To convert a 150W DC motor load into the required H bridge converter continuous current rating at 12V nominal, you need a module rated for 18.75A minimum. The formula used is Ibridge = (P / V) × 1.5, where the 1.5 multiplier accounts for thermal derating and startup inrush. Substituting the values: (150W / 12V) × 1.5 = 18.75A. If you are sizing an H-bridge for a different voltage or power, the baseline nominal current is simply Watts divided by Volts, but selecting a part without the 1.5x safety margin guarantees thermal failure under continuous PWM load.
The Core Assumption and Neighboring Values
The 1.5x multiplier is the assumption that fixes this answer. In a purely theoretical world, a 150W motor at 12V draws exactly 12.5A. However, an H bridge converter operates by switching MOSFETs on and off at high frequencies (typically 10kHz to 20kHz). This introduces switching losses, and the motor's inductive kickback forces the body diodes of the MOSFETs to conduct during the dead-time. Furthermore, a 1.5x margin absorbs the initial rotor lock/inrush current without tripping the bridge's internal overcurrent protection or melting the PCB traces.
Here is how the required H-bridge current rating shifts across a ±20% power range for a standard 12V DC system:
| Motor Power (W) | Nominal Current (A) | Required Bridge Rating (1.5x) | Recommended Module Class |
|---|---|---|---|
| 120W (-20%) | 10.0A | 15.0A | Integrated IC or small discrete |
| 135W (-10%) | 11.25A | 16.9A | Mid-range discrete module |
| 150W (Baseline) | 12.5A | 18.75A | Mid-range discrete module |
| 165W (+10%) | 13.75A | 20.6A | High-current discrete module |
| 180W (+20%) | 15.0A | 22.5A | High-current discrete module |
How the Conversion Shifts for 120V, 230V, and 3-Phase
The (P/V) × 1.5 formula is strictly for low-voltage DC (typically under 48V). When you shift to higher voltages or alternating current, the topology and the math change entirely.
- 120V AC / 230V AC Mains: You cannot use a standard low-side/high-side DC H bridge converter to directly switch AC mains. Doing so will result in catastrophic short circuits due to the lack of isolation and improper gate drive referencing. For AC motor control at these voltages, you are no longer sizing a simple H-bridge; you are sizing a matrix converter or an AC-DC-AC inverter stage. The current calculation must now include Power Factor (PF): I = P / (V × PF × Efficiency). A 150W motor at 120V AC with a 0.75 PF draws 1.66A nominal, requiring a high-voltage IGBT or Triac rated for at least 3A continuous and 600V blocking voltage.
- High-Voltage DC (e.g., 400V EV platforms): The current drops (150W at 400V is only 0.375A), but the switching losses and voltage spikes skyrocket. You must switch from silicon MOSFETs to Silicon Carbide (SiC) MOSFETs to handle the dV/dt stress, and the gate driver must have a high common-mode transient immunity (CMTI) rating, typically >100kV/µs.
- 3-Phase Systems: An H-bridge consists of 4 switches (two half-bridges). A 3-phase brushless DC (BLDC) or AC induction motor requires three half-bridges (6 switches total). The per-phase current calculation remains similar, but the total thermal dissipation is distributed across three legs rather than two, altering your heatsink sizing.
When the Conversion is Meaningless
Converting nominal wattage to H-bridge amperage is completely meaningless if the stall current or the power factor is unknown.
If the datasheet does not list stall current, you must measure it: lock the rotor, apply a low test voltage (e.g., 2V), measure the current, and scale it linearly to your operating voltage. If you cannot measure it, you must abandon the 1.5x multiplier and use a 3.0x to 4.0x multiplier, or select an H-bridge with hardware current chopping (like the Texas Instruments DRV8701) that actively limits the PWM duty cycle when the shunt resistor detects an overcurrent event.
Decision Tree: Picking Your Exact H Bridge Converter Part
Use this decision path to terminate your sizing exercise with a concrete part number based on your calculated continuous current and system voltage.
| Condition (Calculated Ibridge & Voltage) | Topology Required | Concrete Part Pick (2026 Market) | Approx. Cost |
|---|---|---|---|
| < 5A, < 24V DC | Integrated IC (Brushed DC) | TI DRV8871 (H-bridge in a single package) | $2.50 |
| 5A - 30A, < 24V DC | External MOSFET Bridge Module | Infineon BTS7960-based 43A module (e.g., IBT-2) | $9.00 |
| > 30A, < 48V DC | Discrete MOSFETs + Dedicated Gate Driver | IR2110 Driver + 4x IRLB3034 MOSFETs on custom PCB | $14.00 (BOM) |
| > 30A, 3-Phase BLDC | 3-Phase Integrated Driver | TI DRV8316 (Integrated FETs, up to 8A) or DRV8353 (External FETs) | $4.50 - $7.00 |
| > 100V DC (High Voltage) | Isolated Gate Driver + SiC/IGBT | Silicon Labs Si8285 Isolated Driver + Infineon SiC MOSFETs | $25.00+ |
FAQ: H-Bridge Sizing Edge Cases
Why do cheap H-bridge modules claim 43A but burn up at 15A?
Modules based on the Infineon BTS7960 often advertise 43A because that is the absolute maximum pulsed current limit of the silicon die under ideal, infinite-heatsink conditions. On a standard red PCB with thin copper pours and no forced air, the thermal resistance limits continuous current to about 15A-20A before the solder joints reflow or the silicon overheats. Always derate cheap modules by at least 50% from their marketing claims.
Does PWM frequency affect my current rating calculation?
Yes. The 1.5x multiplier assumes a standard 16kHz PWM frequency. If you push the PWM frequency above 30kHz to move the motor whine out of human hearing range, the MOSFET switching losses increase linearly. For every doubling of PWM frequency above 20kHz, add an additional 0.2 to your derating multiplier (e.g., use 1.7x at 40kHz) or upgrade to MOSFETs with a lower gate charge (Qg) specification.
Can I parallel two H-bridges to double the current capacity?
Do not parallel the outputs of two separate H-bridge modules unless they have explicit current-sharing circuitry. Slight differences in gate drive propagation delay (even 50 nanoseconds) will cause one bridge to turn on before the other, forcing it to take the full inrush current and fail. Instead, parallel the discrete MOSFETs within a single half-bridge leg and drive them with a single, high-current gate driver.
