SLUAAX9 Application note

SLUAAX9 June 2025 BQ25756

2.1 Buck Mode Losses

For buck mode, the top FET is the synchronous FET and the bottom FET is the asynchronous FET. The power loss for the top FET can be split into the switching losses and conduction losses and is expressed in Equation 24.

Equation 1.

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

The conduction losses come from the MOSFET being statically-on.

Equation 2.

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

Equation 3.

{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 4.

P_{s w_t o p} = P_{I V_t o p} + P_{Q o s s_t o p} + P_{g a t e_t o p}

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 5.

P_{I V_t o p} = 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}

Equation 6.

I_{v a l l e y} = I_{L_D C} - 0.5 I_{r i p p l e}

Equation 7.

I_{p e a k} = I_{L_D C} + 0.5 I_{r i p p l e}

Where I_{L_DC} is the DC inductor current and I_ripple is the peak-to-peak ripple current of the inductor. The MOSFET turn-on and turn-off times, t_on and t_off , are defined with the following equations:

Equation 8.

t_{o n} = \frac{Q_{s w_t o p}}{I_{o n}}

Equation 9.

t_{o f f} = \frac{Q_{s w_t o p}}{I_{o f f}}

Here, Q_{sw_top} is the switching charge of the top side FET which can be approximated by Equation 10.

Equation 10.

Q_{s w_t o p} = Q_{G D_t o p} + Q_{G S_t o p}

And I_on and I_off are defined with the following equations:

Equation 11.

I_{o n} = \frac{V_{D R V_S U P} - V_{p l t}}{R_{o n}}

Equation 12.

I_{o f f} = \frac{V_{p l t}}{R_{o f f}}

Where V_plt is the Miller plateau voltage, V_{DRV_SUP} is the gate drive voltage on the MOSFETs, R_on is the total turn-on gate resistance, and R_off is the total turn-off gate resistance of the gate drive. The next term in the total loss calculation, P_{Qoss_top}, can be calculated with Equation 13.

Equation 13.

P_{Q o s s_t o p} = 0.5 V_{I N} \times Q_{o s s_t o t a l} \times f_{s w}

Where Q_{oss_total} is the total parasitic output charge.

Equation 14.

Q_{o s s_t o t a l} = Q_{g d_t o p} + Q_{d s_t o p} + Q_{g d_b o t} + Q_{d s_b o t}

The last term in the loss equation can be calculated with Equation 15.

Equation 15.

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

Where Q_{gate_top} is the gate charge of the top MOSFET and V_IN is the input voltage, not the gate drive voltage of the MOSFET. This is to include the losses generated by the internal LDO of the charge controller. If an external gate drive voltage is provided, the LDO losses can be avoided and Equation 16 can be used instead:

Equation 16.

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 MOSFET are comprised of conduction losses and switching losses:

Equation 17.

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}

As for the conduction loss, the loss can be defined for buck mode similar to the top FET with Equation 18.

Equation 18.

P_{c o n_b o t t o m} = (1 - D) \times {I_{L_R M S}}^{2} \times R_{D S_O N_b o t t o m}

Where D is the duty cycle and I_{L_RMS} is the RMS current through the inductor. To minimize the conduction loss, the R_{DS_ON} need to be low. The next and final term in the loss calculation, the switching loss, can be calculated with Equation 28.

Equation 19.

P_{s w_b o t t o m} = P_{R R_b o t t o m} + P_{d e a d_b o t t o m} + P_{g a t e_b o t t o m}

Where the P_{RR_bottom} term is the loss due to the reverse recovery charge of the MOSFET, P_{dead_bottom} is the loss due to the conduction loss of the body diode during dead time, and P_{gate_bottom} is the gate drive loss. Each term can be derived with the following equations:

Equation 20.

P_{R R_b o t t o m} = V_{I N} \times Q_{r r} \times f_{s w}

Equation 21.

P_{d e a d_b o t t o m} = 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 22.

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

Q_{gate_bottom} is the gate charge of the bottom MOSFET, V_SD is the forward voltage of the body diode, and V_IN is the input voltage, not the gate drive voltage of the MOSFET. This is to include the losses generated by the internal LDO of the device. If an external gate drive voltage is provided, Equation 23 can be used instead:

Equation 23.

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