DC-DC MOSFET Loss Calculator

First-order MOSFET loss model for Buck, Boost, and Inverting Buck-Boost (CCM). Choose device role to estimate per-FET losses.

Topology & Operating Point

Topology

Switching Frequency (kHz)

Buck Device

Input Voltage V_in (V)

Output Voltage |V_out| (V)

Output Current I_out (A)

Inductor Ripple (ΔI/I_avg) %

MOSFET & Switching Params

R_DS(on) (mΩ @ T_j)

t_r + t_f (ns)

C_oss (nF)

C_oss factor m (0–1)

C_oss edges / cycle

Switching edges / cycle (on+off)

Synchronous? (dead-time diode)

Dead-time (ns)

Body Diode V_f (V)

Q_g (nC)

V_GS,drv (V)

Key Derived Quantities

Duty Cycle D

—

Switch-off Voltage V_off

—

Inductor Avg I_L

—

Ripple ΔI (pp)

—

MOSFET I_rms

—

Loss Breakdown (per MOSFET)

Term	Equation	Value (W)
Total		—

Model: P_cond=I_rms²R_DS(on); P_sw≈½·V_off·I_edge·(t_r+t_f)·f_sw·Edges; P_coss=½·m·C_oss·V_off²·f_sw·(Coss edges); P_D=V_f·I_D(avg) (dead-time); P_gate=Q_g·V_GS·f_sw·Edges.

How to Use the MOSFET Loss Calculator (DC-DC)

Estimate per-device power loss for Buck, Boost, and Inverting Buck-Boost in CCM. Supports HS/LS roles, gate-drive, C_oss energy, and dead-time diode.

0) Pick the Topology & Device Role

Buck: choose High-Side or Low-Side device.
Boost / Inverting: choose Switch or Synchronous (SR).
Role sets the device duty: Buck HS = D, Buck LS = 1−D; Boost/BB SW=D, SR=1−D.

If you’re sizing both devices, run the calculator twice—once per role—and sum results.

1) Enter the Operating Point

V_in, |V_out|, I_out, f_SW (kHz)
Ripple fraction r = ΔI/I (0.2–0.4 typical CCM)
Duty is computed: Buck ≈ V_out/V_in, Boost ≈ 1−V_in/V_out, Inverting ≈ |V_out|/(|V_out|+V_in).

2) MOSFET & Switching Parameters

R_DS(on) at operating T_j, and t_r+t_f (ns)
C_oss (nF) with multiplier m (0–1) and C_oss edges/cycle (1–2)
Synchronous? If yes, set Dead-time (ns) and diode V_f
Gate-drive: Q_g (nC), V_GS,drv (V)

3) Read the Derived KPIs

D (duty), V_off (V_DS at turn-off), Ī_L, ΔI (pp)
Device RMS: I_rms ≈ √(duty)·√(Ī² + ΔI²/12)
Edge current: I_edge ≈ Ī + ΔI/2

4) Loss Terms (per MOSFET)

P_cond = I_rms²·R_DS(on)
P_sw ≈ ½·V_off·I_edge·(t_r+t_f)·f_SW·edges
P_coss = ½·m·C_oss·V_off²·f_SW·(C_oss edges)
P_D = V_f·I_{dead-time,avg} (sync only)
P_gate = Q_g·V_GS·f_SW·edges

Total device loss is the sum above. For a full phase (e.g., HS+LS) add both devices’ totals.

Quick Checklist

Topology + correct device role selected
V_out vs V_in is valid (Buck: Vout<Vin, Boost: Vout>Vin)
Ripple r within 0.2–0.4 (CCM)
R_DS(on) at operating T_j

Dead-time realistic (20–100 ns typical)
m for C_oss chosen (0–1)
Gate Q_g & V_GS from driver/databook
Compare HS vs LS / SW vs SR totals

FAQ & Tips

Why do LS (buck) losses look high at light load?
Dead-time diode conduction dominates. Reduce dead-time, raise V_GS if allowed, or add adaptive dead-time control.

How to reflect soft-switching?
Use smaller t_r+t_f and set C_oss edges=1 or m<1 to approximate energy recovery.

Gate loss seems large.
High f_SW × large Q_g adds up. Consider lower-Qg FETs or lower V_GS if R_DS(on) penalty is acceptable.

Thermal check?
θ_JA or θ_JC × (P_tot) → ΔT. Verify heatsink/PCB copper can keep T_j within limits.

Mini Workflow

1) Pick topology + device role (Buck HS/LS, Boost/BB Switch or SR)
2) Enter Vin, |Vout|, Iout, fSW, ripple r
3) Set MOSFET params: RDS(on), tr+tf, Coss (with m & edges), gate (Qg, VGS)
4) If synchronous, set dead-time and diode Vf
5) Read KPIs → review loss table; repeat for the companion device
6) Sum device losses, then verify thermal margins