Help on buck converter design

[Joeadeoye]

I am trying to design a simple buck converter that will accept input of upto 150V and output 15V using the above circuit i got from the internet. After carefully checking the circuit, i became a bit confused and i would need some explanations from fellow engineers in this forum.

First is about the mosfet. The mosfet used is a N-Channel mosfet, and to fully turn on an n-channel mosfet, its source pin has to be reference to GND and if its not, then its gate pin has to be bootstrapped. Looking at the circuit diagram above, the source pin of the mosfet has no reference to ground, and the gate is not bootstrapped as well, so how will it work?

Second is the totem-pole at the output of the IC, it seems to be connected in reverse direction, is this intentional or a mistake?

Third is the UC3845 IC itself, the IC and all its external components have no reference to GND, instead the GND of the IC is tied to the source pin of the mosfet (which does not have GND reference either), so how is this suppose to work?

From this same circuit, i have seen two different Youtube videos that claimed to use this same circuit and it worked, but i'm a bit confused how manage it could work. Here are the links to the video for reference:

Please i need some explanations.

 

[Easy peasy]

How will it work indeed ?

When the chip output is high it is shorted through B -> C of Q2 to local chip 0v, so a real bug right there.

There should be at least an 220nF cap and 15V zener across the chip, Vcc to local gnd - else the chances of it working are quite slim.

Lastly there appears to be no bootstrap to lift the gate high enough to fully turn on the fet.

 

[KlausST]

Hi,

don´t trust random internet sources.
Trust the semiconductor manufacturer´s informations.

Klaus

 

[D.A.(Tony)Stewart]

Q3 is clearly incorrect, reversed and ties Vin to Vfb via Q3-c.

It seems they were trying to buffer the current to speed up turn off yet clamp the Vgs pullup to Vin using the 34V internal Zener on Vcc and faked it.

I may have overlooked something though, but it looks wrong.

 

[cupoftea]

yes it looks wrong..use the LT1245 in LTspice like in the attached to sim it first.

--- Updated Aug 16, 2024 ---

LT1245 is pin4pin with ucc3845/uc3845

 

[hiphei]

npn and pnp should be pnp and npn..then it's like normal totem pole..it seems to try to start via 6k8 resistor and after that feed itself into high side drive from output..