Pulse-powered current limiter...

P

Piotr Wyderski

Guest
Hi,

I am experimenting with white LED COBs and wanted to power them from a
constant current source while still having the PWM dimming capability.
The current limiter should be located next to the diode and the lamp
\"interface\" should be regular two wires. In other words, the current
limiter should be capable of being powered from a +12V/open drain source
with the PWM frequency of ~400Hz.

I started my experiments with a boring two-transistor series MOSFET/BJT
current limiter. The voltage at the MOSFET source turns on a BJT that
steals the MOSFET gate voltage, stabilizing the drain current. It works,
but the V_BE tempco makes the current drift way beyond my comfort zone.
OTOH, an opamp-based current limiter + a decent reference would keep the
current rock-solid, but it can go nuts during the PWM edge transients.

Any suggestion on how to make a limiter capable of being PWM-powered and
maintaining stable current during the ON phase down to, say, 3% accuracy
over the 11-15V input voltage range and 25-80 degrees C temperature
would be appreciated. The nominal LED voltage is 9V and the required
current is ~500mA. Are there any particularly forgiving opamps?

Best regards, Piotr
 
On Fri, 18 Feb 2022 22:29:11 +0100, Piotr Wyderski
<bombald@protonmail.com> wrote:

Hi,

I am experimenting with white LED COBs and wanted to power them from a
constant current source while still having the PWM dimming capability.
The current limiter should be located next to the diode and the lamp
\"interface\" should be regular two wires. In other words, the current
limiter should be capable of being powered from a +12V/open drain source
with the PWM frequency of ~400Hz.

I started my experiments with a boring two-transistor series MOSFET/BJT
current limiter. The voltage at the MOSFET source turns on a BJT that
steals the MOSFET gate voltage, stabilizing the drain current. It works,
but the V_BE tempco makes the current drift way beyond my comfort zone.
OTOH, an opamp-based current limiter + a decent reference would keep the
current rock-solid, but it can go nuts during the PWM edge transients.

Any suggestion on how to make a limiter capable of being PWM-powered and
maintaining stable current during the ON phase down to, say, 3% accuracy
over the 11-15V input voltage range and 25-80 degrees C temperature
would be appreciated. The nominal LED voltage is 9V and the required
current is ~500mA. Are there any particularly forgiving opamps?

Best regards, Piotr
Click to expand...

Depletion fet? They aren\'t accurate to 3%, but an LED shouldn\'t need
that sort of precision.

--

If a man will begin with certainties, he shall end with doubts,
but if he will be content to begin with doubts he shall end in certainties.
Francis Bacon
 
fredag den 18. februar 2022 kl. 22.29.33 UTC+1 skrev Piotr Wyderski:
Hi,

I am experimenting with white LED COBs and wanted to power them from a
constant current source while still having the PWM dimming capability.
The current limiter should be located next to the diode and the lamp
\"interface\" should be regular two wires. In other words, the current
limiter should be capable of being powered from a +12V/open drain source
with the PWM frequency of ~400Hz.

I started my experiments with a boring two-transistor series MOSFET/BJT
current limiter. The voltage at the MOSFET source turns on a BJT that
steals the MOSFET gate voltage, stabilizing the drain current. It works,
but the V_BE tempco makes the current drift way beyond my comfort zone.
OTOH, an opamp-based current limiter + a decent reference would keep the
current rock-solid, but it can go nuts during the PWM edge transients.

Any suggestion on how to make a limiter capable of being PWM-powered and
maintaining stable current during the ON phase down to, say, 3% accuracy
over the 11-15V input voltage range and 25-80 degrees C temperature
would be appreciated. The nominal LED voltage is 9V and the required
current is ~500mA. Are there any particularly forgiving opamps?
Click to expand...

MOSFET and something like the lower voltage TL431, TLVH431 ?

maybe a diode and capacitor to provide a constant supply for the gate/reference
 
John Larkin wrote:

> Depletion fet?

I love them, but they have pretty high V_TH and it is
temperature-dependent too. Too much voltage drop and doesn\'t solve the
tempco issue -- a regular 2 BJT limiter performs better.

> They aren\'t accurate to 3%, but an LED shouldn\'t need that sort of precision.

Absolute accuracy is not required, but some value of the current should
remain relatively stable. What\'s the point of PWM dimming if the lamp
current can drift away by 20% just because the lamp has been turned on
for some time?

Below is one idea. The differential amplifier looks good, so at the sim
level it boils down to the tempco of the reference. Google says 5.6V
zeners are particularly good, but I don\'t know how good is good. Looks
like it would be worth torturing with a hotair gun with the Q5 and Q7
being a BCV61 and Q1 and Q2 a BCM847.

Best regards, Piotr

Version 4
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TEXT -344 584 Left 2 !.tran 1
 
Lasse Langwadt Christensen wrote:

> MOSFET and something like the lower voltage TL431, TLVH431 ?

I was experimenting with the MAX8515 from Mouser, but it was a total
disaster. It looks as if the the physical part and the datasheet were
completely unrelated. IMHO the origin excludes counterfeiting, but the
way the part misbehaves makes me think that even the pinout cannot be
right. If I just connect IN and PGND/GND and leave FB and OUT
unconnected, the chip draws 40mA, while the datasheet says 1mA max. When
I connect FB to GND, the current consumption goes to 0. The scope says
it is not oscillating. No idea where the 40mA goes, as the high-power
path is through the OUT pin. When connected to the FET, it jumps to the
40mA mode and starts oscillating at the OUT. 100nF kills the
oscillations, but the chip looks offended and keeps the FET almost off.
Same behaviour with 3 different chips.

I had similar experience last year with some Maxim boost converter --
the chip looked haunted, while a TI part did the job right out of the
box. Not going to use any Maxim part anymore.

I have suddenly started liking discretes.

> maybe a diode and capacitor to provide a constant supply for the gate/reference

I considered this, but keep it a last resort solution.

Best regards, Piotr
 
On Sat, 19 Feb 2022 00:28:54 +0100, Piotr Wyderski
<bombald@protonmail.com> wrote:

John Larkin wrote:

Depletion fet?

I love them, but they have pretty high V_TH and it is
temperature-dependent too. Too much voltage drop and doesn\'t solve the
tempco issue -- a regular 2 BJT limiter performs better.

They aren\'t accurate to 3%, but an LED shouldn\'t need that sort of precision.

Absolute accuracy is not required, but some value of the current should
remain relatively stable. What\'s the point of PWM dimming if the lamp
current can drift away by 20% just because the lamp has been turned on
for some time?

Below is one idea. The differential amplifier looks good, so at the sim
level it boils down to the tempco of the reference. Google says 5.6V
zeners are particularly good, but I don\'t know how good is good. Looks
like it would be worth torturing with a hotair gun with the Q5 and Q7
being a BCV61 and Q1 and Q2 a BCM847.

Best regards, Piotr
Click to expand...

It\'s just an LED! I doubt that you would notice 20% light output
change. LEDs get dimmer as the temp goes up anyhow.

There are cheap LED current limiter chips.

Or maybe an LM1117 and a resistor.



Version 4
SHEET 1 1700 1204
WIRE 0 -224 -64 -224
WIRE 160 -224 0 -224
WIRE 1360 -224 160 -224
WIRE 1536 -224 1360 -224
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SYMATTR Value 1k
TEXT -344 584 Left 2 !.tran 1






--
Click to expand...

If a man will begin with certainties, he shall end with doubts,
but if he will be content to begin with doubts he shall end in certainties.
Francis Bacon
 
On Friday, February 18, 2022 at 1:29:33 PM UTC-8, Piotr Wyderski wrote:

I am experimenting with white LED COBs and wanted to power them from a
constant current source while still having the PWM dimming capability.
The current limiter should be located next to the diode and the lamp
\"interface\" should be regular two wires. In other words, the current
limiter should be capable of being powered from a +12V/open drain source
with the PWM frequency of ~400Hz.
Click to expand...

At lower voltage, there\'s integrated solutions for onesie white-output LEDs, like
AMC7135 (that one\'s about 350 mA, 6V), in three-pin packages. At 9V nominal,
even the venerable uA723 with a pass transistor could do
something close to valid, with its overcurrent limit; maybe 10% accuracy.
 
whit3rd wrote:

At lower voltage, there\'s integrated solutions for onesie white-output LEDs, like
AMC7135 (that one\'s about 350 mA, 6V), in three-pin packages. At 9V nominal,
even the venerable uA723 with a pass transistor could do
something close to valid, with its overcurrent limit; maybe 10% accuracy.
Click to expand...

But would it like pulsed supply? This is the core issue, not how to
build a limiter itself. I have no idea what\'s inside a real
opamp/refrence/LDO and the models probably do no cover this aspect. Or
maybe they do, some randomly selected LTC opamps went monkey in LTspice.

Best regards, Potr
 
On 19/02/2022 7:29 am, Piotr Wyderski wrote:
Hi,

I am experimenting with white LED COBs and wanted to power them from a
constant current source while still having the PWM dimming capability.
The current limiter should be located next to the diode and the lamp
\"interface\" should be regular two wires. In other words, the current
limiter should be capable of being powered from a +12V/open drain source
with the PWM frequency of ~400Hz.

I started my experiments with a boring two-transistor series MOSFET/BJT
current limiter. The voltage at the MOSFET source turns on a BJT that
steals the MOSFET gate voltage, stabilizing the drain current. It works,
but the V_BE tempco makes the current drift way beyond my comfort zone.
OTOH, an opamp-based current limiter + a decent reference would keep the
current rock-solid, but it can go nuts during the PWM edge transients.

Any suggestion on how to make a limiter capable of being PWM-powered and
maintaining stable current during the ON phase down to, say, 3% accuracy
over the 11-15V input voltage range and 25-80 degrees C temperature
would be appreciated. The nominal LED voltage is 9V and the required
current is ~500mA. Are there any particularly forgiving opamps?

    Best regards, Piotr
Click to expand...

A small resistor to swamp the tempco?
 
lørdag den 19. februar 2022 kl. 03.02.24 UTC+1 skrev David Eather:
On 19/02/2022 7:29 am, Piotr Wyderski wrote:
Hi,

I am experimenting with white LED COBs and wanted to power them from a
constant current source while still having the PWM dimming capability.
The current limiter should be located next to the diode and the lamp
\"interface\" should be regular two wires. In other words, the current
limiter should be capable of being powered from a +12V/open drain source
with the PWM frequency of ~400Hz.

I started my experiments with a boring two-transistor series MOSFET/BJT
current limiter. The voltage at the MOSFET source turns on a BJT that
steals the MOSFET gate voltage, stabilizing the drain current. It works,
but the V_BE tempco makes the current drift way beyond my comfort zone.
OTOH, an opamp-based current limiter + a decent reference would keep the
current rock-solid, but it can go nuts during the PWM edge transients.

Any suggestion on how to make a limiter capable of being PWM-powered and
maintaining stable current during the ON phase down to, say, 3% accuracy
over the 11-15V input voltage range and 25-80 degrees C temperature
would be appreciated. The nominal LED voltage is 9V and the required
current is ~500mA. Are there any particularly forgiving opamps?

Best regards, Piotr

A small resistor to swamp the tempco?
Click to expand...

or an NPN with two diodes as reference, all thermally connected
 
On 18/02/2022 11:56 pm, Piotr Wyderski wrote:
I have suddenly started liking discretes.
Click to expand...

Haha! A sign of maturity and useful in a time of supply shortages and no
second sources!

piglet
 
On 18/02/2022 11:06 pm, Lasse Langwadt Christensen wrote:
fredag den 18. februar 2022 kl. 22.29.33 UTC+1 skrev Piotr Wyderski:
Hi,

I am experimenting with white LED COBs and wanted to power them from a
constant current source while still having the PWM dimming capability.
The current limiter should be located next to the diode and the lamp
\"interface\" should be regular two wires. In other words, the current
limiter should be capable of being powered from a +12V/open drain source
with the PWM frequency of ~400Hz.

I started my experiments with a boring two-transistor series MOSFET/BJT
current limiter. The voltage at the MOSFET source turns on a BJT that
steals the MOSFET gate voltage, stabilizing the drain current. It works,
but the V_BE tempco makes the current drift way beyond my comfort zone.
OTOH, an opamp-based current limiter + a decent reference would keep the
current rock-solid, but it can go nuts during the PWM edge transients.

Any suggestion on how to make a limiter capable of being PWM-powered and
maintaining stable current during the ON phase down to, say, 3% accuracy
over the 11-15V input voltage range and 25-80 degrees C temperature
would be appreciated. The nominal LED voltage is 9V and the required
current is ~500mA. Are there any particularly forgiving opamps?

MOSFET and something like the lower voltage TL431, TLVH431 ?

maybe a diode and capacitor to provide a constant supply for the gate/reference
Click to expand...

It can be even simpler, just use the TLV431 as a kind of NPN with
precisely defined Vbe, as in this sketch ...

<https://www.dropbox.com/s/c4l4hnnu6f2j5xx/TLV431_curlim.JPG?raw=1>

piglet
 
piglet wrote:

It can be even simpler, just use the TLV431 as a kind of NPN with
precisely defined Vbe, as in this sketch ...

https://www.dropbox.com/s/c4l4hnnu6f2j5xx/TLV431_curlim.JPG?raw=1
Click to expand...

This is exactly how I wanted to use the MAX8515, but for some reason it
just doesn\'t work. It even does notwork without the FET - it looks like
it needs tons of capacitance to start behaving.

Best regards, Piotr
 
On Fri, 18 Feb 2022 22:29:11 +0100, Piotr Wyderski wrote:

Hi,

I am experimenting with white LED COBs and wanted to power them from a
constant current source while still having the PWM dimming capability.
The current limiter should be located next to the diode and the lamp
\"interface\" should be regular two wires. In other words, the current
limiter should be capable of being powered from a +12V/open drain source
with the PWM frequency of ~400Hz.

I started my experiments with a boring two-transistor series MOSFET/BJT
current limiter. The voltage at the MOSFET source turns on a BJT that
steals the MOSFET gate voltage, stabilizing the drain current. It works,
but the V_BE tempco makes the current drift way beyond my comfort zone.
OTOH, an opamp-based current limiter + a decent reference would keep the
current rock-solid, but it can go nuts during the PWM edge transients.

Any suggestion on how to make a limiter capable of being PWM-powered and
maintaining stable current during the ON phase down to, say, 3% accuracy
over the 11-15V input voltage range and 25-80 degrees C temperature
would be appreciated. The nominal LED voltage is 9V and the required
current is ~500mA. Are there any particularly forgiving opamps?

Best regards, Piotr
Click to expand...

For an op-amp based current limiter, you want one that comes out of
saturation fast, i.e. in a small fraction of your PWM period. Some are
orders of magnitude better than others in this respect, but it\'s rarely
mentioned in the datasheet.

There was a list of good ones in AoE III IIRC. I\'ll have a look in my
copy when I\'m back in the office next week.
 
On 19/02/2022 2:16 pm, Piotr Wyderski wrote:
piglet wrote:

It can be even simpler, just use the TLV431 as a kind of NPN with
precisely defined Vbe, as in this sketch ...

https://www.dropbox.com/s/c4l4hnnu6f2j5xx/TLV431_curlim.JPG?raw=1

This is exactly how I wanted to use the MAX8515, but for some reason it
just doesn\'t work. It even does notwork without the FET - it looks like
it needs tons of capacitance to start behaving.

    Best regards, Piotr
Click to expand...

I am not familiar with the MAX8515 but knowing the way they think it is
probably massively over complicated and expects a proper unvarying
supply rail. The simple \'431 can work to surprisingly high frequency. If
even a \'431 is unbearable then since this is an undemanding application
you could make a two NPN diff amp with a reference somewhere from 0.9 to
1.5V made from a divided down zener?

piglet
 
On 20/02/2022 12:16 am, Piotr Wyderski wrote:
piglet wrote:

It can be even simpler, just use the TLV431 as a kind of NPN with
precisely defined Vbe, as in this sketch ...

https://www.dropbox.com/s/c4l4hnnu6f2j5xx/TLV431_curlim.JPG?raw=1

This is exactly how I wanted to use the MAX8515, but for some reason it
just doesn\'t work. It even does notwork without the FET - it looks like
it needs tons of capacitance to start behaving.

    Best regards, Piotr
Click to expand...

Just thinking - some op amps misbehave when driving a capacative load.
Not any load but only a value between certain values. Does it still
misbehave if you isolate the 8515 and the fet with say 1k?
 
piglet wrote:

> The simple \'431 can work to surprisingly high frequency.

Yes, its datasheet explicitly says it can work without capacitors. So
maybe instead of demanding low threshold voltage I should add 2.5V
offset with a second 431? To be prototyped:

https://i.postimg.cc/pXKzztfs/shifted-current-sense.png

If even a \'431 is unbearable then since this is an undemanding application
you could make a two NPN diff amp with a reference somewhere from 0.9 to
1.5V made from a divided down zener?
Click to expand...

This is exactly what my sim does:

https://i.postimg.cc/1XrRRMFr/discrete.png

Best regards, Piotr
 
On 20/02/2022 7:12 am, Piotr Wyderski wrote:
piglet wrote:

The simple \'431 can work to surprisingly high frequency.

Yes, its datasheet explicitly says it can work without capacitors. So
maybe instead of demanding low threshold voltage I should add 2.5V
offset with a second 431? To be prototyped:

https://i.postimg.cc/pXKzztfs/shifted-current-sense.png
Click to expand...

Sorry but I don\'t understand the need for a second 431?

If even a \'431 is unbearable then since this is an undemanding
application you could make a two NPN diff amp with a reference
somewhere from 0.9 to 1.5V made from a divided down zener?

This is exactly what my sim does:

https://i.postimg.cc/1XrRRMFr/discrete.png

    Best regards, Piotr
Click to expand...

I was thinking of something much simpler, like this ...

Version 4
SHEET 1 1700 1204
WIRE 464 -224 48 -224
WIRE 800 -224 464 -224
WIRE 976 -224 800 -224
WIRE 1184 -224 976 -224
WIRE 464 -192 464 -224
WIRE 1184 -48 1184 -224
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WIRE 608 -32 464 -32
WIRE 976 16 976 -224
WIRE 608 64 608 -32
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WIRE 48 176 48 -224
WIRE 800 192 800 -224
WIRE 976 192 976 160
WIRE 608 240 608 144
WIRE 736 240 608 240
WIRE 1184 240 1184 176
WIRE 1184 240 1040 240
WIRE 800 304 800 288
WIRE 896 304 800 304
WIRE 976 304 976 288
WIRE 976 304 896 304
WIRE 608 352 608 240
WIRE 896 368 896 304
WIRE 1184 384 1184 240
WIRE 48 528 48 256
WIRE 464 528 464 176
WIRE 464 528 48 528
WIRE 608 528 608 432
WIRE 608 528 464 528
WIRE 896 528 896 448
WIRE 896 528 608 528
WIRE 1184 528 1184 464
WIRE 1184 528 896 528
WIRE 48 576 48 528
FLAG 48 576 0
SYMBOL voltage 48 160 R0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V1
SYMATTR Value PULSE(11 15 0 1 0)
SYMBOL res 960 0 R0
SYMATTR InstName R5
SYMATTR Value 22k
SYMBOL zener 480 176 R180
WINDOW 0 24 64 Left 2
WINDOW 3 24 0 Left 2
SYMATTR InstName D1
SYMATTR Value BZX84C6V2L
SYMATTR Description Diode
SYMATTR Type diode
SYMBOL res 448 -208 R0
SYMATTR InstName R6
SYMATTR Value 470
SYMBOL npn 1040 192 M0
SYMATTR InstName Q5
SYMATTR Value BC547B
SYMBOL npn 736 192 R0
SYMATTR InstName Q7
SYMATTR Value BC547B
SYMBOL nmos 1136 80 R0
SYMATTR InstName M1
SYMATTR Value FDS4885C_N
SYMBOL res 1168 368 R0
SYMATTR InstName R8
SYMATTR Value 2.2
SYMBOL res 1168 -64 R0
SYMATTR InstName R9
SYMATTR Value 1
SYMBOL res 592 48 R0
SYMATTR InstName R1
SYMATTR Value 22k
SYMBOL res 592 336 R0
SYMATTR InstName R7
SYMATTR Value 4700
SYMBOL res 880 352 R0
SYMATTR InstName R2
SYMATTR Value 470
TEXT 112 584 Left 2 !.tran 1
TEXT 336 584 Left 2 ;Piotr - EPW SED FEB 2022 - PWM-able current sink

piglet
 
piglet wrote:

> Sorry but I don\'t understand the need for a second 431?

This is to ensure low voltage drop at the current sense resistor.
It moves the 0-0.6V CS range to 2.5-3.1V where the second 431 can start
regulating, as its V_REF is 2.5V.

I have just prototyped this one and its performance is stunning -- it
starts with 497mA and I cannot move it by more than 4mA with the hotair
gun. In the 11-15 V_IN range I can see no current change, it is only
temperature-dependent. Problems: at 250mA it is inherently stable, at
500mA it starts oscillating, so a 22nF cap at the main REF is required
to kill it.

[snip your sim -- thank you, most interesting. It requires >1V reference
though. My PNP diff amp was to allow <0.6V]

Best regards, Piotr
 
On 20/02/2022 11:30 am, Piotr Wyderski wrote:
piglet wrote:

Sorry but I don\'t understand the need for a second 431?

This is to ensure low voltage drop at the current sense resistor.
It moves the 0-0.6V CS range to 2.5-3.1V where the second 431 can start
regulating, as its V_REF is 2.5V.

I have just prototyped this one and its performance is stunning -- it
starts with 497mA and I cannot move it by more than 4mA with the hotair
gun. In the 11-15 V_IN range I can see no current change, it is only
temperature-dependent. Problems: at 250mA it is inherently stable, at
500mA it starts oscillating, so a 22nF cap at the main REF is required
to kill it.

[snip your sim -- thank you, most interesting. It requires >1V reference
though. My PNP diff amp was to allow <0.6V]

    Best regards, Piotr
Click to expand...

Thanks, yes I understood it within a few minutes of my first reply, it
should work fine - as you then proved!

piglet
 
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