Abstract: This paper discusses the protection system of a DC switching power supply, proposes the design principles and overall protection measures, analyzes the characteristics and design methods of various protections in the switching power supply, and introduces several practical protection circuits. Keywords: Switching power supply; Protection circuit; System design 1 Introduction The high-power switching devices used in DC switching power supplies are expensive, and their control circuits are relatively complex. Furthermore, the loads of switching power supplies are generally electronic systems with a large number of highly integrated devices. Transistors and integrated devices have poor resistance to electrical and thermal shocks. Therefore, the protection of switching power supplies should consider both the safety of the power supply itself and the load. There are many types of protection circuits; this paper introduces polarity protection, program protection, overcurrent protection, overvoltage protection, undervoltage protection, and overheat protection circuits. Several protection methods are usually combined to form a complete protection system. 2 Polarity Protection The input of a DC switching power supply is generally an unregulated DC power supply. Due to operational errors or unexpected situations, the polarity may be incorrectly connected, damaging the switching power supply. The purpose of polarity protection is to ensure that a switching regulator only operates when connected to an unregulated DC power supply with the correct polarity. Polarity protection can be achieved using unidirectional conductive devices. The simplest polarity protection circuit is shown in Figure 1. Since diode D carries the total input current of the switching regulator, this circuit is suitable for low-power switching regulators. In higher-power applications, the polarity protection circuit is used as part of a programmed protection mechanism, eliminating the need for a high-power diode and reducing power consumption. For ease of operation and to easily identify correct polarity, an indicator light is connected after the diode in Figure 1. 3. Programmed Protection The circuit of a switching power supply is relatively complex, basically divided into a low-power control section and a high-power switching section. The switching transistor is high-power; to protect the switching transistor during power-on or power-off, the low-power control circuits such as the modulator and amplifier must be activated first. Therefore, a correct power-on procedure must be ensured. The input of a switching regulator is generally connected to an input filter with a small inductor and a large capacitor. At startup, a large inrush current flows through the filter capacitor, which can be several times the normal input current. Such a large inrush current can melt the contacts of a power switch or relay and blow the input fuse. Furthermore, the inrush current can damage the capacitor, shortening its lifespan and causing premature failure. Therefore, a current-limiting resistor should be connected during startup to charge the capacitor. To prevent this resistor from consuming excessive power and affecting the normal operation of the switching regulator, it is automatically shorted by a relay after the startup transient process, allowing the DC power supply to directly power the switching regulator, as shown in Figure 2. This circuit is called the "soft-start" circuit of the switching regulator. The logic components or operational amplifiers in the control circuit of the switching regulator require an auxiliary power supply. Therefore, the auxiliary power supply must operate before the switching circuit. This can be ensured by a startup program control circuit. The typical power-on procedure is as follows: input power polarity identification, voltage protection → power-on program circuit operation → auxiliary power supply operation and charging of the input capacitor C of the switching regulator through the current-limiting resistor R → switching regulator modulation circuit operation, short-circuiting the current-limiting resistor → stable operation of the switching regulator. In a switching regulator, during initial power-on, due to its large output capacitor capacity, it takes a certain amount of time to charge to the rated output voltage. During this period, the sampling amplifier samples the low output voltage, which, according to the closed-loop regulation characteristics of the system, forces the switching transistor to conduct for a longer period. This causes the switching transistor to tend to conduct continuously during this time, making it prone to damage. Therefore, during this power-on period, the pulse width modulation drive signal output by the switching modulation circuit to the base of the switching transistor must ensure that the switching transistor gradually transitions from cutoff to a normal switching state. Thus, power-on protection is added to complement soft-start. 4. Overcurrent Protection When unexpected situations such as load short circuits, overloads, or control circuit failures occur, the current flowing through the switching transistor in the voltage regulator will become excessive, increasing the transistor's power consumption and causing it to overheat. Without an overcurrent protection device, high-power switching transistors may be damaged. Therefore, overcurrent protection is commonly used in switching regulators. The most economical and simplest method is to use a fuse. Due to the small heat capacity of transistors, ordinary fuses generally cannot provide protection; fast-blow fuses are commonly used. This method has the advantage of easy protection, but the fuse specification needs to be selected according to the specific safe operating area requirements of the switching transistor. The disadvantage of this overcurrent protection measure is the inconvenience of frequently replacing fuses. Current limiting protection and current cutoff protection commonly used in linear regulators can also be applied in switching regulators. However, according to the characteristics of switching regulators, the output of this protection circuit cannot directly control the switching transistor; instead, the output of the overcurrent protection must be converted into a pulse command to control the modulator to protect the switching transistor. To achieve overcurrent protection, a sampling resistor is usually connected in series in the circuit. This affects the efficiency of the power supply, so it is mostly used in low-power switching regulators. However, in high-power switching power supplies, considering power consumption, the connection of the sampling resistor should be avoided as much as possible. Therefore, overcurrent protection is usually converted into over/undervoltage protection. 5. Overvoltage Protection Overvoltage protection for switching regulators includes input overvoltage protection and output overvoltage protection. If the voltage of the unregulated DC power supply used by the switching regulator, such as the battery or rectifier, is too high, the switching regulator will not function properly and may even damage internal components. Therefore, it is necessary to use an input overvoltage protection circuit. A protection circuit composed of a transistor and a relay is shown in Figure 3. In this circuit, when the voltage of the input DC power supply is higher than the breakdown voltage of the Zener diode, the Zener diode breaks down, current flows through resistor R, causing transistor V to conduct, the relay to operate, the normally closed contact to open, and the input to be cut off. The Zener diode's voltage regulation value Vz = ESrmax - UBE. The polarity protection circuit for the input power supply can be combined with input overvoltage protection to form a polarity protection identification and overvoltage protection circuit. Output overvoltage protection is crucial in switching power supplies. Especially for a 5V output switching regulator, its load consists of numerous highly integrated logic devices. If the switching transistor of the switching regulator suddenly fails during operation, the output potential may immediately rise to the voltage value of the unregulated DC input power supply, causing significant instantaneous damage. A common method is thyristor short-circuit protection. The simplest overvoltage protection circuit is shown in Figure 4. When the output voltage is too high, the Zener diode breaks down, triggering the thyristor to conduct, short-circuiting the output terminal, causing an overcurrent. This, in turn, cuts off the input through a fuse or circuit protector, protecting the load. The response time of this circuit is equivalent to the thyristor's turn-on time, approximately 5–10 μs. Its disadvantages are that the operating voltage is fixed, the temperature coefficient is large, and the operating point is unstable. Furthermore, the Zener diode exhibits parameter dispersion; even with the same model, the overvoltage trigger value can vary, making debugging difficult. Figure 5 shows the improved circuit. R1 and R2 are sampling circuits, and Vz is the reference voltage. When the output voltage Esc suddenly increases, transistors V1 and V2 conduct, and the thyristor conducts. The reference voltage Vz is determined by the formula, and UBE1 is the voltage drop across the emitter junction (BE) of V1. The operating voltage of this circuit is variable, and the operating point is quite stable. When the Zener diode is 7V, its temperature coefficient and the temperature coefficient of the emitter junction (BE) voltage of transistor V1 can cancel each other out, resulting in a very low temperature coefficient. However, for DC switching regulators with an output of 5–5.5V, the commonly used operating voltage is 5.5–6V. Therefore, the Zener diode voltage must be below 3.5V, and the temperature coefficient of the Zener diode near this voltage is -20 to -30mV/℃. Therefore, the protection circuit may malfunction in situations with large temperature variations. Using an integrated circuit voltage comparator to detect the output voltage of the switching regulator is a commonly used method. By utilizing the change in the comparator's output state in conjunction with corresponding logic circuits, an overvoltage protection circuit is constructed. This circuit is both sensitive and stable. 6. Undervoltage Protection When the output voltage is lower than the specified value, it indicates an abnormality in the input DC power supply, the internal circuitry of the switching regulator, or the output load. When the input DC power supply voltage drops below the specified value, it causes a drop in the output voltage of the switching regulator and an increase in input current, endangering both the switching transistor and the input power supply. Therefore, undervoltage protection is necessary. A simple undervoltage protection method is shown in Figure 6. When the unregulated input voltage is normal, the Zener diode ZD breaks down, transistor V conducts, the relay operates, the contacts close, and the switching regulator is powered on. When the input voltage is lower than the minimum allowable voltage, the Zener diode ZD does not conduct, V is cut off, the contacts open, and the switching regulator cannot operate. Internally, a malfunction in the control circuit or a failure of the switching transistor can cause the output voltage to drop; a short circuit in the load can also cause the output voltage to drop. Undervoltage protection is particularly important in boost or inverting boost DC switching regulators, as it is closely related to overcurrent protection. The implementation method involves connecting a voltage comparator to the output of the switching regulator, as shown in Figure 7. Under normal conditions, the comparator has no output. Once the voltage drops below the allowable value, the comparator flips, driving the alarm circuit; simultaneously, it feeds back to the control circuit of the switching regulator, causing the switching transistor to cut off or disconnect the input power supply. 7. Overheat Protection The high integration and lightweight design of switching regulators significantly increase the power density per unit volume, consequently increasing the temperature requirements for the components inside the power supply. Otherwise, circuit performance will deteriorate, and components will fail prematurely. Therefore, overheat protection should be included in high-power switching regulators. A temperature relay is used to detect the internal temperature of the power supply. When overheating occurs inside the power supply, the temperature relay activates, putting the entire alarm circuit into an alarm state, thus achieving overheat protection for the power supply. Alternatively, the temperature relay can be placed near the switching transistor. Generally, the maximum allowable case temperature for high-power transistors is 75℃, and the temperature setting value is 60℃. When the case temperature exceeds the allowable value, the relay disconnects the circuit, protecting the switching transistor. Thermistor switching devices, specifically the thermal thyristor, play a crucial role in over-temperature protection. They can be used as temperature indication circuits. Based on the characteristics of the p-type controlled-gate thermal thyristor (TT102), the conduction temperature of the device is determined by the value of RT [see Figure 8(a)]. The larger RT is, the lower the conduction temperature. When placed near a power switching transistor or inside a power supply unit, it functions as a temperature indicator. When the case temperature of the power transistor or the internal temperature of the device exceeds the allowable value, the thermal thyristor conducts, causing the LED to light up as an alarm. If combined with an optocoupler, it can activate the overall alarm circuit, protecting the switching regulator. It can also be used for overheat protection of power transistors, as shown in Figure 8(b). The base current of the transistor is bypassed by the n-type controlled-gate thermal thyristor TT201, turning off the transistor and cutting off the collector current to prevent overheating. 8 Conclusion The above discusses various protection methods in switching regulators and introduces some specific implementation methods. For a given switching power supply, the following points should also be considered from the perspective of overall protection: 1) Limit the operation of the switching transistors used in the switching regulator to within the DC safe operating area. The DC safe operating area of ​​the selected switching transistor can be found in the transistor manual. Determine the input overcurrent protection value based on the maximum collector current. However, this instantaneous maximum value should be converted to the average current. Under rated output current and output voltage conditions, the voltage value for input overvoltage protection is the one where the dynamic load line of the switching transistor does not exceed the maximum input voltage of the DC safe operating area. 2) Limit the output of the switching regulator to within the given technical specifications. Within the required operating temperature range, the upper and lower limits of the switching regulator's output voltage are the output overvoltage and undervoltage protection values. Overcurrent protection can be determined based on the maximum output current. To avoid false alarms, the protection value should have a certain margin. 3) After determining the protection method based on the above two points, determine the alarm measures according to the needs of the power supply unit. Generally, alarm measures include audible alarms and visual alarms. Audible alarms are suitable for complex systems where the power supply is installed in an inconspicuous location, providing effective fault warnings to staff. Visual alarms clearly indicate faults and pinpoint their location and type. Protection measures should be determined based on the components being protected. In high-power, multi-power supply applications, AC/DC circuit breakers and highly sensitive relays are typically used to implement automatic protection measures, cutting off the power input to stop the system and prevent damage. Schemes that use logic control circuits to cut off corresponding switching transistors are both sensitive, convenient, and economical. This eliminates the need for bulky, slow-responding, and expensive high-power relays or circuit breakers. 4) Adding protection circuits to the power supply affects system reliability; therefore, the protection circuit itself must be highly reliable to improve the overall reliability of the power supply system and, consequently, the MTBF of the power supply. This requires rigorous protection logic, simple circuitry, and minimal components. Furthermore, the difficulty of repairing the protection circuit itself and the extent of damage to the protected power supply must be considered. Therefore, a comprehensive and systematic approach to all protection measures for the switching power supply is essential to ensure its normal operation, high efficiency, and high reliability.