Detailed analysis of various losses in switching power circuits

Partial loss of input

1. The internal resistance loss T of a common-mode inductor caused by surge current increases

Properly design common mode inductors, including wire diameter and number of turns

2. Discharge resistor losses

Organization of an increase in discharge resistance, taking into account compliance with safety regulations

3. Thermistor losses

Reduce thermistor resistance if other values ​​are met

Start loss

Normal starting method, starting resistance circuit does not turn off after starting switching power supply, and this loss continues to exist.

Improvement method: start with DC start mode, and cut off starting circuit after start to reduce losses.

Losses associated with operation of a switching power supply

Lock chain loss

There is a discharging resistor and mos switch tube will generate discharging losses every time you switch

Improvement method: use TVS clamp as shown in figure below, which can avoid resistive discharge loss (Note: only resistive discharge can be reduced here, and peak loss caused by leakage energy of inductance is unavoidable)

Of course, most fundamental method of improvement is to reduce leakage inductance of transformer.

Losses in supply windings

The power chip requires a certain current and voltage to operate. The higher supply voltage Vcc, greater losses will be.

Improvement Method: Because current drawn inside IC is constant, try lowering Vcc power supply voltage to make sure IC can operate within safe operating voltage range!

Transformer losses

Because effective operating frequency in standby mode is very low and current limiting point is generally small, change in magnetic flux is small and core loss is very small, which has little effect on standby, but winding loss cannot be ignored.

Losses due to transformer windings

Distributed capacitance charge and discharge losses between winding layers (Distributed capacitance is charged when MOS switch lamp is off, and losses caused by discharge when switch MOS lamp is on.)

When testing current waveform of mos tube, there is a current peak when first turned on, which is mainly caused by distributed capacitance of transformer.

Improvement method: add insulation tape between winding layers to reduce interlayer distributed capacitance.

Mosfet Switch Losses

MOS losses include: conduction loss, switching loss, control loss. Among them, biggest standby loss is switching loss.

Improvement method: Reduce switching frequency, use a frequency conversion chip, or even a frequency hopping chip (the chip enters into discontinuous oscillations with no load or very light load)

Absorptive losses in rectifier

The peak current caused by junction capacitance at output rectifier and sink capacitance of rectifier in switching state is reflected in primary circuit, causing switching losses. In addition, there are losses caused by charging and discharging resistance in snubber circuit.

Improvement method: Try to reduce capacitance value of absorbing capacitor and reduce resistance value of absorbing resistor, based on premise that other indicators allow.

Of course, there are also switching losses, conduction losses, and reverse recovery losses on rectifier tube, so you should try to choose a diode with low conduction voltage and short reverse recovery time if allowed.

Loss of output feedback loop