Snubber Networks for ZVS Voltage Overshoot Management

1. The Inevitable Problem: Ringing and Overshoot

When a switch in a power converter turns off, it abruptly interrupts the flow of current. Your experience with inductors tells you that any attempt to suddenly change the current through an inductance ($L$) results in a large voltage ($V=L\frac{di}{dt}$). In a real-world converter, this inductance comes from two main sources:

• Leakage Inductance: In isolated topologies like a flyback or full-bridge, this is the parasitic inductance of the transformer that isn't perfectly coupled between the primary and secondary windings.
• Layout Inductance: The parasitic inductance of the PCB traces themselves, which we worked to minimize in previous lessons.

This inductance forms a resonant LC circuit with the parasitic capacitance of the switch (primarily its output capacitance, $C_{oss}$). When the switch turns off, the energy stored in the inductance is released, causing the voltage across the switch to "ring" and overshoot the expected steady-state voltage.

To see what this looks like and how a snubber helps, let's watch the beginning of this excellent video from Biricha Digital.

This ringing is a major problem for several reasons:

• Component Stress: The peak voltage can easily exceed the MOSFET's breakdown voltage rating ($V_{DS(max)}$), leading to immediate failure.
• EMI: The high-frequency oscillation acts like an antenna, radiating electromagnetic interference (EMI) that can disrupt other electronics.

This is where snubber networks come in. A snubber is a circuit designed to absorb the energy from this parasitic resonance, "snubbing" or damping the ringing and clamping the voltage to a safe level.

2. Passive Snubber Networks: The Workhorses

The simplest and most common solutions are passive snubbers, which use resistors, capacitors, and sometimes diodes to dissipate the unwanted energy as heat.

The RC Snubber

The most basic snubber is a simple resistor and capacitor placed in series across the component you want to protect (e.g., the MOSFET).

• How it works: The capacitor provides a low-impedance path for the high-frequency ringing current. The resistor is chosen to provide critical damping to the resonant LC circuit, dissipating the energy as heat. The capacitor also serves to block DC current, so the resistor doesn't create a short circuit when the switch is on.

The design of an RC snubber involves a trade-off. A larger capacitor provides better damping, but it also increases the power dissipated in the resistor. For your goal of PCB review, this is a key insight: the snubber resistor is a component that is designed to get hot.

When reviewing a PCB, if you see an RC network near a switch, you should check that the resistor package is large enough (e.g., 1206 or larger SMD size) and has sufficient copper area around it to dissipate the expected heat. A blackened or discolored area around a snubber resistor is a red flag indicating a potential thermal issue.

The RCD Snubber (or RCD Clamp)

For higher power applications, especially in flyback converters, an RCD snubber is often preferred.

• How it works: An RCD snubber adds a fast diode to the network. When the switch turns off and the voltage begins to spike, the diode becomes forward-biased and directs the energy into the snubber capacitor. The capacitor charges up, "clamping" the voltage spike. The energy stored in the capacitor is then slowly dissipated by the parallel resistor. This approach is often more effective at clamping peak voltage than a simple RC snubber.

3. The Special Role of Snubbers in ZVS Designs

You might think that since ZVS is designed to eliminate switching losses, snubbers would be unnecessary. This is a common misconception.

1. Managing Turn-Off and Leakage Energy: ZVS primarily targets turn-on losses. Turn-off losses still occur, and the ringing caused by transformer leakage inductance is still a major issue in isolated ZVS topologies like the Phase-Shifted Full-Bridge and LLC converter. Snubbers are still essential for damping this ringing and minimizing turn-off losses.

2. Improving Reliability at Light Load: This is a more subtle but crucial role. In some ZVS converters, particularly the Phase-Shifted Full-Bridge, there can be a reliability problem at light loads. The issue stems from the MOSFET's intrinsic body diode conducting and not having enough time for its stored charge to be removed before a high voltage is re-applied, which can lead to device failure.

A clever solution involves using the snubber circuit not just for protection, but as a strategic tool.

This is a powerful concept for a PCB reviewer. The size of the snubber capacitor across a MOSFET in a ZVS design might not just be for damping; it could be a deliberate choice to ensure reliable operation across the entire load range by forcing "quasi-ZVS" at light loads. This is a hallmark of a well-thought-out, robust design.

4. Active Snubbers: Recycling Energy

Passive snubbers are effective, but they work by converting parasitic energy into heat, which hurts efficiency. For high-efficiency, high-power-density designs, an active snubber (or active clamp) is often used.

• How it works: An active clamp adds a small auxiliary MOSFET and a capacitor. Instead of dissipating the energy from the leakage inductance, the auxiliary switch directs it into the clamp capacitor. This stored energy is then recycled back to the input or delivered to the output on the next switching cycle.

The active clamp is not just a snubber; it's a core part of the Active Clamp Flyback topology, which uses this principle to achieve ZVS. We will explore that topology in detail in a future module. For now, recognize that seeing a small auxiliary switch next to a main switch is a sign that this more advanced, energy-recycling technique is being used.

Conclusion

You have now completed the final piece of our module on PCB layout principles. You can now go beyond simply looking at traces and component placement and assess the more subtle circuits used to ensure a converter's reliability and performance.

Key Takeaways:

• Snubbers Combat Parasitics: Snubber networks are essential for managing the voltage overshoot and ringing caused by the interaction of parasitic inductance and capacitance during switching.
• Passive vs. Active: Passive snubbers (RC, RCD) are simple and dissipate parasitic energy as heat. Active snubbers (active clamps) are more complex but improve efficiency by recycling that energy.
• Snubbers are Critical for ZVS: In ZVS designs, snubbers are not obsolete. They manage turn-off transients, control ringing from leakage inductance, and can be strategically used to modify switching transitions to improve reliability, particularly at light loads.
• PCB Review Checklist: When reviewing a board, look for snubber components near switches. Check for adequate thermal management for snubber resistors and identify if more advanced active clamp circuits are being used.

This concludes our deep dive into PCB layout. You are now equipped with the principles of gate drive layout, power loop minimization, strategic component placement, and snubber implementation. In our next module, we will put all this knowledge to the test in a practical case study, where you will review a complete ZVS converter design from schematic to final PCB.