DIRECT CURRENT What's the difference between AC-DC and DC-DC power supplies?

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Power supplies concert a power source, such as Alternating Current (AC) from an electrical outlet, into the type of electricity needed for an application, such as Direct Current (DC) for a battery-powered device.

And although some of the most well-known devices will convert Alternating Current (AC) to Direct Current (DC), there is also the option of DC-DC power supplies. Let's take a look at the difference between AC-DC and DC-DC power supplies.

If you need to be brought up to speed, we recommend reading our article on the difference between AC and DC power supplies first.

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How does an AC-DC power supply work?

In buildings where many devices need to be powered, AC-DC power supplies are commonly deployed.

AC-DC converters take the AC power from wall outlets and convert it to unregulated DC. These power supplies include transformers that change the voltage of the AC that comes through wall outlets, rectifiers to save it from AC to DC and a filter that removes noise from the peaks and troths of the AC power waves. Typically, the voltage will be stepped down by the transformer to the voltage required by the device being supplied.

In the first step of converting AC to DC, the voltage is rectified using a series of diodes. This transforms the sinusoidal AC wave into a series of positive peaks using a rectifier. At this point, however, there is still waveform fluctuation - the time between the peaks - that needs to be removed.

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To filter this out, a capacitor is used by creating a reservoir of energy that is then applied to the load when its voltage drops. The capacitor stores incoming energy on the rising edge and expands it when the voltage falls, significantly reducing instances of voltage droop. Generally, the higher the storage capacity of the capacitor, the higher the quality of the power supply.

After voltage conversion, output variation is smoothed out by passing the voltage through a regulator to create a fixed DC output.

Unregulated vs. regulated power supply

There are two varieties of AC power supply - unregulated and regulated.

In an unregulated supply, the ripple voltage stays in the output voltage. And because unregulated supplies exhibit ripple voltage, a regulated supply should always be used over an unregulated supply if there is any ambiguity regarding which should be used for a given device or application. This is because the ripple voltage can cause damage to electronic components. Many AC-DC power supplies include a regulator for this reason.

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How does a DC-DC power supply work?

Some devices - especially those powered by batteries or solar cells - require DC power at varying voltage levels, and this is where a DC-DC converter comes in. A DC-DC power supply converts direct current coming from the power source (a battery) from one voltage level to another depending on the point-in-time needs of the device being supplied.

In a portable electronic device, for example, a cellular phone, there are often several sub-circuits, each with its own voltage level requirement that differs from that supplied by the power supply. In addition, battery voltage decreases as stored energy is delayed.

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How a DC-DC power supply works depends on the one in question; there are many different types of DC-DC converter (electronic, magnetic, non-isolated, buck-boost, etc.) and the type most appropriate for an application will depend on the device itself - from automobiles to portable devices.

However, many will include inverters and rectifiers that first convert DC power into AC power, which is then sent through a transformer to change the voltage. After attaining the correct voltage, the current travels back to the rectifier, where it is converted into DC power again.

Just like with AC power supplies, DC-DC power supplies may use regulators to smooth out the signal and eliminate ripple voltage.

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