Power dissipation under no-load conditions

AN1818

3/6

The total value of the secondary-side power dissipation is calculated using:

Equation 1

where

Equation 2

Equation 3

Primary-side power dissipation

Now to turn our attention to the primary side of a typical adapter application. The primary side of an SMPS

application consists of a number of functional blocks (such as the PFC and PWM), each contributing to

the overall power consumption of the device. But in general, the overall power dissipation owing to

primary side functions can be assumed to be about 80mW under no-load conditions (for battery chargers

and adapters in the range 5W).

The key factor concerning the overall power consumption on the primary-side is its efficiency rating,

which is at best about 50%. This means that to transfer 1mW of power to the secondary side, 2mW of

power are generated in the primary side.

Returning to our calculation in Equation 3 above, this means that if we require 200mW of power on the

secondary-side in no-load conditions, 400mW will be generated on the primary side, and an additional

80mW will be necessary to drive the primary-side functions.

This relationship means that for any given reduction in no-load power dissipation on the secondary side,

the beneficial effect will be twice as great on the primary side.

P

out

V

out

I

tot

⋅

=

I

tot

I

cc

I

ref

I

opto

++

=

P

out

V

out

I

tot

⋅

()

V

out

I

ref

I

cc

I

opto

++

()

⋅

()

20V

7mA

1.5mA

1.5mA

++

()

⋅

()

20V 10mA

⋅

() 200mW

=

=

=

=

=