FLYBACK CONVERTER SNUBBERS Verification methods of snubber circuits in flyback converters

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Flyback converters are popular AC-DC solutions for electronic products consuming up to 50 W power. However, while low-cost, the basic circuits have parasitic components which cause harmful ringing and voltage spikes. This article discusses verification methods for possible snubber circuit solutions.

Background

Most modern electronic devices need an AC-DC power supply, which converts AC mains power into the lower-level DC voltages essential to the electronics hardware. Such power supplies, often implemented as wall brick types for consumer applications, must be able to handle the world’s various AC voltages and frequencies, while also providing input to output electrical isolation.

Flyback converter designs are popular for AC-DC power conversion levels below 50 W, because of their simplicity and low cost. However, along with these advantages, such converters have parasitic components which typically produce ringing waveforms with extremely high voltage spikes. If not suppressed, this ringing can adversely affect switching elements or other components, or create unacceptable EMI emissions.
Accordingly, damping circuits, known as snubber circuits, are used to suppress the ringing effect. Different snubber designs are available, each with their own advantages and disadvantages. Developing suitable, reliable solutions involves specific verification methods: this article briefly presents some of these, as recommended and designed by Rohde & Schwarz.

Isolated flyback converter

Buck-boost type flyback converters provide input-output isolation; their active power factor correction is also useful in some applications. The design’s provision of multiple outputs from its single transformer increases its popularity.

The flyback converter design is based (typically) on a MOSFET switch, transformer, rectifier diodes and capacitors. The switch converts the AC input voltage into a pulsating DC voltage, which is transferred through the transformer and output rectifier diode to the capacitor and load. The transformer turns-ratio controls the input-output voltage ratio. The output voltage is reflected to the primary winding during switch off-time, so the ideal maximum voltage at the switch node to ground is the input voltage plus the reflected voltage - sometimes called flyback voltage.

The converter can be operated in various modes, including:

• Discontinuous Conduction Mode (DCM)

• Critical Conduction Mode (CRM) - also known as Transition Mode or Boundary Conduction Mode

• Continuous Conduction Mode (CCM)

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The differences in these modes relate mainly to how the MOSFET is switched throughout the operating cycle, and the resulting input and output voltage and current waveforms. Irrespective of the mode used, the flyback converter’s performance is heavily influenced by inductive and capacitive parasitic components related mostly to the MOSFET switch and transformer. These can cause high voltage spikes and resonance, which limits the converter’s maximum frequency. Additionally, if the voltage spikes are not clamped, the MOSFET will be subject to the sum of these and the input plus the flyback voltage. The device will fail if this sum exceeds its breakdown voltage and/or drives it into avalanche mode.

Accordingly, a snubber circuit is required to clamp or damp the ringing and convert leakage energy into heat. If placed across semiconductor devices or transformers, snubbers can reduce or even eliminate voltage or current spikes, limit dI/dt or dV/dt and can reduce EMI by damping the voltage and current ringing. Of the many different snubber designs, the two most common are the resistor-capacitor (RC) damping network and the resistor-capacitor-diode (RCD) turn-off snubber circuit. They are popular because, as passive designs, they are low-cost.

However, their passive design negatively impacts overall efficiency. Accordingly, because the snubber design should also be optimized for the best overall converter performance in EMI, efficiency, and total system cost, some designers will choose active snubber designs; although more expensive, they can decrease or even eliminate the losses that a passive circuit would cause.

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Passive snubbers

A primary RCD snubber circuit is a dissipative design which is simple and inexpensive, since no additional control circuit is needed. However, the designer must consider and measure different parameters to obtain a proper snubber solution: these include leakage inductance, and the maximum voltage that the MOSFET drain switch node can tolerate.

Running a simulation without a snubber circuit shows the difficulty of finding a suitable MOSFET device if no snubber technique is used. Therefore, snubber circuits are commonly applied to reduce the peak voltage as well as improving EMI emissions.

Running the simulation with a snubber circuit shows that maximum voltage at the MOSFET drain reduces to half of that with no snubbing – a manageable level for modern MOSFETs. However, the simulation also shows that the RCD circuit does not damp EMI emissions, so an RC-Damping circuit may also be used to damp the high frequency oscillation.

The passive circuit simulations should be complemented by measurements, both with and without snubber circuits. These measurements focus on the effect of the snubber in reducing a high voltage peak at the switching element drain, and ensuring device limits are not exceeded.

RC circuits can be used in parallel to the low side MOSFET to damp resonance at the oscillation frequency. Secondary RC damping circuits can be used to protect the secondary side rectifier against voltage stress, and are sometimes required to pass the final EMI test. Like the RCD circuits, these types should be validated by a combination of simulation and measurement.

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Active snubber circuits

Active snubber circuits offer a more efficient, non-dissipative solution, as they clamp using a switch and capacitor rather than dissipating unwanted power as heat. They allow more compact high-frequency, highly efficient designs, with zero voltage switching as a possibility. However, they carry a price premium, due to the cost of the switch plus the intelligent control algorithm needed to manage multiple switch timings. One solution is to use a Texas Instruments controller which features the necessary functionality.

To correctly apply this controller as an active snubber, designers must first understand its various operating modes, and then conduct a set of measurements to validate these modes. The operating modes are summarized below: please refer to the relevant Rohde & Schwarz Application Note (see button above) for details of the measurement and validation methods.

The complex controller provides several operating modes so that it always converts input power to output power as effectively and efficiently as possible, over its entire operating range.

Adaptive Amplitude Modulation (AAM) is used at higher loads where the output level ranges from approximately 2,25 A down to 1,5 A for the highest efficiency.
Adaptive Burst Mode (ABM) is used from medium down to light loads, where output current ranges from approximately 1,5 A down to 0,18 A.
Low Power Mode (LPM) is used in the light load region and ranges approximately from 0,18 A down to 0,08 A.
Standby Power Mode (SPM) is entered at an output current lower than 0,08 A to minimize standby power consumption.

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Conclusion

Flyback topology is well-known and commonly used in consumer and industrial applications where lower power AC-DC conversion with optimized costs is required. Nevertheless, the converter design still requires intelligent validation methods to overcome the anomalies caused by unavoidable parasitic components.

While a simulation may provide good and fast results at the design phase outset, these results must be validated by real measurements in most cases. A validation strategy that combines both approaches is the best way to provide enough confidence to release a reliable product to the market.

As semiconductor manufacturers introduce new controllers that further improve flyback controller efficiency, the validation strategy becomes even more important, due to the controllers’ complexity and variety of operating modes.

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