On 18/02/2022 11:56 pm, Piotr Wyderski wrote:> > I have suddenly started liking discretes. >Haha! A sign of maturity and useful in a time of supply shortages and no second sources! piglet
Pulse-powered current limiter
Started by ●February 18, 2022
Reply by ●February 19, 20222022-02-19
Reply by ●February 19, 20222022-02-19
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/referenceIt 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
Reply by ●February 19, 20222022-02-19
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
Reply by ●February 19, 20222022-02-19
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, PiotrFor 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.
Reply by ●February 19, 20222022-02-19
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, PiotrI 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
Reply by ●February 19, 20222022-02-19
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, PiotrJust 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?
Reply by ●February 20, 20222022-02-20
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?This is exactly what my sim does: https://i.postimg.cc/1XrRRMFr/discrete.png Best regards, Piotr
Reply by ●February 20, 20222022-02-20
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 >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, PiotrI 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 WIRE 464 -32 464 -112 WIRE 608 -32 464 -32 WIRE 976 16 976 -224 WIRE 608 64 608 -32 WIRE 1184 80 1184 32 WIRE 464 112 464 -32 WIRE 976 160 976 96 WIRE 1136 160 976 160 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
Reply by ●February 20, 20222022-02-20
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
Reply by ●February 20, 20222022-02-20
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, PiotrThanks, yes I understood it within a few minutes of my first reply, it should work fine - as you then proved! piglet
