A quality power supply must have: start-up design

A high-quality power supply must have the characteristics of good starting performance and high conversion efficiency, but have you ever thought that the input voltage range of wide-voltage power supply is so wide, and the power IC chip needs a stable working voltage, how can we guarantee the performance of the module? What? This article is for you to answer, let you understand the power module from the essence.

The startup circuit provides energy to the system and poses a risk to the stability of the power supply due to its severe loss in extreme conditions. A good startup circuit only supplies energy to the power system when it is started, and stops when the system is running normally. So how can the startup circuit be safe and reliable and stop working after the output voltage is established? Let me discuss the startup circuit of the switching power supply with me!

First, start circuit design concept

The input voltage range of the DC-DC switching power supply is wide, and the power IC chip needs a stable operating voltage, so the starting circuit needs to provide a safe and stable starting voltage for the IC. As shown in Figure 1 below, it is mainly a simple starting circuit composed of a resistor and a voltage regulator. The power consumption of the startup circuit is large under normal operation, especially when the switching power supply is in a high temperature environment, input high voltage, and output full load. Severe and easy to bring risks to the stability of the system, but also reduce the conversion efficiency of the switching power supply.

Figure 1 simple start circuit

Therefore, the startup circuit is not suitable for providing power to the power IC and the protection circuit for a long time, and generally only provides energy at the time of system startup. When the output voltage is established, the auxiliary windings with less loss provide energy for the chip and the protection circuit, and the startup circuit at this time needs to stop working.

Second, a startup circuit design

As shown in Figure 2 below, it is a commonly used startup circuit in switching power supply. The circuit uses two triodes for secondary amplification, which can be equivalent to a three-terminal linear regulated power supply. It has fast startup speed, safe and reliable performance, and output voltage is established. The advantage of stopping work immediately afterwards.

The input voltage VIN provides the I B current for the NPN transistor Q1. It is in the amplification region, and the I C is the amplification current and the base current of the PNP transistor Q2. By controlling the I C current, the Q2 can be saturated and I E The saturation current charges capacitor C until Q2 is in a half-off or half-saturated state. At this point, the capacitor is equivalent to a constant current source to provide energy for the I C chip. When the capacitor voltage drops to a certain value, the startup circuit continues to charge the capacitor until the auxiliary power supply has a voltage before passing through the resistors R2 and R3. The voltage division causes Q1 to be in an off state, at which point the startup circuit stops working, and then the power supply to the chip is completely provided by the auxiliary winding.

Figure 2 standard startup circuit

As shown in Figure 3 below, the experimental waveform of the circuit of Figure 2, green is the I E current waveform, and yellow is the V DD voltage waveform (the oscilloscope uses zlgZDS2022). It can be seen from the figure that the switching power supply can be started in three stages. In the first stage, I E charges the capacitor C with a current of approximately 1 mA at the time of power-on, and enters the second stage when the V DD voltage reaches the UCC28C40 threshold voltage. When the saturation current is increased to 5mA, the capacitor is continuously charged while supplying power to I C. When the output voltage is established, it enters the third stage. At this time, the IE current is zero, the startup circuit stops working, and the V DD voltage rises to the auxiliary winding voltage. During the entire startup process, the current of I E is relatively small and relatively flat, so the circuit is safe and reliable.

Figure 3 Experimental circuit waveform

The wide-voltage switching power supply products developed by ZLG Zhiyuan Electronics basically adopt the structure of the above-mentioned starting circuit type, which can ensure the reliability of starting and reduce the loss.

Third, how to make the startup circuit safe and reliable

In order for the startup circuit to operate safely and reliably, in addition to the necessary theoretical calculations, more attention should be paid to the choice of the device. Careful device selection can bring the true value of the circuit closer to the theoretical value of the calculation. The Zener diode D1 should select a small dynamic resistance and a low knee point, so that the potential of the base of Q1 maintains a small fluctuation under a large change of the input voltage, thereby making the supply voltage V DD stable. The resistance values ​​of the resistors R1, R2, and R3 are as large as possible under the normal operation of the circuit to reduce the loss of the startup circuit. R4 mainly limits the I E current so that Q2 quickly reaches the saturation point. If the condition allows the Q2 package to be as large as possible to enhance the heat dissipation capability.

The voltage of the auxiliary winding is also a factor affecting the stability of the starting circuit. If the auxiliary winding voltage is low, the starting circuit will not be completely cut off when the switching power supply is loaded. The Q2 tube may be burnt due to overheating under high temperature and high voltage full load. The auxiliary winding voltage is too high. Under certain abnormal conditions, the voltage supplied by the auxiliary winding approaches or exceeds the rated voltage of the power IC, posing a threat to the power source I C . Excessive voltage of the auxiliary winding will also have an impact on the overall efficiency of the switching power supply. The design of the starting circuit will affect the performance of the entire switching system.

Fourth, select high-quality isolated power modules to reduce the design risk of the circuit

ZLG Zhiyuan Electronics independently develops and produces isolated power modules with wide input voltage range, isolated 1000VDC, 1500VDC, 3000VDC series, various package types, compatible with international standard SIP, DIP and other packages. At the same time, ZLG Zhiyuan Electronics can provide users with professional power supply peripheral application circuit design experience reference based on rich power supply design and application experience to improve product reliability.

ZLG Zhiyuan electronic power module is widely used in power supply and industrial power because it has high efficiency, wide input voltage range, small size, high reliability, good impact resistance, good isolation and wide temperature range. Automation, communications, medical, transportation, building automation, instrumentation and automotive electronics.