Because a feedforward capacitor is added to the feedback terminal of the power supply, it forms new zeros and poles with the feedback resistor. Although Cff introduces a gain boost after its zero frequency, this involves complex control theory and will not be elaborated further. The output high-frequency signal can quickly reach the FB pin of the chip through the capacitor, enabling the power supply chip to respond quickly to changes in the downstream load.
Compared to traditional fixed-frequency control, the COT architecture offers faster dynamic response, a significant advantage particularly in low duty cycle applications. When the load changes abruptly, the output voltage drops. If it falls below the reference voltage, the high-side switch immediately opens, unlike fixed-frequency control which requires an external clock signal to trigger the high-side switch. During the TON phase, the inductor current rises. If the feedback voltage remains below the reference voltage, a new TON is initiated after a minimum off-time. This continuous opening and closing of the high-side switch continues until both the output voltage and inductor current reach the expected values. This is the source of COT's fast dynamic response—by altering the switching frequency to influence the duty cycle and achieve stable output. Examples include MPS's MPQ8633A and MPQ8633B chips.
The output ripple of a DC-DC converter is a systemic issue affecting the power supply plane. In most cases, the measures mentioned above can theoretically reduce output ripple. However, careful attention must be paid to PCB layout and routing, such as minimizing power loops and keeping feedback traces short and away from other interfering signal lines. With the development of switching power supplies, more and more traditional linear power supplies are being replaced by their superior performance. However, output ripple in switching power supplies has always been a headache for engineers. So, what is power supply ripple, and how is it generated? We know that the DC output voltage of a switching power supply is obtained by rectifying, filtering, and regulating the AC voltage. Due to imperfect design of the filtering circuit, periodic and random noise will be attached to the DC level, resulting in ripple. Under rated output voltage and current, the peak value of the AC voltage in the output DC voltage is what is commonly referred to as the ripple voltage. Simply put, ripple is the AC component superimposed on the stable DC output.
Methods for testing power supply ripple and suppressing power supply ripple
Ripple in switching power supplies reduces power efficiency. Higher ripple can also generate surge voltages or currents, and in digital circuits, ripple can interfere with circuit logic levels, making it entirely detrimental. In the design of switching power supplies, we not only need to measure ripple correctly, but also to reduce it as much as possible.
The correct method for measuring ripple typically involves the following steps:
1. First, select the appropriate probe setting. Under normal circumstances, it is recommended to use the X1 setting to avoid unnecessary noise attenuation affecting ripple measurement;
2. Select AC coupling as the channel coupling mode to limit the input of DC signals;
3. Enable the oscilloscope's "bandwidth limiting" function and select "20MHz" bandwidth limit to filter out unnecessary high-frequency noise;
4. To avoid interference from electromagnetic radiation and other sources, it is recommended to use a grounding spring during measurement to avoid unnecessary interference from long grounding wires;
5. Adjust the horizontal time base, vertical setting, and offset to display the ripple signal in the center of the screen (the blue box in Figure 2 shows the oscilloscope ripple test results).
1. Low-frequency ripple noise
This is mainly due to the rapid changes in the switching state of the transistor. During the switching process, the rapid changes in current and voltage generate low-frequency ripple noise. This noise typically has a low frequency, but it can have a significant impact on circuit performance.
Suppression methods:
①Increase the inductor and capacitor parameters of the output low-frequency filter to reduce them to the required specifications;
② Adopt a forward feedback control method to reduce low-frequency ripple components.
2. High-frequency ripple noise
This is mainly caused by component parameter mismatch or parasitic effects in the internal circuitry of the switching power supply. For example, parasitic inductance of capacitors and parasitic capacitance of resistors can generate high-frequency ripple noise. This noise typically has a high frequency but a relatively small amplitude.
Suppression methods:
① Employ multi-stage filtering;
② Increasing the output high-frequency filter can suppress high-frequency output ripple;
③ Increase the operating frequency of the switching power supply to increase the high-frequency ripple frequency.
3. Common-mode ripple noise
This noise is generated due to the potential difference between components within the internal circuitry of the switching power supply. When the switching power supply operates at high frequencies, this potential difference causes current to flow between the components, resulting in common-mode ripple noise. This noise significantly impacts circuit performance and stability, requiring measures to suppress it.
Suppression methods:
① Employ specially designed EMI filters;
②Reduce the amplitude of the switch.
4. Ultra-high frequency resonant noise
This noise is generated due to the resonance effect between components in the internal circuitry of the switching power supply. When the switching power supply operates at high frequencies, the resonance effect between components causes fluctuations in current and voltage, thus generating ultra-high frequency resonant noise. This noise has a significant impact on circuit performance and stability, and measures need to be taken to suppress it.
Suppression methods:
① Employs a soft-recovery diode;
② Use switching transistors with small junction capacitance;
③ Reduce wiring length.
5. Ripple noise caused by closed-loop regulation and control
Closed-loop regulation control is one of the commonly used control methods in switching power supplies. However, closed-loop regulation control may cause changes in the component parameters of the internal circuitry of the switching power supply, thereby generating ripple noise. This noise is usually low in frequency, but it can have a significant impact on circuit performance.
Suppression methods:
① A compensation network to ground is added to the regulator output because this compensation can suppress the increase in ripple caused by regulator self-oscillation;
② Select the appropriate open-loop gain and parameters of the closed-loop controller, preferably adjusting them according to the load conditions;
③ Do not add a pure time delay filter in the feedback channel. The advantage of doing so is that the time delay can be minimized, thereby increasing the speed and timeliness of closed-loop regulation, which is beneficial for suppressing output voltage ripple.
