winding in order to create a uniform magnetic field inside the working volume of the transformer) requires in the manufacture of special care and accuracy in assembly. But disassembling a transformer glued with epoxy glue is practically impossible without the use of milling equipment (after cutting the transformer with a milling cutter, it will be necessary to restore the working gap on the central rod by reducing it by the thickness of the cut). Therefore, the only way out in this situation is to desolder the electrostatic (interference-proof) copper foil screen, remove unnecessary windings and wind the required winding in their place using the "shuttle" method, and instead of a large diameter wire, it is more preferable to use several parallel conductors of smaller diameter with an equivalent total cross section.

The device used non-deficient parts. Capacitors C1 K73-17, C2, C3, C10, SP - K73-9, all for a rated voltage of 630 V, C4 - K50-32. If the load of the SMPS exceeds 50 W, in parallel with the capacitor C4, you must connect another one of the same or use K50-35B with a capacity of 220 microfarads (or 330 microfarads) for a voltage of 350 V. Capacitor C6 - K53-30 or another. Oxide capacitors C8, C13 K50-35. The rest are any ceramic ones for a rated voltage of 63 ... 100 V. All fixed resistors are MLT, with the exception of R16 C5-16MV. Trimmer resistor R7 - SPZ-386. We will replace the diode bridge KTS405B, KTS405V or separate diodes with a permissible reverse voltage of at least 400 V and an operating current of 1 A. Diodes VD6 and VD7 are pulse diodes with a nominal frequency of at least 35 kHz, the first of which is for a nominal voltage of at least 600V and current 1 A, the second - 100V and 5 A (for low-voltage power supplies). Instead of an industrial line filter choke L1, a home-made one is used: a 1500NM-2000NM ferrite ring with an outer diameter of about 20 mm is used with windings wound on it in several tens of turns of two MGTF-0,35 conductors.

All elements of the SMPS are mounted on a printed circuit board made of one-sided foil fiberglass with a thickness of 1,5 mm (Fig. 4). A jumper is soldered into holes A and B of the board after the device is set up. Capacitor C4 is fixed parallel to the board with a wire clamp mounted on a recess in the end of the case; the ends of the clamp are soldered into the corresponding holes. To ensure the reliability of electrical contact, the negative terminal of the capacitor is connected to the board through a washer with a petal and a nut on the threaded part of the housing. Capacitor SI and resistor R13 are connected by surface mounting, the second terminal of the capacitor with a soldered mounting lug is connected directly to the metal plate of the transistor case, which is installed on the heat sink. This will significantly reduce the level of radiated interference. For the same purpose, the SMPS is placed in a metal case with ventilation holes for cooling.

The device is connected to the network with a flexible mounting wire: a switch and a fuse are soldered into the gap of one conductor with a trip current twice the operating current measured by an ammeter during adjustment (as noted earlier, it will depend on the load). The secondary winding is connected with flexible insulated conductors, depending on the required voltage value at the output of the SMPS. Transistor VT1 is shifted to the border of the board so that it can be mounted through a mica plate directly to the metal case of the device or to a heat sink with an effective cooling area of ​​100...200 cm2.

It should be remembered that the SMPS is galvanically connected to the network: if handled carelessly, this can cause electric shock. According to the rules of electrical safety, for the time of establishing the IIP, it is necessary to connect to the network through an isolating transformer with a power of at least 300 W.

Literature

1. Chips for switching power supplies and their application: a Handbook. - M.: DODEKA, 1997.
2. Lukin N. V., Koryakin-Chernyak S. L. Knots and modules of modern TV sets (third issue). - Kyiv-M.: Science and technology & Solon, 1995.
3. Potapov A., Kubrak S., Garmash A. Power supply module MP-403. - Radio, 1991, No. 6, p. 44-46.

Authors: V.Kosenko, S.Kosenko, V.Fedorov, Voronezh

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