Easy calculation of calorific value of switching power supply


Switching power supply efficiency

 The higher the “efficiency” of the switching power supply, the less heat it generates, and the lower the efficiency, the more heat it generates. The unit of efficiency is expressed in%. Previously, switching power supplies were about 75-80% efficient, but modern power supplies are 85% or more and less than 90%, and 95% high-efficiency power supplies are not uncommon. The linear power supply (dropper power supply) was 50% to 60%, so the efficiency is much better. The efficiency has improved due to the circuit method, but the efficiency of electronic components has improved.

Let’s take TDK-Lambda’s HWS100A-5 as an example. There is “efficiency” in the input field of the following specifications. The efficiency differs by 2 to 3% between 100VAC input and 200VAC input. A 200VAC input is more efficient. Also, the higher the output voltage (the smaller the output current), the better the efficiency. Efficiency varies by 5-6% between 3.3V and 48V outputs. In other words, a power supply with AC100V input and 3.3V output is 8% less efficient than a power supply with AC200V input and 48V output.


Heat generation amount (electric energy amount) of switching power supply

 Now, let’s calculate the calorific value (electric energy) of the HWS100A-5 when AC100V is input. Since the output is 5V20A, the output power is 100W. According to the specifications, the efficiency is 84%, so 100W ÷ 0.84 = 119W is consumed on the load side (secondary side), which means that 100W is actually used on the load. The difference 119W-100W = 19W is converted to heat.
Since the calorific value of 19W is the electric power generated per second, the value obtained by multiplying this by the time is the calorific value (electric energy) and the unit is J (joule). Or cal.

The calorific value (electric energy) of HWS100A-5 for 1 hour (3600 seconds) is 19W x 3600 seconds = 68,400J. There is also a calorie unit for calories. Since it is decided that 1J = 4.2cal to convert from J (joule) to cal (calorie), it becomes 68400J ÷ 4.2 = 16,285cal.