Flyback Converter Design: Feedback, Creepage and Layout

An isolated supply breaks the ground connection between the input and the output, which allows a measurement to be made across a potential difference and allows a circuit to be referenced to a point that is not ground.

What the Isolation Provides

The isolation prevents a direct current path, so the two sides can sit at different potentials. It also breaks ground loops, which is the reason it is used in instrumentation even where the voltages are modest.

The isolation is not absolute. There is a capacitance across the barrier, and current flows through it at high frequency. The capacitance limits the common mode rejection at high frequency even though the insulation resistance is high.

The barrier must also withstand the test voltage and the creepage requirement, which are physical properties of the transformer and the board layout. Our laminate notes describe the material properties involved.

Flyback Topology

A flyback converter stores energy in the transformer during the on time and delivers it during the off time. It requires only one switch on the primary side and it provides isolation with a single magnetic component.

The transformer carries a direct current in the primary winding, which means the core must have an air gap to avoid saturation. The gap is a design parameter and it affects the inductance and the losses.

The topology has higher losses and more electromagnetic emissions than a forward converter at the same power, and it is used where the power is modest and the simplicity matters. Our switching regulator notes describe the loop area that determines the emissions.

Feedback Across the Barrier

The output voltage must be measured on the secondary and communicated to the controller on the primary. The information crosses the barrier through an optocoupler or a transformer coupled scheme.

An optocoupler’s current transfer ratio varies with temperature and with age, which affects the regulation. A shunt regulator with an optocoupler is the standard arrangement and its behaviour depends on the surrounding component tolerances.

Where the isolation is not required, the feedback can be taken directly, which removes the variation and improves the regulation. Our power integrity notes describe the supply impedance that the load sees.

Layout of the Switching Loop

The primary side switching loop carries a current with a fast edge, and its area determines the radiated emission. The loop is formed by the switch, the transformer primary and the input capacitor.

The loop must be small, which means the capacitor is close to the switch and the transformer. Every millimetre of trace adds inductance and increases the radiated field.

The secondary side has its own loop with the rectifier and the output capacitor, and the same rule applies. The two loops should be on opposite sides of the barrier and should not overlap.

Creepage Across the Barrier

The barrier must maintain the creepage distance between the primary and secondary circuits on the board. The distance is measured through the transformer, around the board edge and along the surface.

A slot milled in the board under the transformer increases the creepage. The slot must be wide enough not to be bridged by contamination and must be placed so that it does not weaken the board mechanically.

The transformer itself must be rated for the isolation, and the pin arrangement determines whether the creepage through the component meets the requirement. Our quality notes describe how the isolation is verified.

Verification

The verification is a dielectric withstand test between the primary and secondary, at the required voltage for the required time, on every unit or on a sample according to the standard.

Where the product must survive over a long life, a partial discharge test reveals whether the insulation has margin rather than merely whether it survived the test.

The functional test should also confirm the regulation and the output ripple at the load extremes, since a barrier that is intact does not guarantee a supply that performs.

Practical Checklist

Keep the primary switching loop small and the secondary loop on the other side of the barrier. Provide a milled slot where the creepage requirement demands more distance.

Keep the feedback path away from the switching node, since the optocoupler’s signal is small and the node is a strong source of interference.

Verify the isolation by test and record the result with the unit. Where the isolation is a safety requirement, the test is part of the production flow rather than a sample check.

Process Control and Verification

On a design of this kind, flyback is the item that decides how the rest of the board is arranged. Reviewing the design before the data is released is cheaper than correcting it after the panel is in the tank, because every step downstream inherits the decision made at the front end. Documenting the assumption is part of the design work, and a short note on the drawing prevents a question that would otherwise arrive a day later and cost a day of schedule.

The process window is set by the narrowest step in the flow, so an improvement anywhere else shows up as margin rather than as yield until that step is addressed. A short note on the drawing about handling, storage or packaging is often worth more than an extra decimal place on a tolerance.

Where a value sits close to a process limit, the drawing should say so, since the shop can then open the process window rather than working to a nominal figure that carries no tolerance. A stack-up that is drawn rather than described removes most of the ambiguity from a quotation, and it lets the fabricator price the board against the dielectric and copper weights that will actually be used.

Process Control and Verification

On a design of this kind, flyback is the item that decides how the rest of the board is arranged. Reviewing the design before the data is released is cheaper than correcting it after the panel is in the tank, because every step downstream inherits the decision made at the front end. Documenting the assumption is part of the design work, and a short note on the drawing prevents a question that would otherwise arrive a day later and cost a day of schedule.

The process window is set by the narrowest step in the flow, so an improvement anywhere else shows up as margin rather than as yield until that step is addressed.

Process Control and Verification

On a design of this kind, flyback is the item that decides how the rest of the board is arranged. Reviewing the design before the data is released is cheaper than correcting it after the panel is in the tank, because every step downstream inherits the decision made at the front end. Documenting the assumption is part of the design work, and a short note on the drawing prevents a question that would otherwise arrive a day later and cost a day of schedule.

FAQ

Does isolation remove the need for filtering? No. The barrier has capacitance across it, and common mode current still flows through it, so filtering is still required.

Can a flyback supply high power? It can, and at higher power a forward or resonant topology is more efficient and quieter. The choice follows from the power level.

What does gopcb provide for isolated supplies? We provide topology and transformer recommendations, primary and secondary loop layouts, creepage and slot design for the barrier, feedback selection with its tolerance effects, and dielectric withstand and partial discharge testing with the results recorded per unit.