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convert this voltage to a well-regulated one as needed by the electronics. Figure 2 shows the block diagram of this concept. The problem with the basic setup in Figure 2 is that the 50 Hz/60 Hz transformer is relatively bulky and expensive. Also, the linear regulator dissipates quite a lot of heat, so the total system efficiency is low and getting rid of the generated heat is difficult with high system power. SWITCH-MODE POWER SUPPLIES TO THE RESCUE To avoid the disadvantages of a power supply as shown in Figure 2, switch-mode power supplies were invented. They do not rely on 50 Hz or 60 Hz AC voltage. They take a DC voltage, sometimes rectified AC voltage, and generate a much higher frequency AC voltage to use a much smaller transformer or, in nonisolated systems, to rec- tify the voltage with an LC filter to generate a DC output voltage. The advantages are small solution size and relatively low cost. The AC voltage being generated does not need to be a sine voltage waveform. A simple PWM signal shape will work just fine and is easy to generate with a PWM gen- erator and a switch. Up until the year 2000, bipolar transistors were the most commonly used switches. They would work well but had relatively slow switching tran- sition speed. They were not very power efficient, limiting the switching frequency to 50 kHz or maybe 100 kHz. Today we use switching MOSFETs instead of bipolar transistors, allowing for much faster switching transitions. This in turn gives us lower switching losses, allowing for switch- ing frequencies of up to 5 MHz. Such high switching frequencies enable the use of very small inductors and capacitors in the power stage. Switching regulators bring a lot of benefits. They generally offer a power efficient voltage conversion, allow volt- age step-up and step-down, and offer relatively compact and low cost designs. The disadvan- tages are that they are not so simple to design and optimize, and they generate EMI from the switching transitions and the switching frequency. The availability of switch-mode power supply regulators, along with power supply design tools such as LTpowerCAD® and LTspice® have greatly simplified this difficult design process. With such tools, the circuit design process of a switch-mode power supply can be semi-automated. ISOLATION IN POWER SUPPLIES When designing a power supply, the first question to answer is whether or not galvanic isolation is required. Galvanic isolation is used for multiple reasons. It can make circuits safer, it allows for floating system operation, and it prevents noisy ground currents from spreading through different electronic devices in one circuitry. The two most common isolated topologies are the flyback and forward converters. However, for higher power, other isolated topologies such as push-pull, half- bridge, and full-bridge are used. If galvanic isolation is not required, in most cases a nonisolated topology is used. Isolated topologies always require a transformer and such a device tends to be expensive, bulky, and often difficult to Figure 2. A line transformer followed by a linear regulator. 5 FEBRUARY 2022 | www.eetimes.eu Cover Story