Design of the feeding circuits of the most popular

Design of several line feed circuits

using line feed circuits in IC cards, multifunctional and other small devices that use lines for communication can save additional external power or batteries. This paper analyzes several circuits that obtain high-power power supply from the line, compares their advantages and disadvantages, and gives the applicable occasions of each circuit

the ordinary machine used in the past only needs very small power to meet the requirements of normal calls. They use the feeder of the switch and a few milliamps of current provided by a simple parallel or series linear voltage stabilizing circuit to drive the call circuit, without paying too much attention to the problems of efficiency and overcurrent protection

however, for many new multi-function machines, public IC card machines or small devices that use public switching for remote communication, our main purpose is to prevent an unqualified experimental machine from flowing to the market, and the power supply current of a few milliamps is far from meeting the demand. If external power supply or battery power supply is used, it will not only increase the cost, but also bring inconvenience to users. The most ideal scheme is to obtain the system power supply from the line feed. However, according to the gb/t standard, the current that can be obtained by the telephone equipment 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. In addition, the variation range of line impedance is also large, which will reach more than 1000 ohms in bad conditions. At this time, if the traditional power supply circuit is used, it will certainly not be able to obtain sufficient power

the switch adopts voltage or current impulse testing machine fault detection and maintenance feeding mode to provide the voltage to the user circuit is generally 48V, the feeding current when hanging up is generally less than 500uA, and the feeding 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 get is quite limited. Fortunately, the low-power technology of single chip microcomputer has made great progress. For example, TI's MSP430 single chip microcomputer can work under the current of dozens of microamps. However, in the hang up state, the online intelligent device still needs to turn off redundant functions (such as LCD) as much as possible to avoid overcurrent, which will lead to the wrong judgment of terminal failure by the switch

the feeding current is also limited in the off hook state. For a certain line impedance, the input power that the user circuit can obtain from the off hook line depends on the voltage taken. If the communication requirements are not considered, when the voltage taken is 1/2 of the power supply voltage of the switch or the input resistance matches the line impedance, the user circuit can obtain the maximum power. However, doing so will lead to excessive loop resistance and affect normal communication, so the actual voltage is much lower. At this time, the power obtained is almost proportional to the voltage taken, so the only way to obtain high-power power supply is to increase the input voltage as much as possible 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 without increasing the current. It also needs to convert the power through a high-efficiency switching dc/dc converter to obtain a low-voltage, high current output power

on hook feeding circuit

there are many feeding modes in the on hook state. The feeding mode of constant current source combined with linear voltage stabilizing circuit is a commonly used one. Figure 1 shows the circuit structure of this feeding mode. In this circuit, the bridge is used for polarity protection, followed by a constant current source circuit connected in parallel with the communication circuit. The current is basically determined by R1, which is about 300uA. This is to ensure the power supply of the hook, and at the same time, the switch will not misjudge the terminal failure due to excessive leakage. Because the current is too small, it is impossible to use a more efficient switching power supply. The communication circuit generally adopts transformer coupling to eliminate the imbalance to the ground, but if the circuit allows, the connection mode of communication circuit and Bridge sharing the ground can also be used to simplify the circuit

Figure 1: hook up feeding circuit (constant current source with linear voltage stabilizing circuit)

off hook feeding circuit

off hook feeding circuit can be roughly divided into two categories. The feeder circuit in parallel with the communication circuit belongs to parallel feeding, and the feeder circuit in series with the communication circuit belongs to series feeding. Figure 2 is a commonly used parallel feeding circuit. The large inductance L1 ensures that the DC feeding will not affect the AC signal. For bad line conditions, such as 5km long user line, if the influence of communication circuit is not considered, the maximum voltage on Zener D5 is 15V, and the power can reach 340mw. When the line condition is worse, reduce the voltage on the zener D5 to 13V, and you can obtain about 300MW power. Using Maxim's high-efficiency dc/dc converter max1685 with switching frequency of 600kHz and efficiency of 95%, 85mA power supply current can be obtained by converting this voltage into 3.3V

Figure 2: off hook parallel feed circuit

this circuit works stably and reliably, but there are also several disadvantages: first, some current flows back to the line through the communication circuit loop, which is not fully utilized by the power module; Second, the inductance in parallel has an influence on the AC signal of the communication circuit; Third, the volume of large inductors is huge, which is unacceptable to many designers of portable devices; In addition, the parasitic resistance of large inductance will also affect the power efficiency. Therefore, the large inductance in Figure 2 is often replaced by the electronic inductance or constant current source in Figure 3. Although this can solve the problem of excessive inductance volume, due to the use of triode, there is inevitably a fixed voltage drop of more than 1V, which reduces the efficiency of the whole power supply

figure 3: (a) equivalent inductance circuit

(b) simplified electronic inductance

(c) constant current source

if the feeding circuit is connected in series with the communication circuit as shown in Figure 4, it becomes a series feeding circuit. The biggest advantage of series feed circuit is that it can use the whole loop current to supply power to the system. In addition, because the circuit is in series, there is no need for large inductance, only tens of milliohms of inductance is needed for power filtering, which can effectively overcome the problem that parallel feeding is difficult to solve, and it is more flexible, so it is used as the power supply of IC and other equipment by most engineers. The circuit can work well in bad line conditions. Even if the equivalent length of the user line exceeds 7km and the voltage is reduced to 10V, the power of about 280mW can be obtained

Figure 4: off hook series feed circuit

because the power circuit is connected in series with the communication circuit, the input impedance is large. In order to minimize the impedance of the communication circuit, the transformer coupling method is usually used. But this also creates a problem, because both AC signal and DC power current flow through the transformer, so higher requirements are put forward for the transformer. The transformer is required not only to have large enough inductive reactance and good linearity, but also to have small DC impedance, good heat dissipation, and can withstand 100mA current, and maintain stable inductive reactance and linearity in a large current range (18 ~ 80mA). Such requirements are very harsh for coil transformers

summary of this paper

several feed circuits analyzed in this paper have their own advantages and disadvantages and scope of application. Usually, different feed circuits need to be selected according to the actual application, and an application generally requires more than one feed circuit, often a combination or variant of the above circuits. Of course, in some applications, the feeding circuit and the communication circuit are cleverly combined to complete the feeding and communication functions. Although this is conducive to reducing the volume and cost, it is not conducive to debugging and reference for future generations. Therefore, this paper does not discuss this kind of feeding because it is biodegradable

Cai Ying, master candidate, southwestjiaotonguniversity email:caiying76@ (end)