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How to design a stable, efficient and safe startup switching power supply circuit?
In the design of a switching power supply, the starting circuit plays a crucial role in determining the overall performance, particularly in terms of startup efficiency, conversion efficiency, and stability under extreme conditions like high temperatures and high voltages. How does ZLG Zhiyuan Electronic ensure the creation of a robust, efficient, and safe starting circuit for their switching power supply modules?
The startup circuit is responsible for supplying initial energy to the system, but it can pose significant risks to the stability of the power supply if it operates inefficiently or continuously under normal conditions. Ideally, a good startup circuit only functions during the initial startup phase and ceases operation once the system stabilizes. But how do we ensure that this circuit operates safely and efficiently while ceasing function post-stabilization? Let’s delve into the details of the startup circuit in a switching power supply.
Firstly, let's consider the concept behind the startup circuit design. Given the wide input voltage range of a DC-DC converter, the power IC requires a stable operating voltage. Thus, the startup circuit must provide a reliable and steady starting voltage to the IC. Typically, this involves a simple circuit consisting of a resistor and a voltage regulator, as depicted in Figure 1 below. However, this basic design results in excessive power loss under normal operation, particularly in high-temperature environments, high-voltage inputs, and full-load outputs. Such inefficiencies can compromise system stability and diminish the overall efficiency of the switching power supply.
Therefore, the startup circuit isn’t designed to supply power to the power IC and protection circuits indefinitely; instead, it only provides energy at the moment of system startup. Once the output voltage is established, the auxiliary windings with lower losses take over to supply energy to the chip and protection circuits, while the startup circuit simultaneously shuts down.
Now, let's explore some common startup circuit designs. Figure 2 illustrates 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 offers rapid startup times, reliable performance, and promptly halts operation once the output voltage is established.
In this circuit, the input voltage VIN provides the base current IB for the NPN transistor Q1, which remains in the active region. The collector current IC amplifies this current and serves as the base current for the PNP transistor Q2. By controlling the IC current, Q2 can be driven into saturation, allowing the emitter current IE to charge capacitor C until Q2 reaches a semi-saturated or semi-cut-off state. At this point, the capacitor functions as a constant current source, supplying energy to the IC chip. When the capacitor voltage falls below a certain threshold, the startup circuit resumes charging the capacitor until the auxiliary power supply voltage stabilizes. The resistors R2 and R3 then divide the voltage, causing Q1 to turn off, thereby halting the startup circuit’s operation. The chip is now fully powered by the auxiliary winding.
Figure 3 shows the experimental waveform of this circuit. The green trace represents the IE current waveform, while the yellow trace depicts the VDD voltage waveform (measured using the zlgZDS2022 oscilloscope). From the diagram, we can observe that the switching power supply initiates in three distinct phases. Initially, IE charges the capacitor C at approximately 1 mA during power-on, transitioning to phase two when the VDD voltage reaches the UCC28C40 threshold voltage, increasing to 5 mA. During this phase, IE continues to charge the capacitor while powering the IC. Once the output voltage stabilizes, the third phase begins. Here, the IE current drops to zero, signaling the shutdown of the startup circuit, with the VDD voltage rising to the auxiliary winding voltage. Throughout the startup process, the IE current remains relatively small and stable, ensuring safety and reliability.
To ensure the startup circuit operates safely and reliably, careful consideration must go into component selection beyond just theoretical calculations. Choosing high-quality components brings the practical performance closer to the calculated theoretical values. For instance, the Zener diode D1 should have a low dynamic resistance and a low knee point to maintain minimal fluctuations in the base potential of Q1 despite large variations in input voltage, thus maintaining a stable supply voltage VDD. The resistors R1, R2, and R3 should be set as high as possible during normal circuit operation to minimize the losses in the startup circuit. R4 primarily limits the IE current, enabling Q2 to reach saturation quickly. If conditions permit, selecting a larger Q2 package enhances its heat dissipation capabilities.
The voltage of the auxiliary winding also impacts the stability of the startup circuit. An insufficient auxiliary winding voltage might prevent complete shutdown of the startup circuit under full load conditions, potentially leading to overheating and damage of the Q2 transistor under high temperature and high voltage scenarios. Conversely, an excessively high auxiliary winding voltage could endanger the power IC under certain abnormal conditions, as the voltage supplied by the auxiliary winding might approach or exceed the rated voltage of the IC, posing a threat. Furthermore, overly high auxiliary winding voltages can negatively affect the overall efficiency of the switching power supply.
When designing a switching power supply, choosing high-quality isolated power modules can significantly enhance circuit efficiency. Zhiyuan Electronics offers a range of independently developed and manufactured isolated power supply modules, available in 1000VDC, 1500VDC, and 3000VDC series. These modules come in various package types, compatible with international standards such as SIP and DIP, and can be customized based on specific project requirements. These tailor-made features ensure that the isolated power supply meets unique application demands.
Zhiyuan Electronic power modules are renowned for their high efficiency, wide input voltage range, compact size, exceptional reliability, excellent shock resistance, strong isolation capabilities, and broad temperature range. They find applications in power supplies, industrial automation, communications, medical equipment, transportation, building automation, instrumentation, and automotive electronics.
By focusing on these design principles and leveraging high-quality components, Zhiyuan Electronics ensures that their switching power supply modules deliver superior performance, reliability, and efficiency across diverse applications.