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Old 04-23-2015, 11:54 PM #17
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Default Re: 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:



(BJTs are easier to draw than FETs )
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Last edited by Cyparagon; 04-24-2015 at 12:15 AM. Reason: sens correction
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Old 04-24-2015, 05:02 AM #18
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Default Re: Inverting buck/boost - Case Positive Diode Driver

Quote:
Originally Posted by Cyparagon View Post
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:



(BJTs are easier to draw than FETs )
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.
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Old 04-26-2015, 01:53 PM #19
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Default Re: Inverting buck/boost - Case Positive Diode Driver

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Originally Posted by rhd View Post
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.
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.

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Old 04-28-2015, 03:21 PM #20
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Default Re: Inverting buck/boost - Case Positive Diode Driver

Quote:
Originally Posted by Cyparagon View Post
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:



(BJTs are easier to draw than FETs )
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.
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Old 04-28-2015, 03:32 PM #21
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Default Re: Inverting buck/boost - Case Positive Diode Driver

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.
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Old 04-29-2015, 05:56 AM #22
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Default Re: Inverting buck/boost - Case Positive Diode Driver

Quote:
Originally Posted by djQUAN View Post
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.
This wouldn't work?

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Old 04-29-2015, 08:53 AM #23
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Default Re: Inverting buck/boost - Case Positive Diode Driver

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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Old 04-29-2015, 06:07 PM #24
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Default Re: Inverting buck/boost - Case Positive Diode Driver

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Originally Posted by rhd View Post
This wouldn't work?

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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Old 04-29-2015, 09:00 PM #25
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Default Re: Inverting buck/boost - Case Positive Diode Driver

Something like this gate drive idea should work. Just switch the PNP and NPN for inversion.

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Old 04-29-2015, 09:05 PM #26
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Default Re: Inverting buck/boost - Case Positive Diode Driver

I have used that before. Very fast and very
effective

Quote:
Originally Posted by Cyparagon View Post
Something like this gate drive idea should work. Just switch the PNP and NPN for inversion.

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Old 05-03-2015, 09:56 PM #27
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Default Re: Inverting buck/boost - Case Positive Diode Driver

Quote:
Originally Posted by Cyparagon View Post
Something like this gate drive idea should work. Just switch the PNP and NPN for inversion.

Quote:
Originally Posted by The Lightning Stalker View Post
I have used that before. Very fast and very
effective
Cyparagon / TLS, What do you think of this part, to accomplish the inversion above?
- Rohm QSZ2TR http://rohmfs.rohm.com/en/products/d...polar/qsz2.pdf

and is there need for the 560 and 22 ohm resistors shown?
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Old 05-03-2015, 11:00 PM #28
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Default Re: Inverting buck/boost - Case Positive Diode Driver

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Cyparagon / TLS, What do you think of this part, to accomplish the inversion above?
- Rohm QSZ2TR http://rohmfs.rohm.com/en/products/d...polar/qsz2.pdf

and is there need for the 560 and 22 ohm resistors shown?
That is non-inverting. For inverting, you
need the collectors tied together. Nice part,
though
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Old 05-03-2015, 11:07 PM #29
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Default Re: Inverting buck/boost - Case Positive Diode Driver

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.
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Old 05-03-2015, 11:34 PM #30
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Default

Quote:
Originally Posted by Cyparagon View Post
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.
Seems like the 560 should be lower. IC can typically handle 500mA gate current.
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Old 05-04-2015, 12:26 AM #31
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Default Re: Inverting buck/boost - Case Positive Diode Driver

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Seems like the 560 should be lower. IC can typically handle 500mA gate current.
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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