SMD Diode Identification

[mbear2k]

Hello - I hope this question is okay to ask here...

I am working on a power supply board made by LG, and have a single shorted SMD diode.  This diode sits across the gate and source of a W9NK70Z FET (Anode to Gate and Cathode to Source) and is dead shorted.  The FET was also bad and has been replaced.

The diode is labeled as "K45L" and there are no other markings other than the anode band.  This number does not come back in any parts searches...

Any thoughts on what this diode is, or an equivalent?  One search brought me loosely to a 40V 5A Schottky Barrier Diode?

Thanks,
Mark

[dannyf]

It is likely a zener. Rated at the max vgs of the mosfet.

[SeanB]

You would not go really wrong with a 15V 400mW zener diode in that position. But as the device anyhow has built in zener diodes it probably would work well without it.

Are you sure the contoller driving the FET is still working, most do tend to die if the fet puts full supply voltage on the output when they die.

[mbear2k]

It looks like the FET has internal back to back zener's, so the external diode must be used for another function?

Clip from the W9NK70Z data sheet:


The controller was damaged and has been replaced.

[SeanB]

Possibly the original design used a non protected device, or the designer wanted to not run the protection diode all the time so used the zener, possibly also to protect the driver from overvoltage ( which did not help fully in this case) on FET failure.

[KJDS]

It's possible that the circuit was designed with a FET that didn't have protection diodes included so the external diode was put in. Then for some reason the FET was changed but the external diode wasn't.

[dannyf]

If that's indeed true, you probably want to figure out why the gate was fried.

[SeanB]

Easy, overvoltage, poor heatsink or dying electrolytic capacitors caused ripple on the rails and boom.

[mbear2k]

I should come clean and mention that this is for a Y-Sustain board on an LG/HP plasma TV.  They have a common failure of one FET, this specific diode and sometimes an open resistor.  There is also a module 'intelligent power module' or IPM that is apparently the general cause for the chain reaction.  The IPM fails due to poor heat control - and probably components too close to rating, and takes out the series of supporting parts above before the 4A VS fuse blows.  However - and I understand why - but nobody wants to share the detail on what the diode is.  Just sell board repair services...

My failed IPM (look closely at left had side):



Y-Sustain Board example.  I've replaced the IPM (under the black heatsink) and the FET (second from left on top):

[SeanB]

If it is the device labelled as FRD01 or FRD02 then those are fast recovery diodes, typically 1000V rated at 5A or so, and with fast recovery. Used to keep the FET body diode from switching on when the fet is turned off, as the body diode is a slow recovery one, and having it off all the time with the fast recovery diode handling the current ( equal to the FET ratings typically) means it will be able to turn on faster.

[mbear2k]

The IPM board that was damaged in previous pic lives under the heat-sink below.  The whole IPM module was replaced.

But I like the thought that the external diode for the W9NK70Z FET was to provide a fast recovery diode to augment the internal slow recovery diodes.  Based on the pics below, is the consensus  1000V rated at 5A or something smaller? 

W9NK70Z data:
VDSS: 700v  ID Pw
RDS(on) < 1.2 ?
ID: 7.5 A
Pw: 115 W

Here's a shot of the board I am working on.  Diode location is D7 in front of the FET, second from the left.


Closer shot of the diode location:


Actual bad diode pulled:

[mbear2k]

I just realized... on my failed IPM module above are a handful of diodes labeled K4J.  Maybe I could harvest one to replace my shorted K4L?