Switch mode power supplies, SMPS provide improved efficiency & space saving over traditional linear supplies, but care has to be taken to ensure noise on the output is low.
Switch mode power supply, SMPS, technology may be referred to by a number of similar terms. While they all look at the same basic technology, they refer to different elements of the overall technology:
- Switch mode power supply, SMPS: The term switch mode power supply is generally used to indicate an item that can be connected to the mains, or other external supply and used to generate the source power. In other words it is a complete power supply.
- Switch mode regulator: This typically refers just to the electronic circuit that provides the regulation. A switch mode regulator will be part of the overall switch mode power supply.
- Switch mode regulator controller: Many switch mode regulator integrated circuits do not contain he series switching element. This will be true if the current or voltage levels are high, because an external series switching element would be able to better handle the higher current and voltage levels, as well as the resultant power dissipation.
The basic concept behind a switch mode power supply or SMPS is the fact that the regulation is undertaken by using a switching regulator. This uses a series switching element that turns the current supply to a smoothing capacitor on an off. The time the series element is turned on is controlled by the voltage on the capacitor. If it is higher than required, the series switching element is turned off, if it is lower than required, it is turned on. In this way the voltage on the smoothing or reservoir capacitor is maintained at the required level.
The use of any technology is often a careful balance of several advantages and disadvantages. This is true for switch mode power supplies which offer some distinct advantages, but also have their drawbacks.
- High efficiency: The switching action means the series regulator element is either on or off and therefore little energy is dissipated as heat and very high efficiency levels can be achieved.
- Compact: As a result of the high efficiency and low levels of heat dissipation, the switch mode power supplies can be made more compact.
- Costs: one of the points that makes switch mode power supplies very attractive is the cost. The higher efficiency and the switching nature of the design means that the heat that needs to be reduced is lower than that of linear supplies and this reduces costs. Long with this, the switching nature of the supply means that many of the components are lower cost.
- Flexible technology: Switch mode power supply technology can be used to provide high efficiency voltage conversions in voltage step up or "Boost" applications or step down "Buck" applications.
- Noise: The transient spikes that occur from the switching action on switch mode power supplies are one of the largest problems. The spikes can migrate into all areas of the circuits that the SMPSs power if the spikes are not properly filtered. Additionally the spikes or transients can cause electromagnetic or RF interference which can affect other nearby items of electronic equipment, particularly if they receive radio signals.
- External components: While it is possible to design a switch mode regulator using a single integrated circuit, external components are typically required. The most obvious is the reservoir capacitor, but filter components are also needed. In some designs the series switch element may be incorporated within the integrated circuit, but where any current is consumed, the series switch will be an external component. These components all require space, and add to the cost.
- Expert design required: It is often possible to put together a switch mode power supply that works. To ensure that it performs to the required specification can be more difficult. Ensuring the ripple and interference levels are maintained can be particularly tricky.
- Filtering: Careful consideration of the filtering for an SMPS because poor design can lead to high levels of noise and spikes on the output.
So, the PSUs in today's PCs use a 100 to 230VAC power stream as input (alternating current, or AC, main input differs according to country or region) and several regulated DC (direct current) outputs (these, of course, are always the same regardless of the country or region). As the switching frequency rises, the size of the energy packets gets smaller, thereby reducing the size of the components (inductors and capacitors) that store and transfer them. Any PSU that utilizes SPC is called a switching mode power supply, or SMPS, and with the abbreviation PSU in PCs, we assume that it is also an SMPS.
The figure below shows the block diagram of an SMPS used in a contemporary system. The first stage is the EMI/transient filter, followed by the active power correction factor (APFC) converter, the main switchers and the power transformer, and finally the output rectification and filtering stage. Between the DC outputs and the main switchers is a voltage feedback loop that constantly checks the output voltages (rails) and makes the necessary corrections by controlling the duty cycle of the main switchers. The pulse-width modulated (PWM) control is implemented by an integrated circuit (IC) that, in some cases (mostly in low-end PSUs) also controls the APFC switchers. However, in some resonant topologies, PWM is used only at light loads and forward modulation (FM) is utilized for higher loads. So, for the APFC switchers, an independent PWM controller has to be used. The part of an SMPS before the power transformer is called the primary side, and the part after it is the secondary side. These two sides must be isolated, but there must still be a type of connection between them in order to pass feedback signals. Usually, optoisolators or optocouplers are used for this purpose.
The two major advantages an SMPS has over a linear power supply (or linear regulator) is that its design is much smaller and lighter, and more efficient. The best SMPS designs can easily exceed 90 percent efficiency and, in some cases, reach up to 95 percent efficiency with 230VAC input. On the other hand, the most significant drawbacks of an SMPS are its complexity and its production of EMI, or radio frequency interference (RFI), that requires an EMI filter (also called the "transient filtering stage" because its role is twofold) and RFI shielding. In the following sections, we will analyze all of the individual stages of an SMPS, or PSU as we call it from now on.