Hi All

can anyone recommend a heatsink I can purchase for the following FET's or let me know if they are even needed:
FDMC86520L Power- 3W
FDMS86500L Power- 0.5W

At what kind of power should I consider using heatsinks?

Kind Regards

Art

 

Hi All

can anyone recommend a heatsink I can purchase for the following FET's or let me know if they are even needed:
FDMC86520L Power- 3W
FDMS86500L Power- 0.5W

At what kind of power should I consider using heatsinks?

Kind Regards

Art

Heatsinks are needed when the power dissipated in a device exceeds what the device itself can radiate in a given ambient temperature. Most solid-state devices have that information someplace on the data sheet. Most heat sink makers have data sheets that describe the thermal resistance of their products. So the application and selection of heat sinks is determined by the power to be dissipated and the allowable temperature rise of the device. So the very first thing to do towards answering this question is to determine how much power is being dissipated in the device. Rather than use all of that wonderful calculus I learned to determine the exact dissipation, my shortcut has been to use the peak values of device current and voltage to determine the max heating power, and then use that to size the heat sink. Next, the style of the device will determine the general types of heat sinks that can be selected. Some heat sink makers have a lot of handy information on their websites. Of course, someplace in the consideration cost of the heat sink must be considered. If you are building a one-off circuit that may not be such a big deal, but for a production run of thousands of assemblies cost does become a major consideration.

 

At what kind of power should I consider using heatsinks?

For those devices you need to consider a heatsink when the dissipation is more than a half watt.

can anyone recommend a heatsink I can purchase for the following FET's

Those are surface-mount devices designed to be soldered on a PCB trace metal pad and cannot be readily mounted on a heatsink.
They are typically used in switching applications, where their power dissipation is low.
If the dissipation is high enough that a heatsink is needed, you should buy MOSFETs in a TO-220 case.

What are these transistors being used for?

 

Also, for SMDs, the data sheet may have layout considerations for PCB heat sinking. For example, the FDMC86520L datasheet in thermal characteristics shows a 53C increase per watt dissipated when mounted on 1 insq of 2 oz copper.

Once you've determined your actual dissipation then you will have better sense of what you need to do.

 

For those devices you need to consider a heatsink when the dissipation is more than a half watt.
Those are surface-mount devices designed to be soldered on a PCB trace metal pad and cannot be readily mounted on a heatsink.
They are typically used in switching applications, where their power dissipation is low.
If the dissipation is high enough that a heatsink is needed, you should buy MOSFETs in a TO-220 case.

What are these transistors being used for?

This is for a boost converter

Kind Regards
Art

 

This is for a boost converter

Then you are likely okay without a heatsink, if you have a proper low-impedance fast driver for the MOSFET which provides at least 5V switching voltage between the MOSFET's gate and source terminals.

What is the converters output current and input to output voltage and polarity?
Post a schematic.

 

Then you are likely okay without a heatsink, if you have a proper low-impedance fast driver for the MOSFET which provides at least 5V switching voltage between the MOSFET's gate and source terminals.

What is the converters output current and input to output voltage and polarity?
Post a schematic.

My specs have slightly changed since the post but there are two converters
1st One:
Output current: 7.5A Input voltage 12V output voltage 40V
2nd One:
Output current: 1.72A, input voltage 40V output voltage 175V

 

Then you are likely okay without a heatsink, if you have a proper low-impedance fast driver for the MOSFET which provides at least 5V switching voltage between the MOSFET's gate and source terminals.

What is the converters output current and input to output voltage and polarity?
Post a schematic.

and switching frequency of 300kHz for both

 

We cannot possibly answer your questions based on the limited information you are providing..

The first step is to calculate how much power your chips are dissipating (worst case)..
Then check the thermal resistance (junction to case or junction to ambient) that is stated in the datasheet..
Then the design of the circuit board can have a dramatic effect on the temperature of the chip..

For example..
Suppose you are dissipating 1W and your chip has a thermal resistance (junction to ambient) of 53 deg C/W and you follow their recommendation for pad layout,etc...)
So that means that when dissipating 1W the junction of the chip will be 53 deg C higher than the ambient..
If your ambient is 30deg C then the junction of the chip will be at 83 deg C...

If you are dissipating 2W then its going to rise 106degC over ambient and its best to find another chip at that point...assuming surface mount and no heat sink capabilities..

You might want to read this then come back and provide more information...
http://sound.whsites.net/heatsinks.htm

 

Output current: 7.5A Input voltage 12V output voltage 40V

So here's a simple power calculation for the MOSFET:
Ignoring inefficiency, those requirements will generate an average input current of at 7.5 (40/12) = 25A average.
The switching duty cycle will be ≈(40-12)/40 = 70% giving a MOSFET average ON current of 25/0.7 = 35.7A .
The FDMC86520L has an ON resistance of 8mΩ giving a dissipation of (35.7² * .008Ω * 70% = 7.2W).
And that doesn't include any switching losses, which can be significant at 300kHz.

So, you definitely need a MOSFET that can be mounted on a heat-sink, even with a very low on-resistance MOSFET.

 

Hi All

can anyone recommend a heatsink I can purchase for the following FET's or let me know if they are even needed:
FDMC86520L Power- 3W
FDMS86500L Power- 0.5W

At what kind of power should I consider using heatsinks?

Kind Regards

Art

One simple experience when the circuit consume more current / amp that's where it started to heat up...just like mobile charging when it goes over 500 mA you can feel heat.

 

There are one very nice method to measure any heatsink ability. Just screw to it those ribbed two-screw type of aluminum 50 Ohm resistor of some 10-20-50 Watt and give a CC PS until it warms up to about +50 C or dT=30 C. Measure the i and U thus You have a P value what is optimal for this particular radiator.

 

There are one very nice method to measure any heatsink ability. Just screw to it those ribbed two-screw type of aluminum 50 Ohm resistor of some 10-20-50 Watt and give a CC PS until it warms up to about +50 C or dT=30 C. Measure the i and U thus You have a P value what is optimal for this particular radiator.

Why is dT=30K the optimal temperature rise?