SLUAAX9 Application note

SLUAAX9 June 2025 BQ25756

2.2 Boost Mode Losses

For boost mode, the top FET is asynchronous and the bottom FET is synchronous. The total power losses for the top and bottom FET can be determined in the same way as in buck mode with the following equations:

Equation 24.

P_{t o p} = P_{c o n_t o p} + P_{s w_t o p}

Equation 25.

P_{b o t t o m} = P_{c o n_b o t t o m} + P_{s w_b o t t o m}

The conduction losses for the top FET are similar the buck mode losses:

Equation 26.

P_{c o n_t o p} = (1 - D) \times {I_{L_R M S}}^{2} \times R_{D S (o n)_t o p}

Equation 27.

{I_{L_R M S}}^{2} = {I_{L_D C}}^{2} + \frac{{I_{r i p p l e}}^{2}}{12}

In boost mode, the top FET has losses from the reverse recovery charge, dead time, and gate capacitance.

Equation 28.

P_{s w_t o p} = P_{R R_t o p} + P_{d e a d_t o p} + P_{g a t e_t o p}

The gate losses, dead time losses, and reverse recovery losses are:

Equation 29.

P_{R R_t o p} = V_{I N} \times Q_{r r} \times f_{s w}

Equation 30.

P_{d e a d_t o p} = V_{S D} \times I_{v a l l e y} \times f_{s w} \times t_{d e a d_r i s e} + V_{S D} \times I_{p e a k} \times f_{s w} \times t_{d e a d_f a l l}

Equation 31.

P_{g a t e_t o p} = V_{I N} \times Q_{g a t e_t o p} \times f_{s w}

As before, if an external gate drive voltage is provided, the following equation can be used instead:

Equation 32.

P_{g a t e_t o p} = V_{D R V_S U P} \times Q_{g a t e_t o p} \times f_{s w}

The losses for the bottom FET can be calculated is much the same way as the synchronous buck FET.

Equation 33.

P_{c o n_b o t t o m} = D \times {I_{L_R M S}}^{2} \times R_{D S (o n)_b o t t o m}

Equation 34.

{I_{L_R M S}}^{2} = {I_{L_D C}}^{2} + \frac{{I_{r i p p l e}}^{2}}{12}

The switching losses come from the current and voltage overlap during the FET turn-on and turn-off; the parasitic gate capacitance; and gate drive losses into the FET.

Equation 35.

P_{s w_b o t t o m} = P_{I V_b o t t o m} + P_{Q o s s_b o t t o m} + P_{g a t e_b o t t o m}

Where P_{IV_top} is the loss in the top MOSFET loss due to the current and voltage overlap, P_{Qoss_top} is the loss due to the MOSFET parasitic output capacitance, and P_{gate_top} is the gate drive loss. P_{IV_top} is given by the following equations:

Equation 36.

P_{I V_b o t t o m} = 0.5 V_{I N} \times I_{v a l l e y} \times t_{o n} \times f_{s w} + 0.5 V_{I N} \times I_{p e a k} \times t_{o f f} \times f_{s w}

The calculations for I_Valley, I_Peak, t_on, and t_off are the same as in buck mode.