Temperature compensation in circuit?

[stef1777]

Hi folks!

This is not really an audio question. I've built linear current led regulator (1A max, 2.5v-6v). It's working very fine and the voltage range is what I was looking for. The loss is really low (60mV).

But the problem is temp. Between 25°C and 70°C, I've an 100mA drift. I'm looking for the best way to ovoid or reduce this thermal drift at minimum. As well with minimum component addition.

The problem is coming from the bipolar transistor. If I cold it, the drift go away. But I can't in the real life.

May be the transistor is not the best to use, may be I should add a temp compensation circuit. No sure, no sure how to to this?

Any advice?

[AndrewT]

Hi,
I'm lost, is this a current or a voltage regulator?

What does lm334 do?

[Conrad Hoffman]

Early morning armchair guess- you don't have to pull much current out of the base of Q1, and if the Hfe is high, it could be below the range for the 334, or the 334 gain could just be too low. I'd add a resistor from the base of Q1 to the supply (emitter) so the 334 runs over a narrower range and so Q1 can better turn off. I'd also look at the 10uF cap. If it's an electrolytic, the leakage current might be getting involved. Use a low leakage film type. Finally, make sure your current sensing resistors are something stable like metal film, not thick film or MOX. Hopefully one of those things will help, but IMO a far better current regulator can be done with an op-amp and MOSFET as originally published by Siliconix many years ago.

[stef1777]

Resistor are all low ppm metal smd. Caps is XR5.

Curently, I've solved the problem using lower Rsense resistors as calculated to take care of the real internal temp in the box. I still have a 100mA difference as well at start.

[richie00boy]

I think the issue is that Ic and Vbe of the transistor are temperature dependent. Thus simply forcing a base current you will be affected by both these temperature-sensitive variables.

Make the current source part of a potential divider or drive current through a voltage generator such as a zener, and feed the base from that.

[stef1777]

Originally posted by Conrad Hoffman but IMO a far better current regulator can be done with an op-amp and MOSFET as originally published by Siliconix many years ago.

Do you have the Siliconix article somewhere for my information?

.

[Conrad Hoffman]

From the Jan 1983 Siliconix MOSPOWER design catalog, the couple extra components make this circuit far more robust than just directly wiring the MOSFET like many people do. Apply whatever voltage to the input you need- their divider is just an example. Ditto the value of the sense resistor- use whatever is reasonable for your application.

[jcx]

you seem to be stuck with a major conceptual error - yes collector current is a multiple of the base current (beta or Hfe), But the ratio is affected by everything, including temperature as you have demonstrated - so virtually no discrete transistor circuit relies on a Q having a specific value of beta (Hfe)

the most robust way to get a costant current from a bipolar tansistor is to use a resistor to 'measure' the emitter current and controlling the Q base voltage to regulate the emitter current sense resistor V, then the collector current is 1-1/beta times the measured and regulated emitter current - which only relies on beta being large for accuracy

Current Source <- how to optimize?

points to some ccs circuits

in

http://www.diyaudio.com/forums/showt...97#post1007597

sijosae' collection 1-11 are fine, 12-16,18 probably don't work, I would automatically use his fig 6,11 "feedback ccs" when it meets the application requirements

[Conrad Hoffman]

I think the idea of the original circuit is the 334 will sense and provide enough gain to swamp out the TC of the transistor, but that probably isn't reasonable. Q1 will certainly have a lot of temperature sensitive gain the way it's being used, and the 334 isn't a good choice for the "error amplifier". BTW, what's this circuit being used for?

[stef1777]

Originally posted by Conrad Hoffman
BTW, what's this circuit being used for?

As I wrote in the first post, this circuit is used to drive a power led. As I need the smaless possible size, minimum component are used. Linear current regulation is used to have something working with a large voltage and current range, minimum dropout voltage, easy to find component, fiability and DX use. Many dedicated step down converter or booster circuits do not have these qualities.

Add 1 or 2 components to the current design, or find a better transistor to reduce thermal drift can be done, not design a completly a new circuit with tens of components added...