# Talking about the design of front-end EMC from the perspective of surge immunity

Dynamic response is an important indicator in power supply testing. To design a power supply dynamic response that meets the requirements, loop problems are bound to be involved. Speaking of loop design, it will make many engineers frown and have no way of starting the dilemma. Now this article talks about dynamic response design ideas from the following aspects to solve the thorny problems encountered in design.

Dynamic response is an important indicator in power supply testing. To design a power supply dynamic response that meets the requirements, loop problems are bound to be involved. Speaking of loop design, it will make many engineers frown and have no way of starting the dilemma. Now this article talks about dynamic response design ideas from the following aspects to solve the thorny problems encountered in design.

1. Why does a switching power supply need a dynamic response test?

Switching power supply is the power supply “heart” of the entire system. When its output load current jumps, it has strict design requirements for the fluctuation of the output voltage: it must be limited to a certain tolerance range to ensure the stable operation of the system circuit. However, with the development of Electronic technology, various electronic devices with low voltage and high current have become the mainstream design ideas. The current demand continues to increase. It is necessary to ensure that the power output fluctuates within a range. This has become a thorny problem in the industry and has been troubled. engineer.

2. Test methods and requirements for dynamic response of power supply

Input: the minimum, nominal and maximum input DC voltage defined in the specification; output: the dynamic load current conditions defined in the specification and the minimum capacitive load allowed by the specification. For example, E2405UHBD-20W, input voltage range: 9-36VDC, select fixed input voltage 9VDC, 24VDC, 36VDC, nominal output voltage 5V, load current 25%-50%-25% and 50%-75 of rated value Test under the change of %-50%, as shown in Figure 1.

Figure 1 Test schematic

There are strict requirements for the dynamic change of the load. The starting point and end point of the load current, the rising and falling rate of the load current, and the change period of the load current are set according to the specifications, as shown in Figure 2. After powering on the product, adjust the load current cycle t12, t23, tr, tf according to the specifications.

Figure 2 Schematic diagram of dynamic load current setting

Three, the test judgment criterion of dynamic response

After connecting the test circuit diagram and adjusting the test parameters according to Figure 1 and Figure 2, the output load changes between 25%-50%-25% and 50%-75%-50% of the rated value (the slope of the current jump is 0.08 ~0.1A/uS, the period is greater than that required for complete dynamic stability), use an oscilloscope to capture the product output ripple and noise waveform, and adjust the waveform to the dynamic waveform Display, record the overshoot and undershoot amplitude and recovery time at this time; adjust the power supply voltage Record the overshoot and undershoot amplitude and recovery time under different combinations with load jump mode.

Judgment condition

a Each output measurement value meets the specification requirements;

b There should be no ringing (under-damped feedback loop);

c The power supply to be tested must not be damaged;

d The power supply to be tested cannot work unstable or even shut down (Shut down);

e The response time meets the requirements.

4. What is the dynamic response related to? How to rectify?

The load dynamic characteristics of the power supply are related to the response characteristics of the loop. Whether it is good or bad is particularly obvious when it is a dynamic load. In short, the rectification strategy guiding idea: “The wider the bandwidth of the feedback loop, the faster the dynamic response speed; the smaller the damping coefficient, the greater the overshoot.”

a Under the conditions of high-frequency characteristics, EMC and efficiency of the device, improve the dynamic response speed by increasing the switching frequency, or gradually increase the duty cycle under dynamic conditions to avoid problems such as sudden current changes;

b Using capacitors and inductors, the larger the capacitance, the smaller the dynamic response; the smaller the inductance, the faster the dynamic response;

c. The wider the bandwidth of the feedback loop, the faster the dynamic response speed, that is, the faster the dynamic response speed of the system under high gain. The zero point will improve the dynamic response speed, and the pole will slow down the dynamic response speed. That is, the leading compensation makes the system response faster, and the lag compensation makes the system response slower.

d The smaller the phase threshold, the faster the dynamic response, but if it is too small, the system will oscillate.

e Using high-reliability power modules, the isolated power modules independently developed and produced by Zhiyuan Electronics have been accumulated in the industry for nearly 20 years. The current products have a wide input voltage range, isolated from multiple series such as 1000VDC, 1500VDC, 3000VDC and 6000VDC, and packaged Various forms, compatible with international standard SIP, DIP and other packages. At the same time, Zhiyuan Electronics has built a first-class test laboratory in the industry to ensure the performance of power products, equipped with the most advanced and complete test equipment. The full series of isolated DC-DC power supplies have passed complete EMC tests, and the electrostatic immunity is as high as 4KV and surge immunity. The disturbance level is as high as 2KV, which can be applied to most complex and harsh industrial sites, providing users with stable and reliable power isolation solutions.