POWER SUPPLY BASICS – Wavelength Electronics

25 Oct.,2022


Telecom Rectifier

Unregulated Power Supply Theory

Because unregulated power supplies do not have voltage regulators built into them, they typically are designed to produce a specific voltage at a specific maximum output load current. These are typically the block wall chargers that turn AC into a small trickle of DC and are often used to power devices such as household electronics. They are the most common power adapters and are nicknamed a “wall wart”.

The DC voltage output is dependent on an internal voltage reduction transformer and should be matched as closely as possible to the current required by the load. Typically the output voltage will decrease as the current output to the load increases.

With an unregulated DC power supply, the voltage output varies with the size of the load. It typically consists of a rectifier and capacitor smoothing, but no regulation to steady the voltage. It may have safety circuits and would be best for applications that do not require precision.

Figure 4: Block Diagram — Unregulated Linear Supply

The advantages of unregulated power supplies are that they are durable and can be inexpensive. They are best used, however, when precision is not a requirement. They have a residual ripple similar to that shown in Figure 3.

NOTE: Wavelength does not recommend using unregulated power supplies with any of our products.

Regulated Power Supply Theory

A regulated DC power supply is essentially an unregulated power supply with the addition of a voltage regulator. This allows the voltage to stay stable regardless of the amount of current consumed by the load, provided the predefined limits are not exceeded.

Figure 5: Block Diagram — Regulated Supply

In regulated power supplies, a circuit continually samples a portion of the output voltage and adjusts the system to keep the output voltage at the required value. In many cases, additional circuitry is included to provide current or voltage limits, noise filtering, and output adjustments.

Linear, Switched, or Battery-based?

There are three subsets of regulated power supplies: linear, switched, and battery-based. Of the three basic regulated power supply designs, linear is the least complicated system, but switched and battery power have their advantages.

Linear Power Supply
Linear power supplies are used when precise regulation and the removal of noise is most important. While they are not the most efficient power source, they provide the best performance. The name is derived from the fact that they do not use a switch to regulate the voltage output.

Linear power supplies have been available for years and their use is widespread and reliable. They are also relatively noise-free and commercially available. The disadvantage to linear power supplies is that they require larger components, hence are larger and dissipate more heat than switched power supplies. Compared to switched power supplies and batteries, they are also less efficient, sometimes exhibiting only 50% efficiency.

Switched Power Supply
Switched mode power supplies (SMPS) are more complicated to construct but have greater versatility in polarity and, if designed properly, can have an efficiency of 80% or more. Although they have more components, they are smaller and less expensive than linear power supplies.

Figure 6: Block Diagram — Regulated Switching Supply

One of the advantages of switched mode is that there is a smaller loss across the switch. Because SMPS operate at higher frequencies, they can radiate noise and interfere with other circuits. Interference suppression measures, such as shielding and following layout protocols, must be taken.

The advantages of a switched power supply is that they are typically small and lightweight, have a wide input voltage range and a higher output range, and are much more efficient than a linear supply. However, a SMPS has complex circuitry, can pollute the AC mains, is noisier, and operates at high frequencies requiring interference mitigation.

Battery-based power is a third type of power supply and is essentially a mobile energy storage unit. Battery-based power produces negligible noise to interfere with electronics, but loses capacity and does not provide constant voltage as the batteries drain.  In most applications using laser diodes, batteries are the least efficient method of powering the equipment. Most batteries are difficult to match the correct voltage to the load. Using a battery that can exceed the internal power dissipation of the driver or controller can damage your device.

Selecting a Power Supply

  • When choosing a power supply, there are several requirements that need to be considered.
  • The power requirements of the load or circuit, including
    • voltage
    • current
  • Safety features such as voltage and current limits to protect the load.
  • Physical size and efficiency.
  • Noise immunity of the system.