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Switching Regulators: Efficiency & Cost vs. Frequency

I see lots of press releases for new switching regulators. The presenting companies usually talk about why their device is better than everyone else's part. The listed criteria are interesting and sometimes contradictory.

Here are some of the interesting, eye-catching claims. These are often what we call open-ended comparisons — “better” — better than what? It's left unstated. We could assume they mean “than the other guy's crappy parts.” Thus, we have this:

  • Lower cost;
  • Higher efficiency;
  • Better transient response;
  • Higher switching frequency;
  • Capable of operation from higher input voltage;
  • External parts are cheaper;
  • Better regulation;
  • Capable of operating with a higher input-to-output ratio;
  • Less EMI/EMC.

Let's consider some of these. “Capable of operation from higher input voltage” would imply that it's built on a higher voltage process. That means it won't be as inexpensive as the devices that operate at around 5V (maximum) input. So if you see “lower cost,” you can assume that you cannot power the device from your intermediate 12V bus.

What about “capable of operating with a higher input-to-output ratio”? That means the input could be pretty high — maybe 12V to 48V in — and the output could be pretty low — maybe 0.9V to 2.5V. Such a high ratio means that the time that the upper switching transistor is on and the lower one is off is a very short amount of time (relatively speaking). If your switcher is operating at a high switching frequency (a few MHz), your upper transistor may need to turn on, settle down, and turn off in tens of nanoseconds. Better make sure they can do that. Of course, those parts cost more.

If your switcher is capable of outputting tens of amps, those output transistors will have a large chip area and more gate capacitance. Good luck getting them to turn on-and-off quickly. So maybe operating at a lower frequency would be better.

At a lower switching frequency, especially below 1.7MHz, and especially in automotive applications, you run the risk of generating interference with the AM radio band. And at lower switching frequencies, the output inductor and filter capacitors must be larger (in value and physically). So that means more PC board space used and increased component cost.

At lower switching frequencies, the regulator's transient response is not as good — it takes longer for the regulator to react to step changes in load current draw, partly due to the larger filter components mentioned above. Maybe you should try for a somewhat higher frequency.

But not too high, because that same problem from three or four paragraphs ago — the transition time for the transistors to turn on and off — will affect efficiency. As they turn on or off (but not when they are full on or completely off), their power dissipation goes way up. Better slow down again. But not too much, or the inductor will get too big, increasing its DC resistance and power dissipation.

So there's the quick overview of evaluating the manufacturers' claims and weighing them against your design requirements. How have you dealt with these issues?

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24 comments on “Switching Regulators: Efficiency & Cost vs. Frequency

  1. RedDerek
    August 30, 2013

    Another conflict that can occur is higher frequency and higher dynamic range. If the rise and fall times are limited, the dynamic range capability will actually be reduced as frequency increases. That is why most high frequency applications have a small input supply range, and/or have a set output power. Otherwise the PWM controller goes into either a pulse skip mode (discontinuouse), or a burst mode.

  2. Netcrawl
    August 30, 2013

    @Brad interesting article thanks for sharing it, efficiency is the main objective when using switching regulators, high efficiency mean less power drain in input source and less heat build up. These are complex stuff to design, and its not popular hobby for an average technical guy. 

  3. SunitaT
    August 31, 2013

    The main advantages of good switching regulators are their control of switch turn on time. At light loads the turn on time is slower to maintain the constant frequency and at heavy loads this is faster for maximizing efficiency.

  4. David Maciel Silva
    August 31, 2013

    I used long series LM257X switching regulators, and now I realize that maybe the cost the best way to power electronic circuits is the use of the applicable regulatory switching power supplies,

    I did a project recently using this iten:

    http://www.powerint.com/sites/default/files/product-docs/tny274-280.pdf

    I really enjoyed mainly by the cost of the project, the components that make up the circuit are very cheap …

  5. samicksha
    September 2, 2013

    This is good Brad, what else you if we have all the below stated points in device, but i am how do you evaluate them that yes they land on what they are claiming..

    • Lower cost;
    • Higher efficiency;
    • Better transient response;
    • Higher switching frequency;
    • Capable of operation from higher input voltage;
    • External parts are cheaper;
    • Better regulation;
    • Capable of operating with a higher input-to-output ratio;
    • Less EMI/EMC.
  6. Davidled
    September 2, 2013

    I am wondering the comparison of power consumption between switching regulator and linear regulator. I think that switching regulator may generate more heat and consume more power than that of linear regulator. Main concern of switching mode regulator is electric noise due to the switching & heating.

  7. samicksha
    September 3, 2013

    You are right Daej, it include greater complexity, the generation of high-amplitude, high-frequency energy , but in linear regulator, risk device damage is Very low, unless a short occurs between the primary and secondary windings.

  8. jonharris0
    September 5, 2013

    This is definitely an interesting read.  Due to the efficiency improvements, switchers are becoming more popular these days.  They can handle higher currents than linear regulators without all that heat.  Of course, no free lunch, there is the switching noise to be aware of!  With a good output filter design and proper layout, the impacts of the switching noise can be minimized…and if you're lucky that switching noise might be out of band or narrow band enough to not cause any problems…

  9. BillWM
    September 5, 2013

    Switchers require a bit more patients to get to work with low noise  — Have seen multi-watt smps that were under 3mV of noise p-p total.   But the layout and parts selection and use of “Black Art” was significant.

  10. etnapowers
    September 9, 2013

    A Switching Regulator has to be accurately designed and tested to meet the requirements of the application board of the customer.

    Many times I tested regulator having great perfomances in terms of efficiency or frequency but generating noise or malfunctioning on the application board.

  11. etnapowers
    September 9, 2013

    You're right samicksha, but linear regulators cannot reach the efficiency of switching regulators at all.

  12. Brad_Albing
    September 9, 2013

    @Netcrawl – quite right – you need to know the design rules for this type of circuit – and you need a clear understanding of the trade-offs to get a switcher to work the way you need it to work.

  13. Brad_Albing
    September 9, 2013

    @JH – yep – that part about no free lunch is the key – this is why we din't let the digital design engineers try to design power supplies. You really need to do a careful balancing act on the high wire (pun intended).

  14. etnapowers
    September 9, 2013

    The design of the switch is very important to ensure an easy and cost effective driver and no return of power from the load to the power supplier

  15. etnapowers
    September 10, 2013

    The input-to-output voltage ratio depends on the duty cycle of the switching power device so it is important a proper control of this parameter , this is another important factor to classify the regulators

  16. etnapowers
    September 10, 2013

    One important phase of the design of a switching regulator is the choosing of load capacitance to ensure low ripple and to determine the mode of operation, whatever continuous or discontinuous

  17. Brad_Albing
    September 21, 2013

    @etnapowers – As you noted, switching regulators are highly efficient. Note however that at very low current draw, the efficiciency difference between linear and switching regulators may be so small as to be inconsequencial. Then other issues come into play – like the fact that the switcher will cost more and take up more PC board space. Just some more issues to deal with if you're the engineer designing the system.

  18. Brad_Albing
    September 21, 2013

    @Will – that's pretty good if you can get the output ripple down that low in a power supply putting out a few watts. Usually you have to follow the switcher with an LDO to get results like that.

  19. Brad_Albing
    September 21, 2013

    @etnapowers – yep – you always need to keep the needs of the customer (or the customer's app) in mind.

  20. Brad_Albing
    September 21, 2013

    And you need to make sure that it does more than just “works on paper.” If it is providing the right output voltage, but is generating way too much EMI/RFI, then you don't really have a good design.

  21. Brad_Albing
    September 21, 2013

    @etnapowers – Hmm… not sure I know to what you are referring here when you say “no return of power from the load to the power supplier.” That would not be an issue in a buck regulator since current won't flow from the lower potential output node to the higher potential input node.

  22. Brad_Albing
    September 21, 2013

    @etnapowers – yep – just another aspect of the design – setting the output voltage in a closed loop system inherently establishes the duty cycle.

  23. Brad_Albing
    September 21, 2013

    @etnapowers – again, picking the output cap is just another aspect of doing a power supply design. As is deciding whether its going to be continuous or discontinuous mode.

  24. etnapowers
    October 21, 2013

    @Brad , you're right talking about space occupation and cost saving but the switching regulators can have functions like duty cycle regulation or frequency response setup, that linear regulators cannot have. It depends on the application, I fully agree with you.

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