Hi,
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
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.
I believe I was wrong in thinking that the 10mA current would be high.
I found this information on the web: https://www.rohm.com/products/faq-search/faqId/231
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.
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.
ABSOLUTE MAXIMUM RATINGS for this one are:
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
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? ]
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?
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.
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.
Leo..
Yes.
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.
Leo..
