Tuesday, November 15, 2011

Should I Use a Switching or Linear DC Power Supply For My Next Test System? (part 1 of 4)

Part 1 of 4: Linear System DC Power Supply Attributes
To kick things off I thought it would be helpful to start with a short series of posts discussing something fundamental we’re often faced with; that is making the choice of whether to use a switching or linear DC power supply to power up our devices under test. In part 1 here I’ll begin my discussion with the topology and merits of linear DC power supplies, as I have heard countless times from others that only a linear power supply will do for their testing, principally due to its low output noise. Of course we do not want to take the chance of having power supply noise affect our devices’ test results. While I agree a linear DC power supply is bound to have very low noise, a well-designed switching DC power supply can have surprisingly good performance. So the choice may not be as simple anymore. The good thing here however is this may give us a lot more to choose from, something that may better meet our overall needs, including size and cost, among other things.

Linear DC Power Supply Topology
A linear DC power supply as depicted in Figure 1 is relatively simple in concept and in basic implementation:
  1. A transformer scales the AC line voltage to a value consistent with the required maximum DC output voltage level.
  2. The AC voltage is then rectified into DC voltage.
  3. Large electrolytic capacitors filter much of the AC ripple voltage superimposed on the unregulated DC voltage.
  4. Series-pass power transistors control the difference between the unregulated DC rail voltage and the regulated DC output voltage. There always needs to be some voltage across the series pass transistors for proper regulation.
  5. An error amplifier compares the output voltage to a reference voltage to regulate the output at the desired setting.
  6. Finally, an output filter capacitor further reduces AC output noise and ripple, and lowers output impedance, for a more ideal voltage source characteristic.

Figure 1: Basic Linear DC Power Supply Topology

Linear DC power supply design is well established with only incremental gains now being made in efficiency and thermal management, for the most part. Its straightforward configuration, properly implemented, has some inherent advantages:

  • Fast output transient response to AC line and output load changes
  • Low output noise and ripple voltage, and primarily having low frequency spectral content
  • Very low common mode noise current
  • Cost competitive at lower output power levels (under about 500 watts)

It also has a few inherent disadvantages:

  • Low power efficiency, typically no better than 60% at full output voltage and decreases with lower output voltage settings
  • Relatively large physical size and weight
  • High cost at higher power (above about 500 watts)

So it sounds like a linear power supply has to be the hands-down winner especially for low power applications. Or not? To make a more-informed choice we need to look at the topology and merits of a switching power supply, which I will be doing in part 2!

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