Jiangsu Aisikai Electric Co.,Ltd.(CE, CCC, ISO9001)

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Several kinds of practical protection circuit of dc switching power supply

The principle and characteristic of switch power supply

Working principle of the 1

Dc switching power supply by input parts, power conversion, output part and control part. Power conversion section is the core of switching power supply, it to high-frequency unstable dc chopper and complete output needed to transform function. It is mainly composed of switch triode and high frequency transformer. Figure 1 draw the schematic diagram of the dc switching power supply and the equivalence principle block diagram, it is by the full wave rectifier, switch tube V, excitation signal, the fly-wheel diode Vp, energy storage inductor and filter capacitance C. In fact, dc switching power supply is a core part of the dc transformer.

2 the characteristics of

In order to meet the needs of users, the switching power supply manufacturers at home and abroad are committed to the synchronous development of new type high intelligent components, especially the loss by improving the secondary rectifier device, and the power ferrite (Mn - zinc) on material increasing scientific and technological innovation, to improve the high frequency and large to earn high magnetic flux density, at the same time, the application of SMT technology makes switch power made great progress, both sides in the circuit board layout components, to ensure that the light of switch power supply, small and thin. So is the development trend of dc switching power supply high frequency, high reliability, low consumption, low noise, anti-interference and modularization.

There are serious shortcomings of dc switching power supply switch interference, are less able to adapt to bad environment and sudden failure. Due to domestic container pieces of production technology and microelectronic technology, resistance magnetic material technology with some advanced countries still have certain gap, so the dc switching power supply technology is difficult, maintenance trouble and the cost is higher,

The dc switching power supply protection

Based on the characteristics of the dc switching power supply and the actual electrical conditions, in order to make dc switching power supply under the condition of bad environment and sudden fault work safely and reliably, according to the different situations many kinds of protection circuit is designed.

1 over current protection circuit

In the dc switching power supply circuit, in order to protect the adjusting tube, in a short circuit current increases without being burned. The basic method is that, when the output current exceeds a certain value, adjust the tube is in reverse biased condition, thus ended, automatic cut off the circuit current. As shown in figure 1, over current protection circuit by the triode BG2 and partial pressure resistance of R4, R5.

Circuit work normally, the pressure function, through the R4 and R5 makes BG2 base voltage higher than emitter voltage, emitter junction reverse voltage. So BG2 in cut-off condition (open), and has no effect on regulating circuit. When short circuit, the output voltage is zero, BG2 emitter is equivalent to the ground, then BG2 in saturated conductivity state (short circuit), so that the adjustment of pipe BG1 base and the emitter to short circuit, and be in by the state, to cut off the circuit current, so as to achieve protection purposes.

2 over-voltage protection circuit

Dc switching power supply switch voltage regulator of over-voltage protection including input over-voltage protection and output over-voltage protection. If used in switching voltage stabilizer is not regulated dc power supply (such as batteries and rectifier voltage if too high, will cause the switching voltage stabilizer can not work normally, even damage to internal components, so it is necessary to use the input over-voltage protection circuit in the switch power supply.

Figure 3 for the use of transistor and relay protection circuit, in the circuit, when the input voltage is higher than that of zener diode breakdown voltage of the dc power supply, voltage regulator tube breakdown, a current flows through resistance R, T make transistor conduction, relay action, the normally closed contacts disconnect, cut off the input. The polarity of the input power source protection circuit can be combined with input over-voltage protection, constitute a polarity protection identification and overvoltage protection circuit.

3 soft start protection circuit

Switch regulated power supply circuit is more complex, switch input voltage regulator generally have low inductance and capacitance of the input filter. In the boot, filter capacitor will flow through a lot of surge current, the surge current can be as the number of input current in normal times. Such large surge current can make ordinary power switch contact or relay contact melting, fusing and input. In addition, the surge current will also damage the capacitor, shorten the life, premature failure.

For this purpose, the boot should be connected to a current limiting resistor, through the current limiting resistor capacitor charging. In order not to make the current limiting resistor consume too much power, so that the normal operation of the switch voltage regulator, and at the end of the boot in the transient process, using a relay automatically sub it, directly to the switch voltage regulator power supply, dc power supply according to the circuit is called the "soft start" of dc switching power supply circuit.

As shown in figure 4 (a), in the moment, electricity input voltage of the rectifier bridge (from D1 to D4) and current limiting resistor R1 to charging capacitor C, limiting surge current. When the charging capacitor C to about 80% of the rated voltage, the normal work of the inverter. The main transformer secondary winding of thyristor trigger signal, the thyristor conduction and short circuit current limiting resistor R1, the normal operation of the switch power supply. In order to improve the accuracy of delay time and jitter oscillation, prevent relay operation delay circuit as shown in figure 4 (b) can be used instead of the RC delay circuit.

4 overheat protection circuit

Dc switching power supply switch voltage regulator of high integration and lightweight is small volume, greatly improve the power density per unit volume to make it, so if the power supply device of internal components without corresponding to the requirement of its working environment temperature increase, will inevitably make the circuit performance deteriorate, premature component failure. So in high power dc switching power supply should be set in the overheat protection circuit.

In this paper, temperature relay is used to detect the temperature inside the power supply device, when the power supply device internal cause overheating, temperature relay action, so the alarm circuit is in alarm condition, realize the overheating protection for power supply. As shown in figure 5 (a), in the heart of the protection circuit P type thermal thyristor control gate placed near the power switch triode, according to the characteristics of TT102 (by the Rr values determine the conduction temperature of the device, the greater the Rr, conducting the lower temperature), when the power tube shell temperature or the temperature inside the unit exceeds allowable values, thermal thyristor conduction, making leds light alarm. If cooperate with photoelectric coupler, can make the machine alarm circuit action, protect the switch power supply.

This circuit can also be designed as shown in figure 5 (b), used as a power transistor overheating protection, crystal switch tube base current were N type thermal thyristor TT201 bypass control gate, switch tube cut-off, cut the collector current, to prevent overheating.

This paper mainly discusses the dc switching power supply different kinds of protect the internal components, and introduces some specific circuit. For a given dc switching power supply, whether perfect protection circuit and a predetermined setting work, is very important to the safety and reliability of power supply device. Because of the diversity of the protection scheme and the circuit structure of switch power supply, so for specific power supply device, should choose reasonable protection scheme and the circuit structure. In practice, usually choose protection ways to combine way to constitute a perfect protection system, to ensure the normal work of the dc switching power supply.

    Update time:12/24/2015 10:23:32 AM  【Print this page】  【Close