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Do Not Operate a Four-Switch Buck-Boost Converter in Buck-Boost Mode

Editor’s note: Haifeng Fan is our guest blogger from TI for this blog on the first day of exhibits at APEC 201

A DC/DC converter converts an input voltage source to a desired voltage level. When the input voltage is higher than the desired output voltage, you need a buck converter. Conversely, when the input voltage is lower than the output voltage, you need a boost converter. In applications where the input voltage could be either higher or lower than the output voltage, what you need is a buck-boost converter.

A variety of topologies exist for realizing non-inverting buck-boost (step-down and step-up) conversion, such as single-ended primary-inductor converters (SEPICs), Zeta converters, two-switch buck-boost converters and four-switch buck-boost converters. However, buck-boost conversion using these topologies is not as efficient as a basic buck or boost converter. What is the reason behind that? Is there any way to improve efficiency?

Let’s answer these questions using a four-switch buck-boost converter as an example. Taking a closer look at the four-switch buck-boost converter topology, you can tell that it is actually a cascaded combination of a buck converter followed by a boost converter. Somehow, you should be able to operate it as either a buck converter or a boost converter. Figure 1 shows the comparison between buck or boost mode and conventional buck-boost mode.

Figure 1

Operation modes comparison: buck-boost mode versus buck or boost mode

Operation modes comparison: buck-boost mode versus buck or boost mode

When operating in conventional buck-boost mode, Q1 and Q2 share a gate-control signal, while Q3 and Q4 share another one. These two gate-control signals are complementary to each other. In buck or boost mode, when VIN is higher than VOUT , Q2 is kept off while Q4 is always off; thus it works like a typical buck converter. In contrast, when V IN is lower than V OUT , Q1 is always on while Q3 is kept off; it then works as a typical boost converter.

In buck-boost mode, all four switches are switching in each period. In contrast, only two switches are switching in each period in buck mode or boost mode. More switches switching in each period essentially generates more switching loss. Furthermore, the average or pedestal current flowing through inductors and switches in buck-boost mode (I IN +I OUT ) is higher than that in both buck mode (I OUT ) and boost mode (I IN ), which results in higher conduction loss. As a result, the efficiency in buck or boost mode is much higher than that in buck-boost mode.

You can implement optimized buck- or boost-mode control using the LM5175, the latest wide-VIN four-switch buck-boost controller from TI. The efficiency measured from the evaluation module shows very high efficiency over the entire VIN range.

Figure 2

Efficiency and power loss of a four-switch buck-boost converter

Efficiency and power loss of a four-switch buck-boost converter

So in order to achieve high efficiency, do not operate a four-switch buck-boost converter in buck-boost mode. Instead, operate it in buck mode or boost mode. What is your choice?

11 comments on “Do Not Operate a Four-Switch Buck-Boost Converter in Buck-Boost Mode

  1. Nickel65
    March 20, 2015

    I think perhaps the author misses the point of using a four switch converter entirely. You need to use the buck-boost operation when the input and the output voltages are nearly equal, otherwise you would just build a buck convertor or a boost convertor (and save complexity)

    As for inefficiency using buck-boost, someone must have forgotten to tell Linear Tech about that. Their four switch convertor LT3790 is about 98% (I have verified the demo – it runs cool pumping out over 160W) with it operating in buck-boost mode.  See here Linear Tech site /product/LT3790. If you look in the documentation efficiency is highest when the input is close to the output voltage.

    What the four switch operation enables is not simply on and off, but an intermediate state where the high input and high output MOSFETS are on, so the input is connected to the load via the inductor. In this state the voltage across the inductor is minimal so the rate of current change in the inductor is also minimal. This allows the inductor ripple to be reduced significantly, so the inductor value can be lower and magnetic losses are also lower.

    Regards,

    Nick

  2. zgu9575
    March 21, 2015

    I agree with Nick.  Why select a buck-boost controller that claims you should not operate it in buck-boost mode.  Buck-boost mode is a necessary mode link between pure buck or pure boost modes, with boost mode having well known problems when Vin and Vout are the same.

     

    This is a confusing advertisement for TI's (actually National's) first buck-boost.  If you read the datasheet for the LTC3789, which this part is attempting to duplicate, there is no caution against using that buck-boost controller in buck-boost mode.

  3. IPower
    March 26, 2015

    Hi Steve,

    The two figures are both wrong, which are not related to this blog. Please help replace them with the correct figures

    Thanks.

    Haifeng

  4. IPower
    March 26, 2015

    Hi Nick,

    Thanks for your comments. It seems that there is some misunderstanding here, please let me try to clarify.

    Basically, this blog is trying to say there are two different schemes to achieve buck-boost (step-up and step down) conversion, and trying to compare these two schemes.

    Scheme 1:  Operate the converter in buck-boost mode regardless the input voltage. No matter VIN is higher or lower than VOUT, it just operate in one operation mode. In this mode, all 4 switches are swtiching during one switching period. (I called this scheme  buck-boost mode in this blog.)

    Scheme 2: Chose different operation mode based on the relationship between VIN and VOUT. In this scheme, there are three different operation region (I called this scheme buck or boost mode):

        a. When VIN is higher than VOUT, operate it in buck mode

        b. When VIN is lower than VOU, operate it in boost mode, and

        c, Certainly when VIN is close to VOUT, there is a short transition region between buck mode and boost mode.  (I believe you think I was referring to this region by “buck-boost mode”, but actually that is not true, which cause some misunderstanding.)

    I was trying to compare Scheme 1 (buck-boost mode only over entire VIN range) and Scheme 2 (optimized 3 modes depends on the relationship between VIN and VOUT). Figure 1 is supposed to help explain the comparison, unfornately, the figure is not correct here (might be wrong link).

    By ” buck-boost mode” I was referring to the Scheme 1, not the short transition region when VIN is close to VOUT as described as opertion mode c of Scheme 2.  

    I agree that when VIN is close to VOUT, the efficiency might be still high (actually Figure 2 is supposed to show the efficiency of Scheme 2 in 3 different regions, unfortunately, the link of Figure 2 is not correct, and should be get fixed). But in scheme 1, the efficiency will not be high when VIN and VOUT are far apart.

    In the right figure 2, you will be able see the pretty high efficiency in the short transtion region. Actually, LM5175 has an optimized transtion mode to achieve better efficiency when compare to existing solutions. 

    Hopefully, I have made my points clear.  Also, I will try to get Steve's help to get the figures corrected, which might also helps.

    Thanks again for your valuable comments.

     

    Haifeng

  5. Steve Taranovich
    March 28, 2015

    The figures are the wrong ones for this article as you stated. I apologize. I am getting IT to make the correction.

  6. Steve Taranovich
    March 29, 2015

    All figures are now correct—sorry about the inconvenience

  7. IPower
    March 30, 2015

    Thanks for the quick response!

  8. Nickel65
    March 30, 2015

    Hi Haifeng,

    Unfortunately, I think the misunderstanding starts right at the title. It is a direct statement which is contradictory to the purpose of having the four switch buck-boost convertor.

    In the TI converter, much like the LT converter, the device switches between modes to improve efficiency. Using all four switches when there is a large difference between Vin and Vout is unnecssary and would increase losses. I understand that point completely. It would have been clearer on stating the intent of avoiding four switch operation except during the transistion period, where it is necessary.

    Cheers,

  9. IPower
    March 31, 2015

    Hi Nick,

    Thanks. I believe I get you point. Actually buck-boost mode is a commonly used term, which is also clearly described in the blog.

    I understand that you are very familiar with existing buck-boost solutions and you might think the LM5175 works exactly the same way. However, that is not true. The LM5175 operates in a proprietary transition buck or boost mode rather than conventional buck-boost mode when VIN is close to VOUT. You might observe siginificant efficiency drop in transition region from other buck-boost solutions. In contrast, if you take a close look at the efficiency curve, you will find the efficiency of LM5175 does not drop that much in the transition region. That is because of the proprietary transtion buck or boost mode.

    Hopefully, it is clear to you now. Thank you again for the detailed discussion.

    Best regards,

    Haifeng 

  10. praneethgoud
    April 15, 2015

    nice post thank you

  11. vikramsurii
    April 23, 2015

    nice one

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