DC Power Supply Basics Tutorial

- summary or tutorial about the basics of DC power supplies, PSU, detailing linear and switching types and their advantages and disadvantages.

Power supplies or power supply units, PSU, form an essential part of very many items of electronics equipment.

The most common form takes in AC power from the mains supply and delivers a DC voltage to the item requiring power.

Accordingly power supplies are widely used in a variety of forms - some large supplying high levels of current, other power supplies, much smaller providing lower levels of power.

Power supply basics

The aim of a DC power supply is to provide the required level of DC power to the load using an AC supply at the input. Different applications require different attributes, but more often than not these days DC power supplies provide an accurate output voltage - this is regulated using electronic circuitry so that it provides a constant output voltage over a wide range of output loads.

In most power supplies there are number of different elements. These may not all be present in every design.

  • Input transformer:   The input transformer is used to transform the incoming line voltage down to the required level for the power supply. Typically the input transformer provides a step down function. It also isolates the output circuit from the line supply.
  • Rectifier:   The power supply rectifier converts the incoming signal from an AC format into raw DC. Either half wave or more commonly full wave rectifiers may be used as they make use of both halves of the incoming AC signal.
  • Smoothing:   The raw DC from the rectifier is far from constant falling to zero when the AC waveform crossed the zero axis, and then rising to its peak. The addition of a reservoir capacitor here fills in the troughs in the waveform, enabling the next stage of the power supply to operate. Large value capacitors are normally used within this stage.
  • Regulator:   This stage of the power supply takes the smoothed voltage and uses a regulator circuit to provide a constant output virtually regardless of the output current and any minor fluctuations in the input level.

Power supply regulation

There are two basic forms of power supply used in electronics equipment:

  • Unregulated:   This form of power supply was the only type used many years ago. It simply consisted of a rectifier section and this was followed by capacitor or capacitor and inductor smoothing. There was no regulation to steady the voltage. If a large current was drawn the voltage would fall as a result of the resistive losses, and also the smoothing would not be as effective and the level of hum would rise.
  • Voltage regulated:   As transistor circuitry became more commonplace, regulated power supplies became more common. Today they are almost universally used. They typically incorporate a voltage reference, and the output voltage is compared to this and altered accordingly by control circuitry within the regulated power supply.

In addition to this, regulated power supplies may be further subdivided:

  • Linear regulated power supply:   Linear regulated power supplies use an analogue approach. A series element - a semiconductor transistor or FET - is controlled allow the correct voltage at the output for any current within the operating range.

    Note on Linear Power Supplies:

    Linear power supplies are widely used for applications where low noise and ripple are required. As the name suggests, they use linear technology - typically a series linear regulator element to drop voltage. As such they dissipate power, but without any switching mode, they are able to offer high levels of eprformance

    Click on the link for a Linear power supply tutorial

  • Switching regulator power supply:   The switching regulator format for a power supply uses a large output reservoir capacitor. A series element - a transistor or FET - is switched on and off to keep the voltage on the capacitor within the required limits.

    Note on Switch Mode Power Supplies:

    Switch mode power supplies and switch mode regulators have many advantages in terms of efficiency, size and weight. Their design can be more involved than might be thought at first. Yet with a good understanding, these switch mode power supplies, SMPSs, switch mode regulators and switch mode controllers can be successfully designed and built..

    Click on the link for a Switch Mode Power Supply tutorial

Each type of power supply regulation technique has its own advantages and disadvantages. As a result different types of regulator are used in different applications, although with technology improving, switching regulators are being used increasingly.

Advantages Disadvantages

  • Very low level of noise
  • Straightforward technology
  • Low level of efficiency
  • High levels of heat may need to be dissipated
  • Large size compared to switching regulator

  • Highly efficient
  • Can be made very compact
  • Low amounts of heat need to be removed
  • Ripple and noise can be higher than linear regulator
  • EMC issues need to be addressed as switching spikes can cause interference

As a result of the different properties of each type of power supply regulator, linear regulators tend to be used in applications where very low levels of noise and ripple are required and heat dissipation may not be such a problem. Hi-fi amplifiers are one such area. Switching mode regulators are used more widely as they can be made very compact, they are very efficient and the levels of ripple, spikes and noise can normally be low enough for most applications.

By Ian Poole

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