Within the late Nineties I used to be working for an organization that manufactures sub-systems for tramways, subways and lightweight rail automotive producers.
In the future my boss tells me that one in all our inverters, a 5 kW DC to three-phase AC unit, has too many service calls and asks me to see what I can do to decrease the failure price.
The inverter in query is used to energy the air brake compressor. It’s roughly the dimensions of two huge microwave ovens caught collectively. The air compressor is crucial to the right operation of subway vehicles that use an air brake system [1]. On this system air stress is used to manage the brakes on all of the vehicles in a practice. When the compressor inverter fails, the compressor can’t pump air into the brake air reservoir and the entire practice needs to be taken out of service for repairs.
The inverters have been returned to our manufacturing unit for repairs. One failure mode stood out and it accounted for 16 % of all repairs. The half that failed probably the most is a small sign transistor—a part of the inverter fault alarm circuit. That circuit drives a practice line and lights up the air compressor fault lamp on the driving force’s standing panel.
A practice line is a line that goes to all of the vehicles in a single practice. There are management practice traces and alarm practice traces. The fault practice line lights up the incandescent lamps (the automotive design is from the mid-Nineties) within the driver cab of all of the vehicles in a practice. Within the practice six lamps in whole are linked to that line.
I went again to the store ground and requested the technicians for the failed components. They instructed me that the failed components are discarded after repairs. All I had entry to have been the schematics.
I analyzed the circuit. An remoted open collector sign drives the bottom of Q1, with a ten kilohm pull-up resistor, R1, linked to the battery voltage. Transistor Q1 is a NPN Darlington just like the TIP122. The Darlington collector, Q1C, is linked to the battery provide, 74 V, and its emitter, Q1E, goes to a present sensing resistor, R21, 700 milliohms. Lastly a diode, D1, connects to the practice line. The bottom of the present limiting transistor, Q2B, is linked to the Q1E-R2 node and Q2’s emitter to R2-D1 anode. Transistor Q2’s collector is tied to Q1’s base. This can be a customary present limiter circuit [2] [3]. The transistor that failed probably the most was Q2 and it was just like the MPSA42.
Incandescent lamps have important turn-on surge present. The ratio between cold and warm filament resistance varies between 12:1 and 18:1 [4]. This present limiting circuit protects transistor Q1 in opposition to quick circuits and lamp turn-on present overloads.
The query is: what was making Q2 to fail?Â
Might or not it’s as a result of the collector to emitter voltage was too excessive? No, Q2’s collector was linked to the bottom of Q1 and Q1’s emitter was linked to the underside of R2. The utmost Vce is thrice Vbe, or about 2.1 V. Transistor Q2’s most Vceo is 300 V. The identical reasoning applies to Q2 Vcbo with solely two Vbe drops.
Might Q2 fail due to an excessive amount of collector present? No, the pull-up resistor mixed with the battery voltage can solely provide a most of seven.4 mA and Q2’s most steady collector present is 500 mA.
Might Q2 fail due to an excessive amount of energy dissipation? No, with a most collector-to-emitter voltage of two.1 V and a most present of seven.4 mA, peak energy dissipation is lower than 20 mW and Q2’s most ranking is 625 mW.
Might Q2 fail due to reverse present or voltage? No, diode D1 protects in opposition to that.
The one reason behind failure left was extra base present into Q2. The utmost allowed base present is never given in producer’s specs. One supply I discovered is for the 2N3904—a transistor in TO-92 bundle—and the utmost worth for peak base present is 100 mA [5].
How might the bottom present into Q2 be so excessive on this circuit? Transistors don’t swap state immediately. With Q1 turned on and saturated, if a brief circuit happens it takes time for Q2 to activate and for Q1 to show off. Apparently, these delays made it doable for the bottom present in Q2 to be larger than 100 mA and trigger everlasting harm. Till Q2 has turned on and Q1 turned off, nothing is limiting the present within the circuit besides the circuit resistance made up of the 0.7 ohm resistor. Including to the issue is the exponential relationship between the base-emitter voltage and the bottom present. A 60 mV improve in base-emitter voltage in Q2 causes a 10-fold improve in its base present [6].
How might I restrict the bottom present into Q2?  The answer is straightforward: insert a small valued resistor in sequence with Q2’s base.
Will this alteration the present restrict setting? Calculations confirmed {that a} 220 ohm resistor in sequence with Q2’s base produced a suitable 6% improve within the limiting present worth. This resistor will even shield Q2 when Q1 fails short-circuit collector to emitter.
To simplify the repairs I chosen a transistor with a built-in base resistor with the identical pin-out as the unique. The one modifications have been to the components checklist and the schematic; no modifications have been wanted on the PCB.
I knowledgeable my boss that I’d require that every one models that got here into the store be retrofitted. He agreed with me. I issued a request that when a unit was returned for service, transistor Q2 was to be modified to the brand new half whether or not Q2 was useful or not. After that, not a single modified unit returned with a failed Q2.
The teachings discovered are: 1) watch out for limitless present getting right into a BJT base terminal; 2) it isn’t as a result of there are not any limits given within the information sheet to a tool functionality, that the ranking is infinite, and; 3) at all times maintain failed components readily available in case additional evaluation is required.
[1] See the article on Railway air brake on Wikipedia, https://en.wikipedia.org/wiki/Railway_air_brake
[2] Horowitz, Paul and Winfield Hill. The Artwork of Electronics. third ed. improved, Cambridge College Press, 2015. Appendix B, determine B3. When you have the second version see appendix E determine E3.
[3] Typical present limiter circuit, https://electronics.stackexchange.com/questions/195173/bjt-current-limiter-for-linear-power-supply
[4] Normal Electrical, SCR Handbook Together with Triacs and Different Thyristors, fifth version, 1972, desk 9.1, web page 245.
[5] Philips Semiconductor, information sheet 2N3904 NPN switching transistor, Product specification 2004 Oct 11. https://datasheet.octopart.com/2N3904-Philips-datasheet-71790.pdf
[6] Tektronix, 577-177-D1 or D2 Operators Instruction Handbook, doc 070-1436-00 dated 1172, web page 23, determine 19. This doc comprises one of many uncommon figures that present measurements of collector present versus base to emitter voltage over 9 a long time from 200 pA per division as much as 20 mA per division. We are able to see the slope of 60 mV of Vbe per decade of collector present and thus base present. The caption refers back to the transistor as a 2N3509; the textual content in paragraph above refers to a 2N3905, so there may be uncertainty in regards to the precise transistor sort used.
Daniel Dufresne is a retired engineer and has labored in telecommunication, mass transit, shopper merchandise, and excessive energy digital design. He additionally was a professor at Cégep de Saint-Laurent. He lives in Montreal, Canada and nonetheless works on digital initiatives.
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