Every DC regulated power supply does the same job - convert incoming AC mains into a stable, adjustable DC output - but it does that job one of two fundamentally different ways. Understanding the difference between linear and switch-mode (SMPS) regulation tells you exactly which one belongs on your bench.
How a Linear Supply Regulates
A linear supply steps mains voltage down with a transformer, rectifies it to DC, then regulates the output using a pass transistor operating in its linear (analog) region - continuously dissipating the difference between the unregulated input and the regulated output as heat. This is a simple, well-understood regulation method that produces very low output ripple and noise, because there's no switching action to generate high-frequency electrical noise on the output rail. The trade-off is efficiency and size: a linear supply wastes a meaningful fraction of its input power as heat, needs a heavy 50/60 Hz transformer and often a substantial heatsink or fan, and typically weighs several times more than an SMPS unit of the same rating.
How an SMPS Regulates
A switch-mode supply rectifies mains directly, then chops the resulting DC at a high frequency (tens to hundreds of kHz) through a small ferrite-core transformer, regulating the output by varying the switching duty cycle rather than dissipating excess voltage as heat. This makes SMPS designs dramatically more efficient and lighter - a compact SMPS supply can deliver several times the power-to-weight ratio of an equivalent linear unit. The trade-off is switching noise: the high-frequency switching action inevitably couples some noise onto the output rail, which shows up as higher ripple and noise specifications than a comparable linear supply.
The practical rule
Reading Ripple and Noise Specifications
Ripple and noise is usually specified in mV RMS or mV peak-to-peak - the residual AC component riding on top of the DC output. A well-designed linear supply typically specifies ripple in the low single-digit millivolts (Vartech's 3005-3 D2, for example, specifies ≤1 mV RMS at moderate current), while SMPS units are commonly specified as a percentage of full-scale output rather than an absolute figure, since the noise floor scales with the switching frequency and topology. Neither number is "bad" in isolation - what matters is whether it's low enough for what you're powering. A digital logic board couldn't care less about a few mV of ripple; a sensitive analog front-end or RF mixer stage very much does.
Efficiency, Heat and Running Cost
Because a linear supply dissipates the voltage difference as heat rather than converting it, running one at a large voltage drop (say, regulating 30 V input down to 5 V output at high current) generates significant heat and wastes real power as a result. An SMPS supply run under the same conditions wastes far less, since its regulation method doesn't depend on dropping excess voltage across a pass element. For any application involving long duty cycles, high current, or limited ventilation, this efficiency difference translates directly into lower running temperature and lower power cost.
Quick Reference
- RF, analog, precision measurement work: linear supply for lowest ripple and noise.
- Production test, battery/charger, motor/drive testing: SMPS for power density and efficiency.
- Portable or space-constrained bench setups: SMPS, for the weight and size advantage.
- Long duty cycles at high current: SMPS runs cooler and more efficiently than an equivalent linear unit.
Vartech's DC power supply range covers both topologies side by side - linear models (3005/3010/3020/6005/6010 series) and SMPS models (the "S" and "SR" suffixed range, plus the PPS programmable series). See our full Vartech DC power supply buying guide for the complete range, or contact us to discuss which topology suits your application. CIE Instruments is an authorised distributor for the full Vartech DC power supply range.