What performance metrics are affected by the output ripple of a power adapter?
Output ripple of a power adapter refers to a periodic oscillation in the output voltage. Output ripple of a power adapter can affect multiple performance metrics, including power quality, efficiency, noise, electromagnetic compatibility, and device reliability.
First, the output ripple of a power adapter affects power quality. Excessive ripple in the power adapter's output voltage can lead to an unstable power supply to the device, causing instability or malfunction. Conversely, lower output ripple ensures a more stable power supply to the device, effectively preventing malfunctions and instability.
Secondly, the output ripple of a power adapter also affects its efficiency. Excessive output ripple leads to increased energy loss during voltage conversion, thus reducing the adapter's efficiency. Conversely, a power adapter with low output ripple can more efficiently convert the input voltage to a stable output voltage, improving efficiency and reducing energy loss.
Furthermore, the output ripple of the power adapter can also affect the noise of the equipment. Output ripple is reflected on the power lines of the equipment and propagates through the circuitry to other parts of the device, causing electromagnetic interference and noise problems. This noise can interfere with the normal operation of the equipment; therefore, the output ripple of the adapter should be kept within acceptable limits to minimize its impact on the equipment. Additionally, the output ripple of the power adapter is also related to electromagnetic compatibility (EMC). Excessive output ripple increases electromagnetic radiation and susceptibility on the power lines, potentially leading to interference and adverse effects between devices. The adapter should meet the relevant EMC standards to reduce radiation and susceptibility, ensuring that devices can operate normally in an environment free from interference.
Finally, the output ripple of the power adapter also affects the reliability of the device. Excessive output ripple can damage the electronic components of the device, shortening its lifespan or causing it to fail. Conversely, lower output ripple can reduce damage to components and improve the device's reliability and lifespan.
In summary, the output ripple of a power adapter affects multiple performance indicators, including power quality, efficiency, noise, electromagnetic compatibility, and equipment reliability. Therefore, when designing and selecting an adapter, it is crucial to control output ripple to ensure adapter performance and proper equipment operation.
Switching power supplies typically consist of a pulse-width modulation (PWM) control IC and MOSFETs, which control the switching frequency ratio to maintain a stable output voltage. Due to the reduced number of turns in the coil and the smaller core size, switching power supplies have very low losses and generally high efficiency, typically reaching 90%. Furthermore, their small size and stable output give them significant advantages in many aspects.
However, from another perspective, its disadvantages are also obvious. Problems such as circuit complexity, high power supply noise, insufficient transient response, complex output ripple, and susceptibility to electromagnetic interference are also present. For some low-noise circuits, switching power supplies are often powerless. Where do these disadvantages originate, and how can they be avoided? Suppressing ripple improves the overall performance of switching power supplies.
Ripple can cause various damages to circuits, and can be extremely detrimental. Firstly, once ripple is generated, it easily induces harmonics, damaging the circuit itself and reducing power efficiency. Higher ripple levels can cause surges, directly damaging the circuit. Even if it doesn't directly destroy the circuit, it can significantly interfere with the logic of digital circuits, affecting their operation.
The output ripple of a switching power supply mainly originates from the residual low-frequency output ripple, high-frequency ripple at the same high frequency as the switching operation, parasitic common-mode ripple noise, ultra-high-frequency resonant noise from power device switching, and ripple noise caused by closed-loop regulation control. The residual low-frequency output ripple stems from an imperfect filter capacitor in the output circuit. Suppressing this type of ripple is relatively simple; it can be achieved by increasing the output capacitor or using parallel connections to reduce ESR. Ripple caused by closed-loop regulation control can be addressed by directly modifying the regulator parameters, which is also relatively easy to implement.
High-frequency ripple, occurring at the same high frequency as switching operation, appears when power devices perform high-frequency switching, rectification, filtering, and then regulated output of the input DC voltage. It is mainly related to the switching power supply's conversion frequency, the structure of the output filter, and its parameters. Suppression can be achieved by addressing these factors. Increasing the output high-frequency filter or using multi-stage filtering can better suppress ripple. From the switching power supply's operating frequency, increasing the operating frequency can raise the ripple frequency, thereby reducing current fluctuations within the inductor.
Parasitic capacitance and parasitic inductance can be found in many places, such as between power devices and transformers, and in wires. Common-mode ripple noise caused by parasitism needs to be eliminated (suppressed) using specially designed EMI filters. Selecting diodes with better reverse recovery performance is also a very useful method.
The ultra-high frequency resonant noise generated by power devices during switching is quite complex, related not only to junction capacitance but also to transformer leakage inductance, distributed parameters of the switching power supply, and so on. A well-designed PCB layout always provides greater stability to the entire circuit system, and this is a crucial principle when addressing this type of ripple.
Reducing distributed capacitance is a major approach to suppressing UHF resonant noise. Specifically, this can be achieved by using a shielded substrate to reduce the distributed capacitance between the switching transistor and the heat sink, or by improving the winding process and structure to minimize the distributed capacitance between windings. The selection of diodes and switching transistors is also crucial; the junction capacitance of the switching transistor directly affects the noise level. Diodes with soft recovery characteristics are preferable to minimize UHF resonant noise.
In addition, temperature changes can alter the parameters of the device, thus affecting the ripple, which also needs to be considered.
EMI damage of switching power supplies
EMI exists in any electronic system. In switching power supplies, transistors and diodes experience significant current changes during the rise and fall times of switching, easily generating radio frequency energy and becoming interference sources. Interference sources are easily generated in components such as switching transistors, diodes, and high-frequency transformers. Moreover, EMI signals from switching power supplies occupy a wide frequency range and have a certain amplitude.
We always try our best to suppress EMI, and there are many technologies used to suppress EMI. In switching power supplies, filtering, shielding, sealing, and grounding technologies are commonly used for EMI control. During the switching process of switching transistors and diodes, voltage spikes can easily be generated due to transformer leakage inductance and line inductance. In this case, an RC/RCD snubber circuit is usually used, which can significantly improve the switching waveform.
Reducing the du/dt ratio during the switching of power transistors is a crucial aspect of suppressing interference in switching power supplies. Adding a small inductor and capacitor—resonant components—to the switching circuit creates a soft-switching circuit. Soft-switching circuits introduce resonance before and after the switching process, eliminating voltage and current overlap and significantly reducing switching losses and interference.
summary
In switching power supply design, ripple and EMI are both significant factors that negatively impact circuitry. Moreover, they originate from numerous sources and are widely distributed; therefore, effectively suppressing them is crucial for improving the stability and efficiency of switching power supplies.
