Heat Dissipation in Circuits Calculator

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Heat Dissipation in Circuits Calculator + Formula, Examples & Applications

If your actuator isn't using the electrical input, that power goes somewhere—usually as heat in your circuit. Knowing where that heat is produced, and how much, will save your MOSFETs, connectors, and power supply from early failure. This calculator covers the two main contributors in actuator circuits: AC-to-DC power supply losses, and the MOSFET losses from switching in your PWM controllers. You'll get wattage, BTU/hr, and the heatsink size you need to avoid thermal surprises. See below for formulas, step-by-step examples, and practical context.

With thermal calculations, it's not about listed efficiency—it's about tracking every lost watt. Anything you can't account for as useful work becomes heat that a component needs to survive.

What Is Heat Dissipation in Circuits?

Heat dissipation is simply the power lost to heat in your circuit components—it's what's left after subtracting useful actuator work from total input power.

How does heat dissipation work in an actuator circuit?

Picture your supply chain like a leaky hose: not everything you pour in reaches the business end. Energy leaks out as heat at AC-DC conversion, and again in your MOSFETs as you switch them thousands of times per second. With an 85% efficient PSU, you're throwing away 15% as heat for every unit of electric power. MOSFETs also leak energy at every transition—at 20,000 Hz, those losses stack up fast. This calculator gives you the numbers so you can size heatsinks and avoid overheating components down the line.

AC to DC Power Supply Loss AC Mains PSU η = 85% Ploss (heat) DC Output Ploss = Pin − Pout Pin = Pout / η Pin = 141.2 W Pout = 120 W Heat = 21.2 W MOSFET PWM Switching Loss MOSFET RDS(on) V+ Load Gate PWM Conduction Loss Switching Loss Pcond = I² × RDS(on) × D Psw = Qg × V × f Ptotal = Pcond + Psw Heat = 0.625 + 7.2 = 7.825 W

Heat Dissipation in Circuits Calculator

120V North America, 240V Europe/UK.
Typically 12V or 24V for actuator systems.
Total current drawn by all actuators.
Typical switch-mode PSU: 80–92%.
Engineering calculation notice

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.

Found a calculation error? Message us

Heat Dissipation interactive visualizer

See live calculations showing exactly how much heat comes from your power supply versus MOSFET switching in actuator controls. This lets you spot which end of your circuit is your thermal bottleneck.

Calculation Mode
AC Input Voltage 120 V
DC Output Voltage 24 V
Output Current 5.0 A
PSU Efficiency 85 %

HEAT DISSIPATED

17.6 W

BTU/HR

60.1

HEATSINK R_TH

3.4 °C/W

FIRGELLI Automations — Interactive Engineering Calculators

🎥 Video — Heat Dissipation in Circuits Calculator

Heat Dissipation in Circuits Calculator

How do you use this heat dissipation calculator?

Getting your heat dissipation figures is a quick task if you know your circuit variables:

  1. Select the calculation mode. Pick "AC to DC Power Supply Loss" to size for PSU heating. Use "MOSFET PWM Switching Loss" if you're planning heatsinks for your PWM actuator MOSFETs.
  2. Input the circuit details. PSU mode needs AC input voltage, DC output voltage, DC output current, and the supply's efficiency. For MOSFETs, enter supply voltage, load current, RDS(on), duty cycle, PWM frequency, and total gate charge (grab values from the datasheet).
  3. Click Calculate. You'll get total heat output in watts and BTU/hr, plus the maximum °C/W value for a heatsink that will limit the MOSFET temperature to 85°C in 25°C air.
  4. Try Example gives you typical actuator numbers, so you can see the method in action before plugging in your own circuit.
  5. Read the heatsink output carefully. If the value is under ~3 °C/W, you need more than a small stick-on heatsink — look for a larger extruded type, or even forced air in tight spaces.

What is the heat dissipation formula for actuator circuits?

AC to DC Power Supply Loss Formulas

Output Power (W) = DC Voltage × DC Current

Input Power (W) = Output Power ÷ (Efficiency / 100)

Heat Dissipated (W) = Input Power − Output Power

MOSFET PWM Switching Loss Formulas

Conduction Loss (W) = I² × RDS(on) × Duty Cycle

Switching Loss (W) = Qg × Vsupply × fsw ÷ 109

Total Heat (W) = Conduction Loss + Switching Loss

Heatsink Thermal Resistance

Rheatsink (°C/W) = (85 − 25) ÷ Total Heat

Targets 85°C maximum junction temperature at 25°C ambient.

Variable Reference

Symbol Variable Unit
VAC AC Input Voltage V
VDC DC Output Voltage V
IDC DC Output Current A
η Power Supply Efficiency %
VS Supply Voltage (MOSFET mode) V
IL Actuator Load Current A
RDS(on) MOSFET On-State Resistance mΩ
D PWM Duty Cycle %
fsw Switching Frequency Hz
Qg Gate Charge nC
Rheatsink Heatsink Thermal Resistance °C/W

What does a simple heat dissipation example look like?

PSU Mode — 12V 10A Power Supply at 85% Efficiency

Suppose your actuator system is 12V and draws 10A total, fed by a switchmode PSU at 85% efficiency. How much heat is produced inside the power supply?

Step 1 — Output Power:
Pout = 12V × 10A = 120 W

Step 2 — Input Power:
Pin = 120W ÷ (85 / 100) = 120 ÷ 0.85 = 141.18 W

Step 3 — Heat Dissipated:
Ploss = 141.18 − 120 = 21.18 W

Step 4 — Convert to BTU/hr:
21.18 × 3.412 = 72.27 BTU/hr

Step 5 — Heatsink Thermal Resistance:
Rheatsink = (85 − 25) ÷ 21.18 = 2.83 °C/W

That 21.18W is always there—it's not a spike, it's continuous. In a closed control box, this heats the air quickly, so you either need airflow or a heatsink that's at least 2.83°C/W or less.

How does heat dissipation affect real actuator control designs?

Every Lost Watt Becomes Heat — Find It and Manage It

Every bit of power that doesn't move your actuator ends up as heat, somewhere: power supplies, MOSFETs, terminals, wiring. In tight enclosures like boats, RVs, or sealed boxes, that heat builds up—often faster than you expect. Skip these calculations, and you risk overheating, reduced part life, or outright failure.

Power Supply Losses Are Bigger Than You Think

On paper, 85% PSU efficiency sounds fine. In reality, a 120W output still throws off over 21W as heat, constantly. That's as much as running a small soldering iron inside your control cabinet. Pump up the current—say 24V at 15A on an 80% PSU—and you'll have 90W of waste heat. That’s plenty to raise ambient temps dangerously unless you provide venting or a suitable heatsink.

MOSFET Heat in PWM Circuits — The Dual Threat

MOSFETs waste heat in two main ways. Conduction loss is straightforward: current through their resistance heats them like a resistor. Switching loss is less obvious but can quickly add up: each on/off transition means a small gate charge is lost. When you switch at 20,000 cycles a second, that small loss stacks up.

In most actuator circuits at 20kHz, switching loss beats conduction loss unless your current is high. Don’t assume most heat is from conduction alone. Check both values in your design.

"Most designers size the MOSFET for conduction loss and forget that switching loss is happening 20,000 times a second regardless of duty cycle. At PWM frequencies above 20 kHz, the gate-charge loss often dominates — and that's the loss that sneaks up on you when the board starts smoking." — Robbie Dickson, Founder and Chief Engineer of FIRGELLI Automations

The RDS(on) vs. Gate Charge Tradeoff

Picking a MOSFET with a lower RDS(on) cuts conduction loss, but there's a catch: lower RDS(on) nearly always means higher gate charge, which increases switching losses at high frequencies. Don't just chase the lowest resistance—always check both specs and use your real switching frequency and load to run the numbers.

When Is Heatsinking Not Optional?

For actuator drivers with PWM over about 5A, you can't skip heatsinking. The °C/W value from the calculation tells you what size to use. Over 5 °C/W, a clip-on might work. 2 °C/W or less, you'll need a proper large extrusion. Below 1 °C/W, forced air or a physically larger MOSFET is wise.

Calculated Rth (°C/W) Heatsink Type Typical Application
> 10 Bare device or PCB copper pour Low-power MOSFET driver, < 2A actuator
5 – 10 Small TO-220 clip-on heatsink Standard PWM controller, 2–5A
2 – 5 Finned aluminum heatsink, natural convection Mid-power actuator driver, 5–10A
1 – 2 Larger finned extrusion, may need forced air High-current PWM, 10–15A
< 1 Forced-air cooling required, or upsize the MOSFET Heavy-duty industrial driver, > 15A

Real-World Scenario: Feedback Actuator at Half Speed

Take a feedback actuator being PWM limited to 50% at 20kHz. When slowing down near the end of travel, it's common for the MOSFET to dissipate more power as heat than what the actuator delivers as work. The duty cycle doesn't change the switching losses—they only depend on frequency, gate charge, and supply voltage. This is exactly the scenario where engineers get caught out by surprise heating.

What does an advanced MOSFET heat dissipation example look like?

MOSFET Mode — 24V 8A Actuator at 75% Duty, 25kHz

Suppose you have a 24V actuator load drawing 8A. The MOSFET is 35mΩ RDS(on) and 60nC Qg. PWM is 25kHz at 75% duty. What's the MOSFET's heat output?

Step 1 — Conduction Loss:
Pcond = 8² × (35 / 1000) × (75 / 100)
Pcond = 64 × 0.035 × 0.75 = 1.68 W

Step 2 — Switching Loss:
Psw = 60 × 24 × 25000 / 109
Psw = 36,000,000 / 109 = 0.036 W

Step 3 — Total Heat:
Ptotal = 1.68 + 0.036 = 1.716 W

Step 4 — BTU/hr:
1.716 × 3.412 = 5.86 BTU/hr

Step 5 — Heatsink Thermal Resistance:
Rheatsink = (85 − 25) / 1.716 = 34.97 °C/W

Design Interpretation: This case is quite different from the lower-current case. The current is higher, so conduction loss now dominates while switching loss is much lower—a result of the specific choice of low RDS(on) but higher Qg. The calculated heatsink requirement is minimal: a bare TO-220 tab is enough. Numbers switch quickly based on PWM frequency, load, and device parameters—always put your actual specs through the formula.

What are common mistakes when using this calculator?

  1. Overlooking switching loss. It's easy to focus only on conduction, but at high PWM frequency, gate charge loss can be the bigger number. Always calculate both.
  2. Picking the wrong gate charge number from the datasheet. Use total gate charge (Qg), not Qgs or Qgd.
  3. Assuming label PSU efficiency is always your real value. Treat 85% as a guess unless measured at your real load—discount brands often do worse.
  4. Chasing the lowest RDS(on) blindly. Lower resistance brings higher gate charge, which can drive up heat at high speeds. Balance the tradeoff.
  5. Skipping heatsinking on a "small" result. Even values under 5 °C/W require a real heatsink, not just extra PCB copper.
  6. Forgetting that ambient matters. The formula assumes 25°C outside air. Warmer environments give you less temperature headroom and require a lower °C/W (bigger) heatsink.

How can you verify the calculator output is reasonable?

  1. Check PSU loss by hand. Multiply V × I for output, divide by efficiency (as a fraction) for input, subtract to get the heat. For 12V × 10A × 85%, you should see about 21 W. If not, review your efficiency field for errors.
  2. Spot-check BTU. 1 W = 3.412 BTU/hr. If your BTU number doesn't track about 3.4 times the watt result, something went wrong.
  3. Predict conduction vs switching loss by your setup. At low currents and high frequencies, expect switching loss to dominate. At high currents or low frequencies, conduction tops the chart. If yours looks off, check your frequency, RDS(on), and gate charge entries.
  4. Compare to datasheet thermal ratings. See if your calculated loss exceeds the MOSFET's listed power rating in free air. If so, a heatsink is required—use the calculated °C/W value for selection.
  5. Confirm with wattmeter and basic measurements. For PSUs, check input and output power directly with real hardware. Use true efficiency in your calculation rather than manufacturer claims.

Frequently Asked Questions

Why does the calculator assume 85°C junction and 25°C ambient for the heatsink calculation? +

85°C is a comfortable ceiling for most MOSFET junctions—well below their absolute maximum (typically 150°C+), leaving margin for aging and brief thermal spikes. 25°C matches normal room temperature. Warmer enclosure? Subtract the actual ambient from 85°C for your new numerator.

Where do I find RDS(on) and gate charge for my MOSFET? +

Check the first page of your MOSFET datasheet. RDS(on) (in milliohms) is shown at a test gate-drive voltage (usually 10V). Qg is total gate charge (nanocoulombs). Use total, not just Qgs or Qgd. If you can't find a datasheet, sites like Digi-Key show them up top in product listings.

Does the MOSFET switching loss formula account for drain-source switching transients? +

This method covers the bulk of switching loss coming from the gate driver (Qg × V × f). It doesn't include losses from voltage/current overlap during switching—a bigger deal at higher voltages or fast transitions. For conservative estimates in higher-voltage circuits, multiply the switching-loss result by 1.5–2.

Can I use this for H-bridge circuits with 4 MOSFETs? +

Yes, just break it down: usually 2 are switching at PWM rate, and 2 are on/off for full cycles. Run loss numbers for one switching device and double it for both. Add conduction loss for all four as needed.

Why does lowering my PWM frequency reduce switching losses but might cause audible noise? +

Switching loss is directly proportional to frequency; halve the PWM rate, you halve these losses. But drop below ~20kHz, and you may hear your PWM as a whine or buzz, since this falls in the top of human hearing. That's why 20kHz is common if you want quiet actuators—if noise isn't an issue you can go lower and save heat.

What if my power supply efficiency isn't listed — how do I estimate it? +

Modern switchmode supplies (lightweight and small) typically manage 85% or above. Cheap units can be lower, 75–80%. Top-tier PSUs can reach up to 92%. Linear supplies are poor—50–65% is normal. If you want to know for sure, measure with a wattmeter (AC side) and a multimeter (DC side), then divide output by input.

Does duty cycle affect switching loss? +

No—switching loss from Qg × V × f happens on every transition, so it's independent of duty. Duty just affects how long the MOSFET passes current in each cycle, changing conduction loss only. That's why even at low duty, switching loss can stay high.

Thermal design problems show up fast in real systems, even if they're easy to overlook when drawing a schematic or punching part numbers. Get your heat load numbers first—it's a lot easier than chasing failures or hunting for last-minute heatsinks after testing. Want actuator controllers and PSUs designed with heat in mind? See our selection — designed factoring in these exact losses, not just on-paper ratings.

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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