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Temperature compensating diode drops

Hi, all,

I'm designing a diode laser controller board for a customer in Scandinavia. It has a sub-Poissonian adjustable current supply, a resistor-linearized thermistor temperature sensor, and of course a thermoelectric cooler driver.

The TEC requires different current limits in different quadrants. (A TEC is actually a four-quadrant device since it generates electrical power from the temperature difference as well as the other way round.)

With a bridged current driver, that can run into quite a few parts--you need one current limiter per polarity, or if you want any short circuit protection, one per leg (4 in all). So simplicity is at a bit of a premium.

The usual two-terminal, two-BJT current limiter suffers from a fairly gross temperature coefficient, like 3000 ppm/K. Protecting a TEC isn't a super high precision application, but something a bit better than that would be good.

I've sometimes got round that by putting a Schottky diode in series with the sense resistor, like this. (The base current for Q1 actually comes from a RRO op amp via R1--it's a class B complementary bridge.)

Q1

0-*--------- -----*---->|---R2R2---*------0 | \ A | | | ------ ------- | | | / V Q2 | *--R1R1-------*---- -------------*

Trouble is, Schottkies don't have the same dV/dT as BJTs--it's more like

1 mV/K at high current, vs. 2 mV/K at low current, so you don't gain that much--about 1500 ppm/K.

A PN diode connected as above plus something like a TLV431 in series with Q2's emitter will do a good job, probably 300-500 ppm/V, which would be fine. However, that costs four more ICs and another 2.4V of headroom (1.2V per side in each polarity).

Seems like an arbitrage opportunity. ;)

My current thinking is to put the diode in series with the base of Q2 and use Q1's V_BE to provide a reasonably stable bias current, like this:

Q1 1ohm

0-*--------- --------*-------R2R2-----*------0 | \ A | | | ------ A Schottky | | | | | *---R1R1------*--R3R3----* | | | | | ------ | | / V Q2 | *------- --------------*

The diode runs at much lower current, and so has a higher dV/dT, and by choosing R2 correctly, I can adjust it to match Q2's a bit better.

One wouldn't want to bet one's firstborn child on the unit-to-unit consistency being too wonderful, but ISTM that I ought to be able to get a factor of maybe 6 this way.

Anybody else done this sort of thing? Better suggestions welcome!

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC 
Optics, Electro-optics, Photonics, Analog Electronics 

160 North State Road #203 
Briarcliff Manor NY 10510 

hobbs at electrooptical dot net 
http://electrooptical.net
Reply to
Phil Hobbs
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Thanks.

You want to run TECs from a current source, so one side is actually a current conveyor (load connected to the collectors) and the other is a voltage source (load connected to the emitters), adjusted so that the sum of the two output voltages is constant.

The totem pole runs off the highest available supply, so getting enough gate enhancement for the voltage-source side might be a bit of a puzzle. One could use two of the current-source versions, I suppose.

Trying to keep the BOM cost low--the BJT thing is much faster than a cheap op amp. I'll have a look though!

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC 
Optics, Electro-optics, Photonics, Analog Electronics 

160 North State Road #203 
Briarcliff Manor NY 10510 

hobbs at electrooptical dot net 
http://electrooptical.net
Reply to
Phil Hobbs

Why not use an opamp and a mosfet? You could reduce the voltage lost in the shunt, and get precise current limiting.

There are beginning to be some decent current-limiter chips, but opamps and resistors are cheap.

An LED and a bipolar make an interesting current limiter (which makes free light!) but not well suited to high currents.

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--

John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com
Reply to
John Larkin

I use LEDs occasionally. Some have d(Vf)/dT pretty close to a silicon BJT's Vbe.

Q1 1ohm O--+--------. .-------R2-----+----O | \ ^ | | ------ LED | | | / | '-----R1------+-----|>|---------'

IR LEDs in my parts bin have drops as low as 900mV. Dunno about the t/c, though.

For maximum headroom put the load between Vcc and Q1(c).

Cheers, James Arthur

Reply to
dagmargoodboat

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This LED d(Vf)/dT paper from an old thread is still available...

===== quote ===== > Has anybody done this?

seen it done since forever for audio amps, I believe some claim they are also less noisy than regular diodes, just remember to shield the LED from light

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looks like somewhere between red and UV will get close to zero tempco

-Lasse ===== /quote =====

From Lasse's reference: "The coefficient dVf /dT is equal to ? 2.3 mV/K for the UV LED sample (? = 375 nm), ? 5 mV/K for the blue LED sample (? = 455 nm), ? 3.8 mV/K for the green LED sample (? = 530 nm), and ? 1.5 mV/K for the red LED sample (? = 605 nm). "

Plus John's measurement from the "Ring of two current source" thread...

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Cheers, James Arthur

Reply to
dagmargoodboat

My emitter resistor alone could explain 100 PPM or more of tempco.

Self-heating of the transistor becomes a serious error at higher currents too.

I'm guessing that the effect of light on the LED will be way down in the noise.

--

John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com
Reply to
John Larkin

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Looks to me like GaN LEDs' dVf/dT is too high to compensate one Vbe, and all the others are pretty close to 'good enough,' for certain values of 'good' and 'enough.'

Cheers, James Arthur

Reply to
dagmargoodboat

If I could stand a Vbe plus a volt or so, I could replace the NPN current limiters with TLV431s and the PNP ones with LM385-ADJs.

What the world needs is a fast LM10 for 20 cents. ;)

Cheers

Phil Hobbs

Reply to
pcdhobbs

The LED-biased current sources need less than a volt of headroom. Half a volt might do.

Cheers, James Arthur

Reply to
dagmargoodboat

I'm not totally following your scheme, but wouldn't you want to _replace_Q2_ with the TLV431 (using drawing _above_)? Then your current limit is 1.2V/R2 with a _very_low_ TC. A TLV431 is transistor-sized... TO-92 pack, unless you're in love with SOT's.

...Jim Thompson

--
| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| STV, Queen Creek, AZ 85142    Skype: skypeanalog |             | 
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
| E-mail Icon at http://www.analog-innovations.com |    1962     |
Reply to
Jim Thompson

Yup. The LM385-ADJ is the same sort of thing except that it holds the 1.2V between ADJ and K, so it works with PNPs. (To make a 1.2V reference with a TLV431 you short ADJ to K, whereas with an LM385 it's ADJ to A.)

Still costs an extra volt-and-a-bit of headroom, and another buck or so on the BOM. Might be worth it.

Cheers

Phil Hobbs

Reply to
pcdhobbs

Yup. The LM385-ADJ is the same sort of thing except that it holds the 1.2V between ADJ and K, so it works with PNPs. (To make a 1.2V reference with a TLV431 you short ADJ to K, whereas with an LM385 it's ADJ to A.)

Still costs an extra volt-and-a-bit of headroom, and another buck or so on the BOM. Might be worth it.

Cheers

Phil Hobbs

Reply to
pcdhobbs

[snip]
[snip]

Depending on the voltages involved... and the switching speed... you might simply make the bridge switches part of the current control.

More info about what you're attempting? ...Jim Thompson

--
| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| STV, Queen Creek, AZ 85142    Skype: skypeanalog |             | 
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
| E-mail Icon at http://www.analog-innovations.com |    1962     |
Reply to
Jim Thompson

It's a fairly simple bridge driver for a thermoelectric cooler. Normally I' d put a single current limiter in series with the input, but in this case I need different current limits for positive and negative.

The most recent iteration has a class B current conveyor on the left side, driven by an op amp. The right side is voltage-driven, so that the outputs are centred on Vcc/2.

Because of the asymmetric limits, I need at least two separate current limi ters, one for each polarity. Seems like I only need to limit one side, righ t?

However, if the left side of the TEC ever got shorted to ground, the right side could supply enough current to turn it to lava in short order. The las ers are expensive, not to mention service calls and down time, so it seems like a good idea to put current limits in all four legs of the bridge.

(Class D is unattractive due to noise coupling into the laser.)

Cheers

Phil Hobbs

Reply to
pcdhobbs

Now I see why you want 4 instead of 2. But I think only 2 limiters need be semi-precise as per topic and the other 2 limiters can be higher current and coarser without Vbe compensation as they only need to limit s/c fault current?

piglet

Reply to
piglet

Won't asymmetric current limits screw with the symmetry implied by voltage drive?

Seems to me a gross limiter up top for shorts to ground, then limits on each lower leg for current control.

If you want to share your schematic off-line I'll provide free advice ;-)

(I'm having trouble envisioning/imagining the current conveyor configuration :-) ...Jim Thompson

--
| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| STV, Queen Creek, AZ 85142    Skype: skypeanalog |             | 
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
| E-mail Icon at http://www.analog-innovations.com |    1962     |
Reply to
Jim Thompson

Sure, but only in fault conditions, and it simulates well as it is.;)

I could maybe do two current conveyors fighting each other, and apply volta ge FB to the slave side to keep that within bounds. I don't think that save s any parts, though.

Cheers

Phil Hobbs

Reply to
pcdhobbs

I'd put a single current limiter in series with the input, but in this case I need different current limits for positive and negative.

, driven by an op amp. The right side is voltage-driven, so that the output s are centred on Vcc/2.

miters, one for each polarity. Seems like I only need to limit one side, ri ght?

t side could supply enough current to turn it to lava in short order. The l asers are expensive, not to mention service calls and down time, so it seem s like a good idea to put current limits in all four legs of the bridge.

My 1996 paper describes a class-D driver, and the filtering that meant that hardly any of the high frequency current made it into the TEC, let alone t he laser.

The original design wasn't temperature-controlling a laser, but there was a diode laser elsewhere in the system, and the boss finally accepted that th at had to be temperature stabilised too, so Paul Buggs replicated the desig n for that job (leaving out some of the space-consuming filtering on the te mperature sensing side).

--
Bill Sloman, Sydney
Reply to
bill.sloman

I didn't have in mind tying ADJ to K, etc, I had in mind some variation of this...

I can envision various switching schemes where this structure becomes part of the H-bridge. ...Jim Thompson

--
| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| STV, Queen Creek, AZ 85142    Skype: skypeanalog |             | 
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
| E-mail Icon at http://www.analog-innovations.com |    1962     |
Reply to
Jim Thompson

Why not use linear regulators as current sources? If the total drop is too high you could look for an LDO that can be pressed into such service. This can reduce the BOM to two parts per leg.

Alternatively build the limiter into your driver and use a comparators as quadrant detectors to switch gears on the limiter.

--
Regards, Joerg 

http://www.analogconsultants.com/
Reply to
Joerg

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