Abstract: In order to switch to emergency power failure in the harsh environment and safe conditions put forward several practical protection circuit, and the operation of the circuit analyzed in detail.
Keywords: switching power supply; protection circuit; reliability
1 Introduction
Evaluation of switching power supply quality indicators should be based on security, reliability, as the first principle. In the electrical specifications to meet the conditions required under normal use, for the unexpected power failure in the harsh environment and work under safe and reliable, must design a variety of protection circuits, such as soft-start anti-surge, anti-over voltage, under voltage , overheating, over current, short circuit, phase loss protection circuit.
2, several commonly used switching power supply protection circuit
2.1 Surge soft-start circuit
Most of the input circuit switching power supply with capacitor filter rectifier circuit, at the closing moment of the line supply, due to zero initial voltage on the capacitor, the capacitor charge will form a large transient inrush current, especially in high-power switching power supply, filter capacitor with larger capacity, so that surge currents up to 100A or more. In the instant power on such a large surge current, serious cases often lead to closing the input switch fuse blown or burned contacts, over-current damage to the rectifier bridge; light switch who also make the air not on the gate together. Shangshu switching power supply will cause the phenomenon are not working, for almost all of the switching power supply are set to prevent the flows in the current soft-start circuit to ensure normal and reliable operation of power.
Figure 1 is the use of thyristor current limiting resistor R1 V and the composition of the anti-surge current circuit. In the power-on transient, the input voltage through the rectifier bridge (D1 ~ D4) and current limiting resistor R1 to the capacitor C charged to limit inrush current. When the capacitor C to charge to about 80% of rated voltage, the inverter work. The main transformer secondary windings thyristor trigger signal to short-circuit thyristor and the current limiting resistor R1, switch power supply in normal operation.
Figure 1 and current limiting resistor with thyristor soft-start circuit consisting of
Figure 2 is the use of relays K1 and current limiting resistor R1 form of anti-inrush current circuit. Power is instant, the input voltage through the rectifier (D1 ~ D4) and current limiting resistor R1 of the filter capacitor C1 charging to prevent the connected moments of the surge current, and auxiliary power supply Vcc through resistor R2 and connected to the relay K1 on the line package charging the capacitor C2, when the voltage on C2 reaches the action voltage relay K1, K1 actions, the contacts K1.1 closed and bypass the current limiting resistor R1, the power to enter normal operation. Limit the delay time depends on the time constant (R2C2), is usually selected as 0.3 ~ 0.5s. In order to improve the accuracy of delay jitter and to prevent the relay oscillation, the delay circuit shown in Figure 3 the circuit can be replaced RC delay circuit.
Figure 2 is limiting resistor relay K1 and soft-start circuit consisting of
Figure 3 Alternative circuit RC delay
2.2 overvoltage, undervoltage and thermal protection circuits
Line supply overvoltage and undervoltage of the harm caused by switching power supply, mainly due to exposure to the device voltage and current stress beyond the normal scope of use of damaged, while electrical performance due to destruction and can not meet the requirements. Therefore the upper and lower input power to be limited, for use of over-voltage, under voltage protection in order to improve power supply reliability and security.
The reliability of power supply temperature is the most important factor. Based on the data analysis shows that the temperature is increased by electronic components 2 ?, reliability, decreased by 10%, temperature 50 ? temperature rise during the working life of only 25 ?, 1 / 6, in order to prevent overheating damage to the power devices in switching power supply also need to set the overheat protection circuit.
Figure 4 is only a 4 LM339 comparator and several discrete components constitute the over-voltage, under voltage, over-temperature protection circuit. Voltage can be sampled directly from the auxiliary control power rectifier obtained after filtering, which reflects changes in the input supply voltage, the comparator a reference voltage common, N1.1 for the voltage comparator, N1.2 overvoltage comparator to adjust the R1 can be adjusted However, the action threshold voltage. N1.3 comparator for the overheating, RT negative temperature coefficient thermistor, it constitutes a voltage divider R7, close to the surface of IGBT power switching devices, the temperature rises, RT resistance decreased, the appropriate selection of R7 resistance, the N1.3 set temperature thresholds in the action. N1.4 for external fault emergency shutdown, when the forward end of the input low, the comparator output block PWM drive signal low. Since four of the comparator output is in parallel, both overvoltage, undervoltage, overtemperature fault any of the comparator output low, the power to stop blocking the work of drive signals to achieve protection. If little change in the circuit and make the comparator output high drive signal blockade.
Figure 4, overvoltage, undervoltage, overtemperature protection circuit
2.3 The phase loss protection circuits
As the input power wiring or power their own reasons not reliable switching power supply phase operation can sometimes be the case, and out-phase operation is not easy to be identified. When the power is in phase operation, an arm of the bridge rectifier without current, while the other arm would be a serious over-current damage to the work of the inverter while unusual, it is necessary to protect phase. Usually detected phase grid current transformer or electronic phase detection circuit. The current transformer testing high cost, large size, so switching power supply in the general use of electronic phase protection circuit. Figure 5 is a simple lack of protection of electronic circuits. When three-phase equilibrium, R1 ~ R3 node H potential is very low, optically coupled output is approximately zero level. When phase loss, H point potential elevation, optocoupler output high, compared by the comparator output low, blocking drive signal. Adjustable reference comparator to adjust the phase action threshold. The lack of protection for three-phase four-wire system, but not for three-phase three-wire system. Little change in the circuit can also be blocked with a high level PWM signals.
Figure 5 Three-phase four-wire system phase protection circuit
Figure 6 is a three-phase three-wire power supply for phase loss protection circuits, A, B, C lack of any one phase, the optocoupler output level is lower than the inverting input of the comparator reference voltage, the comparator output low power level, block PWM drive signal, turn off the power. Little change in polarity of the comparator input, PWM signals can be blocked with a high level. This lack of protection circuit by opto isolated high power, safe, reliable, RP1, RP2 lack of protection for regulating the movement threshold.
Figure 6 three-phase three-phase system protection circuit
2.4 Short Circuit Protection
Switching power supply with other electronic device, is the most serious short-circuit fault, short circuit protection is reliable is the impact on the reliability of an important factor in switching power supply. IGBT (insulated gate bipolar transistors) high input impedance of both field-effect transistor, drive power is small and bipolar transistor voltage, current capacity and pressure drop characteristics of tubes, is currently in high-power switching power supply the most commonly used power electronic switching devices. IGBT can withstand the short circuit depends on its saturation voltage and short circuit current size, typically only a few µs to tens of µs. Short-circuit current is too large not only to short-circuit withstand time shortened, but also off when the current rate of decline di / dt is too large, due to leakage and the presence of lead inductance, resulting in over-voltage IGBT collector, the over-voltage can be generated internally in the device engine IGBT failure to lock the live effect, while the high over-voltage IGBT breakdown will. Therefore, when a short circuit over-current, it must take effective protective measures.
In order to achieve IGBT short-circuit protection, over-current detection must be carried out. For IGBT over-current detection methods, usually by direct detection of IGBT current sensor current Ic, and then compared with the set threshold, the comparator output with the drive signal to control turn-off; or indirect voltage method, IGBT overcurrent detection voltage drop Vce, because the pressure drop with short-circuit current information, through flow Vce increases, and the essentially linear when the Vce overcurrent detection with the threshold set by comparing The output of the comparator control drive circuit off.
In the short-circuit current appears, in order to avoid shutdown current of the di / dt is too large to form over-voltage, IGBT lock is invalid and cause damage, and in order to reduce electromagnetic interference, often with soft down and soft turn-off gate voltage integrated protection technologies. Over-current signal is detected after the first gate into the fall protection procedures to reduce the magnitude of fault current and prolong IGBT's short circuit withstand time. In the drop-gate action, set a fixed time delay to determine the authenticity of the fault current, such as the fault disappears during the delay time, gate voltage is automatically restored if the problem persists then the soft turn-off process, so that the gate voltage down to 0V below the IGBT drive signals off. Due process in the descending phase of the gate voltage collector current is reduced, it will not appear when the soft turn-off excessive rate of decline in short-circuit current and excessive over-voltage. Lower gate voltage and the soft turn-off gate drive protection software to enable fault current magnitude and rate of decline could be limited, over-voltage reduction, IGBT's current, voltage trajectory can guarantee the security zone.
Reduction in the design of the gate voltage protection circuit to correct choice of the magnitude and speed of the gate voltage drop, if a large range of gate voltage drop (eg 7.5V), speed is not too fast down the gate voltage, generally soft-fall time can be reduced 2µs gate voltage , due to lower gate voltage amplitude, the collector current is small, the fault condition can be quickly blocked the gate, do not use the soft turn-off; If a lesser extent, lower gate voltage (eg 5V or less), lower gate speed faster and the blockade must slow the speed of gate voltage, the soft turn-off to avoid over-voltage occurrence.
To make power in the short-circuit fault condition does not interrupt the work, but also in the original operating frequency to avoid the continuous accumulation of short-circuit protection causes heat damage caused by IGBT by gate voltage drop can not protect the sealed off in a short-circuit protection circuit, leaving the operating frequency lower (eg 1Hz or so), the formation of intermittent "hiccup" protection methods and failure to resume normal work after elimination.
Here are some practical IGBT short-circuit protection circuit and working principle.
Figure 7 is the use of IGBT Vce overcurrent protection principle of the increased circuit for dedicated drive EXB841. EXB841 completion of the internal circuit can well fall off the gate and soft, and has an internal delay function to eliminate interference resulting malfunction. IGBT over-current information with Vce not directly sent to the collector voltage monitoring EXB841 6 feet, but by the fast recovery diode VD1, through the output of the comparator IC1 is connected to EXB841 feet 6, the purpose is to eliminate the forward voltage drop VD1 varies with the current, using the comparator threshold to improve the current detection accuracy. In the event of over-current, low-speed drive EXB841 slow off the circuit IGBT, the collector current spikes in order to avoid damage to IGBT devices.
Figure 7 when using IGBT Vce increase over current protection principles
Figure 8 is the use of current sensors for detecting the IGBT over-current protection circuit, current sensors (SC) primary (1 turn) the collector circuit in series with the IGBT, the secondary overcurrent sensing the rectified signal sent by the comparator IC1 the same phase input, and inverting the reference voltage side comparison, IC1's output to the comparator with positive feedback IC2, the output connected to the PWM controller UC3525 output control pin 10. However flow, VAVref, VB is high, C3 charge to VC> Vref, IC2 output high (greater than 1.4V), close the PWM control circuit. In the absence of drive signal, IGBT off, and the power to stop working, no current flows through the current sensor so that VA> t1, to ensure the power to sleep. IC2 positive feedback resistor R7 to ensure that only high and low two states, D5, R1, C3 charge and discharge circuit, the output will not guarantee high IC2, frequent changes between low, that is not frequently opened IGBT turn-off damage .
(A) circuit diagram
(B) PWM control circuit of the output driver waveform
Figure 8, using current detection sensor IGBT over-current protection circuit
3 Conclusion
Stage a power switching power supply protection device electrical performance requirements of additional features, but in the harsh environment and accident conditions, the protection circuit is perfect set at predetermined work on power plant safety and reliability is essential. Acceptance of technical indicators, protection should be verified.
Switching power supply circuit protection programs and diversity, but the specific power devices, a reasonable protection program should be selected and the circuit structure in order to make truly effective in fault condition to achieve protection.
Protection circuit described in the text can be flexibly combined to simplify the circuit structure and reduce costs.