Understanding Switch Mode Power Supplies

Understanding Switch Mode Power Supplies Switch mode power supplies (SMPS) Conventional “linear” power supplies are automobiles. The vibrator “chopped” the have been used for many years in inefficient because they regulate by 6 volt battery voltage into an AC signal industrial and aerospace applications dumping the excess power in to heat. The that could be stepped up and down to where good efficiency, light weight and AC power transformer, operating at 60 Hz, deliver the plate and bias voltages needed small size were of prime concern. Today also contributes to the inefficiency of to power the tube radio. SMPS (often called “choppers” or some power supplies. When all the “switchers” are used extensively in AC inefficiencies are added, conventional, A more modern SMPS that you may be powered electronic devices such as linear power supplies are typically 40- familiar with is the horizontal output stage computers, monitors, television receivers, 50% efficient, while switchers have of a television receiver or computer and VCRs. efficiencies from 60 to 90%. This is very monitor that develops the high voltage. important when the designer wants to Although this “flyback” circuit is not This Tech Tip explains the basic operation reduce generated heat, reduce power commonly called a switch mode supply, it of the typical switch mode power supplies costs, or increase battery life. is a type of switcher. used in consumer electronic equipment. We will cover both the Pulse Width Anther key benefit of a SMPS is their Today’s more sophisticated SMPS still (PWM) and Pulse Rate (PRM) types. ability to closely regulate the output employ the same basic concept used in Refer to Tech Tip #204 for information on voltage. Switchers adjust for changes in the early vibrator supply: a DC voltage is identifying SMPS problems, and to Tech input voltage or load current, with little converted to an AC signal, the AC signal is Tip #205 for information on change in efficiency. Switch mode stepped up by a transformer; and the troubleshooting PWM type SMPS. supplies regulate continuously, and follow stepped-up AC is converted back to a DC load changes almost immediately. In voltage. Today’s supplies use feedback SMPS Benefits addition, switchers have the unique ability and variable switching to provide to maintain the correct output under low regulation, and incorporate a solid state A SMPS offers three main advantages input voltage conditions. In fact switchers switch. Figure 2 illustrates the basic over a conventional linear power supply: can actually produce an output voltage functions of a SMPS. 1) high efficiency & less heat generation that is higher than the DC voltage applied 2) tighter regulation, and to the input. Basic Switcher Operation 3) smaller size and weight. Of these, greater efficiency is the biggest A final advantage of switchers is their Let’s take a closer look at the basic advantage. relatively small size and weight. Because switcher block diagram shown in Figure 2. switchers operate at high frequencies, the The heart of all switch mode supplies is parts are physically smaller than those the switching transistor and switching needed for a conventional, 60 Hz power transformer. Raw (unregulated) B+ is supply of the same power rating. The supplied to the switching transistor transformers, capacitors and coils are through the primary winding of the both physically smaller and lighter. This transformer.The switching transistor makes them ideal for use in portable (SOT) is a switch. When the switch is equipment. closed (the transistor is turned on) it provides a path for current to flow Switchers Aren’t New through the transformer primary to ground. As we’ll see a bit later, changing Fig. 1: The SMPS transformer (left) and the You may think that switch mode supplies how fast or how long the switch remains AC power transformer both provide isolation are relatively “new” technology. But quite closed regulates the output voltages. and various output voltages, but the SMPS the opposite is true. One of the first transformer is smaller, lighter weight, and switch mode power supplies was the The current flow in the primary winding of more efficient. mechanical vibrator circuit used in early the transformer produces an expanding rate modulated (PRM). Television receivers and computer monitors may use either type, while VCR commonly use PRM supplies. Pulse Width Modulated Regulation PWM regulators vary the “on” or conduction time of the switching transistor. Figure 4 shows an example of pulse rate modulation. Note that the frequency of the signal remains constant and only the duty cycle varies. As the width of the pulse is increased, the Fig, 2: A basic SMPS consists of a DC voltage source, a switching device, a step up/down transformer, (flyback) and a switching control stage. magnetic field which couples to the the input voltage. Switch mode power secondary windings through the core. As supplies provide fast, efficient regulation. the transistor is switched on and off the To better understand why, let’s review magnetic field alternately expands and conventional power supply regulation. collapses in all of the transformer windings. By designing the transformer Conventional power supplies regulate by with different secondary turns ratios, using a series-pass regulator operating in various amplitude pulses are produced at its linear mode, as shown in Figure 3. The Fig. 4: The PWM regulates by varying the the output of each secondary winding. series pass regulator maintains a fairly “on” time of the switching transistor The output of each winding is applied to constant output voltage with changes in high speed switching diodes and filters input voltage and load current. But, in switching transistor stays on longer, and which produce the DC output voltages of order to maintain regulation for all load more energy is applied to the switching the SMPS. conditions, more power is applied to transformer. This produces an increase in regulator than is needed by the load. This the DC output voltage (Unless the load is It is important to remember that all of the unused power is dissipated as heat. shorted, or draws excessive current). windings are mutually coupled by the Likewise, as the pulse width is made magnetic field. If more power is applied to The key to making the regulator more narrower, the transistor is on for a shorter the primary winding, more power is efficient is to produce only as much amount of time, and less energy is applied delivered to the secondary windings; if the power as the load consumes. Then there to the transformer. voltage at one secondary winding will be no extra power to dissipate. This increases (or decreases) the voltages at would be easy if the load current never Pulse Rate Modulated Regulation the other windings change by an amount increased or decreased, or if the source Figure 5 shows an example of pulse rate equal to the turns ratio. voltage remained constant. Of course this modulation. The PRM regulator varies the never happens. rate (frequency) at which the switching Achieving Efficient Regulation transistor is turned off and on. Notice, Switchers are very efficient regulators however, that as the pulse rate increases, Regulation is important in most power because they only produce as much supplies. Good regulation is needed to power as is needed by the load. Voltage maintain a constant output voltage for regulation is achieved by sampling the DC changes in load current and changes in output voltage and comparing it to a reference. (If the SMPS has several outputs the main supply is sampled). The resulting correction voltage is used to control the frequency or “on” time of the switching transistor, which in turn delivers more or less power to the load. The regulators used in consumer Fig. 5: The PRM regulates by varying the Fig. 3: A conventional, series-pass regulator equipment fall into two types; 1) pulse width modulated (PWM), and 2) pulse frequency of the pulses, which varies the dissipates the unused power as heat. switching transistor’s conduction time. the “on time” decreases. Thus, if the MIC 1 - Unregulated B+ transistor (FET) is usually used for the output voltage is too high, the switching The SMPS needs a source of power. The switch. FETs offer several advantages transistor is turned on and off at a faster gray shaded blocks in Figure 6 provide over bipolar transistors for SMPS rate. This results in less energy being the unregulated B+ to power the switcher. applications. (A horizontal output applied to the switching transformer. A In an AC operated device the AC line transistor is a bipolar transistor). First, typical PRM supply may operate at voltage is rectified by a conventional full FETs are voltage operated and can be frequencies as high as 90 kHz under a “no wave power supply. (This is why driven directly without a driver stage. load” condition, but slow to 40 kHz under everything on the primary side of the Secondly, FETs are bi-directional (conduct full load. switching transformer is at “Hot ground” with voltage of either polarity applied to potential). Often an RF filter network is the source and drain) which eliminates Putting It All Together used to prevent the high frequencies the need for a damper diode. Thirdly, produced by the switcher from getting on FETs operate efficiently at high switching Figure 6 shows a block diagram of a the AC power line. frequencies. typical SMPS. While specific switchers many vary somewhat from this diagram, Most microprocessor controlled chassis Lastly, FETs have a low “on” resistance all SMPS have these basic functional include a standby power supply to keep which contributes to the efficiency of the blocks. Some switchers, for example, may the micro running when the main power SMPS, and reduces heat build up.

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