simple switcher constant current supply with high side sense resistor

[Fred Bloggs]

Fred Bloggs wrote...



Without an LED load the voltage soars to near Vin, which could be as
high as 45V. What happens the instant the LEDs are attached? First
a short high-current spike, followed by excessive LM317 voltage (if
only one LED is used), etc.? Hmm, I wonder how the LM317 responds
to a high voltage across its Vout and ADJ terminals for a short time?
I see there's a 6V zener in series with 50 ohms inside the chip, but
that resistor isn't a power resistor... Time for some experiments.

I am not getting that- ADJ terminal drops instantly to Vled, and the
42/2.7K voltage divider, through D2, makes for a small 0.016 x (48-Vled)
or 0.75V worst case drop across the 42 ohm or 0.75/42=18mA- so you have
the wideband emitter follower response in the '317. I'm pretty sure most
'317's can tolerate 48V for a few msecs to discharge Cf.

 

[Tony Williams]

I'm trying to design a simple switcher constant current circuit
(using the LM2574-ADJ). It's easy enough if you put the sense
resistor between the load and ground. However the snag is that
the current sense resistor has to be on the high side of the
load (there are 3 current supplies driving 3 devices with a
commoned ground).

Sideways thought.....

If a flyback converter is arranged to dump a fixed
Joules/cycle, then making the Frequency proportional
to Vout should result in a constant Iout.

 

[Fred Bloggs]

Without an LED load the voltage soars to near Vin, which could be as
high as 45V. What happens the instant the LEDs are attached? First
a short high-current spike, followed by excessive LM317 voltage (if
only one LED is used), etc.? Hmm, I wonder how the LM317 responds
to a high voltage across its Vout and ADJ terminals for a short time?
I see there's a 6V zener in series with 50 ohms inside the chip, but
that resistor isn't a power resistor... Time for some experiments.

You may have a better chance of surviving something like this with the
NPN-PNP Vbe compensated buffer because the 317 current drops to zero and
there is no stored charge in the output transistors. If the LM385 was a
bit faster, something like this would survive- a variation on the
standard precision current source with current source pulldown that is
cut-off with shorted load- the PNP power can be saved with enough heat
sink to carry it through the x msecs under non-short but abrupt drop in
output voltage.

View in a fixed-width font such as Courier.


..
..
..
..
.. Vin >--+
.. |
.. +------------+
.. | Vin | Vout = Vleds+3.3V
.. | sw |---+--L1-+-----+----------+----------+----+
.. | | | | | | | _|_+
.. | | - | | | 3.9 ---
.. | | D1^ | | LM385 1W |
.. | | - | | ---/ | GND
.. | | | 2.7K | // \---------+
.. | | GND | | --- |
.. | | | | | |/e
.. | LM2574-ADJ | | | | ,-|
.. | etc | | | | |/e |\ 2N5195
.. | | | | +----| |
.. | | | | | |\ |
.. | | | | | '---+
.. | | | \| | BC856 |
.. | | | BC846 |-------------------+
.. | | | e/| | |
.. | | | | | |
.. | | | | | |
.. | | | | | |
.. | | e\| | | D2 |
.. | | BC856 |---+--------------|>|----+-->>--,
.. | | /| | | 1N4148 |
.. | | 1.23V | | | |
.. | fb |---+-----' | |/ -
.. | gnd | | +-10K-+--| BC846 LEDs V
.. +------------+ 1K | | |\e -
.. | | | \| | |
.. | | | BC846|--+ |
.. | | 4.7K e/| | |
.. | | | | 560 |
.. | | | | | |
.. | | | | | |
.. GND------+----------+-----------+-----+----+------------->>--'
..
..

 

[Fred Bloggs]

Fred Bloggs wrote:
[..SNIP..]

Looks like a Schotty should help recovery from open load immensely:

View in a fixed-width font such as Courier.


..
..
..
..
.. Vin >--+
.. |
.. +------------+
.. | Vin | Vout = Vleds+3.3V
.. | sw |---+--L1-+-----+----------+----------+----+
.. | | | | | | | _|_+
.. | | - | | | 3.9 ---
.. | | D1^ | | LM385 1W |
.. | | - | | ---/ | GND
.. | | | 2.7K | // \---------+
.. | | GND | | --- |
.. | | | | | |/e
.. | LM2574-ADJ | | | | ,-|
.. | etc | | | | |/e |\ 2N5195
.. | | | | +----| |
.. | | | | | |\ |
.. | | | | | '---+
.. | | | \| | BC856 |
.. | | | BC846 |-------------------+
.. | | | e/| | |
.. | | | | | SD |
.. | | | | +---|<|----+
.. | | | | | |
.. | | e\| | | D2 |
.. | | BC856 |---+--------------|>|----+-->>--,
.. | | /| | | 1N4148 |
.. | | 1.23V | | | |
.. | fb |---+-----' | |/ -
.. | gnd | | +-10K-+--| BC846 LEDs V
.. +------------+ 1K | | |\e -
.. | | | \| | |
.. | | | BC846|--+ |
.. | | 4.7K e/| | |
.. | | | | 560 |
.. | | | | | |
.. | | | | | |
.. GND------+----------+-----------+-----+----+------------->>--'
..
..