SLVUBB4B November 2017 – February 2023

2 FET Losses Calculator

The FET Losses Calculator lets the user either compare two different FETs or calculate losses for the main FET and a synchronous rectifier in a hard-switching power stage. #SLVUBB44807 shows the FET Losses Calculator window.

Note:

The Quasi-resonant Flyback, LLC-Half-Bridge, LLC-Full-Bridge, and Phase-Shifted Full-Bridge are resonant topologies. Manual inputs can provide more accurate results.

Figure 2-1 FET Losses Calculator Window

To attain the most accurate results, it is important to determine the gate drive voltage (V_GS) of the power management controller since the values for Q_g (which is relevant for driver losses) and R_DS(on) are dependent on this voltage and must be obtained from graphs in the data sheet of the FET.

The different losses which can be seen in the FET of a power supply are conducted losses, switching losses, C_oss losses, and body diode losses. Reverse recovery losses are neglected, but can become significant at high switching frequencies.

Conductive losses:

Equation 1.

P_{c o n d} = {I_{F E T, r m s}}^{2} \times R_{D S (o n)}

Switching losses:

t_{r i s e} = \frac{(Q_{g s} - Q_{g (t h)}) \times R_{g, t o t a l}}{V_{G S} - \frac{V_{m i l l e r}}{2} - \frac{V_{G S (t h)}}{2}} + \frac{Q_{g d} \times R_{g, t o t a l}}{V_{G S} - V_{m i l l e r}}

t_{f a l l} = \frac{Q_{g d} \times R_{g, t o t a l}}{V_{m i l l e r}} + \frac{(Q_{g s} - Q_{g (t h)}) \times R_{g, t o t a l}}{\frac{V_{m i l l e r}}{2} + \frac{V_{G S (t h)}}{2}}

Equation 2.

P_{s w i t c h i n g} = V_{D S} \times \frac{f_{s w i t c h}}{2} \times (t_{r i s e} \times I_{F E T, m i n} + t_{f a l l} \times I_{F E T, m a x})

C_oss losses:

Equation 3.

P_{C o s s} = C_{o s s} \times {V_{D S}}^{2} \times \frac{f_{s w i t c h}}{2}

Body Diode losses:

Equation 4.

P_{b o d y} = V_{S D} \times f_{s w i t c h} \times (t_{d e a d, o n} \times I_{F E T, m i n} + t_{d e a d, o f f} \times I_{F E T, m a x})

The total losses for the main FET can be calculated as indicated in #SLVUBB44493

Equation 5.

P_{t o t a l} = P_{c o n d} + P_{s w i t c h i n g} + P_{C o s s}

For synchronous rectifiers, the switching losses equal zero due to soft switching, but during the dead time the body diode is conducting. So the total losses result as indicated in #SLVUBB48072:

Equation 6.

P_{t o t a l} = P_{c o n d} + P_{b o d y} + P_{C o s s}

Additionally, driver losses occur in the power management controller, which can be calculated as shown in #SLVUBB44947:

Equation 7.

P_{d r i v e r} = Q_{g} \times V_{G S} \times f_{s w i t c h}

Power management controllers typically have a limited amount of gate drive current they can source and sink. Therefore, it is important to adjust the total resistance in the gate drive path so the resulting gate drive current is equal to or smaller than the limit in the data sheet.