Powerful voltage converter 12/5 volts according to a simple scheme. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Powerful voltage converter 12/5 volts according to a simple scheme. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Voltage converters, rectifiers, inverters

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Such a converter may be needed to power high-current 5-volt circuits from a car battery, charge lithium batteries from it (then the output voltage will have to be reduced to 4 V); in the author's version, it is used to power an external computer DVD-RW (USB) from a car battery. This drive itself heats up quite a lot during operation, so there is simply nothing to cool the linear stabilizer microcircuit with. And pulsers are famous for their efficiency.

A voltage multiplier and a clock generator are assembled on the DD1 chip (Fig. 1.10).

Rice. 1.10 (click to enlarge)

The multiplier is necessary due to the fact that the circuit uses cheaper and more common field-effect transistors with an n-type channel. To completely unlock the field-effect transistor with an insulated gate and an induced channel (all transistors of the IRF series belong to this type), the voltage at its gate must be raised by 3 ... 5 V above the drain voltage - so you can’t do without a multiplier.

The multiplier is assembled on the elements C3, VD1, VD2 and the filter capacitor C4 according to a typical scheme. To limit the voltage (it can rise to 22 V, and for the 555 microcircuit, voltage above 18 V is dangerous), a resistor R5 is added. Thanks to him, the voltage across the capacitor C4 is about 17 ... 18 V, this is enough for the normal operation of the field-effect transistor and not enough to breakdown the microcircuit. Capacitor C3 can be either multilayer ceramic (parallelepiped, for surface mounting), or film, but not disk ceramic! Otherwise, due to the significant internal resistance of the capacitor, the voltage across C4 will not rise above 15 ... 16 V even without resistor R5, and the key transistor will get very hot. Capacitor C4 can be rated at 16V.

The actual pulse-width modulator is assembled on the timer DD2. Through capacitor C2 and transistor VT1, very short clock pulses from the generator output arrive at the input S of the timer; the shorter they are, the better (otherwise the timer output may be excited). A capacitance of 10 pF is enough, it can even be reduced to 5 pF.

The output pulse duration is adjusted through the REF input (pin 5 of the microcircuit). The duration of the output pulse is equal to the time during which the capacitor C5 is charged from zero to the voltage at this input, that is, when the voltage REF decreases, the duration of the pulses (and the output voltage) decreases, at a voltage of less than 1,5 V it becomes equal to zero.

Principle of operation of the device

The voltage converter is built according to the classical scheme on a field-effect transistor VT2 and a choke L1. A VT3 transistor is used as a flyback diode. In powerful step-down pulsers, it is best to install transistors in this place, since the reverse current is almost equal to the forward current, and if the voltage drop across the key transistor ( VT2 according to the scheme) is easy to reduce to a minimum, then with diodes everything is much more complicated. The result is a paradox: the key transistor is cold, the inductor almost does not heat up, but the diode is like an iron! But the lower the heating, the higher the efficiency of the circuit, and there are fewer problems with heat removal.

Transistor VT3 works in antiphase with the key transistor VT2 thanks to the inverter on the DD3 chip. Since the flyback diode should not be open all the time the key transistor is idle, but only a short (otherwise it will close the circuit output through the inductor) time immediately after the key transistor is closed (it is at this time that the reverse current pulse has the largest amplitude), a capacitor C6 and for fine tuning trimming resistor R8. The rest of the time, the VT3 transistor works as a diode due to the built-in powerful protective diode between the drain and source terminals. That is, replacing the diode with a transistor will definitely not make it worse.

The voltage regulator is assembled on a VD3 zener diode and a VT4 transistor. The accuracy and magnitude of the output voltage depend only on the quality and voltage of the stabilization of the zener diode. It can be replaced with a TL431 chip.

Choke L1 can be wound on the frame of the transformer from the old radio. We take a wire with a diameter of 1 mm (for a load current of up to 2 A) and wind it until the frame is filled (about a hundred turns). Since the inductor operates on direct current, a dielectric gap is required between the plates, that is, we put everything in. W-shaped plates in one direction and between them and the "sticks" we lay 1-2 layers of newsprint (or transformer, if you have one), after which we compress the whole thing very well. You can wind the inductor on a ferrite ring with a diameter of about 30 ... 40 mm, but again it is better to cut it and glue it again, or take a special split core (ferrite cups with a diameter of 20 ... 30 mm and a height of 15 ... 20 mm , approximately 50 ... 80 turns).

Establishment

We fully assemble the circuit, do not solder only transistors VT2 and VT3. We connect the power supply voltage at the power supply terminals DD2 should be 4 ... 6 V more than the supply voltage; if it is less convinced of the presence of generation (the voltage at the output of the generator should be equal to half the supply voltage), we reduce the resistance of the resistor R5, if this does not help, we put a better capacitor C3. If the supply voltage DD2 is greater than 18 V, we increase the resistance of the resistor R5. After that, we solder both transistors and reduce the resistance R8 to zero. We connect a powerful load to the output (12 V, 20 W car light is recommended) and supply +12 V power through the connected ammeter. If everything works fine, the voltage on the light bulb will be approximately equal to the stabilization voltage of the zener diode, and the current consumed by the circuit will be half the current through the light bulb (in the author's version 0,5 A). Now turn off the light bulb. The output voltage should increase by no more than 0,2 ... 0,3 V, and the voltage at the REF DD2 input should be within 0,8 ... 2,5 V relative to the common wire. If it is close to zero, the capacitance of the capacitor C5 should be halved.

Turn the load on and off: the throttle should “knock” briefly (this feedback circuit works out a sharp change in the load current), there should not be any whistles (self-excitation). If there is excitement, most likely, the tracks are drawn incorrectly.

After that, you can start setting up the "smart diode" (VT3). Slowly rotate the trimmer resistor R8, the current consumed by the circuit (+12 V) will begin to decrease by about 5 ... 10%. This current used to be spent exclusively on heating the body of the transistor VT3. But at some time, self-excitation of the output stage may occur - the current consumed by the circuit increases sharply by 2-3 times. The R8 engine must be set to a position in which the current consumption has decreased, but it is still far from excitation. Disconnect-enable the load again, disconnect-enable the power: there should be no excitation of the output and a whistle in the throttle (even a very short one!) If this is not the case, you need to slightly reduce the resistance of R8 and repeat the provocation.

Thanks to this switching circuit of the VT3 transistor, although it heats up, it is noticeably weaker than a good Schottky diode (KD213, 1N5822). At a load current of up to 1 ... 1,5 A, radiators for both transistors are not needed, at a current of up to 3 A, a small heat sink plate must be screwed to the VT3 case (ROLL heats up with such force already at a current of 0,2 A).

Instead of 1RFZ46 in the author's version, there are their Belarusian counterparts. KP723A with a channel resistance of 0,1 ohms or less, KT315 transistors can be replaced with any npn silicon structures. It is desirable to collect electrolytes C7 and C8 from several smaller capacitances connected in parallel, in parallel they can include a couple of film or multilayer ceramic capacitors with a capacity of 0,1 μF or more.

When repeating the circuit, special attention should be paid to the power wires, all elements and all wires must be connected exactly as shown in the figure! Do not save on matches, otherwise you will be tormented with the setting! The tracks drawn in the figure with a thicker line should be at least 1,5 ... 2 mm thicker.

Authors: Kashkarov A.P., Koldunov A.S.

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