October 28, 2019

Switched Mode Power Supply

A switched mode power supply (SMPS) is a more modern and efficient take on the older power supplies known as linear supplies.

LINEAR SUPPLIES

Linear supplies would often take in line voltage, transform this via a low-frequency silicon steel core transformer to a lower voltage. Then through rectification using diodes, and filtering using capacitors, offer a DC voltage for use. This DC is often put into/through a voltage regulator as well.

Linear Power Disadvantages

Linear power supplies have several disadvantages over switched mode supplies…

They use a larger, heavier silicon steel core transformer.
The voltage regulator is always losing power in the form of heat to keep the voltage level constant.
The transformer is always “on” such that even if no load is attached to the DC output, losses in the transformer due to excitation current also waste useful power.
Linear power supplies offer a single output voltage, if more than one is required additional voltage regulators are required within the supply.
With only 30% to 60% efficiencies due to heat loss, they are fairly inefficient supplies.
They also require additional heat sinks within.

Linear Power Advantages

Linear power supplies have several advantages over switched mode supplies…

They are simple to design and implement requiring only diodes and capacitors beyond the transformer itself.
They have low ripple and noise issues coupled with high bandwidth.
They are cheap.

SWITCHED MODE SUPPLIES

These supplies are quite small in comparison to linear supplies. They use pulse width modulation (PWM) to regulate the output voltage. They do this at much higher frequencies than line voltage. This does introduce high-frequency noise into the system but often compared with the increase in efficiency and decrease in size this is often seen as minor or tolerable.

First, the line voltage is rectified using diodes and filtered with capacitors. This sets the input voltage to the PRI of a small ferrite core switching transformer. The output of the transformer is a function of the turns ratio and the duty cycle of the switched DC. By pulsing the DC into the PRI at a high frequency only a small transformer is needed because the PRI turns to need to be less in order to avoid core saturation. It also means the core area is much less as well in avoiding the same core saturation issues.

Ferrite cores do have a lower MAX flux density than do silicon steel cores but since the applied volt-sec’s is so much less given the high frequencies used (200kHz to 500kHz typically) compared with 50/60Hz that not only can this be lower but the turns and core area can all be
lower and still satisfy the smaller volt-sec’s.

The output of the switching transformer is then routed through diodes and filtered with capacitors to give a DC output. Feedback is employed that increases and decreases the duty cycle as the load demand changes.