Unusual buck converter configuration?

Thread Starter

SiCEngineer

Joined May 22, 2019
299
I am reading a book by L Sivan called Microwave Tube Transmitters, which shows a strange buck converter configuration that Ive not seen before, however the book is from 1994.

It looks the usual as a normal buck converter, but has some kind of input voltage feedforward and uses a second transistor to drive the gate of the first, input buck transistor. It looks as though its therefore a buck converter that uses output voltage feedback and input voltage feedforward to regulate the output voltage. My questions are, what is the purpose of this converter? Are there any benefits at all to using this configuration, compared to the usual buck? I am unsure, as this is the first time I have seen this and believe if there was much benefit then it would be seen more often.

I welcome any comments that would help me understand this converter more, and what benefits it may pose for any application, bonus points for it's use as a pre-regulator for a high voltage inverter. :) Unfortunately the book introduces the converter but offers little explanation on its performance, application, why it is preferred etc. I have attached an image from my phone of the converter in question.
 

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Thread Starter

SiCEngineer

Joined May 22, 2019
299
Where do you see the input voltage feedfoward? All I see on the input is a crude simplification of a gate driver circuit.
I thought that maybe the resistive divider on the input side that is fed to the FET was feedforward. But on further analysis it looks like you are right .
 

AnalogKid

Joined Aug 1, 2013
9,176
A possible reason for the input voltage divider at the Q1 gate is that the input voltage range eis greater than the safe Vgs range of Q1.

Q1 is shown as an n-channel JFET, and Q2 as a p-channel JFET. If that is the intent of the original designer, it changes how the circuit functions if you are thinking in terms of MOSFETs that are so common these days.

https://en.wikipedia.org/wiki/JFET#Schematic_symbols

ak
 

crutschow

Joined Mar 14, 2008
26,997
Q1 is shown as an n-channel JFET, and Q2 as a p-channel JFET.
I doubt they are really JFETs, since JFETs are normally depletion mode, and there's no negative voltage source to reverse bias Q1's Vgs, and Q2's only if the comparator has a negative supply.
 

ronsimpson

Joined Oct 7, 2019
1,180
The type of transistors is one question. ?
The circled resistor on the left; Why?
The circled resistor in the error amp reduces the gain.

1583863258603.png
The thing will work. I worry that Q1 will be left on all the time.
This is a text book schematic that has not been tried.
 

schmitt trigger

Joined Jul 12, 2010
422
RON,
your observation is correct. The circled resistor is in the incorrect position. Very likely, depending on the resistor ratio, the upper mosfet would be partially turned on.

Additionally, even if it were correctly placed, the Cgs discharging solely thru the upper resistor, which would result in a slow turnoff and significant losses.
 
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