Switching voltage stabilizer with increased efficiency

Encyclopedia of radio electronics and electrical engineering / Surge Protectors

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The article describes a powerful switching regulator with high efficiency, made on a specialized UC3843 microcircuit. The device has overcurrent protection.

The problem of increasing the efficiency of power supplies is especially relevant for high power converters with low output voltage (3...5V). It was solved using modern foreign elements: a specialized control microcircuit; diodes with low voltage drop and recovery time; field-effect transistors with low on-resistance.

The figure shows a diagram of a powerful switching voltage regulator with overcurrent protection.

Main Specifications

• Input voltage, V......8...16
• Output voltage, V.....5
• Maximum load current, A .... 10
• Output voltage ripple amplitude, mV, no more than 100
• The instability of the output voltage with a change in the input voltage, load current and ambient temperature,%, from the nominal value......2
• Ambient operating temperature range, °С......-10...+70
• Conversion frequency, kHz......100
• Average value of efficiency at maximum load current over the entire range of input voltage variation, %......90

The stabilizer uses a UC3843 control chip from UNITRODE CORP (for a detailed description, see the book "Integrated Circuits: Microcircuits for Switching Power Supplies and Their Applications". - M .: DODEKA. 1997). Here we dwell only on the main functions of its nodes involved in the operation of the stabilizer.

The UC3843 microcircuit incorporates a start-up node, which, when the supply voltage is more than 7,5.. .8 V (pin 7), switches all nodes from standby mode to working state. In this case, the exemplary voltage source (pin 8) generates a stabilized voltage of 5 V, and the master oscillator (pin 4) produces a sawtooth voltage, the frequency and ratio of the rise and fall times of which are determined by the ratings of the elements R3 and C10. The output powerful buffer amplifier (pin 6) generates a rectangular control voltage with an amplitude slightly less than the supply voltage of the microcircuit. Its frequency, pulse duration and pauses coincide with similar parameters of the sawtooth voltage of the master oscillator.

The control chip implements a pulse-width method for stabilizing the output voltage. To do this, it includes a comparison node on the OS. a part of the reference voltage (2.5 V) is supplied to one input, and a part of the output from the resistive voltage divider R1R4 is fed to the other. Elements R2C8 - 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). This voltage forms the current transformer T1, the primary winding of which is connected in series with the transistor VT2. When it is open. a current flows through the secondary winding of the transformer, diode VD1 and resistor R5, which is k times less than the current of the primary winding, where k \u1d wp / w1 is the transformation ratio T5. Thus, a voltage is formed on the resistor R2, exactly repeating the shape of the current of the transistor VT5. the instantaneous value of which at each moment of time is determined from the expression: Ur2 = lvT5·RXNUMX/k.

At the beginning of each period, the control output voltage of the microcircuit opens the transistor VT2, and when the voltage at pin 3 reaches 1 V, it is forced to close. During an overload of the stabilizer, this process occurs every period, thereby preventing an increase in current through the transistor VT2. and hence through the load.

IRF4905 - p-channel field-effect transistor from INTERNATIONAL RECTIFIER. Its open resistance is about 20 mΩ. and the delay in opening and closing is less than 0,1 μs. It acquires such characteristics only when controlled from a powerful pulse amplifier, which provides a large (several A) recharging current of the gate-source and gate-drain capacitances. In the voltage regulator under consideration, this amplifier is made on microassembly transistors VT1.1, VT1.2. In addition, it inverts the control signal generated by the microcircuit.

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. Inductor L1 is wound on a K10x6x4.5 ring made of permalloy MP 140 and contains 5 turns in 6 wires PEV 0,5. laid evenly around the entire perimeter of the ring. Choke L2 is made on the K19X11x4,8 ring of the same material and contains 12 turns of 10 wires of the same diameter. Transformer T1 is wound on a K1Ox6x2000 ring made of ferrite 1NM0.2. The secondary winding w„ 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 connect, respectively, the positive terminal of the capacitors C7-C2 and the source of the transistor VTXNUMX. 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 (C 1-C7, C12-C17) in package D (capacitors for surface mounting) from NEC, NICHCON, TDK, etc. are used. From domestic oxide capacitors K53-28, K53-25, 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 of high quality. The peculiarities of the operation of the DA1 microcircuit include the fact that it does not "like" to work with duty cycles of 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

half the frequency of the master oscillator. This feature can be eliminated by connecting between terminals 3 and 4 of a serial circuit microcircuit 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 12 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. The stabilizer layout was made on a double-sided printed circuit board 60x90 mm in size and 2 mm thick. On the upper side of the board there are "high" elements: chokes, a transformer, a microcircuit, and on the "lower" side there are filtering tantalum capacitors, a VT1 transistor assembly, a VT2 transistor and a VD2 diode with flanges outward. Through six holes evenly spaced around the perimeter, the board is screwed to an aluminum heat sink plate of the same size and 3 mm thickness as the board. The result was a flat design with a thickness of 18 mm. For effective cooling, the stabilizer is installed vertically.

Author: A.Mironov, Lyubertsy, Moscow Region

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