An H-bridge is a four-switch circuit arranged like the letter H. The motor sits in the crossbar. Turning on different diagonal pairs of switches drives the motor forward or reverse; turning on both high-side or both low-side switches brakes it; leaving all switches off coasts it.

Why does the calculator use 2 × R_switch in the current formula?

Current always flows through two switches in series with the motor — one high-side and one low-side. Each contributes its R_DS(on) or V_CE(sat), so the total switch drop is doubled.

What is the difference between conduction loss and switching loss?

Conduction loss is I² × R_DS(on) (or I × V_CE(sat)) that heats the switch while it is on. Switching loss occurs during the transition from off to on and back, proportional to V × I × t_rise × f_PWM. This calculator estimates conduction loss, which dominates at lower PWM frequencies.

What ambient temperature should I use for heat-sink sizing?

Use the highest expected enclosure temperature, typically 40–70 °C for indoor electronics. The required thermal resistance is θ_ja = (T_j(max) − T_ambient) / P_total. Most MOSFETs specify T_j(max) = 150 °C; leave a 20–30 °C margin.

Can I use this for stepper motors?

Stepper motors use two H-bridge channels (one per winding), but the per-channel calculations are identical. Run the calculator once per winding using each winding's resistance and the chopper supply voltage.

What does the Brake state do electrically?

In active brake (both low-side switches on), the motor terminals are shorted to ground. Back-EMF drives a braking current through the low-side switches and motor winding, creating a strong regenerative braking torque. The same logic applies if both high-side switches are on (tied to V_supply).

H-Bridge Calculator

Name: H-Bridge Calculator
Availability: InStock
Author: Nham Vu

Enter your supply voltage and motor specs to get H-bridge switching states, peak current, power dissipation, and heat-sink requirements.

Circuit Parameters

Supply Voltage V_supply (V)

Motor Winding Resistance R_motor (Ω)

Switch Type

MOSFET (R_DS(on)) BJT (V_CE(sat))

R_DS(on) per Switch (mΩ)

PWM Duty Cycle (%)

0% 100% 100%

Ambient Temperature (°C)

Switch T_j(max) (°C)

Current & Power

Peak Current

—

Avg Current

—

Switch Drop (×2)

—

Motor Power

—

Loss per Switch

—

Total Switch Loss

—

Heat-Sink Requirement

Max θ_ja per Switch

—

°C/W

T_j at P_loss

—

°C (est.)

Switching Logic Table

State	Q1 (HS-L)	Q2 (LS-L)	Q3 (HS-R)	Q4 (LS-R)
Forward	ON	OFF	OFF	ON
Reverse	OFF	ON	ON	OFF
Brake (Low)	OFF	ON	OFF	ON
Coast	OFF	OFF	OFF	OFF

HS = High-side, LS = Low-side, L = Left leg, R = Right leg. Never turn on Q1+Q2 or Q3+Q4 simultaneously — shoot-through destroys the bridge.

Summary