How to start switching power supply safely and reliably
In the design of switching power supply, the design of starting circuit often affects the starting performance, conversion efficiency and stability of switching power supply under high temperature and high pressure. How does ZLG Zhiyuan electronic switching power supply module design a stable, efficient and safe starting circuit?
While the starting circuit provides energy for the system, it will bring risks to the stability of the power supply because of its serious loss under extreme conditions. A good starting circuit only provides energy for the power supply system when it starts, and stops working when the system runs normally. So how can we make the starting circuit safe and reliable and stop working after the output voltage is established? Let's discuss the starting circuit of switching power supply with me!
Design conception of starting circuit
The input voltage range of DC-DC switching power supply is wide, and the IC chip of power supply needs stable working voltage, so the starting circuit needs to provide safe and stable starting voltage for IC. As shown in Figure 1 below, it is mainly a simple starting circuit composed of resistors and voltage stabilizing tubes. Under normal operation, the starting circuit consumes a lot of power, especially when the switching power supply is under high temperature environment, high input voltage and full output, the starting circuit is seriously overheated, which will easily bring risks to the stability of the system and reduce the conversion efficiency of the switching power supply.
Therefore, the startup circuit is not suitable for providing energy for the power IC and protection circuit for a long time, and generally only provides energy for the system at the startup time. When the output voltage is established, the auxiliary winding with less loss provides energy for the chip and the protection circuit, and the starting circuit needs to stop working at this time.
Common startup circuit design
At present, the starting circuit commonly used in switching power supply uses two triodes for secondary amplification, which can be equivalent to a three-terminal linear regulated power supply. It has the advantages of fast starting speed, safe and reliable performance and immediate stop after the output voltage is established.
Input voltage VIN provides IB current for NPN transistor Q1, which is in the amplification region, and IC is the amplification current and the base of PNP transistor Q2. By controlling the IC current, Q2 can be in a saturated state and charge capacitor C with the saturated current of IE until Q2 is in a semi-closed or semi-saturated state. At this time, the capacitor is equivalent to a constant current source to provide energy for the IC chip. When the capacitor voltage drops to a certain value, the starting circuit continues to charge the capacitor until the auxiliary power supply has a voltage, and then Q1 is turned off through the voltage division between resistors R2 and R3. At this time, the starting circuit stops working, and then the power supply for the chip is completely provided by the auxiliary winding.
The start-up of switching power supply can be divided into three stages. In the first stage, IE charges the capacitor C with a current of approximately 1mA at power-on. When the VDD voltage reaches UCC28C40 mosfet, it enters the second stage, where the saturation current increases to 5mA, and the capacitor continues to be charged while supplying power to the IC. When the output voltage is established, it enters the third stage, when the IE current is zero, the start-up circuit stops working and the VDD voltage rises to the auxiliary winding voltage. During the whole starting process, the current of IE is relatively small and gentle, so the circuit is safe and reliable.
