Powerful switching regulator with high efficiency, 8-16/5 volts 10 amps. Scheme, description

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Powerful switching regulator with high efficiency, 8-16/5 volts 10 amps. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Surge Protectors

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Your attention is invited to a pulse voltage stabilizer with a synchronous rectifier unit. Its scheme is shown in Fig. 5.39.

Rice. 5.39 (click to enlarge)

Main technical characteristics:

input voltage, V.....8...16;
output voltage, V.....5;
maximum load current, A ..... 10;
output voltage ripple amplitude, mV, no more than.....100;
instability of the output voltage when changing the input voltage, load current and ambient temperature,% of the nominal value.....2;
ambient operating temperature range, °С.....-10...+70;
conversion frequency, kHz.....100;
average value of efficiency at maximum load current in the entire range of input voltage variation, %.....90.

The stabilizer uses a UC3843 control chip from Unitrode Corp. The control chip implements a pulse-width method for stabilizing the output voltage. To do this, it includes a comparison node at the op-amp, one input is supplied with part of the reference voltage (2,5 V), and the other is part of the output from the resistive voltage divider R1, R4. Elements R2, C8 are the corrective circuit of this amplifier. During regulation, the duration of the output pulse begins to decrease compared to the original, as soon as the voltage at pin 2 of the microcircuit exceeds 2,5 V. The frequency of the pulses remains constant.

To protect the stabilizer from overcurrent, a high-speed comparator is provided in the microcircuit. An exemplary voltage of 1 V from the built-in source is applied to one of its inputs, and a voltage proportional to the current flowing through the open transistor VT3 is applied to the other (pin 2).

IRF4905 is used as a power element - a p-channel field-effect transistor from International Rectifier. Its open resistance is about 20 mΩ, and the delay in opening and closing is about 80 n.s. The synchronous rectifier assembly is made on the elements VD2, VT3. Transistor VT3 - n-channel field IRF3205 of the same company - is also selected with a low open channel resistance (8 mΩ). Then, at the maximum load current, the voltage drop, instead of the typical 0,5 V for Schottky diodes, will decrease to about 100 mV, which also reduces the power loss in the ISN as a whole.

It acquires such characteristics only when controlled from a powerful pulse amplifier, which provides a large (several amperes) recharging current of the gate-source and gate-drain capacitances. In the voltage regulator under consideration, this amplifier is made on microassembly transistors VT1. In addition, it inverts the control signal generated by the DA1 chip.

The output smoothing filter is formed by capacitors C12 ... C17. Their number (six) and the choice of type are sufficient for high-quality filtering of the output voltage without an additional high-frequency filter. The input U-shaped filter is necessary to suppress high-frequency interference that occurs due to the pulsed nature of the current consumed by the stabilizer. It became possible to reduce switching losses while increasing the efficiency of the stabilizer due to the use of a Schottky diode with a low voltage drop and a recovery time of about 2 μs as VD0,05.

The device is made on standard elements, with the exception of windings. Choke L1 is wound on a ring. K10x6x4,5 from permalloy. MP 140 and contains 5 turns of 6 PEV-0,5 wires laid evenly around the entire perimeter of the ring. Choke L2 is made on a K19x11x4,8 ring of the same material and contains 12 turns of 10 wires of the same diameter. Transformer T1 is wound on a ring. K10x6x2000 from ferrite 1NM0,2 The secondary winding is made with PEV-200 wire and contains 2 turns evenly laid around the entire perimeter of the ring. The primary winding is a wire passing through the hole of the ring, the ends of which are connected, respectively, to the drain of the transistor VT3 and the current connection of the drain of the transistor VT2 with the left-hand output of the inductor LXNUMX according to the circuit. When connecting a transformer, it is necessary to carefully observe the correct phasing of the windings.

For high-quality filtering of high-frequency interference, leadless tantalum capacitors (C1 ... C7, C12 ... C17) in the D package (capacitors for surface mounting) from NEC, Nichicon, TDK, etc. are used. From domestic ones, oxide capacitors K53-28, K53 are suitable -25, K53-22. True, the last two types of capacitors must be sealed after installation. The stabilizer does not need to be adjusted, of course, if its installation is done qualitatively.

The peculiarities of the operation of the DA1 microcircuit include the fact that it "does not like" to work with duty cycles of the control pulses less than 2, i.e. low supply voltage. This is manifested in the fact that the pairs of pulses of adjacent periods have a different, but constant duration for a given supply voltage. In fact, this means that the shape of the output voltage ripple will receive another envelope at a frequency twice as low as the frequency of the master oscillator. This feature can be eliminated by connecting between terminals 3 and 4 of a serial circuit chip from a resistor with a resistance of 0,1 ... 2 kOhm and a capacitor with a capacity of 1000 ... 10000 pF. However, the frequency of these "parasitic" oscillations is high, practically does not increase the amplitude of the output voltage ripples and does not affect the dynamic properties of the stabilizer as a whole.

The switching regulator must be mounted on a printed circuit board with short and wide conductors. The smaller its size, the less induced interference will be, which to a large extent determine the stability of the device as a whole. Transistor VT2 and diode VD2 are installed on a heat sink with an effective surface area of at least 100 cm2, and to reduce induced interference, these elements should be installed through insulating spacers, and the heat sink itself is electrically connected to the negative terminal of capacitors C2 ... C7. The right output of the inductor L2 according to the diagram should be connected to the positive output of the capacitor C12, and the output of the resistor R4, right according to the diagram, to the positive output of the capacitor C17. It also supplies output voltage to the load.

Author: Semyan A.P.

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