ECEstream: Linear regulated dc power supplies

Switch-Mode Power Supplies (SMPS)

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The Switched-Mode Power Supply converts the available unregulated ac or dc input voltage to a regulated dc output voltage. A Switch-Mode Power Supply (SMPS) is an electronic power supply that includes a switching regulator to convert electrical power efficiently. However in case of SMPS with input supply drawn from the ac mains, the input voltage is first rectified and filtered using a capacitor at the rectifier output. The unregulated dc voltage across the capacitor is then fed to a high frequency dc-to-dc converter. Most of the dc-to-dc converters used in SMPS circuits have an intermediate high frequency ac conversion stage to facilitate the use of a high frequency transformer for voltage scaling and isolation. In contrast, in linear power supplies with input voltage drawn from ac mains, the mains voltage is first stepped down (and isolated) to the desired magnitude using a mains frequency transformer, followed by rectification and filtering. The high frequency transformer used in a SMPS circuit is much smaller in size and weight compared to the low frequency transformer of the linear power supply circuit.</div>
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The ‘Switched Mode Power Supply’ owes its name to the dc-to-dc switching converter for conversion from unregulated dc input voltage to regulated dc output voltage. The switch employed is turned ‘ON’ and ‘OFF’ (referred as switching) at a high frequency. During ‘ON’ mode the switch is in saturation mode with negligible voltage drop across the collector and emitter terminals of the switch where as in ‘OFF’ mode the switch is in cut-off mode with negligible current through the collector and emitter terminals. On the contrary the voltage regulating switch, in a linear regulator circuit, always remains in the active region.</div>
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Manufacturers use a SMPS because it is much smaller than an equivalent linear supply, and as noted can include active power factor correction (PFC). This is designed to keep the mains waveform as close to the mains voltage waveform as possible. This minimizes current for the same power, and reduces mains distortion (which is becoming a major problem). More importantly, a SMPS will often be cheaper than a traditional power supply, and of course is a great deal lighter.</div>
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<b><i>Figure 1: SMPS Block Diagram</i></b></div>
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The PFC circuitry uses raw (unsmoothed) rectified AC, so it has a huge ripple component. The ICs are specifically designed to be able to deal with this waveform, and the output of the PFC circuit is DC - usually at around 350 - 420 V. Depending on the application, it may be either well regulated or just have basic regulation. If PFC is not used, the PFC block above is replaced with a conventional rectifier and high voltage filter capacitor(s). The PWM switching circuit is then responsible for all regulation (if used).</div>
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The DC is then chopped at high speed (typically 50 KHz or more) into a pulse width modulated (PWM) square wave signal. This allows the transformer to be very small even for high power systems, due to the high operating frequency. The output of the transformer is rectified and filtered, and then goes to the (sometimes optional) monitoring circuitry. This is usually designed to provide tightly regulated supplies, and/or to monitor the output current for fault conditions, etc.</div>
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The overall circuitry may initially appear simple if you look at the printed board for such a supply, but then you realise that it's almost always completely surface mount devices (SMD), with tiny components on both sides of the PCB. The overall complexity is astonishing, and the possibility of servicing these supplies ranges from dubious to not-a-chance. It might be possible if you have full schematics and a manufacturer supplied test procedure (along with full SMD rework facilities), but in many cases the only option is to replace the PCB.</div>
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It's not at all uncommon for the PCB to be burned when a SMPS fails, because the protection circuits can only function if the circuitry is functional. There are countless failure modes that defeat all attempts at protection.</div>
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SMPS are used because people want gear that's light, powerful and cool running. Manufacturers like them because they are fairly cheap to make, and shipping and handling costs are reduced because of the low weight. No more bulky transformers and huge capacitors.</div>
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Switch-Mode Power Supplies (SMPS)

The Switched-Mode Power Supply converts the available unregulated ac or dc input voltage to a regulated dc output voltage. A Switch-Mode P...

Linear Regulated Power Supply

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In this article we will discuss about the linear regulated power supply. In converts the available unregulated ac or dc input voltage to a regulated dc voltage. Figure 1 shows the basic block for a linear power supply operating from an unregulated dc input. This kind of unregulated dc voltage is most often derived from the utility ac source. The utility ac voltage is first stepped down using a utility frequency transformer, then it is rectified using diode rectifier and filtered by placing a capacitor across the rectifier output. The voltage across the capacitor is still fairly unregulated and is load dependent. The ripple in the capacitor voltage is not only dependent on the capacitance magnitude but also depends on load and supply voltage variations. The unregulated capacitor voltage becomes the input to the linear type power supply circuit. The filter capacitor size is chosen to optimize the overall cost and volume. However, unless the capacitor is sufficiently large the capacitor voltage may have unacceptably large ripple. The representative rectifier and capacitor voltage waveforms, where a 100 volts (peak), 50 Hz ac voltage is rectified and filtered using a capacitor of 1000 micro-farad and fed to a load of 100 ohms is shown in Fig. 2. For proper operation of the voltage regulator, the instantaneous value of unregulated input voltage must always be few volts more than the desired regulated voltage at the output. Thus the ripple across the capacitor voltage (difference between the maximum and minimum instantaneous magnitudes) must not be large or else the minimum voltage level may fall below the required level for output voltage regulation. The magnitude of voltage-ripple across the input capacitor increases with increase in load connected at the output.</div>
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The step down transformer talked above should be chosen such that the peak value of rectified voltage is always larger than the sum of bare minimum voltage required at the input of the regulator and the worst-case ripple in the capacitor voltage. Thus the transformer turns ratio is chosen on the basis of minimum specified supply voltage magnitude. The end user of the power supply will like to have a regulated output voltage (with voltage ripple within some specified range) while the load and supply voltage fluctuations remain within the allowable limit. To achieve this the unregulated dc voltage is fed to a voltage regulator circuit. The circuit in Fig. 1 shows, schematically, a linear regulator circuit where a transistor is placed in between the unregulated dc voltage and the desired regulated dc output. Difference between the instantaneous input voltage and the regulated output voltage is blocked across the collector emitter terminals of the transistor. As discussed previously, in such circuits the lowest instantaneous magnitude of the unregulated dc voltage must be slightly greater than the desired output voltage (to allow some voltage for transistor biasing circuit). The power dissipation in the transistor and the useful output power will be in the ratio of voltage drops across the transistor and the load (here the control power dissipated in the base drive circuit of the transistor is assumed to be relatively small and is neglected). The worst-case series voltage drop across the transistor may be quite large if the allowed variation in supply magnitude is large. Worst-case power dissipation in the transistor will correspond to maximum supply voltage and maximum load condition (load voltage is assumed to be well regulated). Efficiency of linear voltage regulator circuits will be quite low when supply voltage is on the higher side of the nominal voltage.<br />
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<b><i>Figure 1</i></b></div>
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<b><i>Figure 2: Typical voltage output from rectifier-capacitor combination</i></b></div>
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<b><i>Figure 3: A schematic switched mode dc to dc chopper circuit</i></b></div>
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Linear Regulated Power Supply

In this article we will discuss about the linear regulated power supply. In converts the available unregulated ac or dc input voltage to a...

Regulated DC Power Supplies

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Power supply is a broad term but this article is restricted to discussion of circuits that generate a fixed or controllable magnitude dc voltage from the available form of input voltage. Integrated circuit (IC) chips used in the electronic circuits need standard dc voltage of fixed magnitude. Many of these circuits need well-regulated dc supply for their proper operation. In majority of the cases the required voltages are of magnitudes varying between -18 to +18 volts. Some equipment may need multiple output power supplies. For example, in a Personal Computer one may need 3.3 volt, ±5 volt and ±12 volt power supplies. The digital ICs may need 3.3 Volt supply and the hard disk driver or the floppy driver may need ±5 and ±12 volts supplies. The individual output voltages from the multiple output power supply may have different current ratings and different voltage regulation requirements. Almost invariably these outputs are isolated dc voltages where the dc output is ohmically isolated from the input supply.  In case of multiple output supplies ohmic isolation between two or more outputs may be desired. The input connection to these power supplies is often taken from the standard utility power plug point (ac voltage of 115V / 60Hz or 230V / 50Hz). It may not be unusual, though, to have a power supply working from any other voltage level which could be of either ac or dc type.<br />
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<b><i>Figure: Regulated dc power supply</i></b></div>
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There are two broad categories of power supplies:</div>
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<li><a href="http://www.ecestream.com/2015/03/linear-regulated-power-supply.html" target="_blank">Linear regulated power supply</a></li>
<li><a href="http://www.ecestream.com/2015/03/switch-mode-power-supplies-smps.html" target="_blank">Switched mode power supply (SMPS)</a></li>
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In some cases one may use a combination of switched mode and linear power supplies to gain some desired advantages of both the types.</div>
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Regulated DC Power Supplies

Power supply is a broad term but this article is restricted to discussion of circuits that generate a fixed or controllable magnitude dc v...

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Switch-Mode Power Supplies (SMPS)

Linear Regulated Power Supply

Regulated DC Power Supplies

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