Travel Converter: Travel Voltage Converter Design With USB PD

A travel converter accepts any mains supply in the world and produces a regulated output for a laptop, a phone or a small appliance. Its difficulty is not the power level but the input range and the space it has to fit into.

What the Product Has to Do

The input spans ninety to two hundred and sixty four volts at either fifty or sixty hertz. The output is a set of USB standards plus, in some products, a pass through socket for a high power appliance.

The pass through path is the interesting part of the specification. It carries the mains to the appliance unchanged, which means the board must route mains voltage with the clearances that implies, without sharing the isolation of the converter.

The Wide Input Range

A wide input range is a thermal problem for the input stage. At the lowest input voltage the current is at its highest, so the conduction loss and the size of the input capacitor are both set by the low line condition.

The design is therefore rated at the low line, and the efficiency is quoted at the high line where it is best. Both numbers belong in the specification, because a customer comparing products will find whichever is more favourable.

Input Filtering and Protection

A common mode choke and a pair of X capacitors form the filter that attenuates the switching noise travelling back to the mains. A varistor absorbs the surge, and a fuse or a fusible element provides the last line of protection.

The filter is only effective if its return path is clean. A filter placed on one side of a ground plane that the noise can bypass is decoration rather than protection, so the layout of the entry area matters.

Power Factor Correction

Above a certain power the harmonic current drawn by a simple rectifier is limited by the standard, so a correction stage is added. It draws a sinusoidal current and produces a regulated high voltage bus.

The stage is a boost converter with a control loop, and its inductor carries the full input current. Its design and its layout follow the same principles as any switching converter, at a higher current.

<img src="https://www.gopcba.com/wp-content/uploads/2024/09/d72fc51735905611378fa8e31dee527-1536×1152-2.webp" alt="Flyback transformer and synchronous rectifier on a travel adapter PCB” />

The Isolated Stage

A flyback or an active clamp flyback converts the bus to the output voltage and provides the isolation. The active clamp recycles the leakage energy and allows zero voltage switching, which is why it is used where the efficiency and the size both matter.

The transformer sets the turns ratio and the leakage inductance, and it is the largest magnetic component in the product. Its construction also affects the common mode noise that the product emits, which is why the winding arrangement is part of the EMC design.

Synchronous Rectification

The output rectifier is a driven transistor, which reduces the conduction loss at the low output voltage. Its timing has to be derived from the primary waveform, either by sensing or by a dedicated controller.

The transition is the difficult part. The rectifier has to turn off before the primary switch turns on, or the transformer is shorted through both devices, and the design of the blanking and the sensing is what prevents it.

USB Power Delivery and Other Protocols

The output controller negotiates with the device, offering a set of voltages and currents and adjusting the feedback network when a contract is agreed. The negotiation is carried on the data lines while the power lines stay at a safe default.

The implementation has to fail safe. If the negotiation is interrupted, the output must return to the default voltage rather than remain at the last agreed level, or the next device plugged in will see a voltage it did not ask for. The protocol side of the design is similar to any fast charger.

AC Pass Through

The pass through socket is connected to the input through a relay, an overcurrent device and sometimes a thermal cutout. The board carries mains voltage across the full width of the product, so the layout has to keep that section separate and clearly marked.

The relay itself is a mechanical part with a limited life, and the design should assume that a customer will switch an appliance on and off through it. Its contacts and the board area around them have to be rated for the current.

Thermal Design

The product is small and sealed, so the heat has to leave through the housing. The switching devices, the transformer and the rectifier are the sources, and the copper under them is the path.

Thermal vias, thick copper and a thermal interface to the case are the usual measures. In a product this size, the efficiency is the primary thermal tool, because every watt saved is a watt that does not have to be dissipated.

Safety and Certification

The isolation between the mains and the output is tested at a high voltage, and the creepage and clearance across the barrier are measured on the layout. The barrier includes the transformer, the optocoupler and the board itself.

The certification also covers the pass through path, which is a mains connection not separated from the input. The marking on the product and the instruction to the user are part of the requirement.

Electromagnetic Compatibility

Two switching stages and a mains connection produce both conducted and radiated emission. The input filter, the transformer construction and the loop areas in both stages are the levers, and the measures are those described under EMI suppression.

The design is usually measured at both extremes of the input range, because the noise spectrum changes with the bus voltage. A product that passes at one end of the range may fail at the other.

Standby Power

With no device connected, the product must consume very little. The controller is put into a burst mode, the output is monitored at a low frequency and the negotiation circuit is kept alive only long enough to detect a device.

The measurement is made at the plug, not at the output, because the input filter and the controller also consume. The limit is set by regulation, and it is low enough to influence the choice of controller.

Test and Production

Production tests cover the output voltages, the negotiation, the pass through path, the isolation and the standby current. The isolation test is performed on every unit, because it is a safety function rather than a performance parameter.

The transformer and the control device are the two components with the longest lead time. Their specification should be settled early, and a second source identified, because the production schedule depends on them.

FAQ

Why is a power factor correction stage needed? Because the harmonic current drawn by a simple rectifier exceeds the limit at the power levels this product reaches.

What is the purpose of the pass through socket? It allows a high power appliance to use the adapter as a plug converter without passing through the electronics.

Why is the efficiency quoted at the high line? Because the conduction loss is lowest there. The low line is the condition that sizes the input stage and the thermal design.