Power Tips: Trade-offs in Designing a Universal AC Input Power Supply
The alternating current (AC) line voltage around the world varies in terms of voltage and frequency. The U.S. and Japan supply voltages are around 100V, while Europe and China use ~220V. The frequency across the world also varies from ~50 to 60Hz.
Having a device that works all over the world is pretty convenient. Many devices take advantage of this concept, such as mobile phone chargers and laptop supplies. However, when was the last time you took a big-screen TV on a flight with you from the U.S. to Europe? In that case, it does not make much sense. Larger items that require high power and do not need to travel from zone to zone can take advantage of having a power supply that is specifically designed for the input voltage range.
There are a number of factors to consider which involve the AC input range when designing a power supply:
- Size.
- Cost.
- Performance.
- Regional regulatory markings.
- Electromagnetic interference (EMI).
- Power factor correction (PFC).
A supply designed to operate over the universal AC input range (85-265V) will be more expensive, larger and less efficient than one designed to operate over a certain range (high line or low line). It is also costly for the product to pass the regulatory checks in each region where it will be sold. If the device is only going to be used in Europe, why spend the extra money to qualify it in China? There are also some regions which require special qualifications, such as power factor correction, that other regions don’t. Adding a PFC circuit to a product that only gets used in the U.S. can be a large, unnecessary expense.
As an example of power supply configurations, let’s consider a 250W audio amplifier for a home theater. Table 1 lists some of the trade-offs of the power supply configurations.
| 250W supply configuration | Input capacitor value (µF) | Input capacitor voltage (V) | MOSFET voltage (V) | MOSFET Rds(ON) (mΩ) | Relative cost |
| Universal input | 330 | 400 | 650 | 150 | 3x |
| Low line | 330 | 200 | 450 | 150 | 1x |
| High line | 150 | 400 | 650 | 600 | 1.5x |
| Voltage switch | 330 x 2 | 200 | 650 | 600 | 1.7x |
Table 1 takes into account the differences between the configurations. The most important result is in the relative cost. There could be up to a 3x price difference for a universal line configuration. In addition to the cost, there will also be differences in performance, size and complexity.
In some cases it really makes sense to use a universal input, but in many cases you can realize great benefits by using separate supplies.
Additional Resources
- Explore more power-supply topics.
- Find a Power Tips video to help with your next design challenge.
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