# Power Ratings Of Transistors

#### RAMBO999

Joined Feb 26, 2018
256
Hi

Attached a screen dump of the charcteristics of a typical NPN transistor. This is a 2N3904, for what it's worth, but that's by the by.

What would be the maximum power rating of this transistor based on these characteristics?

Cheers

#### Attachments

• 171 KB Views: 13

#### peterdeco

Joined Oct 8, 2019
380
It's on the line Ptot = 625mW. That is absolute maximum. To be safe and ensure long life, never run components at absolute maximum.

#### RAMBO999

Joined Feb 26, 2018
256
It's on the line Ptot = 625mW. That is absolute maximum. To be safe and ensure long life, never run components at absolute maximum.
Ahah!! I see. I wondered what that was. I did Google it but wasn't sure what the word "dissipation" refered to given that the value is expressd in relation to heat ie: Tc = 25C. Is that value calcuable from the rest of the data on the datasheet? What I mean is that the sheet shows mximums Vceo = 60VDC, Vcbo = 40VDC and Ic = 200 mA for example. Am I correct in saying that in practice you should used this data to select a combinmation of these values that ensure that when you deploy this transistor you implement a combination of these values that collectively do NOT exceed the 625 mW Ptot value by using the usual W=Vi formula?

#### peterdeco

Joined Oct 8, 2019
380
Exactly. For example, max collector voltage is 40V, max collector current is 200mA. But at those maximum ratings the device would be dissipating 8 Watts. It would fry in a fraction of a second.

#### ronsimpson

Joined Oct 7, 2019
1,343
Yes, keep the voltage below 40. As current approaches150mA the current gain falls. Tc=25C is for room temp. Many products run hot inside a plastic box and so the power needs to be backed down even more. (no air flow in the box)

#### RAMBO999

Joined Feb 26, 2018
256
Exactly. For example, max collector voltage is 40V, max collector current is 200mA. But at those maximum ratings the device would be dissipating 8 Watts. It would fry in a fraction of a second.
Thanks very much. I needed that confirmation. I fried one earlier so I can confirm that to be the case!!!

#### RAMBO999

Joined Feb 26, 2018
256
Yes, keep the voltage below 40. As current approaches150mA the current gain falls. Tc=25C is for room temp. Many products run hot inside a plastic box and so the power needs to be backed down even more. (no air flow in the box)
Yes. I have looked at the various charts on the datasheets. Thanks for your input. very helpful.

#### RAMBO999

Joined Feb 26, 2018
256
It's on the line Ptot = 625mW. That is absolute maximum. To be safe and ensure long life, never run components at absolute maximum.
Out of curiosity is there an industry standard / rule of thumb that puts a practical cap on implemented Ptot? Something like 70% of Ptot for example? What would be the norm that you would go for when deciding on the capacity / type of transistor to use?

#### dl324

Joined Mar 30, 2015
12,929
What would be the maximum power rating of this transistor based on these characteristics?
Power dissipation is limited by junction temperture.

At an ambient temperature above 25C, you need to derate maximum power. The datasheet you referenced neglected to state that information. The On Semi datasheet referenced gives you more information.

Since you can't measure junction temperature directly, datasheets provide the thermal resistance from ambient and case to junction:

#### dl324

Joined Mar 30, 2015
12,929
Something like 70% of Ptot for example? What would be the norm that you would go for when deciding on the capacity / type of transistor to use?
Sometimes derating factors are specified by an organization/company/department and other times it's personal preference. Some blindly derate by 50% so they don't need to sweat the details. A more competent designer would derate parameters according to manufacturer specifications and the worst case conditions the device could experience.

#### RAMBO999

Joined Feb 26, 2018
256
Power dissipation is limited by junction temperture.
View attachment 195915
View attachment 195911
View attachment 195912
View attachment 195913

At an ambient temperature above 25C, you need to derate maximum power. The datasheet you referenced neglected to state that information. The On Semi datasheet referenced gives you more information.

Since you can't measure junction temperature directly, datasheets provide the thermal resistance from ambient and case to junction:
View attachment 195914
So environmental factors also play a part in selecting an appropriate transistor. Let's say, for example, I was looking for a transistor to switch on a circuit on a TV power board or on an electric oven contol board. Use cases where the environment is likely to hot when in operation.

#### dl324

Joined Mar 30, 2015
12,929
Let's say, for example, I was looking for a transistor to switch on a circuit on a TV power board or on an electric oven contol board. Use cases where the environment is likely to hot when in operation.
When you design a TV or oven, you specify the maximum ambient temperature that you expect under worst case conditions (e.g. the nominal temperature rise due to components of the appliance and the maximum ambient temperature the appliance is specified to operate in). Knowing your worst case ambient conditions, you can select appropriate devices and operating parameters.

To increase the ambient temperature range, you can use heatsinks for more efficient heat dissipation.

#### RAMBO999

Joined Feb 26, 2018
256
When you design a TV or oven, you specify the maximum ambient temperature that you expect under worst case conditions (e.g. the nominal temperature rise due to components of the appliance and the maximum ambient temperature the appliance is specified to operate in). Knowing your worst case ambient conditions, you can select appropriate devices and operating parameters.

To increase the ambient temperature range, you can use heatsinks for more efficient heat dissipation.
Got that. Thanks.

#### RAMBO999

Joined Feb 26, 2018
256
Thanks to everyone. That's cleared a lot up.

#### ci139

Joined Jul 11, 2016
1,696
2N3904 TO-92 don't want to see much more than 50mA Ic ← the temperature starts climbing shortly above that current
but it supposedly can survive 150°C . . . the very approximate table below shows how at 50mA the 40°C ("HOT" threshold for human) gives the alert !
.
 RB Ib Ic Uc RL Us Pd T Ω A A V Ω V W °C 870k​ 5µ​ 1m​ 380m​ 4k6​ 5​ 380µ​ 25​ 593k​ 7µ3​ 1.5m​ 420m​ 3k1​ 5​ 620µ​ 25​ 409k​ 11µ​ 2.2m​ 470m​ 2k1​ 5​ 1.0m​ 25​ 275k​ 16µ​ 3.2m​ 520m​ 1k4​ 5​ 1.7m​ 25​ 187k​ 23µ​ 4.6m​ 580m​ 950​ 5​ 2.7m​ 26​ 128k​ 34µ​ 6.8m​ 640m​ 640​ 5​ 4.4m​ 26​ 87k​ 50µ​ 10m​ 710m​ 430​ 5​ 7.2m​ 26​ 59k​ 73µ​ 15m​ 790m​ 290​ 5​ 12m​ 27​ 40k​ 110µ​ 22m​ 880m​ 190​ 5​ 19m​ 29​ 28k​ 160µ​ 32m​ 970m​ 130​ 5​ 31m​ 31​ 19k​ 230µ​ 46m​ 1.1​ 84​ 5​ 50m​ 35​ 13k​ 340µ​ 68m​ 1.2​ 56​ 5​ 82m​ 41​ 8k7​ 500µ​ 100m​ 1.3​ 37​ 5​ 130m​ 52​ 5k9​ 730µ​ 150m​ 1.5​ 24​ 5​ 220m​ 69​ 4k0​ 1.1m​ 220m​ 1.6​ 16​ 5​ 360m​ 96​ 2k8​ 1.6m​ 320m​ 1.8​ 10​ 5​ 580m​ 141​ 1k9​ 2.3m​ 460m​ 2.0​ 6.4​ 5​ 940m​ 214​

Last edited: