Can I Parallel 2 Mosfets in VVR?

[Ten Over]

No slight intended, I just thought it made an interesting plot.

Cool.

[nworbetan]

With the voltage split equally between two MOSFET's in series, the voltages on the X-axis will be one-half and the power at each step will be one-half. The power will still peak at the mid-point, but it will be half the single MOSFET power.

English isn't my language, please can you explain this thing with other words

Thanks

Franco

p.s.: Do you mean that also power is divided in two parts so if one Mosfet is subject to 10W dissipation having two mosfet each one is subject to 5W dissipation ? ( 5W + 5W = 10W)

Each mosfet will dissipate 5W, that much is true. However, if the two mosfets are mounted close to each other then the power total power handling of the pair of mosfets is usually going to be less than double what a single mosfet can handle (unless you use an oversized heatsink that cools the mosfets closer to ambient temp than they technically need to be cooled). If placed close to each other in open air, the convection and radiation from each mosfet will create a warmer environment that negatively affects the power handling of the other. Closely mounted on a shared heat sink, heat conduction through the heat sink will do the same thing but more efficiently.

[Kagliostro]

Something like the dissipation of the single triode within the same envelope of a dual triode tube

ECC82.png

Franco

[Kagliostro]

The added neon Spy lamp with an in series High Value Resistor and Trimmer (in kohm order) will act as a warning

2 x Mosfet VVR w Neon Spy.png

Lampada Spia Neon.png

if the lamp lights all is fine

if the lamp is dark the first mosfet is gone and must be subbed

Resistor and Trimmer values must be find by experimenting

Franco

[nworbetan]

Something like the dissipation of the single triode within the same envelope of a dual triode tube

ECC82.png

Franco

No, the inside of a vacuum tube isn't a very good comparison. (I could elaborate, but it would take a paragraph or two.)

More like 10 watt resistors mounted in air dissipating 9 watts each. If they have enough space around them they won't burn each other, but if they're too close to each other they will. If they're touching each other they'll burn even faster.

Resistors' and mosfets' ability to dissipate heat decreases as ambient temp increases. When you move two heat generating devices closer to each other it raises the ambient temp for both.

[martin manning]

Something like the dissipation of the single triode within the same envelope of a dual triode tube

ECC82.png

Franco

Yes. The mechanisms of the heat transfer are different, but it’s the same concept.

[nworbetan]

No.

If you don't want to take my word for it, listen to Texas Instruments. Here's a lesson from them in how to understand some of the numbers that show up on nearly every single heat generating solid state device data sheet. These same numbers also tend to show up on high powered resistor data sheets. Thermodynamics is pretty much universal. If you want to safely design power dissipating solid state devices into your amps, I recommend that you at least make an attempt to read and understand what the various heat related ratings on the datasheets mean. I understand that some people in various tube amp echo chambers like to turn their noses up at silicon and don't want to get their hands dirty touching the garbage, but mosfets are super useful and it's worth taking some time to get to know them better.

slva462.pdf

Power dissipation performance must be well understood prior to integrating devices on a printed-circuit
board (PCB) to ensure that any given device is operated within its defined temperature limits.

Read the pdf if you want to see the math I skipped past here.

With the help of θJA and TJMAX, which are mentioned in the TPS54325 data sheet (SLVS932), PDMAX is
calculated. For example, in the data sheet, θJA is mentioned at 44.5°C/W and TJMAX is given as 125°C.
Using this at different ambient conditions of 25°C and 85°C, one can arrive at the values mentioned in the
data sheet of 2.25 W and 0.9 W, respectively. A parameter called derating factor can be derived from this.
The derating factor is linear, so if the dissipation is 2250 mW for a 100°C rise (from 25°C to 125°C), for
each one degree increase in ambient temperature, the power dissipation rating has to be decreased
2250/100 = 22.50 mW/°C.

Or, if you don't want to do any math then the tl;dr is the ability of a device to dissipate heat decreases when its ambient temp increases.

I could try to explain why the inside of a vacuum tube is a different environment than the inside of a chassis, but that comparison is such an "apples to bananas" comparison that I feel like it would be a distraction from the main topic at hand, which is how to understand the heat generated and dissipated by mosfets (and other similar devices operated in similar conditions).

[nworbetan]

I just found a really thorough introduction to Thermal Resistance and Heat Dissipation, and I wish I had found this a few years ago.

basics_of_thermal_resistance_and_heat_dissipation_an-e.pdf

If the Texas Instruments pdf above was too specific for your tastes, this Rohm Semiconductor lesson covers a lot of the foundational principles that the specific examples in the TI writeup doesn't.

[martin manning]

I just found a really thorough introduction to Thermal Resistance and Heat Dissipation, and I wish I had found this a few years ago.

basics_of_thermal_resistance_and_heat_dissipation_an-e.pdf

If the Texas Instruments pdf above was too specific for your tastes, this Rohm Semiconductor lesson covers a lot of the foundational principles that the specific examples in the TI writeup doesn't.

The two references you linked are good for basic concepts and a practical application. Neither one addresses multiple devices in close proximity, and note that there are differential equations behind those simple methods. Heat transfer is a specialist field in engineering, and there are whole textbooks dedicated to heat transfer in electronic design.

I could try to explain why the inside of a vacuum tube is a different environment than the inside of a chassis, but that comparison is such an "apples to bananas" comparison that I feel like it would be a distraction from the main topic at hand, which is how to understand the heat generated and dissipated by mosfets (and other similar devices operated in similar conditions).

The mechanisms of the heat transfer are different, i.e. in a vacuum tube nearly all of the heat transfer to the surroundings is by radiation, but the concept is the same. Two or more objects generating heat internally will have more difficulty maintaining a given temperature by transferring heat to the surroundings if they are placed in close proximity. Isn't that what you said above?