4 switch buck boost converter

[ku637]

Hello Im just checking if it is feasible to implement a 4 switch buck boost using two ICs having integrated half bridge configuration.

Im checking if it is possible to use device like two LMG5200 and drive it separately to achieve 4 switch buck-boost configuration?

Regards

 

[FvM]

Of course it's possible.

 

[ku637]

I'm studying the following document.

https://www.ti.com/lit/an/slyt765/slyt765.pdf?ts=1618331379403&ref_url=https%253A%252F%252Fwww.google.com%252F

Its lists three figures

Fig :1 is buck boost operation, Fig:2 is buck and Fig:3 is boost.

The document explains that in buck-boost mode all four switches are switching and switching losses are high. Alternatively they can be operated in only buck or boost mode depending upon Vin and during that condition either one of the high side switch will be in permanently ON condition.

So my doubts are

1. Is the buck-boost configuration listed in this document is entirely different from either buck or boost mode operation alone. i.e. while keeping all the FETs switching can i generate an output voltage irrespective of the variation in input voltage. If so is there any documents describing the switching pattern available.

2. If i use a bootstrap enabled device i cannot keep the high side FETs continuously ON it seems if I'm operating in buck or boost mode (fig 2& 3 ) alone.

 

[BradtheRad]

1.
The article would have made it easier for us by showing current paths during each half of the buck-boost cycle. And by showing a connected load.

2.
It's hard to be sure exactly when the body diode comes into play during the cycle, and what gating signals must be applied, and what polarity.

3.
The buck-boost can discharge to an output stage which is totally disconnected from the power supply.
Current flows through the inductor in the same direction during both halves of the cycle.
Therefore I think the buck-boost diagram has incorrect labelling as to Vin minus and Vout minus.
I believe Q4 provides a path to power supply ground. Q2 should be included in the output stage only.
The red arrows incorrectly appear bidirectional. Instead they should not have a left-pointing arrow at their ends, but point right only.