Image Credits: Acopian|Conrad
How Linear Power Supplies Work
Linear power supplies accept AC inputs and provide one or more DC outputs for a wide variety of computer and industrial applications. They use an active element (normally a power transistor) operating in its linear region to generate a desired voltage. With linear power supplies, the incoming AC voltage is stepped down to a lower AC voltage. For example, 120-VAC is stepped down to 24-VAC. The 24-VAC is then rectified through a full-wave bridge rectifier, typically with a high-current and low-voltage bridge. A filter capacitor maintains a constant DC level with minimum ripple. A power transistor acts as a variable resistor in series with the load, and receives it control from a circuit that sense the output voltage. The control circuit modifies the transistor bias to maintain a constant voltage output, regardless of changes in the load current.
When selecting linear power supplies, it is important to consider:
- Voltage level
- Output voltage
- AC inputs phase, input voltage, and input frequency
- AC outputs phase and waveform, AC output voltage
- Output frequency
- Output current
- Apparent power
There are many different DC output voltages for linear power supplies. Examples include +0.75 V, +1.0 V, +1.5 V, +1.8V, +2.5 V, +3.3 V, +5 V, -5 V, +12 V, -12 V, +15 V, -15 V, +24 V, -24 V, +28 V, -28 V, +30 V, -30 V, +48 V, and -48 V.
Linear power supplies differ in terms of mounting style and form factor. Modular linear power supplies are designed to mount on desktops, racks, or walls. External linear power supplies, products that mount inside floor-standing cabinets, and units that mount on a standard DIN rail are commonly available. Rack-mounted linear power supplies may have rack mount size of 1U, 2U, 3U, 4U, or 5U. Other mounting options include:
Board-mount - devices use either through-hole technology (THT) or surface mount technology (SMT).
Printed circuit board (PCB)
Open-frame - has exposed circuitry and components.
Enclosed - circuits and components are protected inside a case.
Tool or chamber-mount - mounted on a chassis or to a cabinet.