Learning Library · Circuit basics · 5
Linear vs. switching power supplies
Both designs turn mains power into a steady DC output, but they get there in opposite ways. A linear supply burns off the voltage it does not need as heat. A switching supply chops the input at high frequency and keeps most of the energy. Each choice has a cost.
Top: where the input power goes. Bottom: the output, AC-coupled and greatly magnified, for each design. Noise shapes are illustrative, not the specification of any model.
How a linear supply works
A transformer steps mains voltage down, a rectifier and capacitor turn it into rough DC, and a pass transistor acts as a variable resistor between that DC and the output. The transistor drops whatever voltage is left over, so its heat is the leftover voltage times the load current.
Efficiency is roughly Vout ÷ Vin. At a low output voltage and a high current, most of the power becomes heat. Many linear bench supplies switch transformer taps to keep Vin close to the setting, which is why some click as you turn the voltage up.
How a switching supply works
A switching supply turns a transistor fully on and fully off tens or hundreds of thousands of times a second. A fully on or fully off switch dissipates very little, and an inductor and capacitor smooth the pulses back into DC. Efficiency stays high across the output range, and the high frequency lets the transformer and filter parts be much smaller and lighter.
The trade-off
| Property | Linear | Switching |
|---|---|---|
| Output ripple and noise | Very low, mostly at twice mains frequency | Higher, at the switching frequency plus fast spikes |
| Efficiency | Falls as Vin − Vout grows | High and fairly flat |
| Size and weight for the power | Large and heavy | Compact and light |
| Load-step response | Fast | Slower, set by the control loop |
Choose linear for noise-sensitive work: audio, RF, precision analog, sensor front ends. Choose switching when you need more power, more voltage range, or a smaller footprint, and the circuit can tolerate a few extra millivolts of noise.
Try this
- Set 5 V out at 2 A with a 12 V linear input. The linear supply turns 14 W into heat to deliver 10 W: under 42 % efficient.
- Raise the output to 11 V with the same input. Linear heat falls to 2 W and efficiency passes 90 %.
- Compare the two noise traces. The switching trace repeats at the switching frequency, and the sharp spikes line up with each switching edge.
To see supply noise yourself, use an oscilloscope with AC coupling, the 20 MHz bandwidth limit on, and the shortest ground connection your probe allows. A long ground lead picks up switching spikes from the air and makes any supply look noisier than it is.
Pick the right supply for the job. Linear and switching DC power supplies.
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