Ripple noise is a critical parameter in power supply design. A high-quality power supply must maintain its output ripple within acceptable limits. But what are the effective methods to reduce ripple noise? Here are eight commonly used techniques that can help improve power supply performance.
**1. Power PCB Routing and Layout**
Proper layout and routing play a crucial role in minimizing ripple. The feedback circuit should be kept away from magnetic components, switching transistors, and power diodes to avoid interference. The placement of output filter capacitors and their traces is especially important. In traditional designs, the impedance differences among capacitors may lead to uneven distribution of high-frequency currents. To optimize this, the loop lengths should be equal so that the current is evenly distributed among the capacitors.
If using a multi-layer PCB, consider covering the main current loop layer with a ground plane. This can significantly reduce noise, provided the ground plane remains intact and continuous.
**2. Add RCD Between FET Drain and Input Positive**
To prevent through-current formation caused by the slower reverse recovery time of FETs compared to diodes, an external gate resistor (R4) can be added to slow down the switching speed. However, to maintain fast turn-off, a diode (D2) can be connected in parallel with the gate resistor, as shown in Figure 2.
**3. Parallel RC Across FET DS**
Adding an RC circuit across the FET’s drain and source helps suppress noise. Typically, a 100pF capacitor (C2) is used, with a resistor (R2) around 10Ω. Too large a capacitance increases switching losses, so it's important to choose the right values.
**4. RC Snubber Across Output Diode**
High-speed switching of the diode can cause parasitic inductance and capacitance to generate high-frequency oscillations during reverse recovery. An RC snubber at the diode’s terminals can dampen these oscillations. Common values are 1–100Ω for the resistor (R3) and 100pF–1nF for the capacitor (C3). For low-frequency applications, a diode with slow reverse recovery can also be used.
**5. Add Secondary LC Filter**
An LC filter is highly effective in reducing ripple and noise. Choosing the right inductance and capacitance based on the ripple frequency is essential. Cylindrical inductors are often used due to their cost and size, but they can create magnetic interference. To reduce this, two inductors can be placed side by side with opposing current directions, which helps cancel out magnetic fields.
**6. Add Faraday Shield Between Primary and Secondary**
A Faraday shield between the primary and secondary windings of a transformer can capture noise at the isolation boundary and redirect it back to the input side. The shield should be a thin copper foil strip to minimize eddy current losses and leakage inductance. Importantly, there should be no electrical connection at the end of the shield to avoid creating a magnetic short.
**7. Reduce Transformer Leakage Inductance**
Using a sandwich winding technique can increase coupling between primary and secondary windings, thereby reducing leakage inductance and lowering noise levels.
**8. Proper Parallel Connection of Transformer Windings**
When multiple windings are used in parallel, differences in DC resistance can cause circulating currents and voltage ringing. To mitigate this, a recommended circuit with a middle diode can be used to eliminate the negative effects of mismatched windings.
In summary, there are eight key strategies to reduce ripple noise in power supplies. If you're using a finished power supply—such as a module, switch-mode power supply, or adapter—these optimizations are usually handled by the manufacturer. As a user, you should pay attention to the specified ripple noise level, such as the typical value of 60mV VP-P for Zhiyuan Electronics' E_UHBDD-6W series.
Zhiyuan Electronics has over 20 years of experience in developing isolated power modules. Their products support a wide range of input voltages and offer isolation options up to 6000VDC. They use various package types, including SIP and DIP, to ensure compatibility. With advanced testing facilities and comprehensive EMC testing, their power supplies meet strict industrial standards, providing reliable and stable power solutions for demanding environments.
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Shenzhen Unitronic Power System Co., Ltd , https://www.unitronicpower.com