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Preventing a BJT from exploding using only base current

Started by Aleksandar Kuktin April 25, 2015
Hi all!

TL;DR: when a BJT sits naked (no DC resistances on either the emitter or 
collector, but well controlled base current) between Vcc and GND, is it 
safe to use ONLY the base current to prevent it from releasing magic 
smoke? Details below.

I'm building a transmitter for a wired interface (Ethernet). I chose to 
drive the transmission line itself with a transformer which is in turn 
driven by a common-collector BJT (a voltage follower/buffer), but with no 
DC current limiting resistors.

The inductance of the transformer has been selected to provide a good 
impedance at the low end of the bandwidth, but that left me with such an 
impedance at the high end of the range that I can not really afford to 
put a series limiting resistor to control the current (and thus power) 
through the BJT when the transmitter is idling.

Stated differently, when the transmitter is not sending any frames, the 
output of the voltage follower is pure DC. It passes only through a 
transformer primary whose impedance at DC is near-zero.

I can not put a resistor in series with the transformer because the 
amplitude of the output oscillations would be too low. I can also not put 
a resistor in series with the collector for the same reason.

The option of using a DC resistor with a capacitor AC bypass is, again, 
DOA because during the operation the capacitor will get charged up to 
maybe Vrms of the output signal and will clip the output signal when the 
transistors input signal falls below that value.

I want to avoid the common-emitter conformation if anyhow possible 
because of problems with the signal distortion. I'm also not yet willing 
to experiment with switching the Vcc wire on and off to provide power-on-
demand, so to speak. Same goes with the baseline base voltage.

So I'm basically stuck with a transistor which, when idling, sits naked 
between Vcc and GND. To enable proper voltage following, its base needs 
to be biased to a pretty high voltage - I'm keeping it at 1-1.2V - but 
this opens up a possibility of a short circuit.

The only way I figured out to prevent it from exploding is to use a well-
selected series resistor with the base to moderate the baseline current 
and basically abuse the BJTs core operating principle (moderation of 
resistance via current) for fun and profit.

How reliable do you judge this to be, given random operating conditions 
the users are sure to put the interface in? Does inter-transistor 
difference (especially hFE differences between individual transistors) 
become important? Will I need to tightly match the base series resistance 
for each individual transistor?
On a sunny day (Sat, 25 Apr 2015 17:01:39 +0000 (UTC)) it happened Aleksandar
Kuktin <akuktin@gmail.com> wrote in <mhgh9j$bo6$1@speranza.aioe.org>:

>Hi all! > >TL;DR: when a BJT sits naked (no DC resistances on either the emitter or >collector, but well controlled base current) between Vcc and GND, is it >safe to use ONLY the base current to prevent it from releasing magic >smoke? Details below.
I have done it and it still works. Beta is reliable within reason. Of course above some temperature the thing will melt anyways.
On Saturday, 25 April 2015 18:01:46 UTC+1, Aleksandar Kuktin  wrote:
> Hi all! > > TL;DR: when a BJT sits naked (no DC resistances on either the emitter or > collector, but well controlled base current) between Vcc and GND,
No load would be pointless. In fact that's not what you have.
> is it > safe to use ONLY the base current to prevent it from releasing magic > smoke? Details below.
Well, what are a tr's pertinent limits? Ic Ib Pdiss Vce Theta SOA Does controlling Ib control all the above? No... unless you either a) monitor those things & use that to control Ib. b) don't monitor, just provide a heavily overrated tr
> I'm building a transmitter for a wired interface (Ethernet). I chose to > drive the transmission line itself with a transformer which is in turn > driven by a common-collector BJT (a voltage follower/buffer), but with no > DC current limiting resistors. > > The inductance of the transformer has been selected to provide a good > impedance at the low end of the bandwidth, but that left me with such an > impedance at the high end of the range that I can not really afford to > put a series limiting resistor to control the current (and thus power) > through the BJT when the transmitter is idling.
How about R,C in parallel? Or less good inductance at low end? Or higher transformer resistance?
> Stated differently, when the transmitter is not sending any frames, the > output of the voltage follower is pure DC. It passes only through a > transformer primary whose impedance at DC is near-zero. > > I can not put a resistor in series with the transformer because the > amplitude of the output oscillations would be too low. I can also not put > a resistor in series with the collector for the same reason.
You can't add a supply current limit?
> The option of using a DC resistor with a capacitor AC bypass is, again, > DOA because during the operation the capacitor will get charged up to > maybe Vrms of the output signal and will clip the output signal when the > transistors input signal falls below that value. > > I want to avoid the common-emitter conformation if anyhow possible > because of problems with the signal distortion. I'm also not yet willing > to experiment with switching the Vcc wire on and off to provide power-on- > demand, so to speak. Same goes with the baseline base voltage. > > So I'm basically stuck with a transistor which, when idling, sits naked > between Vcc and GND.
monitor the copper drop across the transformer?
> To enable proper voltage following, its base needs > to be biased to a pretty high voltage - I'm keeping it at 1-1.2V - but > this opens up a possibility of a short circuit.
diode drop or R.... as you suggest. But you either need to monitor the tr's limits or be very conservative.
> The only way I figured out to prevent it from exploding is to use a well- > selected series resistor with the base to moderate the baseline current > and basically abuse the BJTs core operating principle (moderation of > resistance via current) for fun and profit. > > How reliable do you judge this to be, given random operating conditions > the users are sure to put the interface in? Does inter-transistor > difference (especially hFE differences between individual transistors) > become important? Will I need to tightly match the base series resistance > for each individual transistor?
Hfe varies according to Ic, Vce, and varies widely between samples. Vbe varies according to theta, which generally varies Ib too. I don't know what the hfe range is, but perhaps 5:1 or 10:1 for the 1st 2, 5:1 for the 3rd, giving a /very/ wide spread of hfe. So its hopeless unless you select each resistor for each transistor, checking it works for all conditions, and giving the tr lots of room. Given enough Pdiss & Ic ratings I daresay it could work. If you think it couldn't, what failure mode are you proposing? Other than gross financial & energy inefficiency. I'd be looking for another solution. NT
On Sat, 25 Apr 2015 17:01:39 +0000 (UTC), Aleksandar Kuktin
<akuktin@gmail.com> wrote:

>Hi all! > >TL;DR: when a BJT sits naked (no DC resistances on either the emitter or >collector, but well controlled base current) between Vcc and GND, is it >safe to use ONLY the base current to prevent it from releasing magic >smoke? Details below. > >I'm building a transmitter for a wired interface (Ethernet). I chose to >drive the transmission line itself with a transformer which is in turn >driven by a common-collector BJT (a voltage follower/buffer), but with no >DC current limiting resistors. > >The inductance of the transformer has been selected to provide a good >impedance at the low end of the bandwidth, but that left me with such an >impedance at the high end of the range that I can not really afford to >put a series limiting resistor to control the current (and thus power) >through the BJT when the transmitter is idling. > >Stated differently, when the transmitter is not sending any frames, the >output of the voltage follower is pure DC. It passes only through a >transformer primary whose impedance at DC is near-zero. > >I can not put a resistor in series with the transformer because the >amplitude of the output oscillations would be too low. I can also not put >a resistor in series with the collector for the same reason. > >The option of using a DC resistor with a capacitor AC bypass is, again, >DOA because during the operation the capacitor will get charged up to >maybe Vrms of the output signal and will clip the output signal when the >transistors input signal falls below that value. > >I want to avoid the common-emitter conformation if anyhow possible >because of problems with the signal distortion. I'm also not yet willing >to experiment with switching the Vcc wire on and off to provide power-on- >demand, so to speak. Same goes with the baseline base voltage. > >So I'm basically stuck with a transistor which, when idling, sits naked >between Vcc and GND. To enable proper voltage following, its base needs >to be biased to a pretty high voltage - I'm keeping it at 1-1.2V - but >this opens up a possibility of a short circuit. > >The only way I figured out to prevent it from exploding is to use a well- >selected series resistor with the base to moderate the baseline current >and basically abuse the BJTs core operating principle (moderation of >resistance via current) for fun and profit. > >How reliable do you judge this to be, given random operating conditions >the users are sure to put the interface in? Does inter-transistor >difference (especially hFE differences between individual transistors) >become important? Will I need to tightly match the base series resistance >for each individual transistor?
I don't entirely understand what you are doing. Can you post a schematic? Since a transformer doesn't pass DC, why not capacitively couple into the transformer, and avoid frying the transistor? -- John Larkin Highland Technology, Inc picosecond timing laser drivers and controllers jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
On Sat, 25 Apr 2015 11:01:05 -0700, John Larkin wrote:

> On Sat, 25 Apr 2015 17:01:39 +0000 (UTC), Aleksandar Kuktin > <akuktin@gmail.com> wrote: > >>Hi all! >> >>TL;DR: when a BJT sits naked (no DC resistances on either the emitter or >>collector, but well controlled base current) between Vcc and GND, is it >>safe to use ONLY the base current to prevent it from releasing magic >>smoke? Details below.
<< snip >>
> Since a transformer doesn't pass DC, why not capacitively couple into > the transformer, and avoid frying the transistor?
That was my thought. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com
On Saturday, April 25, 2015 at 1:01:46 PM UTC-4, Aleksandar Kuktin wrote:
> Hi all! > > TL;DR: when a BJT sits naked (no DC resistances on either the emitter or > collector, but well controlled base current) between Vcc and GND, is it > safe to use ONLY the base current to prevent it from releasing magic > smoke? Details below. > > I'm building a transmitter for a wired interface (Ethernet). I chose to > drive the transmission line itself with a transformer which is in turn > driven by a common-collector BJT (a voltage follower/buffer), but with no > DC current limiting resistors. > > The inductance of the transformer has been selected to provide a good > impedance at the low end of the bandwidth, but that left me with such an > impedance at the high end of the range that I can not really afford to > put a series limiting resistor to control the current (and thus power) > through the BJT when the transmitter is idling. > > Stated differently, when the transmitter is not sending any frames, the > output of the voltage follower is pure DC. It passes only through a > transformer primary whose impedance at DC is near-zero. > > I can not put a resistor in series with the transformer because the > amplitude of the output oscillations would be too low. I can also not put > a resistor in series with the collector for the same reason. > > The option of using a DC resistor with a capacitor AC bypass is, again, > DOA because during the operation the capacitor will get charged up to > maybe Vrms of the output signal and will clip the output signal when the > transistors input signal falls below that value. > > I want to avoid the common-emitter conformation if anyhow possible > because of problems with the signal distortion. I'm also not yet willing > to experiment with switching the Vcc wire on and off to provide power-on- > demand, so to speak. Same goes with the baseline base voltage. > > So I'm basically stuck with a transistor which, when idling, sits naked > between Vcc and GND. To enable proper voltage following, its base needs > to be biased to a pretty high voltage - I'm keeping it at 1-1.2V - but > this opens up a possibility of a short circuit. > > The only way I figured out to prevent it from exploding is to use a well- > selected series resistor with the base to moderate the baseline current > and basically abuse the BJTs core operating principle (moderation of > resistance via current) for fun and profit. > > How reliable do you judge this to be, given random operating conditions > the users are sure to put the interface in? Does inter-transistor > difference (especially hFE differences between individual transistors) > become important? Will I need to tightly match the base series resistance > for each individual transistor?
Drive the transformer differentially.
On Sat, 25 Apr 2015 10:41:13 -0700, tabbypurr wrote:

> Well, what are a tr's pertinent limits? > Ic Ib Pdiss Vce Theta SOA Does controlling Ib control all the above? > No... unless you either a) monitor those things & use that to control > Ib. > b) don't monitor, just provide a heavily overrated tr
I was going to use a BFR93A, but I've since changed my mind and now I'm going with 2N3904. The BFR93A is too fragile to be used without continuous oversight (version a above), and 2N3904 should be able to withstand even high temperatures and high betas without overheating (version b above)
>> The inductance of the transformer has been selected to provide a good >> impedance at the low end of the bandwidth, but that left me with such >> an impedance at the high end of the range that I can not really afford >> to put a series limiting resistor to control the current (and thus >> power) through the BJT when the transmitter is idling. > > How about R,C in parallel?
Haven't tried it. I'm worried it will produce ringing with some signals (link test pulses, in particular).
> Or less good inductance at low end? > Or higher transformer resistance?
Lower inductance doesn't help with idling. I can't control transformer resistance.
>> Stated differently, when the transmitter is not sending any frames, the >> output of the voltage follower is pure DC. It passes only through a >> transformer primary whose impedance at DC is near-zero. >> >> I can not put a resistor in series with the transformer because the >> amplitude of the output oscillations would be too low. I can also not >> put a resistor in series with the collector for the same reason. > > You can't add a supply current limit?
I might, but it would increase complexity.
>> I want to avoid the common-emitter conformation if anyhow possible >> because of problems with the signal distortion. I'm also not yet >> willing to experiment with switching the Vcc wire on and off to provide >> power-on- >> demand, so to speak. Same goes with the baseline base voltage. >> >> So I'm basically stuck with a transistor which, when idling, sits naked >> between Vcc and GND. > > monitor the copper drop across the transformer?
Too complicated. Easier and cheaper to just overrate the transistor.
>> To enable proper voltage following, its base needs to be biased to a >> pretty high voltage - I'm keeping it at 1-1.2V - but this opens up a >> possibility of a short circuit. > > diode drop or R.... as you suggest. But you either need to monitor the > tr's limits or be very conservative. > >> The only way I figured out to prevent it from exploding is to use a >> well- >> selected series resistor with the base to moderate the baseline current >> and basically abuse the BJTs core operating principle (moderation of >> resistance via current) for fun and profit. >> >> How reliable do you judge this to be, given random operating conditions >> the users are sure to put the interface in? Does inter-transistor >> difference (especially hFE differences between individual transistors) >> become important? Will I need to tightly match the base series >> resistance for each individual transistor? > > Hfe varies according to Ic, Vce, and varies widely between samples. > Vbe varies according to theta, which generally varies Ib too. > I don't know what the hfe range is, but perhaps 5:1 or 10:1 for the 1st > 2, 5:1 for the 3rd, giving a /very/ wide spread of hfe.
Hmm... I've read about 3x differences, but 5x or 10x seems a little much..
> So its hopeless unless you select each resistor for each transistor, > checking it works for all conditions, and giving the tr lots of room. > Given enough Pdiss & Ic ratings I daresay it could work. If you think it > couldn't, what failure mode are you proposing? Other than gross > financial & energy inefficiency. I'd be looking for another solution.
Well, the only failure mode I'm really worried about is thermal runaway. There is no negative feedback loop anywhere in Ib control (unless I specifically add it) so if for any reason power to be dissipated - during idling - overcomes the ability of the transistor and circuit to dissipate it, the transistor will enter a vicious circle (higher temperature increases Ic leakage, which increases temperature). Normally, there would be a resistor somewhere and it would limit the current and prevent the transistor from running off, but because of transformer problems, during idling, the transistor sits alone between Vcc and GND and has no current limiting scheme other than the thermally vulnerable Ib-Ic relationship. For the record, by using a 2N3904 and low Ib, I was eventually able to construct a circuit which is - on paper - not supposed to smoke even on silicon temperatures exceeding 100 degC (which works out to 50-60 degC ambient). Now my only problem is that the transmitter output is not high enough.
On Sat, 25 Apr 2015 18:27:23 -0700, bloggs.fredbloggs.fred wrote:

> On Saturday, April 25, 2015 at 1:01:46 PM UTC-4, Aleksandar Kuktin > wrote: >> >> [snip long post] > > Drive the transformer differentially.
Thinking about it, and trying it a bit. Unfortunately, a true differential circuit greatly increases both the complexity and part count and - so far - hasn't given me a noticeable benefit.
On Sat, 25 Apr 2015 17:34:30 +0000, Jan Panteltje wrote:

> On a sunny day (Sat, 25 Apr 2015 17:01:39 +0000 (UTC)) it happened > Aleksandar Kuktin <akuktin@gmail.com> wrote in > <mhgh9j$bo6$1@speranza.aioe.org>: > >>Hi all! >> >>TL;DR: when a BJT sits naked (no DC resistances on either the emitter or >>collector, but well controlled base current) between Vcc and GND, is it >>safe to use ONLY the base current to prevent it from releasing magic >>smoke? Details below. > > I have done it and it still works. > Beta is reliable within reason. > Of course above some temperature the thing will melt anyways.
Thanks!
On Sat, 25 Apr 2015 11:01:05 -0700, John Larkin wrote:

> On Sat, 25 Apr 2015 17:01:39 +0000 (UTC), Aleksandar Kuktin > <akuktin@gmail.com> wrote: > >> >> [snip long post] > > I don't entirely understand what you are doing. Can you post a > schematic?
Umm.. I don't really have a ready place on the 'net where I could post it. Is a SPICE netlist fine? Vcc 1 0 3.3 Cin 100 110 680e-12 Rbias_up 110 1 15000 Rbias_dw 110 0 18000 Qdriver 1 110 200 Q2N3904 Xbalun 200 0 1024 1025 BALUN l=20e-6 c=0.998 Balun consists of two 20uH coils with a coupling of 99.8%. Turn ratio 1:1.
> Since a transformer doesn't pass DC, why not capacitively couple into > the transformer, and avoid frying the transistor?
Not entirely sure what you're suggesting... You mean have a capacitor in series with the transformer? I can't do that because the capacitor charges up during transmission. Then, when Vb falls below the charge voltage on the capacitor, the transistor stops conducting and the output waveform is has one of its amplitudes cut. Even if I break the standard and let an asymmetric waveform onto the line, the cut is below (actually above, because the downstroke is cut) the critical 0.6V level. The receiver will reject such a signal.