Bipolar NPN Transistor fails in conductive state

I,m currently learning and experimenting with transistors and have the following circuit which destroys the transistor but I don't know why? I use DC electronic load and I increase the current in small steps until the current reaches about 0.6A. Base current is ~11mA. Than the transistor fails in conductive state without even overheating. The surface temperature stays below 50°C all the time and the voltage difference between the Collector and the Emitter is less than 0.2V at 0.6A Collector current flow.
In my understanding this transistor should be able to conduct continuously 2A of current at any voltage up to 45V.

I am using ZTX690B/ 1W /NPN 45/45V 2A hfe 400 Ucesat 0.5V CBE

ZTX690B Transistor data sheet

Here is the schematic:

Thank you!

let’s see your actual wiring.

Hi @electronic_hobbyist .
I'm finding the base current too high.
In the datasheet it presents a case with base current around 10 mA but only for 300 microseconds
" *Measured under pulsed conditions. Pulse width=300μs. Duty cycle ≤2%"
and 50 mA for 1300 nanoseconds.

Constant 10mA base current will not hurt the transistor.

Thank you but I'm confused.
How can I find the maximum continuous Collector current which the transistor is able to handle?

Why the datasheet doesn't presents the maximum continuous current ratings?

Hi @electronic_hobbyist

I believe I was wrong in thinking that the 10mA current would be high.
I found this information on the web:
rohm is a is a semiconductor manufacturer.

I'm not sure, but one possibility to be investigated is the value of Vbe at the moment the power is turned on.
In the datasheet it says that the maximum Vbe is 5V.

RV mineirin

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Faulty electronic load? Try the same experiment with a 40 Ohm, 25 W resistor.

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Thank you good people!
I will try without the electronic load and I will post the result.

Is it safe to use the transistor close to or at the ABSOLUTE MAXIMUM RATINGS specified in the datasheet? Let say for example if want to use it as a switch which can withstand 2A at 45V. I'm trying to understand how to determine the MAXIMUM SAFE RATINGS based on the datasheet.

Collector-Base Voltage VCBO 45 V
Collector-Emitter Voltage VCEO 45 V
Emitter-Base Voltage VEBO 5 V
Peak Pulse Current ICM 6 A
Continuous Collector Current IC 2 A

Never operate at the MAXIMUM ratings !

24 ÷ 40 ≈ 600mA load current; this is well below the MAXIMUM continuous collector current rating, you’ll be safe.

Confirm with a DMM the load is 40Ω.

Show us your wiring.

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Is there a rule of thumb about how much below the MAXIMUM ratings we should keep the voltage and the current?

Well if you need one, do not go over 2/3s of the ABSOLUTE MAXIMUM RATINGS.

If you adhere to this you should be fine.

BTW, always confirm the operating voltages and currents expected with a DMM.

For example, if you expect Vce(sat) will be .5v, your voltage collector to emitter should be less than or equal to .5v.

Let’s say Vce is .3v at a collector current of 600mA.

The power dissipation of the transistor will be .3V * .6A ≈ .18W at room temperature.

Your transistor can dissipate 1.5W with infinite heat sinking.

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The surface temperature stays below 50°C - but its the junction temperature thats important.

However if Vce = 0.1V and Ic = 600mA P = 60mW so the junction should only be about 10C above the case temperature;

No - the REVERSE breakdown is >5V; when forward biased Vbe depends on Ib and is generally 0.6 - 0.9V

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You're not saturating the transistor. You need about 50mA in the base to saturate it with 600mA collector current. The gain of 400 mentioned in the datasheet doesn't apply to saturation.

With only 11mA base current you'll be seeing a volt or two between the C/E of the transistor and thus a watt or so dissipation - without good heatsinking that's enough to fry it.

You simulation is nonsense apparently - transistor models are often pretty poor in simulators, and simulators may not be handling dissipation limits anyway

[ on further inspection the datasheet claims this transistor ought to work OK with base current = 1% of collector current - perhaps you have a counterfeit transistor, or the cause is something else? ]

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Is there a formula for calculating the saturation current if I use the transistor in continuous conductive state as a closed switch for longer periods of time?
Why in the data sheet they specify the values in pulsed conditions only?

Base-Emitter Saturation Voltage/ VBE(sat)/ 0.9 V/ IC=1A, IB=10mA*
*Measured under pulsed conditions. Pulse width=300μs. Duty cycle ≤2%

Is it a good practice to confirm the saturation state by checking if the voltage drop across the Collector and the Emitter is less enough at given current flow in order to keep the transistor cool?
For example ​if Vce = 0.1V and Ic = 0.6A then P = 60mW seems to be a good value for my particular transistor.

You can easily prove if the transistor is saturated.

Measure the voltage collector to emitter.

If the voltage has reached VCE(sat) then it is saturated.

Also when Base-Collector junction is forward biased you are in saturation.

You should have the transistor well into saturation when Ib is 1/10th Ic.

Always confirm operation with a DMM, full stop !

It's hard to fully saturate (collector voltage lower than base voltage) a bjt transistor.
A lot of base current is needed (Hfe has nothing to do with saturation).
At least 5% of the collector current is needed to saturate a medium power transistor.
Datasheets usually state max saturation at 10% of collector current (the 1:10 in the graph).
That means 30-60mA for 600mA collector current.

If you don't saturate the transistor, the chip will internally overheat (every collector volt is 0.6Watt).

Nowadays we grab a mosfet for switching more than a few hundred milliamps.
No gate current needed (after switching).
Study this page.


Can you show a link to the electronic load you are using?

Intuitively I'd be very uneasy about using a tiny E-line transistor to handle 1 amp of Ic.

175 °C/W is one of the limiting factors of the TO-92 package.
Just 1volt remaining on the collector is driving the internal chip 105°C above ambient.

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Hi, I use this one -> DL24 150W 2.4" DC USB tester APP electronic load lithium battery capacity monitor discharge charge power meter supply checker|Battery Testers| - AliExpress

Purely to avoid thermal effects - you'll need to separately evaluate any heatsinking needed.

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