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How to design a stable, efficient and safe startup switching power supply circuit?
When designing a switching power supply, the startup circuit plays a crucial role in determining the performance, efficiency, and stability under extreme conditions like high temperatures and voltages. How does ZLG Zhiyuan Electronic manage to create a startup circuit that is both stable and efficient?
The startup circuit serves as the initial power provider for the system. However, it can pose risks to the overall stability of the power supply, especially when it experiences significant losses under extreme conditions. Ideally, a well-designed startup circuit should only supply energy during the system's startup phase and cease functioning once the system operates normally. But how do we ensure that the startup circuit works safely and reliably, ceasing operation once the output voltage stabilizes? Let’s explore the startup circuit design of a switching power supply.
Firstly, the concept behind the startup circuit design. The input voltage range of a DC-DC switching power supply is broad, requiring the power IC chip to have a stable operating voltage. Thus, the startup circuit must provide a safe and stable startup voltage for the IC. As depicted in Figure 1, a basic startup circuit typically consists of a resistor and a voltage regulator. While this setup is simple, it incurs substantial power loss during normal operation, particularly in high-temperature environments, under high input voltages, and when the output is fully loaded. Such severe conditions not only endanger the system's stability but also reduce the overall efficiency of the switching power supply.
Therefore, the startup circuit isn’t suitable for continuously powering the power IC or the protection circuits. Instead, it should only provide energy during system startup. Once the output voltage stabilizes, an auxiliary winding with minimal loss takes over to supply energy to the chip and the protection circuit, at which point the startup circuit should stop functioning.
Secondly, let’s look at common startup circuit designs. As illustrated in Figure 2, this is a widely-used startup circuit in switching power supplies. This circuit employs two transistors for secondary amplification, effectively acting as a three-terminal linear regulated power supply. It boasts rapid startup speed, safe and reliable performance, and promptly stops working once the output voltage is established.
The input voltage VIN provides the base current IB for the NPN transistor Q1. Being in the amplification region, the collector current IC flows into the base of the PNP transistor Q2. By controlling the IC current, Q2 can reach saturation, and its emitter current IE charges the capacitor C until Q2 is semi-saturated. At this point, the capacitor acts as a constant current source, supplying energy to the IC chip. When the capacitor voltage drops below a certain level, the startup circuit continues charging the capacitor until the auxiliary power supply generates a voltage. Then, through the voltage division by resistors R2 and R3, Q1 turns off, halting the startup circuit, and the chip is powered solely by the auxiliary winding.
As shown in Figure 3, the experimental waveform of this circuit demonstrates the startup process in three stages. Initially, IE charges the capacitor C with approximately 1 mA during power-on. When the VDD voltage reaches the UCC28C40 threshold voltage, it increases to 5 mA, continuing to charge the capacitor while powering the IC. Once the output voltage stabilizes, the third stage begins. Here, the IE current becomes zero, signaling the end of the startup circuit’s operation, and the VDD voltage rises to the auxiliary winding voltage. Throughout the startup process, the IE current remains relatively small and steady, ensuring safety and reliability.
To ensure the startup circuit operates safely and reliably, besides necessary theoretical calculations, careful component selection is critical. Selecting high-quality components ensures the actual circuit values closely match the calculated theoretical values. The Zener diode D1 should have a small dynamic resistance and a low knee point, maintaining a stable base potential for Q1 despite large fluctuations in input voltage, thus stabilizing the supply voltage VDD. Resistors R1, R2, and R3 should have as high values as possible during normal circuit operation to minimize startup circuit losses. R4 primarily limits the IE current, enabling Q2 to quickly reach saturation. If conditions permit, choosing a larger Q2 package can enhance heat dissipation capabilities.
The voltage of the auxiliary winding also impacts the startup circuit’s stability. If the auxiliary winding voltage is too low, the startup circuit won't fully deactivate under heavy load conditions, potentially overheating and damaging the Q2 transistor under high temperature and voltage. Conversely, if the auxiliary winding voltage is too high, under certain abnormal conditions, the voltage supplied by the auxiliary winding could approach or exceed the power IC’s rated voltage, posing a threat. Additionally, excessive voltage on the auxiliary winding negatively affects the overall efficiency of the switching power supply.
Lastly, choosing high-quality isolated power modules enhances circuit efficiency. Zhiyuan Electronics’ independently developed and manufactured isolated power supply modules feature a wide input voltage range and are isolated at 1000VDC, 1500VDC, and 3000VDC levels. These modules come in various package types, including SIP, DIP, and others, and can be customized to meet specific project requirements. Due to their high efficiency, wide input voltage range, compact size, high reliability, excellent shock resistance, strong isolation properties, and wide temperature range, Zhiyuan’s power modules find applications in power supply, industrial automation, communications, medical devices, transportation, building automation, instrumentation, and automotive electronics.