Bogus protection circuitry on LED driver circuit?

[Flyback]

Hello,
Please could you help us because we believe an electronics design contractor company is trying to embezzle money out of us. We believe they deliberately designed a 140W LED power lamp for us with “problems” in it so that it would fail on thermal grounds. We believe that the electronics design contractor has pre-disposed the LED driver to fail on thermal grounds, knowing that since only they hold the PCB Layout files, we will have to go back and pay them to re-do the PCB to make it thermally acceptable.
There is also a microcontroller on the PCB and some signal circuitry, and he has layed this out so incredibly tightly that even though we have the schematic, we cannot replicate the PCB ourselves. We cannot make the PCB any bigger because all the enclosures have now been made.

The “problems” in the circuit relate to a lack of thermal spreading copper on the small LED driver PCB –this will likely lead to thermal failure…..the reason there is little thermal spreading copper on the PCB is because the designer added a large number of bogus “protection” components to the LED driver circuit, taking up lots of PCB space. These bogus components provide what the designer tells us is a “LED protection circuit”, which shuts off the LED load with a transistor switch whenever an overcurrent surge flows into the LEDs…..the designer tells us that if the LED connector ever becomes intermittent due to corrosion…..then the LED load will keep going open and then reconnecting…the designer tells us that when the LEDs go open, the output capacitors will over-charge (as the LED driver is a current source) and then when the connector re-makes, the overcharged output capacitors will deliver a surge of damaging current into the LEDs.

Thus the designer has added all of the components that you can see in the below schematic which he says, prevents damage to the LEDs and allows the microcontroller to detect that the connectors have become intermittent.

I believe this circuit is bogus….certainly running the simulation with and without the “led protection” components makes no difference….(the intermittent connector is simulated with a voltage controlled switch added to the simulation)
I admit that the comparator would trip on overcurrent, but does anyone seriously add such components to LED drivers?…I mean…how common is connector corrosion?…to the point where the contacts become intermittent?…I suspect bogus circuitry here.
I don't believe that the opamp that he uses as a "comparator" would trip quickly enough to stop the surge of current that he speaks about...and the simulation bears this out.
The designer also did not add much thermal spreading copper even where he could have done…and used no thermal vias to bottom spreading copper, even though he could have done this as the bottom layer is unpopulated. He also uses a 1206 high side sense resistor that dissipates 310mW.

We believe that he predisposed this circuit to fail, in the hope that since he holds the (Easy PC) PCB files (we do not have them), we will go back and offer to pay him to improve the PCB thermally.
What do you think? Do you think the “LED protection components” are bogus?
Why would the LED connectors be likely to corrode anyway?……even if they could, wouldn’t it be better to stop the corrosion by squirting silicone over them at assembly time.?

Schematic of LED driver with suspected “bogus” led protection components attached, as well as an LTspice file, showing the workings of the LED driver with the suspected “bogus” led protection circuitry.

 

[kubeek]

Which caps should overcharge? The two 4u7 ones? Charged to 48V they contain about 10mJ of energy, which certainly is no problem for LEDs that dissipate 150J every second.
Second thing, the FB pin should be used for the purpose of limiting voltage., as per the schematic on the first page. Why do you hire someone who can´t even connect the bloody IC like the datasheet tells him to, and adds a pointless circuitry instead?

 

[Flyback]

yes but that's 10mJ of energy delievered in very short time...surely it could do much damage?....(yes it is the two parallel 4u7's)

..you can't really limit the voltage...the highest Vf led string voltage is too close to the 48V input voltage.

 

[kubeek]

I seriously doubt that, they will have a large thermal mass and should have no trouble with such pulse. Either way, say what kind of LEDs it is and you should find the info on peak current vs duty cycle in the datasheet.

 

[moffy]

I agree with kubeek, the caps don't have enough energy to cause damage. I don't think you are looking at a deliberate conspiracy, just inexperienced designer.

 

[alec_t]

Did your brief to the designer include thermal or performance requirements?

 

[Flyback]

leds are mounted on mcpcb they are said to be 350mA nominal 500mA max and 700mA pulse at some duty cycle....we assumed the designer would be up on thermal issues and do it himself.

 

[kubeek]

You are driving the leds with ten times the rated current?

 

[crutschow]

It seems odd that you don't have the right to the layout files if you paid for the design(?).

 

[Flyback]

leds are 8 parallel strings of 12 in series....so its not ten times overcurrent in the leds.

 

[moffy]

How then do you prevent thermal runaway of one of the strings?

 

[Flyback]

matched leds straight from the foundry and very closely mounted on mcpcb.

 

[steveB]

It's not easy to detect and protect for an intermittent connection. I used to design and build current controllers for laser diodes. It was relatively easy to include precise current control, current limit protection, soft-start functions, static protection, voltage surge protection ... etc., but disconnects were not easy to protect against and the decision was made to use soldering of the laser and assembly care to make sure disconnects were unlikely, rather than trying to design circuits to protect against it. There were a few cases where manufacturing testing of the lasers, which used a temporary mechanical clamp rather than soldering, resulted in a connection failure and some expensive lasers were destroyed. In the end, the cost of damaged lasers, even expensive ones, was cheaper than trying to design and implement the protection circuits.

I have not studied the given circuit to see if it does what the designer claims, but there is a bigger issue here. The designer has not been properly managed. He should have been contracted to do particular tasks and to demonstrate critical design performance results. He should have been contracted to deliver all design materials too. I doubt there is a deliberate attempt to make a bogus system that will fail. That approach does not lead to further work. It leads to getting fired and then sued.

 

[moffy]

matched leds straight from the foundry and very closely mounted on mcpcb.

But for thermal matching they need a very good common heatsink to ensure that all the LED's are near one another temperature wise. Even so the LEDs at the edges will tend to run cooler than those in the center.

 

[Flyback]

They are all close on mcpcb....the led foundry are being closely consulted...mind you, you have a point.
The leds (165w worth) are clumped together on an area about 3cm by 3cm on mcpcb..the mcpcb is on a water cooled heetsink

 

[schmitt trigger]

"I doubt there is a deliberate attempt to make a bogus system that will fail."

My thought s exactly. I would rather think the designer is incompetent and/or ignorant.

 

[moffy]

They are all close on mcpcb....the led foundry are being closely consulted...mind you, you have a point.
The leds (165w worth) are clumped together on an area about 3cm by 3cm on mcpcb..the mcpcb is on a water cooled heetsink

Do you mean to say that the LEDs are soldered to the PCB and that it is the PCB that is heatsunk not the LEDs directly? Or am I misunderstanding the situation? That is a lot of power in a small area.

 

[Flyback]

the led pcb is mcpcb...and yes, it is fixed to a metal heetsink also...its a water cooled metal heetsink