AC-DC|Design
Designing Isolated Flyback Converter Circuits: Addressing EMI and Output Noise
2016.09.08
Points of this article
・Switching power supplies are potential EMI sources, and measures must be taken with respect to both conductive noise and radiated noise.
・From the standpoint of EMC, measures principally address emission (noise radiation).
・Countermeasures begin with installation of noise filters, but the measures taken will be related to the board layout and basic components.
table of contents
In this section, measures to address noise in circuits are explained. Noise evaluation and countermeasures are essential when designing a switching power supply.
To guard against misunderstanding, we begin with a short review of noise-related terms.
・EMI (Electro-Magnetic Interference)
Radio waves and high-frequency electromagnetic waves impacting electronic equipment, and
electromagnetic waves that exert influence.
-Conductive noise: Noise transmitted via cables and circuit board wiring
> Differential (normal) mode noise: Noise that flows in the same direction as a current that
occurs between power supply lines
> Common mode noise: Noise that passes through stray capacitance etc. via a metal case and
the like to return to a signal source
-Radiated noise: Noise that is emitted through the air
・EMS (Electro-Magnetic Susceptibility)
The ability to resist or avoid damage even when there has been interference and disturbance by
electromagnetic waves (EMI: conductive noise and radiated noise)
・EMC (Electro-Magnetic Compatibility)
EMI + EMS. Both emission countermeasures and immunity measures
As EMI, in relation to pathways there are conductive noise and radiated noise; conductive noise is further classified as differential noise and common mode noise according to the manner of propagation. The above is somewhat rough-and-ready, but is the bare minimum that one must know in advance.
EMI countermeasures
When the EMI of a switching power supply circuit affects other circuits, EMI countermeasures are taken. In essence, capacitors or resistor-capacitor circuits for impedance matching or to serve as bypass/filter elements are added to nodes and lines where large currents are switched.
1) C12, R17: RC snubber added to an output rectifying diode
As with input snubbers, these components reduce the spikes occurring during on/off switching. Please refer to this for information on input snubbers. C12 is set to 1000 pF at 500 V, and R17 is set to about 10Ω for 1 W.
2) C10: Y-capacitor added between primary and secondary sides
A capacitor called a Y-capacitor is added between ground on the primary side and secondary side. This is one typical method for reducing the common mode noise caused in the secondary side by primary side switching noise via the capacitance across the windings of an isolation transformer. The voltage rating of the Y-capacitor must be equal to the insulation voltage of the transformer. A capacitance of about 2200 pF is chosen.
3) C11: Capacitor added across the drain and source of the MOSFET Q1
There is a method of adding a capacitor across the drain and source of a MOSFET in order to reduce surges at turn-off due to high-speed switching. This is also one kind of snubber. However, greater losses result, and so attention must be paid to temperature increases. Here, a 10 to 100 pF capacitor with a withstand voltage of 1 kV is used.
The above component constants are reference values that should be considered as starting points. They should be adjusted while checking the effect on noise.
Output noise countermeasures
Of course the output voltage of a switching power supply includes ripples, which depend on the switching frequency; in addition, there is also noise which originates in harmonics, inductances and capacitances. When such noise components are a problem, adding an LC filter to the output is effective.
Starting with an inductance L of 10 μH and C10 of approximately 10 to 100 μF, the values are adjusted while observing the noise components.
The above are the main countermeasures to noise. It will be necessary to measure noise and confirm the influence of noise on other equipment. To make proper noise measurements, a measurement environment and measurement devices are essential. When such quantitative measurements cannot be made, it may still be possible to ascertain whether noise has an effect on performance, through the S/N ratio for the equipment, say.
The measures described here are countermeasures for a power supply circuit configuration. The occurrence of noise is also related to the circuit board layout, component arrangement, component performance, and so on. In some cases, it may be necessary to expand an LC filter from a simple L-type to a π-type or a T-type, or provide a shield for the circuit board, or otherwise modify the design.
Moreover, depending on the equipment specifications, for example the standards instituted by the International Special Committee on Radio Interference (CISPR) or some other noise-related standards should be satisfied. When conformance to standards is necessary, it is extremely important to keep such matters in mind from the design stage.
With this section, we conclude our explanation of circuit design with the title {Designing Isolated Flyback Converter Circuits}. Next we will begin [Board Layout Examples].
【Download Documents】Design Example for PWM Flyback Converter
ROHM’s seminar materials provided at the seminar venue. Explanation how to design a flyback converter using a power supply IC.
List of articles related to the「Designing Isolated Flyback Converter Circuits: Addressing EMI and Output Noise」
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- Want are Isolated Flyhback Convertors?
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Download Technical Documents
Basic of AC-DC Conversion
Basic studies to understand AC-DC converters and to go designing.
AC-DC
- Basic
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Design
-
Overview of Design Method of PWM AC-DC Flyback Converters
- Want are Isolated Flyhback Convertors?
- Isolated Flyback Converter Basics: What is Switching AC-DC Conversion?
- Isolated Flyback Converter Basics: What are Characteristics of Flyback Converter?
- Isolated Flyback Converter Basics: Flyback Converter Operation and Snubber
- Isolated Flyback Converter Basics: What are Discontinuous Mode and Continuous Mode?
- Design Procedure
- Determining Power Supply Specifications
- Choosing an IC for Design
- Designing Isolated Flyback Converter Circuits
- Designing Isolated Flyback Converter Circuits: Transformer Design (Calculating numerical values)
- Designing Isolated Flyback Converter Circuits: Transformer Design (Structural Design) – 1
- Designing Isolated Flyback Converter Circuits: Transformer Design (Structural Design) – 2
- Designing Isolated Flyback Converter Circuits: Selecting Critical Components ? MOSFET related – 1
- Designing Isolated Flyback Converter Circuits: Selecting Critical Components ? MOSFET related – 2
- Designing Isolated Flyback Converter Circuits: Selecting Critical Components ? CIN and Snubber
- Designing Isolated Flyback Converter Circuits: Selecting Critical Components ? Output Rectifier and Cout
- Designing Isolated Flyback Converter Circuits: Selecting Critical Components ? VCC of IC
- Designing Isolated Flyback Converter Circuits: Selecting Critical Components – IC Settings Etc.
- Designing Isolated Flyback Converter Circuits: Addressing EMI and Output Noise
- Example Board Layout
- Summary
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Overview of Design Examples of AC-DC Non-isolated Buck Converters
- What are Buck Converters? – Basic Operation and Discontinuous Mode vs. Continuous Mode
- Selection of Power Supply ICs and Design Examples
- Selecting Critical Components: Input Capacitor C1 and VCC Capacitor C2
- Selecting Critical Components: Inductor L1
- Selecting Critical Components: Current Sense Resistor R1
- Selecting Critical Components: Output Capacitor C5
- Selecting Critical Components: Output Rectifying Diode D4
- EMI Countermeasures
- Board Layout and Summary
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Introduction
- Design Procedure
- IC Used in Design
- Power Supply Specifications and Replacement Circuit
- Synchronous Rectifying Circuit Section: Selection of Synchronous Rectifying MOSFET
- Synchronous Rectification Circuit Section: Power Supply IC Selection
- Synchronous Rectification Circuit Section: Selection of Peripheral Circuit Components-C1, R3 at MAX_TON Pin, and VCC Pin
- Synchronous Rectification Circuit Section: Selection of Peripheral Circuit Components-D1, R1, R2 at DRAIN Pin
- Shunt Regulator Circuit Section: Selection of Peripheral Circuit Components
- Troubleshooting ①: Case When Secondary-Side MOSFET Suddenly Turns OFF
- Troubleshooting ②: Case When Secondary-Side MOSFET Turns On Due to Resonance Under Light Loading
- Troubleshooting ③: Case When, Due to Surge, VDS2 Rises to Above Secondary-Side MOSFET VDS Voltage
- Comparison of Efficiency of Diode Rectification and Synchronous Rectification
- Points to Note Relating to PCB Layout
- Summary
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Introduction
- Power Supply ICs Used in Design: Optimized for SiC MOSFETs
- Design Example Circuit
- Transformer T1 Design – 1
- Transformer T1 Design – 2
- Selecting Critical Components: MOSFET Q1
- Selecting Critical Components: Input Capacitor and Balancing Resistor
- Selecting Critical Components: Switch Setting Resistors for Overload Protection Points
- Selecting Critical Components: VCC-Related Components of Power Supply ICs
- Selecting Critical Components: Components Related to Power Supply IC BO (Brownout) Pins
- Selecting Critical Components: Components Related to Snubber Circuits
- Selecting Critical Components: MOSFET Gate Drive Adjustment Circuit
- Selecting Critical Components: Output Rectifying Diode
- Selecting Critical Components: Output Capacitors, Output Setting and Control Components
- Selecting Critical Components: Current Sense Resistors and Components Related to Detection Pins
- Selecting Critical Components: Components for Dealing with EMI and Output Noise
- PCB Layout Example
- Example Circuit and Component List
- Evaluation Results: Efficiency and Switching Waveform
- Summary
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Overview of Design Method of PWM AC-DC Flyback Converters
- Evaluation
- Product Information
- FAQ