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.
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.
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.
Figure 1: SMPS Block Diagram
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).
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.
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.
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.
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.
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