Question

How do you calculate the input current of an SMPS?

Answer 1

To calculate the input current of a Switch Mode Power Supply (SMPS), you can follow these steps:

Step-by-Step Method:

1. Know the Output Power:
   

   First, you need to know the output power of the SMPS. This can usually be found in the specifications and is typically given in watts (W). Let’s assume the output power \( P_{\text{out}} \) is 100W.

1. Find the Efficiency of the SMPS:
   

   The efficiency (\( \eta \)) of the SMPS is a key factor, as it determines how much of the input power is converted to output power. Efficiency is typically between 80% to 95% for most SMPS units.

   Let’s assume the efficiency is 90% (or 0.90 in decimal form).

1. Calculate the Input Power:
   

   The input power \( P_{\text{in}} \) can be calculated using the formula:
   
   \[
   P_{\text{in}} = \frac{P_{\text{out}}}{\eta}
   \]
   
   For example, with an output power of 100W and an efficiency of 90%, the input power is:

   \[
   P_{\text{in}} = \frac{100W}{0.90} = 111.11W
   \]

1. Know the Input Voltage:
   

   SMPS can be designed for different input voltages. Let’s assume the input voltage \( V_{\text{in}} \) is 230V AC or 12V DC (depending on the type of SMPS).

   For example, if the input is 230V AC, the average DC voltage after rectification (if it’s an AC input) would be around 325V (considering a full-wave rectifier).

1. Calculate the Input Current:
   

   The input current \( I_{\text{in}} \) is calculated by dividing the input power by the input voltage:
   
   \[
   I_{\text{in}} = \frac{P_{\text{in}}}{V_{\text{in}}}
   \]
   
   - For a DC input (e.g., 12V DC):
     \[
     I_{\text{in}} = \frac{111.11W}{12V} = 9.26A
     \]

   - For an AC input (e.g., 230V AC):
     \[
     I_{\text{in}} = \frac{111.11W}{230V} = 0.48A
     \]
   

Summary:

1. Input Current Formula:  
   

  \[
  I_{\text{in}} = \frac{P_{\text{out}}}{\eta \times V_{\text{in}}}
  \]

Where:

1. \( P_{\text{out}} \) = Output power (in watts)
   

1. \( \eta \) = Efficiency of the SMPS (in decimal form)
   

1. \( V_{\text{in}} \) = Input voltage (in volts)
   

By following these steps, you can calculate the input current for an SMPS based on its output power, efficiency, and input voltage.