Introduction and design of several telephone line feeding circuits - Power Circuit - Circuit Diagram

In this paper, several circuits for obtaining larger power supplies from telephone lines are analyzed, their advantages and disadvantages are compared, and the applicable occasions of each circuit are given. The use of telephone line feed circuits in IC card phones, multifunction phones, and other small devices that communicate using telephone lines eliminates the need for additional external power or batteries.

Conventional telephones that were previously used require only a small amount of power to meet the requirements of normal calls. They use the feed of the switch and a few milliamps of current provided by a simple parallel or series linear regulator to drive the talk circuit without too much concern for efficiency and overcurrent protection.

However, for many new multi-function phones, public IC card phones, or small devices that use the public switched network for remote communication, a few milliamps of power supply current can not meet the demand. If you use external power or battery power, it will not only increase the cost, but also bring inconvenience to users. The most ideal solution is to get the system power from the line feed. However, according to the GB/T 15297-2002 standard, the current that the telephone device can obtain in the on-hook state is only about 500uA, and the current that can be obtained in the off-hook state is generally between 18mA and 80mA, and the line impedance varies greatly. Under severe conditions, it will reach more than 1,000 ohms. If you use traditional power circuits, you will not get enough power.

Basic principle of feeding

The voltage supplied by the switch to the user circuit by voltage or current feed is generally 48V, the feed current when hanging up is generally less than 500uA, and the feed current when off-hook is between 18mA and 80mA. It can be seen that in the on-hook state, the current that the user circuit can obtain is quite limited. Fortunately, today's microcontrollers have made great strides in low-power technology. For example, TI's MSP430 microcontroller can operate at currents of tens of microamps. However, in the on-hook state, the smart device on the telephone line still needs to turn off redundant functions (such as liquid crystal display) as much as possible to avoid overcurrent, which causes the switch party to misjudge the telephone terminal.

The feed current in the off-hook state is also limited. For a given line impedance, the input power that the user circuit can obtain from the off-hook telephone line depends on the voltage drawn. If the communication requirements are not considered, the user circuit can obtain the maximum power when the voltage taken is 1/2 of the switch power supply voltage or the input resistance matches the line impedance. However, this will cause the loop resistance to be too large, affecting normal communication, so the actual voltage taken is much lower. The power obtained at this time is almost proportional to the voltage taken, so the only way to obtain a high-power power supply is to maximize the input voltage while meeting the loop-impedance requirements of normal communication.

However, the input power obtained by this method cannot be directly used by the circuit because it only increases the voltage, does not increase the current, and needs to be converted by a high-efficiency switching type DC/DC converter to obtain a low voltage. , high current output power.

On-hook feed circuit

Figure 1: On-hook feed circuit (constant current source with linear regulator circuit).

There are a variety of feeding modes in the on-hook state. The constant current source and the feeding mode of the linear regulator circuit are commonly used. Figure 1 shows the circuit structure of this feeding mode. In this circuit, the bridge is used for polarity protection, and a subsequent constant current source circuit is connected to the communication circuit. The current is basically determined by R1, which is about 300uA. This is to ensure that the switch party misjudges the telephone terminal failure because the leakage current is too large while ensuring the power supply of the on-hook. Since the current is too small, a more efficient switching power supply cannot be used. The communication circuit generally uses transformer coupling to eliminate the imbalance of the ground, but if the circuit allows, the communication circuit and the bridge can also be used to simplify the circuit.

Off-hook feeding circuit

Figure 2: Off-hook parallel feed circuit.

The off-hook feeding circuits are roughly classified into two types. The feeding circuit and the communication circuit are connected in parallel to the parallel feeding, and the feeding circuit and the communication circuit are connected in series to belong to the series feeding. Figure 2 is a commonly used parallel feed circuit. The large inductor L1 ensures that the DC feed does not affect the AC signal. For poor line conditions, such as 5km long subscriber lines, the voltage on the Zener D5 is 15V and the power can reach 340mW without considering the influence of the communication circuit. When the line condition is even worse, reduce the voltage on the Zener tube D5 to 13V, and then obtain about 300mW of power. Using MAXIM's MAX1685, a high-efficiency DC/DC converter with a switching frequency of 600KHz and an efficiency of 95%, this voltage can be converted to 3.3V to achieve a supply current of 85mA.