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Adjustable protection circuit technology solutions DC constant current switching power supply

Issuing time:2015-07-17
SHENZHEN Branch Power Technology Co., Ltd.-Adjustable protection circuit technology solutions DC constant current switching power supply

Power switching device DC switching regulator used in more expensive, the control circuit is also more complex, in addition, switching regulators are generally used to load a large number of high degree of integration of electronic system components installed. Power transistors and integrated device tolerance, less able to thermal shock. Thus switching regulator should take into account the safety regulator to protect itself and the load. Many different types of protection circuits, here polarity protection, program protection, over current protection, over voltage protection, under voltage protection and thermal protection circuits. Some protection is usually chosen to be combined to form a comprehensive protection system.

A polarity protection

DC switching regulator input are generally not regulated DC power supply. Due to operational errors or unexpected cases it is wrong polarity will damage the switching power supply. Polarity protection purposes, is to make the switching regulator only when connected with the correct polarity unregulated DC power supply to operate. Use unidirectional devices can be achieved through the power of polarity protection. Since the diode D to the total current flowing through the input switching regulator, so this circuit is used in the low-power switching regulator is appropriate. In the case of high power, put the polarity protection circuit protection as a part of the program, can save the required power polarity protection diodes, power consumption will be reduced. For ease of operation, easy to identify the polarity is correct or not, after the diode Figure 1, then lights.

Second, the program protected

Switching power supply circuit is more complicated, basically can be divided into the control section and part of the small high-power switching power. Then it is high-power switching transistors, in order to protect the safety of the switching transistor is turned on or shut off the power, you must let modulators, amplifiers and other low-power control circuit. Therefore, to ensure the correct boot. Input of the switching regulator is generally connected with a small inductance and large capacitance of the input filter. In the instantaneous power, filter capacitor flows through a large inrush current, the inrush current can be many times the normal input current. Such a large inrush current will contact the general power switch or relay contact melting, and the input fuse blown. In addition, the inrush current can also damage the capacitor, so that shortened life expectancy, premature damaged. To this end, access should be a limiting resistor boot through the current limiting resistor to charge the capacitor. In order not to make the current limiting resistor consume too much power, which affects the normal operation of the switching regulator, and transient process after the end of the boot, with an automatic relay shorting it, the DC power supply directly to the switching regulator , as shown in Figure 2. This circuit is called a switching regulator "soft start" circuit.

Switching regulator control circuit logic components or operational amplifiers required auxiliary power supply. To this end, the auxiliary power supply switch circuit must precede. This can turn the program control circuit to be guaranteed. General boot process are: identification of the input power supply polarity voltage protection circuit → → boot process and auxiliary power to work through the current limiting resistor R input modulation circuit switching regulator charging capacitor C → switching regulator, Short circuit current limiting resistor → switching regulator stability.

In the switching regulator, when just machines, because of the large output capacitance, charged to the rated output voltage value will take some time. During this time, the sampling amplifier input low output voltage sampling, according to the system loop adjustment feature will force the switch transistor conduction time longer, so that the switching transistors will become continuous conduction during this period, and easily damaged. To this end, the requirements in the boot this time, switching modulation circuit output to the switching transistor base of the PWM drive signal, to ensure that the switching transistor switches off gradually become normal state, and therefore to increase the protection provided to cope with soft power start up.

Third, over-current protection

When there is a load short-circuit, overload or control circuit failure and other unforeseen circumstances, it will cause flow through the regulator switching transistor current is too large, the tubes increase in power consumption, heat, if there is no current protection devices, high-power switching transistors It may be damaged. Therefore the switching regulator over-current protection is commonly used. The most economical and easy way is to use a fuse. Due to the small heat capacity of the transistor, ordinary fuse generally can not play a protective role, commonly used is fast-acting fuse. This method has the advantage of easy protection, however, the need to specifically switch transistor safe operating area requirements to select the fuse specifications. The disadvantage of this measure is to bring current protection fuse replacement often inconvenient.

Linear regulator commonly used in protection and current limiting protection can apply off the switching regulator. However, depending on the characteristics of the switching regulator, the output protection circuit can not directly control the switching transistor, and overcurrent protection must be converted to a pulse output instruction, to control the modulator in order to protect the switching transistor. In order to achieve the over-current protection generally require sampling resistor in series with the circuit, which affects the efficiency of the power supply, so much for the low-power switching regulator applications. In the high-power switching power supply, taking into account the power consumption should be avoided sampling resistor access. Thus, in general overcurrent protection switch too, under-voltage protection.

Fourth, over-voltage protection

Switching regulator over voltage protection including input and output over-voltage protection over-voltage protection. Unregulated DC power supply switching regulator is used, such as batteries and rectifier voltage is too high, the switching regulator is not working properly, or even damage to the internal components, and therefore, it is necessary to use an input over-voltage protection circuit. And relay protection circuit consisting of the transistors shown in FIG.

In this circuit, when the input DC power supply voltage is higher than the breakdown voltage of the zener diode, Zener breakdown, a current flows through the resistor R, the transistor V conduction, relay, normally closed contacts open cut input. Wherein the Zener voltage of Vz = ESrmax-UBE. Polarity power input protection circuit can be input with overvoltage protection together constitute the differential polarity protection and overvoltage protection circuit.

Output over-voltage protection is critical in switching regulator power supply. Especially the output of 5V switching regulator, its load is a lot of highly integrated logic devices. If at work, switching regulator switching transistor sudden damage output potential may immediately rise to unregulated input voltage DC power supply, transient caused great losses. Commonly used method is thyristor short circuit protection. The easiest overvoltage protection circuit shown in Figure 4.

When the output voltage is too high, the regulator breakdown, triggered thyristor, the output short circuit, over-current caused by a fuse or circuit breaker will cut off the input to protect the load. This circuit is equivalent to the response time of the thyristor turn-on time, about 5 ~ 10μs. The disadvantage is that the operating voltage is fixed, temperature coefficient, the operating point unstable. In addition, the regulator there is discrete parameters, the same model, but over-voltage start value but different, makes it difficult to debug. Figure 5 is a circuit improved. Wherein R1, R2 is a sampling circuit, Vz is the reference voltage.

Esc sudden increase in the output voltage, the transistor V1, V2 conduction, the thyristor is turned on. The reference voltage Vz by the formula

To determine, UBE1 emitter junction of V1 (BE) voltage drop. Variable operating voltage of the circuit, and the action points quite stable. When the regulator is 7V, its temperature coefficient and the emitter of the transistor V1 (BE) temperature coefficient of the offset voltage, it can drop very low temperature coefficient. However, the output of 5 ~ 5.5V DC switching regulator, its usual operating voltage is 5.5 ~ 6V. So will the 3.5V voltage regulator below this voltage temperature coefficient close to the regulator is -20 ~ -30mV / ℃. Therefore, the temperature variation in the case where the protection circuit malfunction will occur. Integrated circuit voltage comparator to detect the switching regulator's output voltage, is the more common way, to change the use of the comparator output state with the corresponding logic circuit with, constitute an overvoltage protection circuit that both sensitive and stable.

Fifth, under voltage protection

Output voltage is below a predetermined value, reflecting the input DC power supply, switching regulator output load inside or abnormality occurs. When the input DC power supply voltage drops below a predetermined value will cause the switching regulator's output voltage drops, the input current is increased, not only endanger the switching transistor, but also endanger the input power. Therefore, to be set up under voltage protection. Simple undervoltage protection shown in Figure 6.

When the unregulated input voltage is normal, the regulator ZD breakdown, transistor V conduction, relay, contact sticking, power-switching regulator. When the input voltage is below the minimum allowed, the regulator ZD barrier, V cutoff, contacts jump, switching regulators can not work.

Switching regulator inside, the control circuit malfunction or failure of the switching transistor in the output voltage drops; load short-circuit output voltage will also drop. Especially in the boost or inverting step-up DC switching regulator undervoltage protection is closely related with over-current protection, and therefore more important. This is done by switching regulator output termination voltage comparator, shown in Figure 7.

Normal, the comparator does not output, once the voltage drops below the allowable value of the comparator to flip, drive alarm circuit; and feedback to the switching regulator control circuit, the switching transistor off or disconnect input power.

Sixth, overheat protection

Switching regulator and high integration lightweight small size, making it the unit power density within the volume is greatly improved, the internal components of the power supply device requires its work is also a corresponding increase in the ambient temperature. Otherwise, make the circuit performance deterioration, premature failure of components. Therefore, high-power switching regulator should be set overtemperature protection.

Using temperature relay to detect the temperature inside the power supply device, when the internal power supply overheating, temperature relay on the action, so that the whole alarm circuit is in alarm state, to achieve the power of thermal protection. The temperature can also be placed near the relay switch transistor, generally high-power tube maximum allowable case temperature is 75 ℃, adjust the temperature setting is 60 ℃. When the case temperature exceeds the allowable value after the relay to cut off electrical appliances, to switch protection.

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