Can SOT23 transistor dissipate 175mW when on minimal size pads on the PCB?

[Flyback]

https://industrial.panasonic.com/www-data/pdf/ABA0000/ABA0000CE138.pdf
the 22uF cap is EEHZC1V220R, as above datasheet.

I think I have neglected something...the concerns over inrush into the 22uF electrolytic capacitors are meaningful after all?.......ESR = tan(d)*Xc........and inrush involves a "step" input of current into the capacitor....."steps" have steep sides, which infers "High frequency", and therefore the actual ESR , seen by the "step" input of current will be very very low......in the milliOhms range, so there *will* be massive inrush into the 22uF capacitor after all...and this inrush slams through the BE junction of the PNP transistor........so do we all now agree that the circuit of the top post in this thread is actually very bad/dodgy?

 

[alec_t]

the concerns over inrush into the 22uF electrolytic capacitors are meaningful after all?

I wouldn't have thought so. Spice reckons the integral of the power dissipated in the PNP due to the cap inrush current is only ~1mJ !

 

[Flyback]

yes but is that 1mJ with 7 ohms esr, or a more realistic 50 milliohms esr?

 

[crutschow]

With 50 milliohms ESR I get a peak power of 41W dissipated in the transistor due to the startup surge but, because of its short duration, the total energy dissipated is <700μJ, which is far below any point of damage.

Edit: Even with an ESR of zero, the total energy dissipated is still <900μJ.

 

[alec_t]

I used an esr value of 2.4 Ohms for the sim. But even if you take the esr down to 1mOhm the integral is only ~1.6mJ.

 

[Flyback]

Anyway, there was no concensus that a SOT23 cannot be allowed to dissipate more than 150mW.

So here is the reasoning behind it...which I am sure you may agree with?.........

..........regarding the allowable dissipation of an sot23 device...
Comparing it with SMD resistors is helpful..
A sot23 (3mmx1.3mm) body size is slightly smaller than a 1206 resistor (3.2mmx1.6mm).
A SOT23's pad contact area is, however, a mere 1.2mmx0.4mm.
A 1206's pad contact area is 1.6mm x1mm....much more than a sot23.

...so we can say that a sot23 wouldn't be expected to have a power rating as much as a 1206 resistor (250mW)

In fact, the pad area for a 0805 resistor (1.25mm x 0.8mm) is more than for a sot23.
An 0805 resistor, does however have a smaller body area(2mm x 1.25mm) than a sot23.

So I would say that a sot23 could dissipate no more than an 0805 resistor, i.e. 125mW.

At my last company, a Consultant Engineer, who runs his own electronics consultancy, has a range of lighting electronics products on sale to the commercial lighting world, and has completed a number of electronics projects for the military, plus used to design the neon lights for picadilly circus, plus used to design audio amplifiers for commercial sale, told me never to dissipate more than 100mW in a SOT23 package. (He was referring to my use of SOT23 BJTs in a battery charge circuit., for one of his sister companies)

Is there concurrence with the 100mW maximum for a sot23?

Is there at least concurrence that a sot 23 is at most, equivalent to an 0805 resistor in terms of dissipation allowance.?
Its worth noting that resistors don't ever come in SOT23 packages....and this suggests that the SOT23 package is not as good at dissipating heat as a SMD chip resistor package.?

 

[alec_t]

Is there at least concurrence that a sot 23 is at most, equivalent to an 0805 resistor in terms of dissipation allowance.?

I don't think a comparison of a SOT-23 with a SMD resistor is conclusive. As I understand it (and I'm sure someone will correct me if I'm wrong ) a resistor is a body of material which has a reasonably uniform distribution, whereas a transistor has a less uniform distribution so may have local hot-spots (junctions, wire-bonds) at a temperature much greater than the package as a whole.

Its worth noting that resistors don't ever come in SOT23 packages

Probably because resistors don't have 3 legs!

 

[crutschow]

...........................
Probably because resistors don't have 3 legs!

Except some current shunt resistors which have 4 leads for a Kelvin connection.

 

[Flyback]

OK alec_t, but I note that your words tend to suggest that a sot23 would be a less capable dissipator than a SMD resistor of the same size?

I appreciate that sot23 has an unwanted extra leg for use as a resistor, but the point I am making is that a sot23 is a worse dissipator than a smd resistor of the same size.....do you agree with this?

 

[crutschow]

OK alec_t, but I note that your words tend to suggest that a sot23 would be a less capable dissipator than a SMD resistor of the same size?

I appreciate that sot23 has an unwanted extra leg for use as a resistor, but the point I am making is that a sot23 is a worse dissipator than a smd resistor of the same size.....do you agree with this?

A resistor can operate at a higher internal temperature than a transistor so it's somewhat comparing apples and oranges when you try to compare the maximum dissipation of a resistor in one package with a transistor in a different package. I believe you have enough information about the dissipation characteristics of a SOT-23 package to determine whether it's adequate for your purposes. It would seem to serve no good technical purpose to try to compare the two.

 

[Flyback]

Surely if we are saying that a resistor can always dissipate more than a transistor , then that's fantastic news....because it means we can *definitely" say that a transistor cannot dissipate as much as a resistor package of the same size......and that is a valuable finding.
All I have on the sot23 is a figure of 263 degC per watt, from a load of vaguely written app notes...ive no idea if they are right or not, or what they mean by "standard footprint"......does that mean absolute minimum pad sizes?....who knows?

 

[crutschow]

................................
All I have on the sot23 is a figure of 263 degC per watt, from a load of vaguely written app notes...ive no idea if they are right or not, or what they mean by "standard footprint"......does that mean absolute minimum pad sizes?....who knows?

If you referring to my reference in post #13 it doesn't seem vague to me... it knows. It states the exact board type and land pattern (pad size) used to determined the thermal resistance values.

 

[Flyback]

yes, thanks, I saw that..... by the graph it says at 70degC, it is allowing power dissipation of about 230mW in the SOT23-3.......that is very odd, since an 0805 resistor, which has twice the footprint pad contact area of a sot23-3 , is only rated to 125mW at 70degC.

My small "engineering sense" just tells me that the package with the bigger contact area should have the higher power dissipation capability.

 

[ronv]

Maybe surface area plays a part.

 

[Flyback]

https://www.fairchildsemi.com/ds/2N/2N3904.pdf
..thanks, I looked at that datasheet, and the sot23-3 is being said to have better power capability than the to92 version, which doesn't make sense, surely?
..also, it really is not clear that that sot23 is on minimum pads, or if the pads are part of copper lands the full size of the 1.6" x 1.6" pcb that it speaks of....this is typical, the datasheets just leave us up in the air.

Maybe surface area plays a part.

yes I take the point, but then why don't they make power resistors in bigger surface area parts like the sot23 body?.....all SMD resistors are flatter.

 

[alec_t]

While surface area undoubtedly plays a part (for both conductive and radiative dissipation), the thermal resistance between hot spots and the terminations, and hence the internal topology of the device, is also a factor. For a smd resistor heat has to travel to the ends of the resistor to dissipate by conduction, whereas for a SOT-23 there are two end terminations and one intermediate one.
Without knowing the internal structure of the SOT-23 and the various thermal resistance components it's guesswork as to whether that package can or can't handle more power than an equivalent-sized resistor. As crutschow says, apples and oranges.

 

[crutschow]

https://www.fairchildsemi.com/ds/2N/2N3904.pdf
..thanks, I looked at that datasheet, and the sot23-3 is being said to have better power capability than the to92 version, which doesn't make sense, surely?
....................

I don't understand your comment. That data sheet states that the power rating of the TO-92 is 625mW and the SOT-23 is
350mW...

 

[Flyback]

sorry, my bad, I thought that figure was the degc/watt