Switching stabilizer for telephone. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Telephony

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In multifunctional telephones with a rated current consumption of 60 ... 200 mA (through a 5 V supply circuit), linear voltage stabilizers on the KR142EN5A chip or its analogues are most often used. It happens that the heat sink of the stabilizer is calculated according to the "lower limit", and the microcircuit is very hot. To improve the thermal regime and increase reliability, I propose to replace the linear stabilizer with a pulse stabilizer that is easy to assemble and configure, made on a specialized microcircuit MC3406ZA f. Motorola.

The scheme of the upgraded network adapter with a switching voltage regulator installed in it is shown in Fig. 1.

(click to enlarge)

The MC34063A chip is available in two versions: in the DIP-8 package (with index P) and in the miniature SO-8 package (with index D). The microcircuit remains operational at an input voltage of 3...40 V and allows you to create step-up, step-down and inverting voltage converters [1].

In this case, this chip is included as a buck converter. Using it in this mode is rational if the input voltage exceeds the stabilized output voltage by more than 2 times. With a smaller difference between the input and output voltages, the efficiency of the stabilizer decreases, approaching the efficiency of linear ones. The minimum difference between the input and output voltage required for the normal operation of the buck converter is 3 V.

The mains voltage (220 V) through the fuse FU1 is supplied to the primary winding of the step-down transformer T1. The transformer was used from an adapter from the telephone set, and a fuse was introduced during its refinement. Unfortunately, most manufacturers of network adapters are stubbornly unwilling to install fuses in their products, thereby endangering the safety of equipment and homes. Varistor R1 protects the mains transformer from insulation breakdown of the primary winding wire during power surges.

The voltage from the secondary winding of the transformer through the safety resistor R2 is supplied to the bridge rectifier on diodes VD1...VD4. Capacitor C1 smooths out the ripple of the rectified voltage. Resistor R3 is needed to protect the DA1 chip from overload. The oscillator frequency of the microcircuit is set by the capacitor C11. The Schottky diode VD5 and the inductor L1 are involved in converting the high input voltage to a low stabilized output voltage, the value of which depends on the resistances R4 and R5. Since the comparator of the microcircuit seeks to maintain a voltage of about 5 V at pin 1 of DA1,25, the greater the resistance of R5, the higher the output voltage. Inductors L2...L4 are part of the LC filters. smoothing the ripple of the output voltage. The VD6 zener diode protects the load from damage in the event of a stabilizer malfunction (in this case, the protective resistor R2 burns out).

Details and construction. Most of the parts are installed on a 44x32 mm circuit board (Fig. 2).

The outputs of the parts are connected by wire jumpers. Transformer T1 is suitable for anyone with an overall power of at least 4 W and a voltage on the secondary winding of 10 ... 17 V. The higher the voltage at the output of the rectifier, the less current the stabilizer consumes at a constant load current. Varistor R1 can be any of the TNR, FNR series for a rated voltage of 430 (470) V, for example, FNR-07K471. Resistor R2 is used in SMD version. In this place, you can use a resettable fuse with a rated current of 0,2 A [2] or a similar fuse.

Resistor R3 is a homemade wirewound resistor, made from a thin, high-resistance enameled wire wound around a low-power resistor. The remaining resistors are small-sized, of any type. Oxide capacitors - imported analogues of K50-35, non-polar - ceramic, in SMD version. In place of the VD5 diode, you can use any low-power Schottky diode, for example, 1N5819. Zener diode KS162A can be replaced by KS162V, 1N5341. A bridge rectifier usually fits one. which is installed in the adapter initially. You can assemble it from four diodes of the 1N4000 ... 1N4007 series. KD243. KD247, KD208. Low-power Schottky diodes are also suitable, while the efficiency of the stabilizer will increase slightly.

All the chokes I used are small-sized, industrial-made, with a winding resistance of no more than 0.1 Ohm. The efficiency of the stabilizer is primarily affected by the characteristics of the inductor L1, which can be an inductance of 100 ... 500 μH or more. To begin with, I took a choke, consisting of 35 turns of winding wire 00.37 mm. wound on a ring of low-frequency ferrite M2000NN with an outer diameter of 16 mm, and then picked up a choke from the available ready-made (small-sized) with an H-shaped ferrite core (with which the efficiency of the stabilizer did not deteriorate). To increase its reliability, a small U-shaped radiator made of a brass plate measuring 34063x34063x30 mm is glued to the MC4AO (MC0,3AR) microcircuit to increase its reliability with the help of heat-conducting glue.

My stabilizer is set to an output voltage of 5,45 V, taking into account losses in the connecting cord (0,05 V), since the telephone set powered by the upgraded adapter has a backup power supply recharging unit and a processor board power circuit changed for this reason. If an output voltage of 5 V is required, then the resistance R5 must be reduced to 3,6 kΩ.

Prior to the upgrade of the network adapter, the voltage regulator on the KR142EN5A chip, with a connected load, consumed a current of about 110 mA from the secondary winding of the transformer. and when replacing it with a pulsed current consumption decreased to 70 mA. At the same time, the heating of the adapter case has noticeably decreased. The voltage ripple amplitude at the output of the stabilizer at a load current of 100 mA does not exceed 40 mV at the frequency of the converter.

Literature

1. Switching stabilizer on the MC34063A chip. - Radio constructor. 2008. No. 3, p.12.
2. modern fuses. - Radiomir, 2008. No. 3-4.

Author: A. Butov, village of Kurba, Yaroslavl region.

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