cupoftea
Advanced Member level 5
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Hi Guys,
Its a pretty normal setup that our northern UK customer has.
Common as a Drapers Loaf, many would say.
But our Northern UK customer sees
failures of their main_PCB which seem to occur after the product has
been 4kV Flash tested on their production line. (They literally 4kV flash test every product on production.)
The product is basically 4 single phase mains trailing edge switches on a Main_PCB.
The switches are each cascaded TO247 FETs (source to source).
So single phase mains goes into the product, and gets put back out through these 4 channels...and so these switched
mains channels then go out of the product.. and to other things.
The gate drive for the 4 FET pair switches is via optocoupler. Each opto switches a 14v rail...onto the
gate to turn the FET switches ON.
The 14V rails (obviously four of them, one for each FET switch pair), come from a multi-output offline flyback
This flyback opto_regulates one of the 14v rails, and the others are supposed to just follow suit..
..because they are on the same transformer. This offline flyback is courtesy of an LNK3604.
The Main_PCB also has a zero crossing circuit, which reports zero crossings to the micro via opto. This multi output
flyback also gives isolated bias power to the mains zero cross cct. In fact...this is the only regulated rail of the multi output
flyback, and its opto regulated.
The micro drives the spoken_of opto's which phase switch the FET switchs.
(The micro is perched on a wee daughter board which is ribbon_cable_fed from the main_PCB.)
Bias power for the board comes from an offline 24V flyback secondary. This flyback
is on a different PCB. On the Main_PCB that we speak of, this 24v gets bucked down to give the 3v3
for the micro.
A CAN bus also comes to the PCB.
The Main_PCB sits in an earthed metal enclosure. The spoken of TO247 FETs are screwed to this earthed metal enclosure and
have insulation pads between them and this enclosure.
I am baffled at the isolation testing, because the product puts out raw mains (albeit phase switched).
So how can you make it safe by isolation hipot testing on production? It puts out raw mains(!)
We are going to visit their factory tomorrow in Northern UK, to see how they are (4kV) isolation testing the product.
Presumably they are testing for 4kV of isolation between the 24V rail and all the comms input lines etc
to the raw mains Line and neutral.
From speaking to them, I get the impression that they are also testing for 4kV of isolation between the earthed
enclosure and all the other nets on the PCB (line , neutral, +24v, GND_of_the_+24v, etc)
So have you any tips for us on this? And why are they 4kV isolation testing every product on production?
By the way, the FET switches don't have Zener protection of the (isolated) FET gates....
Its a pretty normal setup that our northern UK customer has.
Common as a Drapers Loaf, many would say.
But our Northern UK customer sees
failures of their main_PCB which seem to occur after the product has
been 4kV Flash tested on their production line. (They literally 4kV flash test every product on production.)
The product is basically 4 single phase mains trailing edge switches on a Main_PCB.
The switches are each cascaded TO247 FETs (source to source).
So single phase mains goes into the product, and gets put back out through these 4 channels...and so these switched
mains channels then go out of the product.. and to other things.
The gate drive for the 4 FET pair switches is via optocoupler. Each opto switches a 14v rail...onto the
gate to turn the FET switches ON.
The 14V rails (obviously four of them, one for each FET switch pair), come from a multi-output offline flyback
This flyback opto_regulates one of the 14v rails, and the others are supposed to just follow suit..
..because they are on the same transformer. This offline flyback is courtesy of an LNK3604.
The Main_PCB also has a zero crossing circuit, which reports zero crossings to the micro via opto. This multi output
flyback also gives isolated bias power to the mains zero cross cct. In fact...this is the only regulated rail of the multi output
flyback, and its opto regulated.
The micro drives the spoken_of opto's which phase switch the FET switchs.
(The micro is perched on a wee daughter board which is ribbon_cable_fed from the main_PCB.)
Bias power for the board comes from an offline 24V flyback secondary. This flyback
is on a different PCB. On the Main_PCB that we speak of, this 24v gets bucked down to give the 3v3
for the micro.
A CAN bus also comes to the PCB.
The Main_PCB sits in an earthed metal enclosure. The spoken of TO247 FETs are screwed to this earthed metal enclosure and
have insulation pads between them and this enclosure.
I am baffled at the isolation testing, because the product puts out raw mains (albeit phase switched).
So how can you make it safe by isolation hipot testing on production? It puts out raw mains(!)
We are going to visit their factory tomorrow in Northern UK, to see how they are (4kV) isolation testing the product.
Presumably they are testing for 4kV of isolation between the 24V rail and all the comms input lines etc
to the raw mains Line and neutral.
From speaking to them, I get the impression that they are also testing for 4kV of isolation between the earthed
enclosure and all the other nets on the PCB (line , neutral, +24v, GND_of_the_+24v, etc)
So have you any tips for us on this? And why are they 4kV isolation testing every product on production?
By the way, the FET switches don't have Zener protection of the (isolated) FET gates....