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FrozenGate by Avery

Inverting buck/boost - Case Positive Diode Driver

I don't see why the inversion is necessary. Why not just flip the cell and put the switching and sense elements "after" the load?

Something like this crude paint sketch:

attachment.php


(BJTs are easier to draw than FETs ;))
 

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Cyparagon said:
I don't see why the inversion is necessary. Why not just flip the cell and put the switching and sense elements "after" the load?

Something like this crude paint sketch:

attachment.php


(BJTs are easier to draw than FETs ;))
Click to expand...

That's smart. I think it would work in theory.

One slight glitch is that if your IC's FB is say 1.25V, that could very well be higher than the voltage at the current sense monitor's SENSE+ terminal. It certainly won't work with a ZXCT1009 that takes its power from the line being sensed, but there are plenty of current sense monitors that have independent power pins.
 
rhd said:
The challenge with that IC is that it disrupts the common (+). The AMC doesn't, which is why it works well here. I can't see a way to make that Linear chip common positive.
Click to expand...

If you still wish to pursue the original linear reg after the switching reg, I think the LTC part is the better option at higher currents and adjustability than the AMC chips.

47458d1429805222-inverting-buck-boost-case-positive-diode-driver-led.png
 
Cyparagon said:
I don't see why the inversion is necessary. Why not just flip the cell and put the switching and sense elements "after" the load?

Something like this crude paint sketch:

attachment.php


(BJTs are easier to draw than FETs ;))
Click to expand...

Many buck ICs are meant for driving P-Mosfets. What would you suggest for using them in your circuit? Drive a P-Mosfet that pulls the gate of the N-Mosfet high? I'm wondering whether the extra switching may add too much delay when dealing with high frequency (ie 1.5 to 2 MHz) ICs.
 
An open drain P channel driving the gate of an N mos is not ideal due to not enough pull down current capability.

You could rig a boost controller IC in buck mode with the negative side switched rather than the positive supply.
 
djQUAN said:
An open drain P channel driving the gate of an N mos is not ideal due to not enough pull down current capability.

You could rig a boost controller IC in buck mode with the negative side switched rather than the positive supply.
Click to expand...

This wouldn't work?

attachment.php
 

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It looks like you may be in need of either a
gate clamp or the MC34063.

It's funny reading over this thread and
seeing nearly the exact design issues I
have been dealing with for the past couple
years.
 
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rhd said:
This wouldn't work?

attachment.php
Click to expand...

Wouldn't work because the N-channel's gate would drain far too slowly with only a 10k resistor as the pulldown.

That's why most gate driver IC's use a complementary push pull pair, so that the gate can be pushed high by one switch, and then pulled low by the other with nearly zero resistance both ways.
 
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Something like this gate drive idea should work. Just switch the PNP and NPN for inversion.

ET_Flyback_MKII.1-full.jpg
 
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The flyback driver example just amplifies the gate drive; it doesn't invert it. That device won't invert the gate drive either. The BJTs have common collectors, and need to be either separate, or have common emitters.

22 probably isn't necessary, but 560 is certainly needed to limit the base current. The IC's gate drive is basically powering a short without that. It may also need to be higher or lower, depending on what sort of gate current the switching IC can typically handle.
 
Cyparagon said:
The flyback driver example just amplifies the gate drive; it doesn't invert it. That device won't invert the gate drive either. The BJTs have common collectors, and need to be either separate, or have common emitters.

22 probably isn't necessary, but 560 is certainly needed to limit the base current. The IC's gate drive is basically powering a short without that. It may also need to be higher or lower, depending on what sort of gate current the switching IC can typically handle.
Click to expand...

Seems like the 560 should be lower. IC can typically handle 500mA gate current.
 
rhd said:
Seems like the 560 should be lower. IC can typically handle 500mA gate current.
Click to expand...

Don't forget BJTs amplify, Typically 50-100 times. If 500mA gate current was sufficient to drive the FET, 10mA should be sufficient to drive the BJTs. You're wasting power if you go too high on the base current.
 
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