Actual Tech Question -MP24943DN switching regulator output s

[John Robertson]

In our pinball games we sometimes run across a small SMD switcher
(MP24943DN) whose output Pin 8 shorts to GND even when operated within
load specs.

https://www.dropbox.com/s/s1r8gmxacjmss54/520-6998-72C-4-COIL-PLFD-NODE-BOARD-Plus-Regulator.png?dl=0

What also happens is the Shotky output diode also shorts - MBR760-E3/45.

http://www.vishay.com/docs/88680/mbr7xx.pdf

Spec sheet for MP24943:

https://www.monolithicpower.com/pub/media/document/MP24943_r1.0.pdf

Looking at Monolithic's web site design pages:

https://www.monolithicpower.com/en/design-tools/design-tools/dc-dc-designer-online.html?PN=MP24943

and putting in the parameters 48VDC max 55VDC, and output 6VDC @ 3A it
does generate a different schematic BOM.

I was looking for the original Application Notes (Evaluation Board) for
this product but was unable to find them online. The MP24943 data sheet
refers one to them of course... "For more possible applications of this
device, please refer to related Evaluation Board Data Sheets." but good
luck finding them (at least so far). I like to use the application notes
to compare to what the PCB designer cooked up to see if they made an
error and if so, fix it properly.

My question is - does anyone see any obvious issues? The regulator
'should' be current limiting, but I think where the error is related to
either VO tied to FB or BST not having a 0R in series with C26 to SW. Is
that zero ohm resistor of any real use? As you can see I am not a design
engineer, just an amateur trying to make things work better by getting
closer to factory requirements.

Thanks,

John :-#)#

--
(Please post followups or tech inquiries to the USENET newsgroup)
John's Jukes Ltd.
MOVED to #7 - 3979 Marine Way, Burnaby, BC, Canada V5J 5E3
(604)872-5757 (Pinballs, Jukes, Video Games)
www.flippers.com
"Old pinballers never die, they just flip out."

 

[Martin Riddle]

On Fri, 14 Jun 2019 11:57:49 -0700, John Robertson <spam@flippers.com>
wrote:

In our pinball games we sometimes run across a small SMD switcher
(MP24943DN) whose output Pin 8 shorts to GND even when operated within
load specs.

https://www.dropbox.com/s/s1r8gmxacjmss54/520-6998-72C-4-COIL-PLFD-NODE-BOARD-Plus-Regulator.png?dl=0

What also happens is the Shotky output diode also shorts - MBR760-E3/45.

http://www.vishay.com/docs/88680/mbr7xx.pdf

Spec sheet for MP24943:

https://www.monolithicpower.com/pub/media/document/MP24943_r1.0.pdf

Looking at Monolithic's web site design pages:

https://www.monolithicpower.com/en/design-tools/design-tools/dc-dc-designer-online.html?PN=MP24943

and putting in the parameters 48VDC max 55VDC, and output 6VDC @ 3A it
does generate a different schematic BOM.

I was looking for the original Application Notes (Evaluation Board) for
this product but was unable to find them online. The MP24943 data sheet
refers one to them of course... "For more possible applications of this
device, please refer to related Evaluation Board Data Sheets." but good
luck finding them (at least so far). I like to use the application notes
to compare to what the PCB designer cooked up to see if they made an
error and if so, fix it properly.

My question is - does anyone see any obvious issues? The regulator
'should' be current limiting, but I think where the error is related to
either VO tied to FB or BST not having a 0R in series with C26 to SW. Is
that zero ohm resistor of any real use? As you can see I am not a design
engineer, just an amateur trying to make things work better by getting
closer to factory requirements.

Thanks,

John :-#)#

IF for some reason you needed to adjust the loop gain, you could play
with R4, There are fixed Feedback components inside around the Error
amp.

I've seen these things butchered from the origanl design notes in
those 12v car ligher plug chargers. Like a high school project gone
wrong. They work better when wired up properly.

Cheers

 

[John Robertson]

On 2019/06/14 3:48 p.m., boB wrote:

On Fri, 14 Jun 2019 11:57:49 -0700, John Robertson <spam@flippers.com
wrote:

In our pinball games we sometimes run across a small SMD switcher
(MP24943DN) whose output Pin 8 shorts to GND even when operated within
load specs.

https://www.dropbox.com/s/s1r8gmxacjmss54/520-6998-72C-4-COIL-PLFD-NODE-BOARD-Plus-Regulator.png?dl=0

What also happens is the Shotky output diode also shorts - MBR760-E3/45.

http://www.vishay.com/docs/88680/mbr7xx.pdf

Spec sheet for MP24943:

https://www.monolithicpower.com/pub/media/document/MP24943_r1.0.pdf

Looking at Monolithic's web site design pages:

https://www.monolithicpower.com/en/design-tools/design-tools/dc-dc-designer-online.html?PN=MP24943

and putting in the parameters 48VDC max 55VDC, and output 6VDC @ 3A it
does generate a different schematic BOM.

I was looking for the original Application Notes (Evaluation Board) for
this product but was unable to find them online. The MP24943 data sheet
refers one to them of course... "For more possible applications of this
device, please refer to related Evaluation Board Data Sheets." but good
luck finding them (at least so far). I like to use the application notes
to compare to what the PCB designer cooked up to see if they made an
error and if so, fix it properly.

My question is - does anyone see any obvious issues? The regulator
'should' be current limiting, but I think where the error is related to
either VO tied to FB or BST not having a 0R in series with C26 to SW. Is
that zero ohm resistor of any real use? As you can see I am not a design
engineer, just an amateur trying to make things work better by getting
closer to factory requirements.

Thanks,

John :-#)#


When you say that pin 8 "shorts to gnd" do you mean that part breaks ?
Lets its smoke out ?

Yes, resistance essentially reaches zero ohms Pin 8 to common/GND and we
lifted the leg (P8) to confirm short.

If not, that switch node will usually go to zero volts when the input
transistor inside the chip turns off when there is current in the
inductor. Actually, the SW node voltage will go slightly negative, to
the forward drop voltage of the diode drop.

It looks like a buck converter.

I think you meant a f..ked converter!

John :-#)#

 

[boB]

On Fri, 14 Jun 2019 11:57:49 -0700, John Robertson <spam@flippers.com>
wrote:

In our pinball games we sometimes run across a small SMD switcher
(MP24943DN) whose output Pin 8 shorts to GND even when operated within
load specs.

https://www.dropbox.com/s/s1r8gmxacjmss54/520-6998-72C-4-COIL-PLFD-NODE-BOARD-Plus-Regulator.png?dl=0

What also happens is the Shotky output diode also shorts - MBR760-E3/45.

http://www.vishay.com/docs/88680/mbr7xx.pdf

Spec sheet for MP24943:

https://www.monolithicpower.com/pub/media/document/MP24943_r1.0.pdf

Looking at Monolithic's web site design pages:

https://www.monolithicpower.com/en/design-tools/design-tools/dc-dc-designer-online.html?PN=MP24943

and putting in the parameters 48VDC max 55VDC, and output 6VDC @ 3A it
does generate a different schematic BOM.

I was looking for the original Application Notes (Evaluation Board) for
this product but was unable to find them online. The MP24943 data sheet
refers one to them of course... "For more possible applications of this
device, please refer to related Evaluation Board Data Sheets." but good
luck finding them (at least so far). I like to use the application notes
to compare to what the PCB designer cooked up to see if they made an
error and if so, fix it properly.

My question is - does anyone see any obvious issues? The regulator
'should' be current limiting, but I think where the error is related to
either VO tied to FB or BST not having a 0R in series with C26 to SW. Is
that zero ohm resistor of any real use? As you can see I am not a design
engineer, just an amateur trying to make things work better by getting
closer to factory requirements.

Thanks,

John :-#)#

When you say that pin 8 "shorts to gnd" do you mean that part breaks ?
Lets its smoke out ?

If not, that switch node will usually go to zero volts when the input
transistor inside the chip turns off when there is current in the
inductor. Actually, the SW node voltage will go slightly negative, to
the forward drop voltage of the diode drop.

It looks like a buck converter.

 

[Winfield Hill]

John Robertson wrote...

In our pinball games we sometimes run across a small
SMD switcher (MP24943DN) whose output Pin 8 shorts ...

https://www.dropbox.com/s/s1r8gmxacjmss54/520-6998-72C-4-COIL-PLFD-NODE-BOARD-Plus-Regulator.png?dl=0

What's your maximum load? The chip says it can do 3A,
are you using it all the away at 3A, no safety margin?
The catch diode will be on most of the time. At 3A it
drops about 0.55 volts, or 1.6 watts. If it doesn't
have a small clip-on heat sink, its junction may be
running pretty hot, and eventually fail, as a short.
Shortly after it shorts, it'll take out the switcher.


--
Thanks,
- Win

 

[John Robertson]

On 2019/06/14 7:56 p.m., Winfield Hill wrote:

John Robertson wrote...

In our pinball games we sometimes run across a small
SMD switcher (MP24943DN) whose output Pin 8 shorts ...

https://www.dropbox.com/s/s1r8gmxacjmss54/520-6998-72C-4-COIL-PLFD-NODE-BOARD-Plus-Regulator.png?dl=0

What's your maximum load? The chip says it can do 3A,
are you using it all the away at 3A, no safety margin?
The catch diode will be on most of the time. At 3A it
drops about 0.55 volts, or 1.6 watts. If it doesn't
have a small clip-on heat sink, its junction may be
running pretty hot, and eventually fail, as a short.
Shortly after it shorts, it'll take out the switcher.

Hi Win,

Thanks, that may well be it. There is no heat sink on that diode.

I will monitor the temperature of the catch/Schottky diode as see how
high it gets in normal use as well as monitoring the load current.This
circuit is used in a commercial pinball game for the networked driver boards

Much appreciated!

John :-#)#

 

[Winfield Hill]

John Robertson wrote...

On 2019/06/14 7:56 p.m., Winfield Hill wrote:
John Robertson wrote...

In our pinball games we sometimes run across a small
SMD switcher (MP24943DN) whose output Pin 8 shorts ...

https://www.dropbox.com/s/s1r8gmxacjmss54/520-6998-72C-4-COIL-PLFD-NODE-BOARD-Plus-Regulator.png?dl=0

What's your maximum load? The chip says it can do 3A,
are you using it all the away at 3A, no safety margin?
The catch diode will be on most of the time. At 3A it
drops about 0.55 volts, or 1.6 watts. If it doesn't
have a small clip-on heat sink, its junction may be
running pretty hot, and eventually fail, as a short.
Shortly after it shorts, it'll take out the switcher.

Hi Win,

Thanks, that may well be it. There is no heat sink on that diode.

I will monitor the temperature of the catch/Schottky diode as see
how high it gets in normal use as well as monitoring the load
current. This circuit is used in a commercial pinball game for
the networked driver boards.

Networked driver boards, what's that?

If there's enough space next to the TO-220 diode, the
upgrade will be very easy, just slide on a heatsink.

My go-to heatsink is the Wakefield 233-60AB. It takes
about 0.5-inch square out from the body of the TO-220,
and at 1.6 watts, the heatsink temp rises only 45 deg.
The MBR760 diode junction is another 8 deg = 103 C, if
the box ambient is say 50C. It should last forever.



--
Thanks,
- Win

 

[John Robertson]

On 2019/06/15 6:57 a.m., Winfield Hill wrote:

John Robertson wrote...

On 2019/06/14 7:56 p.m., Winfield Hill wrote:
John Robertson wrote...

In our pinball games we sometimes run across a small
SMD switcher (MP24943DN) whose output Pin 8 shorts ...

https://www.dropbox.com/s/s1r8gmxacjmss54/520-6998-72C-4-COIL-PLFD-NODE-BOARD-Plus-Regulator.png?dl=0

What's your maximum load? The chip says it can do 3A,
are you using it all the away at 3A, no safety margin?
The catch diode will be on most of the time. At 3A it
drops about 0.55 volts, or 1.6 watts. If it doesn't
have a small clip-on heat sink, its junction may be
running pretty hot, and eventually fail, as a short.
Shortly after it shorts, it'll take out the switcher.

Hi Win,

Thanks, that may well be it. There is no heat sink on that diode.

I will monitor the temperature of the catch/Schottky diode as see
how high it gets in normal use as well as monitoring the load
current. This circuit is used in a commercial pinball game for
the networked driver boards.

Networked driver boards, what's that?

They are using their own twisted pair network in the game - much like
automobile CAN (Controller Area Network) - to communicate with all the
switch, lamp (LED), and solenoid driver boards scattered about.

If there's enough space next to the TO-220 diode, the
upgrade will be very easy, just slide on a heatsink.

Yes, this should be easy to install.

My go-to heatsink is the Wakefield 233-60AB. It takes
about 0.5-inch square out from the body of the TO-220,
and at 1.6 watts, the heatsink temp rises only 45 deg.
The MBR760 diode junction is another 8 deg = 103 C, if
the box ambient is say 50C. It should last forever.

Mouser has those under $0.40USD each, much appreciated!

John :-#)#


--
(Please post followups or tech inquiries to the USENET newsgroup)
John's Jukes Ltd.
MOVED to #7 - 3979 Marine Way, Burnaby, BC, Canada V5J 5E3
(604)872-5757 (Pinballs, Jukes, Video Games)
www.flippers.com
"Old pinballers never die, they just flip out."

 

[John Robertson]

On 2019/06/15 6:23 p.m., bloggs.fredbloggs.fred@gmail.com wrote:

On Saturday, June 15, 2019 at 9:57:51 AM UTC-4, Winfield Hill wrote:
John Robertson wrote...

On 2019/06/14 7:56 p.m., Winfield Hill wrote:
John Robertson wrote...

In our pinball games we sometimes run across a small
SMD switcher (MP24943DN) whose output Pin 8 shorts ...

https://www.dropbox.com/s/s1r8gmxacjmss54/520-6998-72C-4-COIL-PLFD-NODE-BOARD-Plus-Regulator.png?dl=0

What's your maximum load? The chip says it can do 3A,
are you using it all the away at 3A, no safety margin?
The catch diode will be on most of the time. At 3A it
drops about 0.55 volts, or 1.6 watts. If it doesn't
have a small clip-on heat sink, its junction may be
running pretty hot, and eventually fail, as a short.
Shortly after it shorts, it'll take out the switcher.

Hi Win,

Thanks, that may well be it. There is no heat sink on that diode.

I will monitor the temperature of the catch/Schottky diode as see
how high it gets in normal use as well as monitoring the load
current. This circuit is used in a commercial pinball game for
the networked driver boards.

Networked driver boards, what's that?

If there's enough space next to the TO-220 diode, the
upgrade will be very easy, just slide on a heatsink.

My go-to heatsink is the Wakefield 233-60AB. It takes
about 0.5-inch square out from the body of the TO-220,
and at 1.6 watts, the heatsink temp rises only 45 deg.
The MBR760 diode junction is another 8 deg = 103 C, if
the box ambient is say 50C. It should last forever.

heatsink is 30oC/W? Makes that 32x increase MTBF. Maybe forever depends on what he has now.

Currently forever is around one to five years...

John ;-#)#

--
Thanks,
- Win

 

On Saturday, June 15, 2019 at 9:57:51 AM UTC-4, Winfield Hill wrote:

John Robertson wrote...

On 2019/06/14 7:56 p.m., Winfield Hill wrote:
John Robertson wrote...

In our pinball games we sometimes run across a small
SMD switcher (MP24943DN) whose output Pin 8 shorts ...

https://www.dropbox.com/s/s1r8gmxacjmss54/520-6998-72C-4-COIL-PLFD-NODE-BOARD-Plus-Regulator.png?dl=0

What's your maximum load? The chip says it can do 3A,
are you using it all the away at 3A, no safety margin?
The catch diode will be on most of the time. At 3A it
drops about 0.55 volts, or 1.6 watts. If it doesn't
have a small clip-on heat sink, its junction may be
running pretty hot, and eventually fail, as a short.
Shortly after it shorts, it'll take out the switcher.

Hi Win,

Thanks, that may well be it. There is no heat sink on that diode.

I will monitor the temperature of the catch/Schottky diode as see
how high it gets in normal use as well as monitoring the load
current. This circuit is used in a commercial pinball game for
the networked driver boards.

Networked driver boards, what's that?

If there's enough space next to the TO-220 diode, the
upgrade will be very easy, just slide on a heatsink.

My go-to heatsink is the Wakefield 233-60AB. It takes
about 0.5-inch square out from the body of the TO-220,
and at 1.6 watts, the heatsink temp rises only 45 deg.
The MBR760 diode junction is another 8 deg = 103 C, if
the box ambient is say 50C. It should last forever.

heatsink is 30oC/W? Makes that 32x increase MTBF. Maybe forever depends on what he has now.

--
Thanks,
- Win

 

On Saturday, June 15, 2019 at 9:57:51 AM UTC-4, Winfield Hill wrote:

John Robertson wrote...

On 2019/06/14 7:56 p.m., Winfield Hill wrote:
John Robertson wrote...

In our pinball games we sometimes run across a small
SMD switcher (MP24943DN) whose output Pin 8 shorts ...

https://www.dropbox.com/s/s1r8gmxacjmss54/520-6998-72C-4-COIL-PLFD-NODE-BOARD-Plus-Regulator.png?dl=0

What's your maximum load? The chip says it can do 3A,
are you using it all the away at 3A, no safety margin?
The catch diode will be on most of the time. At 3A it
drops about 0.55 volts, or 1.6 watts. If it doesn't
have a small clip-on heat sink, its junction may be
running pretty hot, and eventually fail, as a short.
Shortly after it shorts, it'll take out the switcher.

Hi Win,

Thanks, that may well be it. There is no heat sink on that diode.

I will monitor the temperature of the catch/Schottky diode as see
how high it gets in normal use as well as monitoring the load
current. This circuit is used in a commercial pinball game for
the networked driver boards.

Networked driver boards, what's that?

If there's enough space next to the TO-220 diode, the
upgrade will be very easy, just slide on a heatsink.

My go-to heatsink is the Wakefield 233-60AB. It takes
about 0.5-inch square out from the body of the TO-220,
and at 1.6 watts, the heatsink temp rises only 45 deg.
The MBR760 diode junction is another 8 deg = 103 C, if
the box ambient is say 50C. It should last forever.

Not sure about that. The thermal for his package should be similar to this one, which they list as 62.5oC/W RsubJ-A:

https://goodarksemi.com/docs/datasheets/featured/10A/mbr(f)1545.pdf

Your heat sink buys less than 20oC/W margin, which should put him at about 8x increase in MTBF compared to without it.

--
Thanks,
- Win

 

On Saturday, June 15, 2019 at 9:28:07 PM UTC-4, John Robertson wrote:

On 2019/06/15 6:23 p.m., bloggs.fredbloggs.fred@gmail.com wrote:
On Saturday, June 15, 2019 at 9:57:51 AM UTC-4, Winfield Hill wrote:
John Robertson wrote...

On 2019/06/14 7:56 p.m., Winfield Hill wrote:
John Robertson wrote...

In our pinball games we sometimes run across a small
SMD switcher (MP24943DN) whose output Pin 8 shorts ...

https://www.dropbox.com/s/s1r8gmxacjmss54/520-6998-72C-4-COIL-PLFD-NODE-BOARD-Plus-Regulator.png?dl=0

What's your maximum load? The chip says it can do 3A,
are you using it all the away at 3A, no safety margin?
The catch diode will be on most of the time. At 3A it
drops about 0.55 volts, or 1.6 watts. If it doesn't
have a small clip-on heat sink, its junction may be
running pretty hot, and eventually fail, as a short.
Shortly after it shorts, it'll take out the switcher.

Hi Win,

Thanks, that may well be it. There is no heat sink on that diode.

I will monitor the temperature of the catch/Schottky diode as see
how high it gets in normal use as well as monitoring the load
current. This circuit is used in a commercial pinball game for
the networked driver boards.

Networked driver boards, what's that?

If there's enough space next to the TO-220 diode, the
upgrade will be very easy, just slide on a heatsink.

My go-to heatsink is the Wakefield 233-60AB. It takes
about 0.5-inch square out from the body of the TO-220,
and at 1.6 watts, the heatsink temp rises only 45 deg.
The MBR760 diode junction is another 8 deg = 103 C, if
the box ambient is say 50C. It should last forever.

heatsink is 30oC/W? Makes that 32x increase MTBF. Maybe forever depends on what he has now.

Currently forever is around one to five years...

Okay, heat sink will make it forever. The original design almost got there, they must have underestimated operating ambient air temp.

John ;-#)#





--
Thanks,
- Win

 

[John Robertson]

On 2019/06/17 8:06 a.m., legg wrote:

On Fri, 14 Jun 2019 11:57:49 -0700, John Robertson <spam@flippers.com
wrote:

In our pinball games we sometimes run across a small SMD switcher
(MP24943DN) whose output Pin 8 shorts to GND even when operated within
load specs.

https://www.dropbox.com/s/s1r8gmxacjmss54/520-6998-72C-4-COIL-PLFD-NODE-BOARD-Plus-Regulator.png?dl=0

What also happens is the Shotky output diode also shorts - MBR760-E3/45.

http://www.vishay.com/docs/88680/mbr7xx.pdf

Spec sheet for MP24943:

https://www.monolithicpower.com/pub/media/document/MP24943_r1.0.pdf

Looking at Monolithic's web site design pages:

https://www.monolithicpower.com/en/design-tools/design-tools/dc-dc-designer-online.html?PN=MP24943

and putting in the parameters 48VDC max 55VDC, and output 6VDC @ 3A it
does generate a different schematic BOM.

I was looking for the original Application Notes (Evaluation Board) for
this product but was unable to find them online...

My question is - does anyone see any obvious issues? The regulator
...

Thanks,

John :-#)#


The pin short and the diode short shouldn't be found in the same unit.
A dry solder joint in one might explain the other.

The diode is a through hole device, so not much chance of dry solder
joint there...

Sync buck converters may ( repeat: 'may') have undocumented behavior
at shut-down that produces an abnormally long conduction interval in
the internal shunt mosfet (ie pin8 to ground). If the system load has
a lot of capacitance, the abnormal conduction period can cause reverse
current in the output filter choke. When the switch turns off, you get
a load dump effect, with voltages limited by the node Z under these
conditions. The current can be insanely large w/r to normal operating
conditions.

The typical load for this 6V3A supply are a string of colour LEDs. I
don't think they have any back EMF potential. However your suggestion is
interesting. I do recall linear supplies with a back diode from the
output to the input, when there were large capacitors on the load side,
to avoid this problem.

The amount of energy absorbed may not be 'much', though it scales to
LxIp^2, but neither diode nor chip are designed to do that. Your
schematic has a TVS on input that may be too high a clamping voltage.
An external diode in parallel with the upper switch might do
something.

You could spend the rset of your life trying to capture this event on
a scope, if a particular unit will oblige - temperature and conditions
of use may dominate both behavior and results, so keeping things cool
can't hurt.

RL

Good suggestions for further research on the shut-down side.

Thanks!

John :-#)#

 

[legg]

On Fri, 14 Jun 2019 11:57:49 -0700, John Robertson <spam@flippers.com>
wrote:

In our pinball games we sometimes run across a small SMD switcher
(MP24943DN) whose output Pin 8 shorts to GND even when operated within
load specs.

https://www.dropbox.com/s/s1r8gmxacjmss54/520-6998-72C-4-COIL-PLFD-NODE-BOARD-Plus-Regulator.png?dl=0

What also happens is the Shotky output diode also shorts - MBR760-E3/45.

http://www.vishay.com/docs/88680/mbr7xx.pdf

Spec sheet for MP24943:

https://www.monolithicpower.com/pub/media/document/MP24943_r1.0.pdf

Looking at Monolithic's web site design pages:

https://www.monolithicpower.com/en/design-tools/design-tools/dc-dc-designer-online.html?PN=MP24943

and putting in the parameters 48VDC max 55VDC, and output 6VDC @ 3A it
does generate a different schematic BOM.

I was looking for the original Application Notes (Evaluation Board) for
this product but was unable to find them online. The MP24943 data sheet
refers one to them of course... "For more possible applications of this
device, please refer to related Evaluation Board Data Sheets." but good
luck finding them (at least so far). I like to use the application notes
to compare to what the PCB designer cooked up to see if they made an
error and if so, fix it properly.

My question is - does anyone see any obvious issues? The regulator
'should' be current limiting, but I think where the error is related to
either VO tied to FB or BST not having a 0R in series with C26 to SW. Is
that zero ohm resistor of any real use? As you can see I am not a design
engineer, just an amateur trying to make things work better by getting
closer to factory requirements.

Thanks,

John :-#)#

The slow start capacitor size has also been known to influence chip
behavior during shutdown, as well as start-up.

RL

 

[legg]

On Fri, 14 Jun 2019 11:57:49 -0700, John Robertson <spam@flippers.com>
wrote:

In our pinball games we sometimes run across a small SMD switcher
(MP24943DN) whose output Pin 8 shorts to GND even when operated within
load specs.

https://www.dropbox.com/s/s1r8gmxacjmss54/520-6998-72C-4-COIL-PLFD-NODE-BOARD-Plus-Regulator.png?dl=0

What also happens is the Shotky output diode also shorts - MBR760-E3/45.

http://www.vishay.com/docs/88680/mbr7xx.pdf

Spec sheet for MP24943:

https://www.monolithicpower.com/pub/media/document/MP24943_r1.0.pdf

Looking at Monolithic's web site design pages:

https://www.monolithicpower.com/en/design-tools/design-tools/dc-dc-designer-online.html?PN=MP24943

and putting in the parameters 48VDC max 55VDC, and output 6VDC @ 3A it
does generate a different schematic BOM.

I was looking for the original Application Notes (Evaluation Board) for
this product but was unable to find them online. The MP24943 data sheet
refers one to them of course... "For more possible applications of this
device, please refer to related Evaluation Board Data Sheets." but good
luck finding them (at least so far). I like to use the application notes
to compare to what the PCB designer cooked up to see if they made an
error and if so, fix it properly.

My question is - does anyone see any obvious issues? The regulator
'should' be current limiting, but I think where the error is related to
either VO tied to FB or BST not having a 0R in series with C26 to SW. Is
that zero ohm resistor of any real use? As you can see I am not a design
engineer, just an amateur trying to make things work better by getting
closer to factory requirements.

Thanks,

John :-#)#

The pin short and the diode short shouldn't be found in the same unit.
A dry solder joint in one might explain the other.

Sync buck converters may ( repeat: 'may') have undocumented behavior
at shut-down that produces an abnormally long conduction interval in
the internal shunt mosfet (ie pin8 to ground). If the system load has
a lot of capacitance, the abnormal conduction period can cause reverse
current in the output filter choke. When the switch turns off, you get
a load dump effect, with voltages limited by the node Z under these
conditions. The current can be insanely large w/r to normal operating
conditions.

The amount of energy absorbed may not be 'much', though it scales to
LxIp^2, but neither diode nor chip are designed to do that. Your
schematic has a TVS on input that may be too high a clamping voltage.
An external diode in parallel with the upper switch might do
something.

You could spend the rset of your life trying to capture this event on
a scope, if a particular unit will oblige - temperature and conditions
of use may dominate both behavior and results, so keeping things cool
can't hurt.

RL

 

[John Robertson]

On 2019/06/15 6:37 p.m., bloggs.fredbloggs.fred@gmail.com wrote:

On Saturday, June 15, 2019 at 9:28:07 PM UTC-4, John Robertson wrote:
On 2019/06/15 6:23 p.m., bloggs.fredbloggs.fred@gmail.com wrote:
On Saturday, June 15, 2019 at 9:57:51 AM UTC-4, Winfield Hill wrote:
John Robertson wrote...

On 2019/06/14 7:56 p.m., Winfield Hill wrote:
John Robertson wrote...

In our pinball games we sometimes run across a small
SMD switcher (MP24943DN) whose output Pin 8 shorts ...

https://www.dropbox.com/s/s1r8gmxacjmss54/520-6998-72C-4-COIL-PLFD-NODE-BOARD-Plus-Regulator.png?dl=0

What's your maximum load? The chip says it can do 3A,
are you using it all the away at 3A, no safety margin?
The catch diode will be on most of the time. At 3A it
drops about 0.55 volts, or 1.6 watts. If it doesn't
have a small clip-on heat sink, its junction may be
running pretty hot, and eventually fail, as a short.
Shortly after it shorts, it'll take out the switcher.

Hi Win,

Thanks, that may well be it. There is no heat sink on that diode.

I will monitor the temperature of the catch/Schottky diode as see
how high it gets in normal use as well as monitoring the load
current. This circuit is used in a commercial pinball game for
the networked driver boards.

Networked driver boards, what's that?

If there's enough space next to the TO-220 diode, the
upgrade will be very easy, just slide on a heatsink.

My go-to heatsink is the Wakefield 233-60AB. It takes
about 0.5-inch square out from the body of the TO-220,
and at 1.6 watts, the heatsink temp rises only 45 deg.
The MBR760 diode junction is another 8 deg = 103 C, if
the box ambient is say 50C. It should last forever.

heatsink is 30oC/W? Makes that 32x increase MTBF. Maybe forever depends on what he has now.

Currently forever is around one to five years...

Okay, heat sink will make it forever. The original design almost got there, they must have underestimated operating ambient air temp.

Testing two of these TO-220 diodes after running all day in the game
they run at least 50C with ambient temp of 24C. The derating curve
doesn't cut in until the device reaches about 105C. I will put a
thermocouple on the tab and let it run for a day and see how high it
gets when I do not have the cabinet open to do a temp test.

John :-#)#