Continue to Site

Welcome to EDAboard.com

Welcome to our site! EDAboard.com is an international Electronics Discussion Forum focused on EDA software, circuits, schematics, books, theory, papers, asic, pld, 8051, DSP, Network, RF, Analog Design, PCB, Service Manuals... and a whole lot more! To participate you need to register. Registration is free. Click here to register now.

[SOLVED] What are the diodes for in this power supply sequencing circuit?

Status
Not open for further replies.

d123

Advanced Member level 5
Joined
Jun 7, 2015
Messages
2,505
Helped
494
Reputation
992
Reaction score
525
Trophy points
1,393
Location
Spain
Activity points
27,148
Hi,

What do the diodes do in this circuit?

power supply sequencing diodes.JPG

It looks like some sort of hysteresis section, but simulating it and changing the resistor to ground or either of the resistors in series with the diodes seems to alter either the rise time and/or final fall time (similar to changing the capacitor to the right of the diode/resistor stage).

powerup sequencing with diodes pdf.JPG

Removing the diodes didn't really change any voltage or current values, it only removed the slow tail after the fall from the NMOS load being on.

View attachment power-supply-sequencing-in-spaceflight-systems.pdf

Thanks.
 

I'm guessing they are there to provide seperate on/off time.
 
  • Like
Reactions: d123

    d123

    Points: 2
    Helpful Answer Positive Rating
Hi,

The document says:
...and the ability to delay both the power up and power down...

Klaus
 
  • Like
Reactions: d123

    d123

    Points: 2
    Helpful Answer Positive Rating
Delaying power supplies by putting RCD networks in the current
path seems like a pretty wasteful way to go. And to get a long
delay at amperes of through-current probably means sub-ohm
resistance and ridiculously-sized delay caps as a result (maybe
the input filter of the POLs is big enough, but I've never put
more than a couple of millifarad on any board).

Gate control of a MOSFET would at least afford you the option
that series R approaches zero, when the sequencing is done.
Op amp control of gate can give you whatever risetime (and,
if a second independent amp and shunt FET replaces the
reverse diode and discharge shunt R, fall time) you want, as
long as it respects the op amp bandwidth / slew rate.

If you look at sequencer / supervisor ICs internal and/or
application schematics, they don't put any series R in the
power path.
 
  • Like
Reactions: d123

    d123

    Points: 2
    Helpful Answer Positive Rating
The 'POL' devices, whatever they are show the delayed signal going to the 'ENABLE' pin so it probably isn't the main current path but some kind of on/off control for the device. I would guess the main power path isn't shown. To place components in the main current path could involve some awkward values but at small signal level it would be quite easy.

Brian.
 
  • Like
Reactions: d123

    d123

    Points: 2
    Helpful Answer Positive Rating
What's confusing is that the high side fet topology shown is often used for the main power path.
 
  • Like
Reactions: d123

    d123

    Points: 2
    Helpful Answer Positive Rating
Hi,

Thanks for your replies. Sorry about this, what I'm failing to understand is what function those diodes serve that makes it necessary they be there. In simulations they make next to no difference whether included or not in each respective path. Are they blocking each path (blocking the lower path from the MOSFET to enable, and blocking the upper path from enable back to the MOSFET) for a reason or doing something else?
 

As previously explained, the diodes are required to set on- and off-delay independently. Some parameters of your simulation schematic in post #1 should be corrected however.

POL regulator enable input should have a threshold
R13 << R12
R11 and R12 (respectively R12+R13) should be sufficiently different to show a diode effect
The simulation timescale should somehow fit the delay time order of magnitude, 10 ms in your circuit.
 
  • Like
Reactions: d123

    d123

    Points: 2
    Helpful Answer Positive Rating
Status
Not open for further replies.

Similar threads

Part and Inventory Search

Welcome to EDABoard.com

Sponsor

Back
Top