DESIGN AND OPERATION

POWER SUPPLY DESIGNS

There are three basic types of power supplies based on

different design principles:

►

Linear power supplies

►

Switched-mode power supplies (SMPS)

►

Mixed architecture power supplies

Linear power supplies

Linear regulated power supplies provide highly con stant

output voltage, low ripple and noise and fast regulation,

even with high line and load transients. They produce sig-

nificantly less electromagnetic interference than switched-

mode power supplies.

A conventional mains transformer isolates the power-line

from the secondary circuits (output stages). It is followed

by a rectifier that supplies the unregulated voltage to a

series actuator. Capacitors at the input and output of the

regulator circuit serve as buffers and decrease the ripple.

A high precision reference voltage controls the analog

output amplifier. This amplifier is generally fast and allows

very short recovery times for load changes.

Linear power supply

Reference voltage

Mains Transformer

Actuator

Rectiﬁer

Output

voltage

Ground

Analog control

–

voltage

–

Switched-mode power supplies (SMPS)

SMPS have much higher efficiency than li near regulated

power supplies. In a first step, the line voltage is rectified.

Due to the high input voltage, buffer capacitors with a

small capacitance can be used. In a second step, the DC

voltage to be converted is chopped at a high frequency.

This takes place in the switching transistor and requires

only comparatively tiny and light ferrite chokes or trans-

formers with low losses. The switching transistor is swit-

ched fully on and off, hence switching losses are low. The

output voltage is usually regulated by changing the duty

cycle of the switching transistor. A rectifier and low-pass

filter improve the output quality.

Switched-mode power supply

Mains

rectiﬁer

Filter

Output

voltage

Transformer Rectiﬁer

Switching

transistor

Potential

separation

Ground

Control

–

Ground

Compared to linear power supplies, SMPS achieve consid-

erably higher efficiency of approx. 70 % to over 95 %. They

are lighter and smaller. The capacitor(s) on the output(s)

of an SMPS may be quite small due to the high switch-

ing frequency, but the choice of the right capacitor(s) also

depends on other factors such as the required energy

storage capacity and the AC ripple induced by the load

(e.g. motors). The size of the major components generally