Power dynamic response test, what kind of waveform is qualified

Dynamic response generally refers to the response of the control system's output from the initial state to the final state under the action of a typical input signal. When a unit step input x(t) is added to a certain link (system), its response y(t) gradually rises until it stabilizes at a certain value. The changing state of the response y(t) before reaching a certain value is called the transition state (dynamic). This is called dynamic response.
When engineers design power supplies, dynamic response is an indispensable test index. As the loop problem has always been the heart disease of many engineers, let's talk about dynamic response from several aspects and hope that it will be helpful to everyone:
Why does the switching power supply need to test the dynamic response?
General test methods and requirements for power supply dynamic response
Test conditions Test data and schematic diagram
test steps
What kind of result is qualified?
What is the dynamic response related to? How to rectify?
Why does the switching power supply need to test the dynamic response?
Switching power supplies supply power to various electronic devices. Electronic circuits generally need a voltage source that can maintain the output voltage within a specific tolerance range even when the load current is transient to ensure the normal operation of the circuit. Design engineers must understand the principles of transient response and use correct design ideas to improve the transient response performance of the power supply at a lower cost. With the increase in the operating speed and current demand of various electronic devices or microprocessors, when the load current undergoes transient changes, the ability of the regulator to maintain the output voltage within the specified range has become a widespread problem. The power supply specification of a typical CPU chip requires that even if the load current changes by 10 or 20A within a few hundred nanoseconds, the power supply voltage must remain stable. It is not easy to achieve this performance index. It is also a difficult problem encountered by many power supply engineers.
General test methods and requirements for power supply dynamic response
The input and output voltage is a certain value, such as AC220 input power supply, when tested at AC176V, AC220V, AC264V, the output load varies between 25%-50%-25% and 50%-75%-50% of the rated value, and the recovery time When ≤200us, the output voltage overshoot ≤±5% of the output voltage setting value; recovery time>200us, the output voltage overshoot ≤ the output voltage setting value load regulation rate (that is, it is required not to exceed the output voltage setting value ±0.5%). Of course, some of the waveforms are too bad or the sound is too loud, and customers will not accept them. In addition, some require strict temperature limits or input and frequency tests.
Note: The recovery time refers to the period of time from the point where the DC output voltage change rises to greater than the accuracy of the voltage stabilization, and ends when the accuracy of the voltage stabilization is less than or equal to or less than the accuracy of the voltage stabilization.
Test conditions Test data and schematic diagram
input: defined in the specification and input AC/DC voltage, and AC frequency
output: the dynamic load current conditions defined in the specification and the capacitive load allowed by the specification
Temperature: working temperature, normal temperature and working temperature
Oscilloscope sampling mode: generally set to Sample or Hi-res mode
The test diagram is as follows

load setting (as shown below)
Set the start and end points of the load current, the rise and fall rate of the load current (Slew Rate) and the change period of the load current according to the specifications; the general rise and fall speed of the load current are set to 2.5A/uS, and the change period is generally 20ms.
After starting up, adjust the cycle of load current change (by changing t1, t2) according to the specifications.
Output Dynamic Response Test of the power supply test series (Output Dynamic Response Test)
The data in the following figure:
Voltage value: Vo-max, Vovershoot, Vo-stable1, Vo-stable2, Vo-undershoot, Vo-min,
Response time: tR1, tR2
Reference value: t1, t2, Iomax, Io-min; or load duty cycle, frequency, Iomax, Io-min;

Measure waveform data
The data in the following figure:
Voltage value: Vo-max, Vovershoot, Vo-stable1, Vo-stable2, Vo-undershoot, Vo-min,
Response time: tR1, tR2
Reference value: t1, t2, Iomax, Io-min; or load duty cycle, frequency, Iomax, Io-min;

test steps
1) Set the ambient operating temperature, input voltage/frequency; for the output that needs to be tested for dynamic response, set the start and end points of the load current, the rise and fall rate of the load current (Slew Rate) and the load current according to the specifications The change period of the output load; other output loads are set according to the requirements of the Regulation Table;
2) After starting up, adjust the change period (t1, t2) of the load current according to the specification requirements, and observe the change of the output waveform;
3) Record the test conditions of Vo-max, Vovershoot, Vundershoot and Vo-stable1, Vo-min and Vo-stable2, measure the corresponding values of the output voltage and output response time, and save the waveform;
4) Under the dynamic current change cycle of step 3, change other output load conditions to make Vo-max, Vovershoot, Vundershoot and Vo-stable1, Vo-min and Vo-stable2, measure and record the corresponding data;
5) Take the differential load conditions found in steps 3 and 4 as the load, and turn on the power using various power-on methods provided by the power supply to be tested (such as AC on, PS_ON on);
6) Change the test conditions in turn (dynamic load starting point, input voltage/frequency and ambient temperature), repeat steps 2, 3, 4, and 5;
7) Test the dynamic response of other outputs in the same way.
What kind of result is qualified?
Each output measurement value meets the specification requirements:
There should be no ringing (ringing, under-damped feedback loop) phenomenon,
The power supply to be tested cannot be damaged (Damaged/Broken down),
The power supply to be tested cannot work unstable, or even shut down (Shut down),
The response time meets the requirements.
Judgment legend 1
The output measured values in the following figure meet the specification requirements; although there is excessive damping, it is acceptable;


Judgment Legend 2
Although the measured output values in the following figure meet the specification requirements, the feedback loop is under-damped (unstable), so it is unacceptable.

The measured waveform of the last power supply dynamic load

What is the dynamic response related to? How to rectify?
The dynamic load characteristic is related to the response characteristic of the loop. The good and the bad are obvious when the dynamic load is applied.
The relevant materials for assistance explain the characteristics of dynamic characteristics and loop requirements:

The adjustment time of dynamic characteristics is related to the bandwidth. The narrower the bandwidth, the faster the adjustment; the output overshoot is related to the damping coefficient of the circuit, the smaller the damping coefficient, the larger the overshoot
Reference of countermeasures to improve dynamic response:
Appropriately improve the feedback response rate (such as appropriately reducing the capacitance in the RC circuit on the 431, increasing the optocoupler current, and reducing the capacitance in the RC circuit on the current detection PIN pin), but pay attention to noise and heavy load startup problems; in addition; , This plan is also subject to the choice of actual design plan:
The PWM mode is limited by the duty cycle (Flyback: about 0.8, single-ended forward 0.5, others such as Push-pull, Half-bridge, Full-bridge, etc. are 0.8, Boost is 0.9, etc.), so the selection of the duty cycle at the initial design stage A certain margin should be reserved;
The PFM method is also subject to the limitation of the operating frequency to avoid noise or EMI problems;
Under allowable conditions (lower capacitor voltage), try to increase the duty cycle or switching frequency gradually under dynamic conditions to avoid problems such as increased current stress;
Increase the output capacitor capacity or the number of parallel connections, and appropriately reduce the inductance of the output energy storage inductor
The current in the inductor cannot be abruptly changed. This is the key to affecting the output dynamic response, especially in CCM mode. Therefore, appropriately reducing the inductance can improve the dynamic response, but the feedback stability problem at light load needs to be considered (CCM is converted to DCM). (Will cause system instability)
The current of the capacitor can change abruptly. Therefore, it may be considered appropriate to increase the capacity or number of capacitors to improve it, if the Layout space allows.
Multiple converters are used in parallel, but the cost will be higher. This is in situations where the current rate of change requires a higher rate (such as a 3~6 phase V-core circuit powered by a CPU);
Increase the switching frequency to transfer energy at a faster speed, but the frequency characteristics, EMC and efficiency of the components need to be considered;
The above schemes need to be considered comprehensively in actual application. Of course, there may be other solutions that need to be studied.