What is the formula to calculate power dissipation in a switching regulator?

[zenniz]

What is the formula to calculate power dissipation in a switching regulator?

Here is the circuit design. The input voltage is 12V, output voltage is 5V @ 1A max

 

[tpetar]

First you need to check datasheet provided by manufacturer to see efficiency.

Difference between input and output power will be dissipation.

The output will be 5V x 1A = 5W. If the switcher have 85% efficiency, then the total input power will be 5W / 0.85 = 5,88W. The difference between the output power and the input power is the what we looking for, which in this example is 0,88W.

 

[Ice-Tea]

Typical efficiency is on page 5, 77% for your case. Obviously also depends on your layour and choice of coil and diode.

 

[zenniz]

So it is not as troublesome as calculating the power dissipation for linear voltage regulator...
IIN = IL + IG
PD = (VIN − VOUT) IL + (VIN) IG

 

[Ice-Tea]

If the datasheets provides an efficiency indication for your specific case, no. However, the efficiency is load dependent. Some datasheets provide graphs, others do not. IN addition, a linear device is what it is, switching regulators often require a number of external components which have their influence on efficience. In practice, linear devices are easier and more predicatable.

 

[tpetar]

So it is not as troublesome as calculating the power dissipation for linear voltage regulator...
IIN = IL + IG
PD = (VIN − VOUT) IL + (VIN) IG

Calculation for linear regulator dissipated power is :

Pd = (Vi - Vo) x I

Pd(max) = (Vi(max) - Vo) x Io(max)



Pi = Vi x Ii

Po = Vo x Io

Pd = Pi - Po



Its very important to have good heatsink for linear regulator.

Since power dissipation (heat) in linear regulators is a must, some regulators will require heatsinks to function within their parameters. To get the highes safe power dissipation your regulator can stand, you need to consult its datasheet. The specification you are looking for is the Thermal resistance for Junction-to-Ambient, for the selected package. This number tells you by how many degrees Celsius the regulator will heat up with each Watt of power dissipation. This coupled with the maximum operating temperature, and the maximum expected ambient temperature can be used to figure out how many watts you can dissipate safely.

Use this formula Pdm = (Tm-Ta)/Rj.

Where
Tm is the maximum operating temperature for the device,
Ta is the maximum expected ambient temperature,
Rj is the Junction to Ambient coefficient.

Example:

Rj = 50 (TO-220 case)
Tm = 125C (TO-220 case)
Tm = 40C maximum expected ambient temperature

Pdm = (125-40)/50 = 1.7W.

Maximum expected ambient temperature, is the maxim temperature at which the air around the device is. Keep in mind that if the device is enclosed, and even if it isn't, the air around it will heat up.

When using heatsinks, everything is the same, with the exception that you use the junction-to-Case Thermal resistance specification from the datasheet. This added to the Thermal resistance of the heatsink, gives you the total Thermal resistance you can use in the formula above instead of Rj.


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