What is a transistor?
Selecting transistors to ensure safe operation
Whan a TR is operated, it naturally accompanies electric and thermal loads. The life of TR becomes short if such loads are too much and may be broken in the worst case. To avoi such events, it is highly recommended to check beforehand if the conditions of driving TR should have problem or not. In this section, we explain as to the method of judging whether a selected TR can be used or not. Please go through this over this material to avoid possible future trouble and use TRs in safety.
Before Judgement : From selection to mounting
1.Selection of transistor
search the possible candidate from web & short form catalogue.
2.Get samples and data-sheets
Some sample can be purchase also from ROHM Website. Availability of samples for net purchase is now expanding.
3.Mount a TR on the evaluation board or function check.
■Check points !
・if a chosen TR surely works safely on the actual circuit.
・if a chosen TR functions for a long time stably
(=reliability)after its starting of work.
・if there are enough electrical margin for such TR drive.
Judgement : Usable or Not
Judgement as to whether it is OK to use a selected TR or not shall be done based on the following steps. (Details are referred in the next page and onwards.)
1.Measure the actual current & voltage waveform
Confirmation of current and voltage
First, check to see the current and the voltage applied to a TR with the oscilloscope. Although all the listed ratings on the data sheets need to match the measured values, the parameters listed below should be given priority.
|Bipolar Transistor||Collector-Emitter Voltage :Vce||Collector current : Ic|
|Digital Transistor||Output voltage :Vo(GND-OUT)||Output current：Io|
|MOSFET||Drain-Source Voltage : Vds||Dorain current : Id|
Example : Waveform when switching 2SD2673(100μs / div)
2.Is the absolute maximum rating observed all the time ?
Confirmation of absolute maximum rating
Check if the current and the voltage confirmed in(1.Confirmation of current and voltage)are not exceeding the absolute maximum rating stipulated in the data sheet.
There are items not checked in (1.), but all such unchecked items also need to stay below the absolute maximum rating. The use of chosen TR is not possible if the peak current or the voltage surge could go beyond the absolute maximum rating range even for a moment. Otherwise, there is possibility of performance deterioration of destruction of device if driven beyond the absolute maximum
Example : Data sheet of 2SD2673 (Description of absolute maximum rating)
Example : A case the absolute maximum rating is violated momentarily (NG)
3.Is the use of TR within SOA?
●Confirmation of Safety Operating Area (SOA*1) - Part.1
SOA specifies the area where a TR can be driven safely. However, SOA is merely based on single (One) pulse alone. Therefore, it is necessary also to check if all the pulses stay within SOA if the TR is driven by repetitive pulses also the averaged applied current to be calculated in (4. Confirmation of Safety Operation Area (SOA) - Part.2) stays within the rated power.
*1 Also called ASO (Area of Safe Operating).
SOA compliance check
Check if waveform confirmed in (1.Confirmation of current and voltage)
stays withn SOA. NG (TR Not Usable) judgement must be concluded if inrush / peak current or voltage surge goes beyond the absolute maximum rating even for a moment.
Also, please be careful that there are cases that SOA is violated even if the waveform (2.Confirmation of absolute maximum rating)is within the absolute maximum rating. (See the example below.)
Example : 2SD273 SAFETY OPERATING AREA
4. Is the use of TR within SOA derated per the ambient temperature*1?
*1 The ambient temperature where the TR is used or the temperature of die when the temperature of TR is elevated by its heat.
●Confirmation of Safety Operation Area (SOA) - Part.2
Normally, SOA is defined subject to the room temperature / 25 deg.
Please refer to "Calculation method of die temperature" which is prepared
separately for details of die temperature calculation.
Addendum 1 Temperature Derating Method of SOA
< Bipolar TR / Digital TR >
1.SOA (Safety Operating Area)
SOA(Safety Operating Area) need temperature derating when the ambient temperature is more than25 deg. or the tempearture of die is elavated by the heating of TR itself. Temperature of derating is the ambient temperature for the former, and the die temperature for the latter. To be specific, SOA line needs to be shifted toward the direction where the current is smaller. THe derating ratio varies depending on the area as per the Fig.1
1-1. Heat-restriction area
In this area, SOA line has 45°downward inclination. ( Power constant line)
In this area, the derating ratio is 0.8% /deg.
1-2. Secondary down-slope area
For bipolar TR, there is secondary down-slope area due to thermal overdrive.
SOA in this area has the slope is more than at 45°and the derating ratio is 0.5% /deg.
2.Example Ta=100 deg.
2-1. Derating in the area of heat-restriction area
In case of ambient temperature of 100 deg.
Therefore, the judgement should be made by considering SOA line shift by 60% toward the smaller current direction.
2-2. Derating in the area of secondary down-slope
Likewise as above, the calculation is done as below.
Accordingly, the judgement should be made by considering SOA line shift by 37.5% toward the smaller current direction.
Addendum 2 Temperature Derating Method of SOA
1.SOA (Safety Operating Area)
SOA of MOSFET does not have secondary down-slope line unlike SOA of bipolar TRs has, and there is lineof heat-restriction area (power constant line) only.
The derating ratio in this heat-restriction area is 0.8% / deg. like bipolar TR.
Incidentally, in case of MOSFET, the maximum current needs derating as well since ON-resistance goes up as the temperatur rises..
About the maximum current derating by ON-resistance increase
Current consumption "P" is express as below.
The maximum current consumption is constant even if the ambient temperature rised, but ON-resistance increases.
Therefore, if ON-resistance shifted from R1 to R2 upward per the ambient temperature rise, the maximum current I at 25 deg. will be derated as I ' per the temperature increase, then the following formula / calculation can be applied.
2.Example Ta = 75 deg.
2-1. Derating per SOA line
In case the ambient temperature is 75 deg.,
Therefore, we should check the safety by considering SOA line shift by 40% horizontally toward the smaller current direction.
2-2. Derating of Maximum current
In case of RSS100N03,
Maximum current Id = 10A, Idp = 40A, and ON-resistance is
10m ohm at 25 deg. (Vgs = 10V, Id = 3A)
16m ohm at 75 deg. (Vgs = 10V, Id = 3A) (each is typ. value) , so that
the maximum current allowable at 75 deg. is,
5.Continuous pulse? Single pulse?
●Cofirmation of Power / Heat generation
When an Inrush current come only once like when the power is switched on or off, it is called
Unlike single pulse, it is called "continuous pulse" if the pulses are loaded repeatedly. In that case, check if
6.Is Averaged power Consumption below the rated power at the ambient temperature?
●Confirmation when below the rated power
Below the rated power under the ambient temperature = Die temperature below the maximum rating 150 deg.
The rated power is decided as the power which heats up the die at 150 deg.
(For details, please refer to"Calculation method of die temperature" prepared separately.
Power calculation method
In principle, the averaged power is the value obtained as a result of dividing the integration of Current and Voltage by the time. i.e.,
Now, thinking the following switching behavior for instance.
In this case, the calculation should be done by dividing one cycle in four.
(1) When OFF ---> ON
(3) When ON ---> OFF
(4) We can regard that there is almost no current when OFF (there should be some nA to 10nA level of leak current should exist in reality) and the current consumption as 0 (zero) while OFF state.
From the above calculation, if we divide all the total of integrattion result per each zone by 400μs which is the length of cycle, the the averaged current consumption is figured out as
We took an example of bipolar transistor 2SD2673 earlier in order to do integration calculation of collector current Ic and Collector-Emitter voltage Vce. The integration calculation in case of other TR types can be as below for obtaining the averaged current consumption.
-> Digital TR : Output Current Io and Outpout Voltage Vo
-> MOSFET : Drain Current Id and Drain-Source Voltage Vds
After getting the averaged current consumption, we should check with Pc (Collector-loss) for digital TR (Drain-loss for MOSFET).
Example : Data sheet of 2SD2673
In this case, the averaged applied power is 0.415W and Colloetor-loss is 0.5W (Recommended land : glassepoxy circuit board), so that the use of this TR under the ambient temperature of 25 deg. is OK. (In the strict sense, Pc should differ depending on the difference of heat dissipation conditions as from the types of circuit board or as from land-pattern area. However, we assume that the the recommended lnd-patterns are employed.)
When the ambient temperature is above 25 deg., please consider the temperature derating in designing per the derating curve. For example, Pd (power dissipation) becomes 80% under ambient temprature 50 deg. compared with the one at 25 deg.. Therefore, Pd = 0.5W 80% = 0.4W In this case, the averaged current consumption is considered as 0.415W which is not matching to the derating curve at the point of ambient temperature50 deg.. Thus, the TR can not be used.
For the details of Die Temperature Calculation, please refer to the separate documents "Calculation method of die temperature".
Power calculation integration formula
Calculation of power between a - b by integration with Current I and Voltage V.