If you choose the wrong MOSFET for an H-bridge motor driver, you risk overheating and possibly destroying the device. Most of these failures are avoidable if you run some basic thermal and electrical calculations before committing to a design. This H-Bridge Motor Driver Calculator lets you estimate conduction loss, switching loss, and total heat generated based on motor voltage, current, the MOSFET's on-resistance, and PWM frequency. These numbers matter in automation, robotics, and vehicle systems, where the MOSFETs are often running near their limits. Below, you'll find the core equations, a worked example, some thermal management options, and answers to common problems.
What is H-Bridge MOSFET Power Dissipation?
Power dissipation in H-bridge MOSFETs comes from two main sources: loss due to the device's resistance when it's fully on (conduction loss), and the extra heat generated each time the MOSFET switches states (switching loss).
Simple Explanation
Think of a MOSFET like a water valve: when it's open, flow faces some resistance—this is conduction loss. When you open or close it, the process isn’t perfectly smooth; energy is wasted during each transition. That’s the switching loss. If you switch it more times per second (higher PWM), those small losses add up noticeably.
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H-Bridge Motor Driver Circuit
H-Bridge Motor Driver Calculator — MOSFET Selection Interactive Visualizer
Calculate conduction loss, switching loss, and total power dissipation for H-bridge motor drivers. See how motor voltage, current, MOSFET Rds(on), and PWM frequency affect heat generation and thermal management requirements.
CONDUCTION LOSS
1.92W
SWITCHING LOSS
0.24W
TOTAL DISSIPATION
2.16W
HEATSINK REQ'D
SMALL
FIRGELLI Automations — Interactive Engineering Calculators
How to Use This Calculator
- Input your motor voltage (V) and current (A).
- Enter the MOSFET's Rds(on) from its datasheet in milliohms.
- Set your PWM frequency in kHz.
- Click Calculate for results.
H-Bridge Motor Driver Calculator
This calculator is intended for education, concept evaluation, and preliminary design. Results are based on the equations and assumptions described on this page, but cannot account for every real-world load case, tolerance, material property, environmental condition, installation detail, safety factor, code, or regulatory requirement. Verify all inputs, assumptions, units, and results independently before selecting components or using the result in a real application. Safety-critical, structural, medical, lifting, transportation, or regulated applications must be reviewed by a qualified engineer.
📹 Video Walkthrough — How to Use This Calculator
Power Dissipation Equations
Conduction Loss
Use the formula below to calculate conduction power loss.
Pcond = I2 × Rds(on)
Where:
- Pcond = Conduction power loss (W)
- I = RMS motor current (A)
- Rds(on) = MOSFET on-resistance (Ω)
Switching Loss
Use the formula below to calculate switching power loss.
Psw = 0.5 × V × I × (tr + tf) × f
Where:
- Psw = Switching power loss (W)
- V = Supply voltage (V)
- I = Motor current (A)
- tr + tf = Rise and fall times (s)
- f = PWM frequency (Hz)
Total Power Dissipation
Use the formula below to calculate total MOSFET power dissipation.
Ptotal = Pcond + Psw
Simple Example
Motor voltage: 12V. Motor current: 4A. MOSFET Rds(on): 20 mΩ. PWM frequency: 10 kHz.
Conduction loss = 2 × 4² × 0.020 = 0.64 W
Switching loss = 2 × 0.5 × 12 × 4 × 50×10⁻⁹ × 10,000 = 0.024 W
Total dissipation = 0.64 + 0.024 = 0.664 W — no heatsink required.
Understanding H-Bridge Motor Driver Power Dissipation
H-bridge motor drivers are everywhere—automation, robotics, and anyplace you need to drive a DC motor in both directions. Picking the right MOSFET is mostly about balancing cost with thermal reliability so your circuit doesn’t cook itself under load. This calculator quickly gives you the main loss figures you need to compare MOSFETs and get your thermal side sorted out before you pick parts.
How H-Bridge Circuits Work
H-bridges use four switches (often MOSFETs) arranged like an “H.” By toggling which pairs of switches are on, you control both the direction the motor spins and whether the motor brakes or coasts to a stop.
Typical switching logic:
- Forward Direction: Q1 and Q4 ON, Q2 and Q3 OFF
- Reverse Direction: Q2 and Q3 ON, Q1 and Q4 OFF
- Braking: Q3 and Q4 ON (or Q1 and Q2), for dynamic braking
- Coast: All switches OFF, so the motor freewheels through diodes
Power Loss Mechanisms in MOSFETs
There are two main ways you’ll generate heat with H-bridge MOSFETs: conduction losses and switching losses.
1. Conduction Losses
When the MOSFET is on, it acts as a resistor—Rds(on). You get classic I²R loss. Because two MOSFETs are typically on at once in the current path, you double this term for total conduction loss per leg.
Watch out for:
- Motor Current: Higher current ramps losses up fast—it’s squared
- Rds(on): Use the lowest value you reasonably can
- Temperature: Rds(on) climbs as you heat up, typically 0.5–0.8% per °C
- Gate Drive Voltage: Too low and Rds(on) goes up
2. Switching Losses
When the MOSFET changes state, you briefly get high voltage and current at once—producing a spike of wasted power. It happens quickly, but if you switch often enough (high PWM), it adds up.
Key points:
- PWM Frequency: Higher frequency = more switching events = more loss
- Gate Drive Strength: Stronger/faster drivers reduce loss duration
- Load: Inductive loads are tougher to turn off and cause higher losses
- Supply Voltage: More volts, more energy dissipated every switch
Practical Design Example
Here’s a practical example for a FIRGELLI linear actuator setup:
Example: 24V Linear Actuator Drive
Given:
- Motor Voltage: 24V
- Motor Current: 8A
- MOSFET Rds(on): 15 mΩ
- PWM Frequency: 25 kHz
Calculations:
Conduction Loss = 2 × I² × Rds(on) = 2 × 8² × 0.015 = 1.92 W
Switching Loss ≈ 2 × 0.5 × 24 × 8 × 50×10⁻⁹ × 25,000 = 0.24 W
Total Power Dissipation = 1.92 + 0.24 = 2.16 W
Result: For this setup, some heatsinking is necessary to keep the MOSFET junction at a safe temperature during continuous use.
MOSFET Selection Criteria
When you plug values into this calculator, pay attention to:
Voltage Rating (VDSS)
Pick a MOSFET that can handle at least 2–3 times your supply voltage for margin against spikes. On a 24V system, 60V is a common choice.
Current Rating (ID)
The MOSFET needs to handle 1.5 times your max motor current, more if the heatsinking is poor or ambient is high. Always check the fine print on current ratings: they usually assume heavy heatsinking.
On-Resistance (Rds(on))
Lower is generally better for conduction loss, but low Rds(on) parts can carry a higher price or have larger gate charges, which push up switching losses and require stronger gate drivers.
Gate Charge (Qg)
Lower gate charge means easier, faster switching—worth considering if you run high PWM frequencies or have a low-power gate driver circuit.
Thermal Management Considerations
Once you know your total dissipation, you can estimate how much (if any) heatsinking you’ll need:
Junction Temperature
Tj = Tambient + Pdissipated × Rth(j-a)
Rth(j-a) depends a lot on package and cooling. Ballpark figures (real values from datasheets and test setups can differ):
- No heatsink (TO-220): ~50–80°C/W
- Small heatsink: ~15–25°C/W
- Large heatsink: ~5–10°C/W
- Forced air: ~2–5°C/W
Advanced Design Considerations
Dead Time and Shoot-Through Protection
Always design in some dead time—usually 100–500ns—to avoid both MOSFETs on one leg conducting at once (“shoot-through”), which can destroy them instantly.
Gate Drive Circuit Design
You need a gate driver strong enough to fully turn the MOSFET on and off quickly. For high-side MOSFETs, a bootstrap driver is often the straightforward solution.
PCB Layout and Thermal Design
Keep high-current paths short and thick. Use large copper pours and, if you need to move heat to the backside, thermal vias. Small PCB traces can cause both electrical and cooling issues at higher power.
Applications in Linear Actuator Systems
H-bridge drivers make bi-directional and position control possible for FIRGELLI linear actuators and similar setups. They’re common in:
- Industrial Automation: Positioners, robot axes, material handling
- Medical Devices: Adjustable beds and patient equipment
- Automotive: Seats, windows, tailgates
- Home Automation: Desks, lifts, smart mechanisms
Rough operating conditions, long cycles, and minimal downtime mean you need to make sure the MOSFETs will survive continuous power loss and heating. The calculator helps weed out devices that run too hot or inefficient for your job.
Related Calculations and Tools
If you’re designing a motor drive, you’ll probably need other calculations as well. Check the engineering calculators for things like:
- Motor torque and required power
- How to pick a heatsink for your loss
- PWM filter sizing for EMI
- Current sense resistor selection
Reliable design comes from covering all these angles—not just the power loss, but the system as a whole, including cooling and layout.
Frequently Asked Questions
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About the Author
Robbie Dickson
Chief Engineer & Founder, FIRGELLI Automations
Robbie Dickson brings over two decades of engineering expertise to FIRGELLI Automations. With a distinguished career at Rolls-Royce, BMW, and Ford, he has deep expertise in mechanical systems, actuator technology, and precision engineering.
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