Re: Constant current for 16 white LEDs?


dagmargoodboat@xxxxxxxxx wrote:
miso@xxxxxxxxx wrote:
wrongaddr...@xxxxxxx wrote:
dagmargoodboat@xxxxxxxxx wrote:
wrongaddress@xxxxxxx wrote:

Yes, I have a similar circuit but the problem is getting enough
inductor current at low voltage. If I raise the supply voltage to 9
volts, I can get 60 volts out into 2700 ohms, or 1.3 watts which is
enough for this application. Input current is about 190mA, so the
efficiency is about 1.3/1.7 = 76%

But it won't work at low voltage of 4 volts, and that's the problem.
Why does Vce Sat increase as supply voltage decreases?

-Bill

Bill,
Your MOSFET's the problem -- 4V is not enough to turn an IRF620 fully
on. If you look at Fig. 1 on page 3 of the data ***, you'll see that
it's not even good for 200mA at Vgs=4.5v.
(http://www.irf.com/product-info/datasheets/data/irf620.pdf)

I'm sure I have a suitable FET I could spot you, or you could
substitute an appropriate bipolar transistor.

Best regards,
James

I don't think the problem is the FET since I am driving the gate with a
0-8 volt
square wave from an external generator. The on-resistance is 180
milliohms.
But the same problem occurs with a bi-polar transistor driven with
100mA of base current. The inductor current never ramps high enough
before the voltage drop on the whatever transistor used starts climbing
and overheating the device. It won't stay saturated with a low Vce drop
using a 4 volt supply. But it will work at higher voltages.

The question is, how do i get 1.5 amps or more into an inductor of low
resistance (200 milliohms or less) using a 3 to 4 volt supply without
overheating the transistor?

Something funny is going on with the inductor at low voltage, but I
don't know what it is. I have tried several values from 3 to 25 mH
inductors.

-Bill

If the on-resistance of the switch is low, it shouldn't get very hot.

Are you trying to make a discontinuous conduction boost mode switcher?

I wondered that too, but took Bill's earlier response to Paul Schoen
to mean Bill's got some sort of current-limiting or duty cycle control
is his actual circuit.

Bill, your setup is perfectly ordinary and should work. Something is
wrong.

Driving with a signal generator you'd better be sure the inductor
fully discharges during each discharge pulse, otherwise current will
accumulate from one cycle to the next--reaching very high levels--until
something breaks (or screams).

The inductor-charging time needed increases as supply voltage falls.

Vce(sat) does _not_ increase as Vcc falls; it increases with higher
collector current (or Vds/drain current). Therefore, either you're not
driving it hard enough, it's oscillating, or you're passing a lot more
current than you think.

Maybe you're hitting continuous mode. What's your drive's
duty-cycle? Frequency?

Maybe your inductor is saturating. Substitute a power resistor, and
see if your transistor can drive it. Or, leave the inductor in and
watch the drain current with a 'scope and see if it spikes after
ramping smoothly. That's very possible--your inductance is huge.

Maybe you've underestimated the peak inductor current needed to get
~60v @ 20mA from 4V ? You'll only be discharging at most 1/15th of the
time, so you'll have to discharge at 15 x 20mA (300mA) to get the
current you want in continuous mode, possibly much more in
discontinuous mode.

In this situation I'd prefer a few series LED strings in parallel --
it improves reliability (redundant LED chains, reduces peak currents,
eases switching stress).

Hey, speaking of switching stress, please tell us you're using a
suitable fast-recovery rectifier -- NOT a 1n4001 ;-)

Best,
James Arthur

Actually, I just noticed he said he was using a square wave. In a boost
converter, discharge will take less time than charge, thus there is a
need to change the duty cycle.

I did a boost converter control scheme that used a current sense
circuit to limit the charge cycle, and a comparator to detect the end
of the discharge cycle. The end of the inductor that flies high can be
used to detect the end of discharge when it falls lower than the
battery voltage. It is simple, though there are plenty of gotchas I
could go into if he picks such a scheme.

Now in the case of driving leds, I think you could build a controller
strictly based on current sensing. I haven't done this, but here is the
scheme. Pick a peak current limit to charge the inductor. When the peak
current limit is reached, let the inductor fly. The end of the
discharge cycle would be detected by a lower current limit being
reached. Say the upper limit was 20ma, and the lower limit was 10ma.
Then you would generate a triangle wave of current in the inductor
between 10ma to 20ma.

All that said, I'm more comfortable using a voltage limit on the boost,
with some trick to limit the current in the leds. Discontinuous boost
converters are stable beasts. I still like using the boost output to
feed a LDO which in turn controlls the gate drive of the power fet.

.

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