Maybe I'm wrong, but in terms of power led, I am more concerned about over-current than for the overvoltage. It is the current that can burn them out. An example. You have a source that provides a constant voltage of 10 V and a current of 10 A. 10 LEDs that can handle 1 A. With a parallel connection, each chip would get its FV of 10 V and its maximum current of 1A. One fails and then the 10 A divided in 9 instead of 10 and each of the remaining chips will receive 1.1 A, and so on.
The multi-chip of 10 watt consists of 9 individual LEDs connected in 3 parallel loops with 3 LED in serie. This mean that if one LED is broken, it will not affect much, but if the whole chip break, the remaining will be affected with a current increases. Theoretically your setup will give a current about 770 mA (calculated from 250/12) to each chip and each chip manage 1 A. So the danger is not that great. It would require that it broke down 7 whole cip for you to have a current over 1 A. But as you pointed out, they have different FV and this can also affect how the current is distributed between the different chips.
So this is how I understand the problems associated with power LED. They are designed for a constant current, and not for a constant voltage
But your design is "up and running" so your design also seems to work.
Sincerely Lasse
Not sure your rationale of increasing current to the other LEDs in a parallel circuit if one shorts out works that way. In a circuit with parallel resistors yes, but I'm fairly certain LEDs don't behave the same way as resistors in a parallel circuit. My understanding is that provided each LED continues to see the same voltage, which is dictated by my power supply & dimmers, it shouldn't matter if 4 out of 5 LEDs in the parallel circuit short out, the 5th LED still sees 10 volts & will conduct only the current designed into it when manufactured (900mA @ 10 volts), no more. In a similar scenario with resistors in a parallel circuit, yes, if 4 out of 5 resistors short out, the 5th resistor now conducts way more current than when it still had it's buddies to share volts & amps with. When I did current measurements a while back, with 12VDC forward voltage & a 3ohm resistor in series with a white LED, there was only a minor variance in current between the four that I checked. Same idea with the Royal Blues.
When I first started looking at using these 10 watters I also noted as you explain that they consist of 3 rows of single LEDs in series/parallel configuration. Aside from the voltage required to drive them, I prefer them over the 20 or higher watt arrays since the individual LEDs on the chip have a little more room, i.e. they're not as tightly spaced as the higher wattage arrays. My theory there is, they should last longer since overheating is less likely. Still, I assume if one of the LEDs on the array shorts out, regardless of whether it's a 10, 20, 30 or 100 watt LED, it won't be long before there's a chain reaction (thermal runaway) & the whole thing burns up. One more reason I prefer the 10 watt LEDS, less costly to replace one than if a 100 watter cooks. Also why I have some confidence in my parallel set up & don't worry too much about current. Let the LEDs take care of their current based on the voltage they see.
If one of my dimmers fails open circuit, the LEDs will go out due to loss of power. No harm done, I simply get a new dimmer. If a dimmer fails short circuit, the LEDs hooked up to it will see the full power supply voltage which I have set at 10 volts, still within acceptable range of all the LEDs I'm using. Before finding the dimmers, I considered fine tuning the current to each LED by using a fixed value resistor & balancing them all out, but that's just way too much work & too many more components to be worthwhile. At the price of these 10watt LEDs, I'm fine with them slightly out of balance & if one burns out, no biggy, I'm only out a few $$s & they're simple to replace.
