Re: High brightness white LEDs damaged by custom switcher


"Fred Bloggs" <nospam@xxxxxxxxxx> wrote in message
news:463DD22E.2000707@xxxxxxxxxxxxx


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


That is exactly what I contend. The 16F684 is a very versatile and
inexpensive chip, which has most if not all of the capabilities needed for
the safeguards listed above.

The power on reset (POR) and power on timer (PWRT), and oscillator start-up
timer (OST) should eliminate any problems when power is first applied, and
it is highly unlikely that the 12 VDC battery will be too low to provide
regulated 5 VDC. The circuit is used in a dedicated application where input
supply and output load will always be known.

The brownout detect (BOD) assures that Vdd must be above VBOD=2.1V for the
chip to get out of Reset. When reset, the output PWM is disabled. Once the
device is running, the A/D converters monitor the input and output
voltages, and output current, to assure they are within normal range. I am
using the 5 VDC supply as reference, so erroneous readings could happen if
that voltage was way off, but there is minimal chance of that. The most
critical parameters of output voltage and current are fail-safe if supply
voltage reference is too low.

While running, the watchdog timer will reset the circuit if a glitch causes
a software lockup. The WDT can be set as fast as 1 mSec, but even that is
not quite fast enough to prevent excess output current if the PWM is maxed
out. However, the only relay in the circuit is the one which turns the
supply on and off, so transients are unlikely during operation. The unit
will be housed in a strong aluminum cylinder, surrounded by water, and
powered from a battery pack which is also submerged, so there is little
chance of external RF or voltage spikes.

Overcurrent in the power MOSFET is protected by the battery fuse, which is
20 amps. The MOSFET should be able to withstand that. The circuit does not
have saturation detection, but that is unlikely if the duty cycle is
limited and there are no component failures. The circuit will be
encapsulated, and not designed to be repaired. It is just a $5-$10
component in a high-tech flashlight that has $50 to $100 worth of LEDs and
a total package cost of $200 or so. Reliability is very important, but
protection of the LEDs is essential.

I will agree that a dedicated, pretested SMPS chip might be more reliable,
especially if there are errors in the PIC code or the associated circuitry.
That puts the burden on me to test the performance under all possible
conditions. A dedicated chip could still malfunction if an external circuit
element fails or is not properly chosen. I appreciate the words of warning,
but Fred's positive response leads me to believe my choice of a PIC is not
unreasonable.

Thanks,

Paul


.

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