In the switching regulator, the machines just because the output capacitance, and charge to the rated output voltage value of the need for a certain period of time。 During this time, sampling the output amplifier with low input voltage sampling, closed-loop regulation characteristics of the system will force the switching of the transistor conduction time lengthened, so that switching transistor during this period will tend to continuous conduction, and easily damaged。 To this end, the requirements of this paragraph in the boot time, the switch to switch the output modulation circuit transistor base drive signal of the pulse width modulation, can guarantee the switching transistor by the cut-off switches are becoming more and more normal state, therefore the protection of the setting up of a boot to tie in with the soft start。 

4、over-current protection   

  When the load short-circuit, overload control circuit failure or unforeseen circumstances, such as would cause the flow of switching voltage regulator transistor current is too large, so that increased power tubes, fever, if there is no over-current protection device, high power switching transistor may be damaged。 Therefore, the switching regulator in the over-current protection is commonly used。 The most economical way is to use simple fuse。 As a result of the heat capacity of small transistors, general fuse protection in general can not play a role in the rapid fuse common fuse。 This method has the advantage of the protection of vulnerable, but it needs to switch transistor in accordance with specific security requirements of the work area to select the fuse specifications。 This disadvantage is over-current protection measures brought about by the inconvenience of frequent replacement of fuses。

  Linear voltage regulator commonly used in the protection and current limiting to protect the cut-off in the switching regulator can be applied。 However, according to the characteristics of switching regulators, the protection circuit can not directly control the output transistor switches, and over current protection must be converted to pulse output commands to control the modulator to protect the transistor switch。 In order to achieve over-current protection are generally required sampling resistor in series in the circuit, this will affect the efficiency of power supply, so more for low-power switching regulator of occasions。 In the high-power switching power supply, by taking into account the power consumption should be avoided as far as possible access to the sampling resistor。 Therefore, there will usually be converted to over-current protection, and under-voltage protection。

5、over-voltage protection 

  Switching regulator's input over-voltage protection, including over-voltage protection and output over-voltage protection。 Switching regulator is not used in DC power supply voltage regulator and rectifier, such as battery voltage, if too high, so switching regulator is not working properly, or even damage to internal devices, therefore, it is necessary to use the input over-voltage protection circuit。 Using transistors and relays protection circuit。

  In the circuit, when the input DC power supply voltage higher than the voltage regulator diode breakdown voltage value, the breakdown voltage regulator tube, a current flowing through resistor R, so that V turn-on transistor, relay, normally closed contact off open, cut off the input。 Voltage regulator voltage regulator which controls the value of Vz = ESrmax-UBE。 The polarity of input power with the input protection circuit can be combined with over-voltage protection, polarity protection constitute a differential circuit and overvoltage protection。   

  Output over-voltage protection switching power supply is essential。 In particular, for the 5V output of the switching regulator, it is a lot of load on a high level of integration of the logic device。 If at work, switching regulator sudden damage to the switch transistor, the output potential may be increased immediately to the importation of non-regulated DC power supply voltage value, causing great loss instantaneous。 Commonly used method is short-circuit protection thyristor。 The simplest over-voltage protection circuit。 When the output voltage is too high, the regulator tube breakdown triggered thyristor turn-on, the output short-circuit, resulting in over-current through the fuse or circuit protective device to cut off the input to protect the load。 This circuit is equivalent to the response time of the opening time of thyristor is about 5 ~ 10μs。 The disadvantage is that its action is fixed voltage, temperature coefficient, and action points of instability。 In addition, there is a voltage regulator control parameters of the discrete, model over-voltage start-up the same but has different values, difficult to debug。 Esc a sudden increase in output voltage, transistors V1, V2 conduction, the thyristor conduction。 Reference voltage Vz by type。 开关直流稳压电源保护英文文献和中文翻译(2):http://www.youerw.com/fanyi/lunwen_99228.html