H-Bridge MOSFET Selection
Calculate H-bridge MOSFET requirements including peak current, conduction losses, and minimum current rating for DC motor drivers.
Formula
How It Works
This calculator determines MOSFET ratings and gate driver requirements for H-bridge motor control circuits. Power electronics engineers, robotics designers, and EV builders use it to select components that handle motor current with adequate voltage and thermal margins. Proper H-bridge design prevents catastrophic shoot-through failures and ensures reliable bidirectional motor control.
Per power electronics fundamentals (Mohan, 'Power Electronics', 3rd ed.), an H-bridge uses four switches to enable forward, reverse, and braking by controlling current direction through the motor. Key selection parameters per manufacturer guidelines: V_DS rating ≥ 2× supply voltage (accounts for inductive voltage spikes), I_D continuous ≥ 1.5× motor rated current, and R_DS(on) low enough to limit conduction losses to acceptable thermal budget.
MOSFET voltage derating is critical: per JEDEC reliability guidelines, voltage stress should be <80% of V_DS rating for long-term reliability. A 24V motor system with inductive spikes reaching 1.5× supply requires MOSFETs rated ≥ 60V (24×1.5÷0.80 = 45V minimum, use next standard rating). Conduction losses scale with R_DS(on): modern power MOSFETs achieve 1-10 mΩ at 30-60V ratings, enabling 10A continuous with only 0.1-1W loss per FET. Total H-bridge losses at 10A: 0.4-4W for quality MOSFETs versus 30-40W for integrated drivers with 300-500 mΩ internal switches.
Worked Example
Design an H-bridge for a 36V e-bike throttle controller. Motor specifications: 500W rated, 15A continuous, 45A peak inrush for 0.5 seconds.
Step 1 — Determine MOSFET voltage rating: Inductive spike estimate: 1.5× supply = 54V With 80% derating: V_DS_min = 54 / 0.80 = 67.5V Select 80V or 100V rated MOSFETs (standard values)
Step 2 — Determine current rating: Continuous: I_D ≥ 1.5 × 15A = 22.5A minimum Peak (pulsed): I_D_peak ≥ 45A for 0.5s Select MOSFETs rated 40-60A continuous / 120A+ pulsed
Step 3 — Select specific MOSFET and calculate losses: IRFB4110 (100V, 120A, R_DS(on) = 3.7 mΩ at 25°C, 5.5 mΩ at 100°C) Conduction loss at 15A per FET (2 conducting): P_cond = 15² × 0.0055 × 2 = 2.48W total
Step 4 — Calculate thermal requirements: IRFB4110 in TO-220: R_θJC = 0.65°C/W With heatsink R_θCS = 0.5°C/W, R_θSA = 2°C/W: R_θJA_total = 0.65 + 0.5 + 2 = 3.15°C/W per FET ΔT per FET: 1.24W × 3.15 = 3.9°C rise—excellent
Step 5 — Select gate driver: Gate charge Q_g = 150 nC, at 20 kHz: I_gate_avg = Q_g × f = 3 mA Peak gate current for 50ns switching: I_peak = Q_g / t = 3A Select IR2104 or similar half-bridge driver with 0.5-1A peak drive capability
Result: IRFB4110 (100V/120A) MOSFETs with IR2104 gate drivers. Total conduction loss 2.5W enables operation without heatsinking at 15A continuous. Add 100 ns dead-time and 47Ω gate resistors to prevent shoot-through.
Practical Tips
- ✓Per Texas Instruments application notes, use integrated H-bridge driver ICs (DRV8876, DRV8874) for motors <5A where convenience outweighs efficiency; discrete MOSFETs with <10 mΩ R_DS(on) for >5A where 5-10% efficiency gain matters
- ✓Place 100 nF ceramic capacitors within 10mm of each MOSFET drain-source per EMC guidelines to suppress 10-100 MHz switching transients; add 100-470 µF electrolytic across DC bus for inrush energy
- ✓Implement dead-time of 50-200 ns between high-side off and low-side on per MOSFET turn-off specifications—IR2104 and similar drivers include automatic dead-time insertion
Common Mistakes
- ✗Selecting MOSFETs at exactly supply voltage: Per JEDEC, inductive kickback reaches 1.5-2× supply voltage; a 24V system needs 60V MOSFETs minimum—48V MOSFETs will fail from voltage stress within weeks to months
- ✗Omitting freewheeling diodes on discrete builds: MOSFET body diodes conduct during dead-time but have slow reverse recovery (50-200 ns); add external Schottky diodes for currents >10A to reduce switching losses by 20-40%
- ✗Using single gate resistor for all four MOSFETs: Each gate needs individual resistor (10-47Ω typical) to prevent parasitic oscillation and allow independent tuning per Infineon gate driver guidelines
Frequently Asked Questions
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