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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Flyback cascaded SMPS, 220/12,6 volts 0,5 amps. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Power Supplies In the article brought to the attention of readers, a description of the principle of operation and a practical diagram of an SMPS based on a flyback cascaded voltage converter are given. The absence of voltage surges on the drain of the field-effect switching transistor makes it possible to reduce the requirements for its maximum allowable voltage. Network switching power supply, the circuit of which is shown in fig. 1 refers to the so-called flyback converters, but the scheme of its output stage differs significantly from the "classical" flyback. It can be seen that in the drain circuit of the transistor VT1 there is no damping diode-resistive-capacitor circuit, a separate winding of a pulse transformer is not required to power the controller, and instead of it, two separate energy storage devices are used - an L2 choke and an isolating transformer T1.
In the output stage, a simplified diagram of which is shown in Fig. 2, the energy transfer process can be conditionally divided into two stages. On the first one, energy is accumulated in the magnetic field of the inductor L1 during the forward stroke and transmitted through the open diode VD1 to the capacitor C1 during the reverse stroke. By the end of the reverse stroke, the capacitor C1 will be charged as follows. that its upper lining according to the scheme will acquire a negative potential relative to the lower one. At the second stage, during the next forward stroke, the charged capacitor C1 through the open transistor VT1 and the diode VD2 will be connected to the primary winding of the transformer T1 and will begin to discharge. At this time, energy is accumulated in the magnetic field of the transformer T1 and, finally, during the next reverse stroke, the energy is transferred to the output through a rectifier on the diode VD3 and with a smoothing capacitor C2.
Now let's analyze the work of the SMPS (see Fig. 1) in more detail. A bridge rectifier is assembled on diodes VD1-VD4, capacitors C4, C5 smooth out the ripple of the rectified voltage. Since the processes of energy accumulation are combined, the total current of the inductor L1 and the primary winding of the transformer T2 flows through the open transistor VT1. The current in L2 increases linearly under the action of a rectified voltage, and the rate of its rise is determined by this voltage and the inductance of the inductor. The current also increases linearly in the primary winding T1, and the capacitance of the capacitor C9 is chosen such that during the forward stroke the voltage across it changes slightly. This current component of the transistor has the same direction as the inductor current, since a negative polarity voltage is applied to the upper terminal of the primary winding of the transformer T1. When the current through the transistor VT1 reaches a certain value, the voltage across the resistor R9, which acts as a current sensor, will cause the control controller DA1 to switch and the field effect transistor will close. We note as a disadvantage the fact that at the same power level with the "classic" flyback converter, the transistor current is higher here. The advantages of the device are manifested at the reverse stage - as the field-effect transistor VT1 closes, the inductor current L2 charges the capacitor C9 due to the self-induction EMF. Since the voltage on this capacitor cannot change instantly, the transient process at the drain of the transistor proceeds smoothly, there is no voltage surge, so there is no need to use a damping diode-resistive-capacitor circuit, which significantly reduces the efficiency of the power supply at low output powers. With the start of the reverse stroke, the process of energy accumulation in the transformer T1 stops, and the voltage on its primary winding, which was negative during the forward stroke, will become positive due to self-induction - the VD6 diode will open, providing the DA1 controller with supply voltage. and diode VD9. feeding the load. When connected to the mains, the controller supply voltage initially comes through the R6C8 circuit and is limited by the VD5 zener diode at 15 V. The R10 resistor limits the current of this zener diode in steady state, and the L1 choke additionally protects the controller power supply circuits from voltage surges. The conversion frequency is set by the elements R4, C3 and is about 62 kHz. The output voltage is controlled by an optocoupler U1 and is regulated by changing the duty cycle of the control pulses supplied to the gate of the transistor VT1. The SMPS provides an output voltage of 12,6 V at a current of up to 0,5 A. The instability of the output voltage does not exceed ± 2,5%, and its ripple at the conversion frequency does not exceed 100 mV. The efficiency at an output power of 6 W is at least 0,72. When the load is disconnected, the SMPS operates in the restart mode, while the output voltage does not increase. The minimum load at which it enters the stabilization mode can be the indication LED. The current consumed from the network in this mode is reduced to a few milliamps. The device was assembled on two breadboard printed circuit boards. On one of them - the DA1 controller with related elements, on the second - the rest. The boards are interconnected by wires of the minimum possible length. The controller board uses size 1206 surface mount resistors and capacitors. Capacitors C5, C9 - K73-17, C4, C11 - oxide capacitors suitable in size and operating voltage. Inductor L1 - EC24, resistor R9 is composed of two connected in parallel, and R5 - two connected in series. We replace the IRF830 transistor with another field switching transistor with a permissible drain-source voltage of 500 V, a current of 4,5 A and an open channel resistance of not more than 1,5 Ohm. No heat sink is required for the transistor. The device uses a UCC38C44D chip from Texas Instruments. With minor changes in the circuit, you can use similar controllers from other families, including the UC3844A. It is important that the maximum duty cycle of the output pulses is 50%. For the manufacture of the inductor L2 and the transformer T1, a small-sized W-shaped magnetic circuit EFD15 from Epcos, material No. 87, complete with a standard frame, was used. the inductance of one turn is 100 nH. Inductor L2 contains 130 turns of PEV-2 0,2 wire, laid in four layers and has an inductance of 1,7 mH. You can also use a ready-made choke with a saturation current of 0,3 ... 0,4 A, for example, SDR1006-152KL from Bourns. Transformer T1 contains two windings of 36 turns of wire PEV-2 0,35, isolated from each other by two layers of polyester tape. the inductance of each of the windings is 0,12 mH. The use of these magnetic circuits makes it possible to obtain a height of the mounted device of about 10 mm. For the transformer, it is also permissible to use an annular magnetic core made of MP-140 material with an outer diameter of 18 mm, the efficiency will decrease by 2 ... 2,5%. In this case, the number of turns should be increased to 50, and it is more convenient to wind the windings with a double-folded wire with high-quality insulation, for example, MS16-14 or MP37-12. The transformer made in this way has a lower leakage inductance, and the device works with it more stably. Since most of the elements of the device are under mains voltage, it is advisable to use an isolating transformer of suitable power for adjustment and verification by connecting a load equivalent to the output of the SMPS being adjusted. First you need to make sure that the controller and its circuits are in good condition, for which, without connecting the device to the network, a constant voltage of 13 ... The device does not require selection of elements and adjustment. you can change the output voltage within a small range by selecting a resistor R11 (1,2 kOhm). Having connected the rated load to the output, the output voltage is checked and, without turning off the SMPS, its output is short-circuited. In this case, the average current consumed from the network should decrease, which indicates the normal operation of the protection circuits. Author: V. Sokol, village of Chashnikovo, Solnechnogorsk district, Moscow region.
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