Re: High brightness white LEDs damaged by custom switcher



miso@xxxxxxxxx wrote:
On May 5, 3:44 pm, "Paul E. Schoen" <pst...@xxxxxxxxx> wrote:

<m...@xxxxxxxxx> wrote in message

news:1178400948.897551.99850@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx




Just so I'm reading this correctly, you are not paralleling strings of
leds. That is, you only drive one string of leds. If that is not the
case, I would expect problems.

When I think of the effort it takes to make a bullet-proof DC/DC chip,
I just shake my head at the idea of doing it in software. In a chip,
events take place simultaneously, while a uP is a step at a time using
polling.

The LEDs are in series, so all see the same current, and the voltage
required is about 26 VDC for 7 and 49 VDC for 13. The PIC can respond to
certain events within a few microseconds, by using interrupts. The
difficulty is in generating the interrupt signal outside the PIC. That is
why I plan to put a transistor on the current sense to detect an
overcurrent. I could also add a similar circuit to detect output
overvoltage, and generate the same interrupt.

The first thing to do is disable the PWM output, which can be done in a few
clock cycles. Less than 1 uSec for an 8 MHz clock. Then the A/Ds can be
used to see what caused it, and act accordingly. All three analog inputs
are now read within 1 mSec, but could be within 60 uSec. The A/D can make a
reading in 18 uSec.

As long as the external circuitry has a sufficiently slow response, I don't
see any problem implementing a switcher with a PIC. The real advantage is
that the hardware can be built in a simple, straightforward way, and then
changes can be implemented in PIC code. As requirements change, the same
circuit can be used with little or no change, and the PIC can be reflashed
to the new parameters.

I think the PIC is perfectly suited to this application. It may not be so
for situations where the input voltage may change suddenly, or output loads
are constantly changing. The main problem here seems to have been
identified, and several possible fixes should eliminate it. Extensive
testing should prove that.

Thanks,

Paul


It is one thing to get a circuit to work. It is another thing to turn
it loose on the general public. This is where the controller chips
shine over home brew designs. For instance, what happens if the user
inserts weak batteries. That is, how good is the undervoltage lockout.
What about an intermittent battery? Both at start up and during
operation. There is quite a bit of engineering in a DC/DC chip that
the user never sees, but it makes the design robust. Oh, and all this
has to work over temperature.

The typical start-up circuit work like this. First, you have enough
supply voltage to exceed a VT. One you have a VT, then you have trust
worthy logic. Next up, you would wait for the voltage reference to
exceed some simple reference, often just a N-fet fed with a current
source. The bandgap can take microseconds to start up, to maybe
hundreds if it is very low current. Once you trust the reference, you
will measure the supply voltage to see if it is suitable. If the
voltage is too low, the logic can be flaky. Once all conditions are
met, you start a timer circuit because just maybe the voltage source
is not steady (switch bounce, whatever). The you fire up the DC/DC,
there are other safety circuits. For instance, a relay could fire and
glitch the chip. [Probably not your situation.] A watch-dog timer will
insure the logic gets reset if the pulse width is well out of spec.
There are other safety features, typically over current protection on
the power fet.

Basically, the off the shelf chip is (or perhaps should be) bullet
proof. I just can't see doing this in a pic. The controller chip you
buy has the history of a few in the field failures.



Your concerns are way over the top. A RISC PIC endows the circuit with far more flexibility than a dedicated switching chip, which is made from the exact same type of logic elements and reference circuits as the PIC uses.

.

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