How to improve the fast transient response of switching power supplies?

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How to improve the fast transient response of switching power supplies? [Copy link]

Transient response describes the ability of DCDC to respond to rapidly changing loads. For CPU core voltage or RF power amplifier circuit, transient response is a very important indicator. In the IEEE 802.11 standard, the time for the output power of the device to rise from 10% to 90% is specified. In order not to affect the performance of the product, we certainly hope that the rise time is as short as possible. The RF circuit itself often does not limit the rise time, but this puts higher requirements on the power supply circuit: it must respond in a very short time and provide the maximum output power. Observing the topological structures of the two DCDC feedback circuits in the figure below, it is not difficult to find that the feedback circuit on the right has an additional 10pF capacitor than the feedback circuit on the left. This capacitor is not simple. We will see the purpose of this capacitor later. This capacitor is called a "feedforward capacitor."

Adding capacitor to the feedback circuit

When there is no feedforward capacitor, the feedback network of the DCDC consists of two resistors, which are used to set the output voltage of the DCDC converter. Its transfer function is shown in the figure below.

Transfer function without feedforward capacitor

With the addition of feedforward capacitor, the DCDC converter can respond to high frequency disturbances more efficiently.

Transfer function with feedforward capacitor

From the perspective of the loop, incorporating a capacitor means introducing a zero and a pole, where the zero is lower than the pole. This will make the phase smoother, and of course the bandwidth will be wider accordingly. Comparing the two graphs, it is not difficult to find that the two feedback structures have consistent response curves in the low frequency band; in the mid-to-high frequency region, the reduced path impedance of C1 minimizes the output power supply disturbance, effectively providing higher gain and phase. In order to suppress transient ripple more, the size of the feedforward capacitor can be increased. For a running DCDC power converter, the increase in gain and phase prompts the converter to respond faster to transient loads.

lblgjcxm

I want to know, if adding this capacitor has any negative impact, if not, why not seal it directly inside the DCDC power supply?

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lblgjcxm posted on 2019-12-23 14:23 I want to know if adding this capacitor will have any negative impact, if not, why not seal it directly in the DCDC power supply?

The use of feedforward capacitors is different with or without them. The difference between low frequency band and medium and high frequency band has been explained in the article.

lblgjcxm

Screenshot from the datasheet, voltage response when load current changes suddenly, it seems that there is no negative impact, in most cases it is better to add

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lblgjcxm posted on 2019-12-24 10:47 The screenshot from the data sheet shows the voltage response when the load current changes suddenly. It seems that there is no negative impact. In most cases, it is better to add it

In fact, many DCDC chip outputs, especially in the feedback output, the manual specification recommends

For example, TI's LMR70503

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MP2233