I am currently researching on the application of the Four Switch Buck Boost Converter on a Fuel Cell Hybrid Vehicle, and I would like to ask about the transition mode of the buck boost converter. I have found 2 methods of implementation for the transition mode.

circuit duty cycle

Figure 1: First implementation of the FSBBC (a) circuit (b) PWM signals

The first implementation uses two different duty cycles. The buck operation is achieved by the application of the buck duty cycle on Q1 (complementary buck duty cycle is applied on Q2) and the boost operation is achieved by the application of the boost duty cycle on Q3 (complementary boost duty cycle is applied on Q4). The buck duty cycle is fixed to 0.8, and the boost duty cycle varies from 0.05 to 0.45, depending on the desired Vout from the applied Vin


Figure 2: Second implementation of the FSBBC

The second implementation uses one duty cycle and has the transistors working in pairs. The A-D pair turns on during the first sub-interval of the duty cycle, and the B-C pair turns on during the second sub-interval of the duty cycle. The duty cycle is around 0.5 depending on the desired Vout from the applied Vin.

  • The simulation of each implementation works, but are each implementation possible from a physical implementation standpoint?
  • What are the advantages or disadvantages to using the first implementation over the second implementation and vice versa? It seems that the second implementation is more common compared to the first implementation, despite the higher current flowing through the inductor during its operation. I say "common", because most of the papers I've found online use the second implementation.
  • Why is the current flowing through the inductor in the second implementation higher (input current + output current) as detailed in this link: https://e2e.ti.com/blogs_/b/powerhouse/archive/2015/03/16/do-not-operate-a-4-switch-buck-boost-converter-in-buck-boost-mode

I have attached the 2 papers I've referenced in the google drive link below for further information: https://drive.google.com/drive/folders/1Z25cREcIzBCCv4K6PO2a87GD-ruKcjmQ?usp=sharing

Please correct me if my understanding is incorrect, or there are gaps in my explanation. I would greatly appreciate any help you can provide. Thank you.

  • \$\begingroup\$ " I am currently researching on the application of the Four Switch Buck Boost Converter on a Fuel Cell Hybrid Vehicle" - Why? \$\endgroup\$ Commented Aug 8, 2020 at 18:48
  • \$\begingroup\$ @BruceAbbott It's the topic of my bachelor thesis \$\endgroup\$ Commented Aug 8, 2020 at 21:05
  • \$\begingroup\$ All of your questions above should be answered with research. Bachelor thesis means you should have already acquired enough knowledge to do your own research and come to an educated conclusion based on the results. If these papers go against what your results are showing then it is highly probable you have done something wrong. Keep in mind this is a bachelor's thesis, not a masters or Ph.D. thesis. So you are expected to use what's already known not to make new discoveries. \$\endgroup\$
    – JoeyB
    Commented Aug 11, 2020 at 0:14

1 Answer 1


You don't mention it, but this converter is bidirectional. The energy and net current can flow either way. This is a good feature as it allows regenerative braking to recharge the battery (accumulator).

Neither switching scheme is ideal.

The second implementation does not conduct current directly from input to output. This means that all current flows through all four MOSFETS and that the inductor does more work, as it must convert all the power

The first implementation is more efficient when bucking (left side voltage higher than right) as the inductor is called upon to store less energy. The inductor is only transferring the voltage difference when bucking but when boosting with a fixed duty-cycle on the right side, it degenerates towards the first implementation.

Ideally you would drive whichever side wants the higher voltage with PWM and have the top mosfet on the other side on all the time.

Essentially, you want to make a buck converter with an ideal diode or a boost converter with an ideal diode, according to what voltage ratio you need.


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