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Mosfet power dissipation help.

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tracidfish

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Can someone please explain to me how to choose a mosfet.

I need one to controll a DC motor using a 555 timer with > 50% duty cycle.

The motor is 8amps and 12 volts.

I am specifically lacking understanding on the Power dissipated (Pd) Paramter.

The on resistance is 0.012 ohm

Is the Pd the load current * supply voltage?
 
If u are running PWM as opposed to linear speed control the calcs are simpler.

There are a few parameters u must look at in the spec sheet.

RDSon = tells u the resistance of the device when it's fully 'on', of course RDSoff is infinite for our calcs => zero heat.

Thus u determine the voltage drop across the device using Vds=current x RDSon. U want this as small as possible.

For .012 ohm , Vds = .096V.
Power=heat= V*I = .096 * 8 = 0.768 Watt.

Thus the device generates 0.768 Watt under 100% duty.

50 % is 0.5 x .768 = 0.384 W

If you are running a relatively high frequency (>50Khz) then u will have to consider switching losses/heating. Also, that will require using a mosfet driver chip to reduce the switching time (Miller and Q effects)
I guess u know how to determine heat sinking requirements from that.? Note the Deg C/W of the Heat sink u want to use and cater for the loss of the efficiency due to the thermal paste.

Have a look here:
https://www.electro-tech-online.com/threads/heat-sink-sizing.119699/
 
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Mosaic's analysis ignores the largest contributor to FET heating which must be considered even at low kHz switching rates. It is what happens as the gate driver (555) switches high or low. The 555 can supply only a finite current to charge and discharge the FETs gate capacitance, which means that the FET spends time in a state where it is dissipating 8*12/2=48W.
 
Mosaic's analysis ignores the largest contributor to FET heating which must be considered even at low kHz switching rates. It is what happens as the gate driver (555) switches high or low. The 555 can supply only a finite current to charge and discharge the FETs gate capacitance, which means that the FET spends time in a state where it is dissipating 8*12/2=48W.

I did mention the Miller effect. However, it matters more at higher frequencies.
I also mentioned the Q...which is the charge required to switch the FET...this covers capacitance. The 555 can supply 200mA drive current., which isn't as low as a uC and not as high as a mosfet driver 2-6Amps. The time the FET will be dissapating the 48W is on the order of < 5 microseconds, if so much. Therefore at a 5Khz frequency the 48W is equivalent to 48/1000 or .048W averaged over a second. Fairly insignificant.
 
Agreed. I generally ignore switching losses for a power FET when it is driven from a digital push-pull output and freq is lowish (ie under 25kHz).

A CMOS 555 will really outperform a normal TTL 555 for driving a FET gate, as it's output will switch faster with better voltages etc to overcome the FET gate capacitance.
 
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