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High brightness white LEDs damaged by custom switcher

Discussion in 'Electronic Design' started by Paul E. Schoen, May 4, 2007.

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  1. "Fred Bartoli"
    According to my simulation, the circuit enters continuous mode only for an
    input of a bit less than 10 VDC. The LEDs have an ESR of about 2 ohms each,
    so a couple volts of ripple can be tolerated. They are rated for 1.8 A
    maximum pulse current. The ASCII file following reflects some recent
    diddling, but is approximately what I have.

    Regarding the PIC, the honest answer is probably that I *like* using them,
    but there are other reasons. The SMPS controllers I am familiar with use
    reference voltage of 2.5 V or more, so I would need to add an op-amp to
    boost the current sense voltage. I would like to retain the features of (1)
    shutdown for output overvoltage (to avoid blowing the cap if the load is
    disconnected), (2) input undervoltage detection, which sets the PWM to a
    low current power saving mode, and (3) the unique dual brightness feature.
    I could use a PIC to control the SMPS controller, but that would add
    another component on an already tight board.

    I am convinced that the precautions I have come up with will eliminate the
    main problem of excess current surges. I will need to do some actual
    testing to be sure, of course.

    Many thanks to all who have responded.

    Paul

    LTSpice file:
    =============================================================================

    Version 4
    SHEET 1 1216 680
    WIRE -128 -240 -128 -256
    WIRE -128 -240 -304 -240
    WIRE -64 -240 -128 -240
    WIRE 224 -240 -64 -240
    WIRE 592 -240 224 -240
    WIRE -304 -208 -304 -240
    WIRE 224 -192 224 -240
    WIRE 592 -144 592 -240
    WIRE -304 -48 -304 -128
    WIRE -192 -48 -304 -48
    WIRE 592 -32 592 -64
    WIRE 688 -32 688 -48
    WIRE 688 -32 592 -32
    WIRE 736 -32 688 -32
    WIRE 816 -32 800 -32
    WIRE 928 -32 816 -32
    WIRE 944 -32 928 -32
    WIRE 1072 -32 944 -32
    WIRE -64 -16 -64 -240
    WIRE 592 0 592 -32
    WIRE 928 0 928 -32
    WIRE 352 16 112 16
    WIRE 480 16 480 -16
    WIRE 480 16 432 16
    WIRE 1072 16 1072 -32
    WIRE -192 48 -192 -48
    WIRE 224 48 224 -128
    WIRE 320 48 224 48
    WIRE 816 64 816 -32
    WIRE 816 64 752 64
    WIRE 224 80 224 48
    WIRE 480 80 480 16
    WIRE 544 80 480 80
    WIRE 752 96 752 64
    WIRE 816 96 816 64
    WIRE 320 112 320 48
    WIRE 928 112 928 80
    WIRE -304 128 -304 -48
    WIRE 1072 128 1072 96
    WIRE 1152 128 1072 128
    WIRE 480 144 480 80
    WIRE 112 160 112 16
    WIRE 1072 160 1072 128
    WIRE 1152 176 1152 128
    WIRE 592 192 592 96
    WIRE 752 192 752 160
    WIRE 816 192 816 160
    WIRE 816 192 752 192
    WIRE 1152 192 1152 176
    WIRE 928 208 928 176
    WIRE 1072 272 1072 240
    WIRE 1152 272 1152 240
    WIRE 1152 272 1072 272
    WIRE -304 304 -304 208
    WIRE -192 304 -192 112
    WIRE -192 304 -304 304
    WIRE -64 304 -64 48
    WIRE -64 304 -192 304
    WIRE 112 304 112 240
    WIRE 112 304 -64 304
    WIRE 224 304 224 144
    WIRE 224 304 112 304
    WIRE 320 304 320 192
    WIRE 320 304 224 304
    WIRE 480 304 480 224
    WIRE 480 304 320 304
    WIRE 592 304 592 272
    WIRE 592 304 480 304
    WIRE 720 304 592 304
    WIRE 784 304 720 304
    WIRE 816 304 816 192
    WIRE 816 304 784 304
    WIRE 832 304 816 304
    WIRE 928 304 928 272
    WIRE 928 304 912 304
    WIRE 1008 304 928 304
    WIRE 224 320 224 304
    WIRE 720 336 720 304
    WIRE 784 416 784 304
    WIRE 1072 416 1072 272
    WIRE 1072 416 784 416
    WIRE 720 448 720 400
    WIRE 1008 448 1008 384
    WIRE 1008 448 720 448
    FLAG 224 320 0
    FLAG 480 -16 Vg
    FLAG 112 16 SigIn
    FLAG 944 -32 Vout
    FLAG 688 -48 Vsw
    FLAG -128 -256 Vsupply
    SYMBOL voltage 112 144 R0
    WINDOW 0 37 59 Left 0
    WINDOW 3 -304 182 Left 0
    WINDOW 123 0 0 Left 0
    WINDOW 39 0 0 Left 0
    SYMATTR InstName V1
    SYMATTR Value PULSE(0 12 20n 20n 20n 7.5u 10u 2000)
    SYMBOL res 336 32 R270
    WINDOW 0 32 56 VTop 0
    WINDOW 3 0 56 VBottom 0
    SYMATTR InstName R5
    SYMATTR Value 10
    SYMBOL voltage -304 112 R0
    WINDOW 123 0 0 Left 0
    WINDOW 39 24 132 Left 0
    SYMATTR SpiceLine Rser=.05
    SYMATTR InstName V2
    SYMATTR Value PULSE(0 10 .1m 100n 100n 8.6966m 10m 2)
    SYMBOL nmos 544 0 R0
    WINDOW 3 -54 102 Left 0
    SYMATTR Value SI7454DP
    SYMATTR InstName M1
    SYMBOL ind 576 -160 R0
    SYMATTR InstName L1
    SYMATTR Value 10µ
    SYMATTR SpiceLine Ipk=6 Rser=0.015 Rpar=30000 Cpar=17.04p mfg="Gowanda"
    pn="121AT1002V"
    SYMBOL schottky 736 -16 R270
    WINDOW 0 32 32 VTop 0
    WINDOW 3 0 32 VBottom 0
    SYMATTR InstName D2
    SYMATTR Value MBR20100CT
    SYMATTR Description Diode
    SYMATTR Type diode
    SYMBOL polcap 800 96 R0
    WINDOW 3 24 64 Left 0
    SYMATTR Value 47µ
    SYMATTR InstName C1
    SYMATTR Description Capacitor
    SYMATTR Type cap
    SYMATTR SpiceLine V=63 Irms=600m Rser=0.13 MTBF=20000 Lser=0 mfg="Nichicon"
    pn="UPH1J470MRH" type="Al electrolytic" ppPkg=1
    SYMBOL cap 736 96 R0
    SYMATTR InstName C2
    SYMATTR Value 0.47µ
    SYMATTR SpiceLine V=50 Irms=22m Rser=3.9 MTBF=2000 Lser=0 mfg="Nichicon"
    pn="UPL1HR47MAH" type="Al electrolytic" ppPkg=1
    SYMBOL cap -208 48 R0
    SYMATTR InstName C3
    SYMATTR Value .47µ
    SYMATTR SpiceLine V=50 Irms=5.62 Rser=0.007 MTBF=0 Lser=0 ppPkg=1
    SYMBOL res 912 -16 R0
    SYMATTR InstName R2
    SYMATTR Value 5
    SYMBOL res 464 128 R0
    SYMATTR InstName R6
    SYMATTR Value 500
    SYMBOL res 576 176 R0
    SYMATTR InstName R8
    SYMATTR Value .02
    SYMATTR SpiceLine pwr=2
    SYMBOL zener 944 272 R180
    WINDOW 0 24 72 Left 0
    WINDOW 3 -69 0 Left 0
    SYMATTR InstName D1
    SYMATTR Value DFLZ33
    SYMATTR Description Diode
    SYMATTR Type diode
    SYMBOL zener 944 176 R180
    WINDOW 0 24 72 Left 0
    WINDOW 3 -108 6 Left 0
    SYMATTR InstName D3
    SYMATTR Value BZX84C6V2L
    SYMATTR Description Diode
    SYMATTR Type diode
    SYMBOL res 928 288 R90
    WINDOW 0 0 56 VBottom 0
    WINDOW 3 32 56 VTop 0
    SYMATTR InstName R1
    SYMATTR Value 1
    SYMATTR SpiceLine pwr=1
    SYMBOL res 992 288 R0
    SYMATTR InstName R3
    SYMATTR Value 1k
    SYMBOL cap 704 336 R0
    SYMATTR InstName C5
    SYMATTR Value .1µ
    SYMBOL res 1056 0 R0
    SYMATTR InstName R4
    SYMATTR Value 10k
    SYMBOL res 1056 144 R0
    SYMATTR InstName R9
    SYMATTR Value 499
    SYMBOL cap 1136 176 R0
    SYMATTR InstName C6
    SYMATTR Value .01µ
    SYMBOL ind -320 -224 R0
    SYMATTR InstName L2
    SYMATTR Value 100n
    SYMATTR SpiceLine Ipk=8 Rser=0.00087 Rpar=9.4 Cpar=0 mfg="Coilcraft"
    pn="SLC7530D-101MX"
    SYMBOL diode 208 -192 R0
    SYMATTR InstName D5
    SYMATTR Value MMSD4148
    SYMBOL polcap 208 80 R0
    WINDOW 3 24 64 Left 0
    SYMATTR Value 100µ
    SYMATTR InstName C7
    SYMATTR Description Capacitor
    SYMATTR Type cap
    SYMATTR SpiceLine V=25 Irms=145m Rser=0.62 MTBF=1000 Lser=0 mfg="Nichicon"
    pn="UPR1E101MPH" type="Al electrolytic" ppPkg=1
    SYMBOL res 304 96 R0
    SYMATTR InstName R10
    SYMATTR Value 1k
    SYMBOL polcap -80 -16 R0
    WINDOW 3 24 64 Left 0
    SYMATTR InstName C4
    SYMATTR Value 100µ
    SYMATTR Description Capacitor
    SYMATTR Type cap
    SYMATTR SpiceLine V=35 Irms=460m Rser=0.16 MTBF=3000 Lser=0 mfg="Nichicon"
    pn="UPL1V101MPH" type="Al electrolytic" ppPkg=1
    TEXT -224 504 Left 0 !.tran 6m startup
    TEXT -8 376 Left 0 ;R4 not required
    TEXT -8 440 Left 0 ;91% efficiency possible
     
  2. 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
     
  3. Guest

    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.
     
  4. Fred Bloggs

    Fred Bloggs Guest

    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.
     
  5. 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
     
  6. Al

    Al Guest

    Fred Bartoli

    Ypu're right! It was a bad design. But it was on a military project. The
    cost of the ECP would be way beyond the cost of some transistor matching.

    Al
     
  7. HapticZ

    HapticZ Guest

    another amen!

    as with all projects, it's 99 % in the preparation.

    with electronics, its 99% design smarts that yields the best fit for the
    end result!!

    so we get (buy) a $120,000 4 year education for our kids, who then spend 99
    percent of thier time figuring out how to pay for thier own kids $240,000
    education.

    it may seem smart, but this country has a lot to learn from asia and others!
    they pick the right ones to do the education scene, not throw money at bad
    success rates!

    hmmmmmmmm?
     
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