Output transient response

One of the important characteristics of output voltage is transient response. The transient refers to the output current, that is, the load current changes rapidly, so it is the output voltage load transient response characteristics. Katakana is sometimes used directly in English terminology, and the transient response is called transient response.

This feature is important because it is closely related to the stability of the output voltage for the load current, which can be optimized by the component constant of the external circuit, in other words, it can be adjusted to a better characteristic by confirming the characteristic.

To confirm the transient response, observe the voltage waveform after combining the following conditions, and use the current probe to confirm the waveform of oscilloscope, load device and load current.

The confirmation condition is > 0

Input voltage: minimum and maximum

· load current: Min → max, min ← Max

Environment: upper and lower limits of temperature conditions

When the load is continuously switched on and off, it is possible to observe a waveform similar to the lower left waveform. When the load current decreases sharply, the output voltage will rise instantaneously and return to the set voltage after a certain time. When the load current increases rapidly, the situation is just the opposite, and the output voltage will first drop and then return to the original state. The main points of observation are

1) After the output voltage changes, how long will it take to return to the stable state

2) After the output voltage changes, whether there are ringing, overshoot, undershoot and other waveform disorderThese two points.

1) It refers to the characteristics of small variable voltage and fast recovery time of stable state, which can also be said to be high-speed transient response, and the change will end soon. 2）。 As shown in the oscillogram, the best case is that no ringing occurs. The lower left waveform is an example of a good waveform.

If the transient response characteristics do not meet the requirements, the phase margin and gain margin of the feedback loop will be adjusted. Specifically, it is to adjust the component constant of the relevant external circuit in the upper right circuit diagram. Work and verify these components repeatedly and adjust them at the same time, but if you don’t have relevant experience, you can’t foresee how to adjust them. It is a feasible method to measure the quantitative phase and gain state, confirm the margin and then adjust it. Using fra (frequency response analyzer) is not only convenient but also easy to complete the operation.

In the adjustment, because the stability will be reduced by accelerating the response speed, it is necessary to maintain the phase margin while adjusting the fastest response speed. To remind you, the output change caused by load transient cannot be reduced to zero.

Output voltage rise waveform

At the same time, the output voltage waveform is confirmed, but the rising characteristic of output voltage is observed when the input power is on. The method and measuring instrument are almost the same as measuring the transient response characteristics. The evaluation can be carried out more efficiently by setting up a series of confirmed items. The only difference is that it is not necessary to switch the output load current continuously.

The confirmation condition is > 0

Input voltage: minimum and maximum

Environment: upper and lower limits of temperature conditions

* confirm whether there is ringing, overshoot or undershoot.

The confirmation should be carried out in combination with the above conditions. For example, set the minimum input voltage, maximum load current and lower temperature limit, input power on and confirm the waveform of output voltage. These are confirmed in one breath. The key point of observation is whether the output waveform produces ringing, overshoot or undershoot. When this kind of phenomenon occurs, it will not only take a lot of time to adjust the output voltage until it is stable. If the change is too large, even the power supply equipment may malfunction, and even must be reset.

In fact, we can regard it as one of the output reaction characteristics. This represents a balance between the transient response and the transient response, which can be confirmed in a series of steps. The optimization of ringing, overshoot or undershoot is the same as the transient response, which is realized by adjusting the phase margin and gain margin. In addition, the relationship between soft start and load capacitance will also cause rise waveform variation. Therefore, it is necessary to explore whether the waveform observed in a certain way is affected by these two factors.

The waveform shown is a good characteristic display.

Key points:

The transient response of the output load should be optimized by adjusting the phase compensation circuit of the feedback circuit.

The optimization of output rising waveform is the same.

The assessment and adjustment are interrelated and belong to a series of operations.