What are the losses inside the switching power supply?
The main loss inside the switching power supply To improve the efficiency of the switching power supply, it is necessary to distinguish and roughly estimate various losses. The internal loss of switching power supply can be roughly divided into four aspects: switching loss, conduction loss, additional loss and resistance loss. These losses often occur together in lossy components and are discussed separately below.
Losses associated with power switching
The power switch is one of the two main sources of loss inside a typical switching power supply. Losses can basically be divided into two parts: conduction losses and switching losses. Conduction loss is the loss when the power switch is in the conduction state after the power device has been turned on and the driving and switching waveforms have been stabilized; switching loss occurs when the power switch is driven and enters a new working state, driving and switching Loss when the waveform is in transition. These phases and their waveforms are shown in Figure 1.
The conduction loss can be measured by the product of the voltage across the switch and the current waveform. These waveforms are approximately linear, and the power loss during conduction is given by Equation (1).
A typical approach to controlling this loss is to minimize the voltage drop during the conduction period of the power switch. To achieve this goal, the designer must make the switch work in saturation. These conditions are given by equation (2a) and equation (2b), through the base or gate overcurrent drive, ensure that the collector or drain current is controlled by external components rather than the power switch itself.
Switching losses during power switching transitions are more complex, with both their own factors and the effects of related components. Loss-related waveforms can only be observed with an oscilloscope connected to the drain-source (collector-emitter) end of the voltage probe, and the AC current probe can measure the drain or collector current. When measuring the loss at each switching instant, a shielded short-lead probe must be used, because any length of unshielded wire may introduce noise from other power sources, and thus cannot accurately display the real waveform. Once a good waveform is obtained, the area enclosed by these two curves can be roughly calculated by the simple method of summing triangles and rectangles. Turn-on loss can be calculated by formula (3).
This result is only the loss value during the turn-on period of the power switch, plus the turn-off and conduction losses to get the total loss value during the switching period.
Losses associated with the output rectifier
In the total loss inside a typical non-synchronous rectifier switching power supply, the loss of the output rectifier accounts for 40%-65% of the total loss. So understanding this section is very important. From Figure 2 you can see the waveforms associated with the output rectifier.
