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Power is the backbone of any electronic system, and the power adapter is what feeds the system. Choosing the right supply can be the critical difference between a device working at optimum levels and one that may deliver inconsistent results.
In addition to alternating current (AC) to direct current (DC) power supplies, DC to DC converters are also available. If DC is already available in your system, a DC to DC converter may be the better design choice than AC, discussed below.
Direct current power supplies are either unregulated or regulated. Regulated supplies come in several options including linear, switched and battery-based.
AC-DC Conversion Basics
A Power transformer takes the AC from the wall outlet, converts it to unregulated DC, and reduces the voltage using an input power transformer, typically stepping it down to the voltage required by the load. For safety reasons, the transformer also separates the output power adapter from the mains input.
Power Supply Comparisons
AC power adapters come in two varieties, unregulated and regulated. Unregulated is the most basic type of power transformer and does not have the ability to supply consistent voltage to a load, while regulated power supplies do and have many design options.
Linear converters are the least complex but also create the most heat, while switched converters are more intricate and cooler but create more noise. Batteries are typically switched converters. Each has advantages and drawbacks, but which to use will be based mostly on the type of application and the conditions under which it will be run.
Table 1 illustrates how the types of power supplies are categorized and summarizes many of the pros and cons of each type.
| UNREGULATED | REGULATED |
Pro:
|
Pro:
|
Con:
|
Con:
|
| LINEAR | SWITCHED | BATTERY |
Pro:
|
Pro:
|
Pro:
|
Con:
|
Con:
|
Table 1: Types of Power Supplies
Unregulated Power Supply Theory
Because unregulated power transformers 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.
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 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.
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.
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 are 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
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
Important Specifications
While all power supply specifications are valuable, some are more critical than others. A few specifications of note are:
Output Current: The maximum current that can be supplied to the load.
Load Regulation: The load regulation is how well the regulator can maintain its output with a load current change, and usually is measured in millivolts (mV) or as a maximum output voltage.
Noise & Ripple: Noise is any added and unwanted electronic interference, and ripple is the small variation in voltage when AC is transformed into DC. These are typically combined into one measurement. In switching power supplies, the measurement is given in peak-to-peak, showing the extent of the noise spikes that arise from the switching.
Overvoltage Protection: Sometimes output voltages can exceed their nominal values and can damage the load. Overvoltage protection is a circuit that shuts down the power supply should the voltage limits be exceeded.
Overload Protection: Overload protection is a safety measure used to prevent damage in the event of a short circuit or overcurrent event. Much like the circuit breaker in a house, the overload protection shuts off the power supply so the load will not be damaged.
Efficiency: Efficiency is the ratio of power being pulled from the power grid that is effectively being converted to DC power. A good SMPS power supply will operate with at least 80% efficiency and, with a proper system design, can operate at even higher rates. An efficient system will reduce heat generation and can save energy.
Noise & Ripple
Noise and ripple are artifacts of the transformation of AC to DC and are the byproduct of rectification and switching. During conversion, the alternating sine wave cannot be completely suppressed. These artifacts are typically combined into one specification, given in peak-to-peak voltage, showing the extent of the noise spikes that arise from switching, which can negatively affect sensitive instrumentation.
The small voltage variations are called ripple. Many times, the amount of fluctuation depends on how well the power supply is matched to the load.
Noise is the unwanted additions that occur outside the normal ripple. It comes from many other sources, including switching and electronic noise generated outside the power supply, such as from nearby electronics. Noise usually occurs in conjunction with ripple and is much more variable and unpredictable. Switching noise typically occurs at very high frequencies.
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