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Current clamp oscillating at 50MHz

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Flyback

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Hi
We have a DALI power bus comprising a 15mA current clamp in a 15V rail. (as in attached schematic)
As a test, I turned on the transistor, Q4.

When I clipped a TA041 differential probe across the 82R resistor, I saw a 50MHz sinusodal oscillation across this resistor, of about 250mV pk2pk (I was using a 50MHz scope). The current in the 82R was on average less than 15mA, …about 11mA on average.

When I unclipped the Diff Probe, and instead attached a normal scope probe across the 82R, the oscillation was gone, and I saw a perfect 15mA of current flowing in the 82R resistor as it should. (1.24V across the 82R resistor).

I may be being pedantic here, but I want to solve this oscillation. (even though it only happened when the Diff Probe was connected). Do you agree that a good way to damp out this oscillation is to put an RC series circuit from Collector to base of Q3? ( Values about 10n, 10R)

I’ll provide more info later when I can get back to the lab if it's ok.

NXJ1S1215 datasheet
https://power.murata.com/data/power/ncl/kdc_nxj1.pdf

TLV431 datasheet
https://www.diodes.com/assets/Datasheets/TLV431.pdf

TA041 Differential probe
https://www.picotech.com/accessorie...ential-probes/25-mhz-700-v-differential-probe
 

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rjenkinsgb

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Most Helpful Member
I'd guess that you were effectively adding a small capacitance across the ground and sense of the TLV431, causing a lag in it's response to changes in current so it was over-compensating for the variation each time it "caught up".
A very small added base-emitter cap, or the RC from base to collector, should reduce it, by slowing the transistor response to match.

Why do you need such a complex current limiter for a simple task? It does not appear to relate to any standards compliance as the official DALI standard is 24V and 250mA.

Wouldn't the basic emitter resistor and two diodes from the base work just as well, if not better?
eg. this style: http://www.ecircuitcenter.com/Circuits_Audio_Amp/BJT Current_Source/BJT_Current_Source.htm
A 39 ohm resistor for RE should give somewhere around 15mA. You would also get less voltage loss at the load when the current is less than 15mA.
 

Flyback

Well-Known Member
Thanks but the tolerance on the current setting resistor inside the BCR401 is too loose, we need to be more in control of the current level.
Tolerance is also loose with the "BJT and two diode method", since Vf and Vbe have too loose tolerance.
 

ronsimpson

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50mhz? That is too fast for the 431.
I would try Ra in the range of 100 to 1000 ohms. This will slow down Q3. It is the only thing that fast. Not slowing down by much, just enough that it has no gain at 50mhz.
I would also try Rb. The input of the 431 should not be driven by a low impedance source. Some resistor in the range of 1k to 10k. If the input of the 431 is driven hard above the reference voltage it will work strange.
1542215929261.png
 

rjenkinsgb

Well-Known Member
Most Helpful Member
we need to be more in control of the current level
???
I do not understand this; anything claiming DALI compatibility must be able to switch 250mA minimum - and even if it's a "local" interface between two separate devices or items of equipment, you should surely be designing with eg. +/- 20% tolerance in mind to allow for component aging and less-than-perfectly performing items?

The general point of using some type of industry-standard interface system is that it should "just work", as long as any two devices are anywhere within those interface specifications, even under worst case mismatched tolerances.

I just cannot comprehend, as an industrial electronics designer for decades, how that current source could be in any way critical?
 

Flyback

Well-Known Member
Thanks, this is just one remote device. As long as the DALI bus can be pulled up and down, then it doesnt matter in our case. We only have one device on the DALI bus. ..well , two if you incluse the micro. The DALI device that we attach draws only 2mA or so....it can pull the DALI rail down no problem.
 
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