1 Reply Latest reply on Jun 2, 2011 10:37 AM by Ron Reiners

# Setting limits on heat generation formula

I am using Flow to produce thermal models of pc boards and enclosures. My goal is to determine how much power (watts) I need to apply to a power transistor that will raise the temperature of the surrounding components to a certain temperature.

I have attached a simple test model and some screen shots of GOALS and HEAT SOURCE definitions. In this model I am trying to bring the temperature of the thermistor component up to 50 degrees from a 25 degree ambient temperature. When you start the simulation the power builds up until the thermistor temperature is reached then it starts reducing power as the temperature overshoots. As the temperature falls, power is added back as it undershoots. Eventually the model rings out and stabilizes at 50 degrees on the thermistor with the power transistor slightly hotter. This model works as it should in real life basically because it is so simple. When I tried it on an actual product I had huge swings because there is so much delay in heating up other mass objects. I couldn't use it because I was getting +400 watts and -250 watt swings.

The Equation goal POWER ADJUSTMENT is used continuously adjust the power level based on the temperature of the thermistor.

The Equation goal CURRENT WATTAGE is used extract the current wattage and pass it back to the previous goal.

I then define the heat source as a dependency formula based on the POWER ADJUSTMENT goal.

My question. Is there a way to limit the amount of power calculated in the POWER ADJUSTMENT goal? I want to use the POWER ADJUSTMENT goal unless it is above 4 watts or below 0 watts. I was thinking something like an "IF" statement

Thanks

Ron

• ###### Re: Setting limits on heat generation formula

I was able to work this out. The solution is to set the heat source to a dependency F(goal) - table using equation goal formula as the goal. The goal table is set up using 4 goals, 2 are the upper and lower limits and two are large positive and negative numbers. The large positive and negative number are set to be equal to the upper and lower limit numbers. This will effectively limit the result from the formula equation to the upper and lower limits. See attached file.