The metrics for measuring the stability of a switching power supply are phase margin and gain margin. Phase margin refers to the phase at which the gain drops to 0dB. Gain margin refers to the gain at which the phase is zero (actually, it's attenuation). In practical switching power supply design, gain margin is only considered when designing flyback converters; it is generally not used when designing other converters.
In switching power supply design, phase margin has two independent functions: one is to dampen the dynamic processes that occur in the converter during load step changes; the other is to ensure system stability even when component parameters change. Phase margin can only guarantee "small-signal stability." During load step changes, the power supply inevitably enters the "large-signal stability" range. In engineering practice, we consider the loop phase margin to be greater than 45° under room temperature, standard input, and normal load conditions. This phase margin is sufficient to ensure system stability under various parameter variations and errors. If the load changes or the input voltage range varies greatly, the loop and phase margin should be greater than 30° under all loads and input voltages.
In the following sections, we will delve into the testing methods, conditions, and interpretation of results for abnormal outputs during power supply operation. Through comprehensive testing and analysis, we aim to provide reliable data support for power system design and optimization, ensuring that the system maintains a stable and controllable output state under various load variations. These tests are typically part of the design and evaluation of power integrated circuits or power systems. The following are the main contents of these tests:
Voltage rise and fall channel tests:
Objective: To evaluate the response characteristics of a power supply during voltage rise and fall.
Test content: Test whether there are glitch spikes and negative slopes in the voltage rise and fall channels of the power supply, which may have adverse effects on the system.
Overshoot and Undershoot Tests:
Objective: To evaluate whether the system generates instantaneous overshoot and undershoot exceeding or falling below target values when voltage changes.
Test content: Measure the overshoot and undershoot of the actual output relative to the steady-state value, and evaluate the time required to recover to the steady-state value.
Dip, sag, and surge tests:
Objective: To assess whether the power supply exhibits fluctuations, power-down drops, or overvoltages after reaching a steady state.
Test content: Measure the power supply's fluctuation drops, power-down drops, and overvoltages in steady state, as well as the amplitude and duration of these phenomena.
These tests are designed to ensure that the power supply provides a stable and reliable voltage output under varying operating conditions and load changes. In practical applications, the performance of the power supply is crucial for the proper functioning of the electronic equipment connected to its output. By conducting these tests, designers can optimize power supply systems to ensure stable and reliable operation under diverse working conditions.
The purpose of the test is to verify whether the output voltage and signal of the power supply under test meet the specifications when it is turned on/off (to examine whether the feedback design is underdamped or overdamped): whether there is voltage overshoot, voltage drop, oscillation, or ringing.
Input: Minimum and maximum input AC/DC voltage, minimum and maximum AC frequency as defined in the specifications.
Output: Minimum and maximum output loads as defined in the specifications.
Temperature: Minimum operating temperature, ambient temperature and maximum operating temperature
1. Test Procedure
1) Set the minimum ambient operating temperature, minimum input voltage/frequency, and maximum load according to the specifications;
2) Power on/off the device using various power-on/off methods provided by the power supply under test (e.g., AC on/off, Remote on/off), observe the status of each output and signal line, and record the test waveforms (as shown in the figure below):
Does the voltage exhibit glitch (pulse spike) before it enters the rising channel and after it drops to 10%?
"Glitch" refers to a transient, brief malfunction or interference in an electronic or digital system that may cause malfunction or unusual behavior. A glitch typically describes a brief, unexpected signal fluctuation or system behavior that may be caused by electromagnetic interference, power supply noise, inter-signal interference, or other factors.
Overshoot and Undershoot
Overshoot Testing:
Objective: The main objective of overshoot testing is to evaluate whether the system generates a momentary overshoot exceeding a target value when the input signal changes rapidly.
Performance evaluation: Test whether the system generates instantaneous overshoot before reaching a new steady-state value when the input signal changes.
Control system design: For control systems, overshoot testing focuses on whether the system response is properly controlled to avoid exceeding the target value.
Signal processing: In signal processing systems, overshoot testing evaluates the system's ability to process instantaneous signal changes.
Undershoot Testing:
Objective: The main objective of undershoot testing is to evaluate whether the system produces a momentary undershoot below the target value when the input signal changes rapidly.
Performance evaluation: Test whether the system experiences transient undershoot before reaching a new steady-state value when the input signal changes.
Control system design: For control systems, undershoot testing focuses on whether the system response is properly controlled to avoid falling below the target value.
Signal processing: In signal processing systems, undershoot testing evaluates the system's ability to process instantaneous signal changes.
Both tests involve the system's dynamic response to instantaneous signal changes. In electronic system design, through reasonable control system design, controller algorithms, and stability analysis, the effects of overshoot and undershoot can be minimized, ensuring that the system performs stably and reliably when faced with rapidly changing input signals.
"Sag" and "Dip" are two terms used in the field of power systems to describe voltage drops, typically referring to instantaneous voltage decreases. While they may be used synonymously in some contexts, they can have subtle differences in certain situations.
Sag: "Sag" refers to a brief drop in voltage, typically visible over several cycles. Sag can be caused by power failure, transient overload, equipment malfunction, or other unexpected events. It is a transient voltage instability. Generally, Sag refers to a voltage drop below 5% of Vout.
Dip: "Dip" also refers to a brief drop in voltage, similar to Sag. However, "Dip" may sometimes be used more broadly to indicate transient voltage instability, including short-term voltage drops and fluctuations.
The Chinese translations for "Overshoot" and "Surge" are "过冲" and "过压力" respectively.
Overshoot:
"Overshoot" is primarily used to describe phenomena in control systems and signal processing, referring to a situation where the response briefly exceeds a target value before reaching a steady state or target value. It may also be used to describe similar phenomena in circuits and mechanical systems. This is related to the stability and response characteristics of control systems. In control systems, excessive overshoot can lead to system instability or oscillations.
Overvoltage ("Surge"):
"Overvoltage" is primarily used in the power system field to describe the phenomenon of a momentary rise in voltage. Overvoltage can be caused by sudden power outages, sudden load changes, lightning strikes, or other reasons, resulting in transient voltage fluctuations. This is related to equipment protection and stability in power systems. It is commonly used to describe the phenomenon of a momentary rise in power supply voltage in a power system. This can be caused by sudden power outages, sudden load changes, or other reasons. In power systems, overvoltage can damage electronic equipment; therefore, measures are usually taken to suppress overvoltage during power system design.
3) Change the test conditions (output load, input voltage/frequency and ambient temperature) in sequence and repeat step 2.
2. Judgment conditions
(1) The power supply under test can be turned on/off normally and will not be damaged;
(2) No glitch should appear on any output or signal line;
(3) The overshoot, undershoot, and adjustment time Tr1 (including oscillation/ringing) of each output meet the design specifications;
(4) The Dip/Sag/Surge on each output and signal line still meet the design specifications (such as voltage regulation requirements and logic signal high and low level specifications).
3. Improvement Measures
(1) For Glitch, the startup timing of the power supply's internal control chip needs to be checked; sometimes, this problem may also be caused by poor chip design.
(2) Regarding the occurrence of negative slopes:
① Check the startup sequence of different current loops inside the power supply (such as fan startup).
② Observe the changes in the PWM pulses to determine if the feedback circuit needs adjustment;
(3) Adjust the damping coefficient of the feedback circuit for Overshoot and Undershoot.
Reading test results: After adjusting the Slew Rate, select the test item in the test worksheet, turn on the load, and test whether the voltage output overshoots. Fill in the test results (overshoot voltage Vovershoot, overshoot time tos) in the table.
How to test the stability of a DC-DC power module?
Stability is a crucial indicator for evaluating DC power modules, as it directly impacts the operational stability of power products and equipment. The stability of a DC-DC power module can be assessed by measuring parameters such as output voltage, output current, load, waveform, and efficiency.
1. Static Testing Methods
Static testing involves using a DC voltmeter and a load ammeter to measure the power supply's output voltage and current, respectively. Before testing, the power supply needs to be thermally stabilized. During the test, the load current can be varied to obtain the maximum output current and voltage. In addition to output voltage and current, some static test parameters include ripple, fluctuations, wave characteristics, and static output voltage accuracy.
2. Dynamic Testing Methods
Dynamic testing methods primarily assess the load capacity of a power supply. Before dynamic testing, the test time and frequency need to be set. The indicators characterizing load capacity are load capacity and load capacity rise time. During dynamic testing, the test load value and frequency need to be changed to obtain the output voltage and current under normal operating conditions.
3. Load testing methods
First, set the test load, which should meet the power supply specifications.
Next, apply the rated voltage to the power supply and record the output current and voltage values.
Then, change the value of the test load and test the output current and voltage again.
The output current and voltage values under different test conditions are obtained, and the data are analyzed and compared.
In addition to the above testing methods, short-circuit protection tests and temperature tests can also be performed. After the tests are completed, the data can be analyzed to evaluate the stability and reliability of the DC-DC power module.
