Re: a lot of switching mode power supply design questions




This design aims to keep current adjustable yet constant with low
ripple. I couldn't use low freq. dimm. adjustment.
I wonder if I use a much bigger inductor, e.g. a 1000 uH compared to
a theoretical 330 uH, may I get much cleaner
current? Large inductor means large ESR, ESC. I'm using a homemade
toroid inductor. 1000 uH with 176 ohm and
26 uF @1k Hz.




On May 27, 2:23 pm, "Paul E. Schoen" <pst...@xxxxxxxxx> wrote:
<bigca...@xxxxxxxxx> wrote in message

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On May 27, 12:18 pm, "Paul E. Schoen" <pst...@xxxxxxxxx> wrote:
<bigca...@xxxxxxxxx> wrote in message

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the goal is 0-10A current source for led driving
max chip: high side current sensing, 20x gain
ucc chip: pwm, 400khz, width varies with input voltage
ixdd chip: mosfet driver
opamp: current amplifier

You will have problems using a proto-board at 400 kHz. And the IRL3803,
with 230 nSec rise time, will very likely overheat due to switching
losses.
The main reason for using high frequencies is to reduce the size of the
inductive and capacitive components. If you have room, you would do much
better to run this at about 50 kHz and use something like a 100 uH
inductor. At that frequency, you might be able to use a breadboard.
I do use lower frequency but ripple is way too large. I think high
frequency can lower ripple?

If the LED is used for lighting purposes, the human eye will integrate PWM
above about 100 Hz. If there is some reason you need to keep LED current
constant with low ripple, you can use a larger inductor, or add more
filtering to get it as low as you wish. But that is probably not necessary.
If you see any flicker, it is because there is some other instability in
the circuit, probably a poorly compensated feedback loop. This is one
reason why most brightness modulators run the LED at full brightness when
it is ON, and use a fixed PWM to adjust the on/off ratio in a 100-200 Hz
fixed frequency, as I posted previously.

Paul

.

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