Undervoltage protection (UVP) of a switching power supply is a crucial safety feature. Its main functions can be summarized as follows:
A,Protect the power supply itself and its components:
Preventing overcurrent damage to power devices: When the input voltage is too low, in order to maintain a stable output voltage, the switching power supply (especially the PWM controller) will try to compensate by increasing the on-time (duty cycle) or frequency of the switch tube. This will cause the current flowing through the switch tube (MOSFET/IGBT), rectifier diode, inductor, transformer and other power devices to increase significantly. Long-term high current will cause these components to overheat and eventually burn out.
Preventing Core Saturation: In topologies like flyback and forward, the flux change (ΔB) of the transformer or inductor is proportional to the input voltage and on-time. When the input voltage is too low, the controller will extend the on-time to maintain output. Excessively long on-times can cause the transformer or inductor's core to enter saturation. Once saturated, the inductance drops sharply, causing the current to increase uncontrollably (limited only by the line resistance), resulting in a huge instantaneous current spike that can easily damage the switch.
Maintaining control loop stability: Extremely low input voltage may cause the power supply's control loop (such as the PWM comparator) to malfunction or enter an unexpected state, resulting in abnormal output or component damage.
B,Protect input source:
Preventing deep battery discharge: When a switching power supply is powered by a battery (such as a lead-acid battery or lithium battery), undervoltage protection promptly disconnects the load to prevent the battery voltage from dropping too low. Deep battery discharge can severely impair performance (reducing capacity and shortening lifespan) and even cause irreversible damage (particularly for lithium batteries, potentially posing safety risks).
Reducing the burden on weak power grids: In scenarios where the grid voltage is unstable or power supply is weak in remote areas, if the power supply continues to operate at too low a voltage, it will draw more current from the already weak grid, which may further lower the grid voltage, forming a vicious cycle, affecting other equipment, and even causing grid protection devices to operate (such as tripping). Undervoltage protection disconnects the load in time, helping to maintain grid stability.
C,Protect subsequent load devices:
Preventing abnormal operation or damage: While undervoltage protection primarily protects the power supply itself, it also indirectly protects downstream devices. When the input voltage is too low, the power supply may be unable to stably output the required voltage and power, resulting in output voltage drops, increased ripple, or abnormal fluctuations. This unstable power supply can cause downstream electronic devices (such as microprocessors, motor drivers, and displays) to malfunction, lose data, reset, or even damage. Promptly shutting off the power output can prevent load devices from operating under adverse power conditions.
D,Preventing hiccups:
Without undervoltage protection, when the input voltage is critically low, the power supply may repeatedly attempt to start (because the voltage is barely high enough to start the circuitry). However, once it begins operating under load, the input voltage is pulled even lower, causing the power supply to protect itself or shut down. After shutdown, the input voltage rises again, and the power supply attempts to start again, repeating this cycle, creating a "hiccups" phenomenon. This frequent start-stop cycle can not only damage power supply components but also cause downstream loads to repeatedly power on and off, which is very detrimental. Undervoltage protection prevents this oscillation by setting a clear shutdown threshold (usually with a certain hysteresis).
In summary, the core role of the switching power supply undervoltage protection is like a "safety gatekeeper":
Internally (power supply itself): Prevents high current, overheating, and magnetic saturation caused by low input voltage, thereby protecting key power devices (switches, diodes, magnetic components) from damage.
External (input source): Prevent the power supply battery from deep discharge and damage, and avoid dragging down the weak power grid.
For the subsequent stage (load): Prevent the load equipment from operating under unstable or insufficient power supply conditions to reduce the risk of failure.
For the entire system: Provide a clear and stable shutdown point to prevent the system from oscillating (hiccuping) below the critical voltage and ensure controllable behavior under fault conditions.
Therefore, undervoltage protection is an essential safety mechanism in switching power supply design, crucial for improving power supply reliability, durability, and overall system stability. Its threshold (the voltage at which protection is triggered) and hysteresis voltage (the voltage at which operation is restored) require careful design based on the specific application (input source type and load characteristics).
