Heat Dissipation in Circuits Calculator

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Heat Dissipation in Circuits Calculator + Formula, Examples & Applications

Every watt your actuator doesn't use turns into heat — somewhere in your circuit. If you don't know where that heat is coming from and how much there is, you'll burn out MOSFETs, melt connectors, or watch your power supply slowly cook itself. This calculator handles the 2 main heat sources in actuator control systems: AC-to-DC power supply conversion losses and MOSFET switching losses in PWM controllers. You'll get exact watt figures, BTU/hr conversions, and the heatsink thermal resistance you need. Formulas, worked examples, and practical guidance included below.

In thermal design, the catalog efficiency number isn't the answer — the wasted watts are. Every watt unaccounted for ends up as heat in a component that has to survive it.

What Is Heat Dissipation in Circuits?

Heat dissipation is the power wasted as thermal energy in your electrical components — the difference between what goes in and what actually reaches your actuator as useful work.

How does heat dissipation work in an actuator circuit?

Think of it like water flowing through a leaky hose. Your power supply and MOSFETs are the leaky sections — some energy escapes as heat instead of making it to the actuator. A power supply rated at 85% efficiency loses 15 cents of every dollar you put in. MOSFETs in a PWM circuit leak energy every single time they switch on and off — at 20,000 times per second, that adds up fast. This calculator tells you exactly how many watts you're losing so you can size your heatsinks and avoid thermal failures.

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%.

Heat Dissipation interactive visualizer

See exactly where power becomes heat in your actuator control systems. Compare PSU conversion losses vs MOSFET switching losses with real-time thermal calculations.

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 numbers takes about 30 seconds. Here's how:

  1. Select your calculation mode. Choose "AC to DC Power Supply Loss" if you want to know how much heat your power supply generates, or "MOSFET PWM Switching Loss" if you're sizing heatsinks for a PWM actuator controller.
  2. Enter your circuit parameters. For PSU mode, you need the AC input voltage, DC output voltage, DC output current, and the PSU's efficiency rating. For MOSFET mode, enter the supply voltage, load current, MOSFET RDS(on), PWM duty cycle, switching frequency, and gate charge — all from your MOSFET datasheet.
  3. Hit Calculate. The calculator returns heat dissipated in watts and BTU/hr, plus the maximum heatsink thermal resistance you need to keep junction temperature at 85°C in a 25°C ambient environment.
  4. Use the "Try Example" button to fill in typical actuator system values and see how the math works before entering your own numbers.
  5. Interpret the heatsink result. A lower °C/W number means you need a beefier heatsink. If the result is under about 3 °C/W, you're looking at a finned aluminum heatsink with forced air — not just a small clip-on.

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

You're running a 12V actuator system drawing 10A total through a switch-mode power supply rated at 85% efficiency. How much heat does the PSU generate?

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 of heat is real and constant. In an enclosed actuator control cabinet, that's enough to raise the internal temperature significantly. You need adequate ventilation or a heatsink rated at 2.83 °C/W or lower.

How does heat dissipation affect real actuator control designs?

Every Lost Watt Becomes Heat — Find It and Manage It

Here's the uncomfortable truth about actuator control systems: every watt that doesn't reach your actuator becomes heat somewhere in the circuit. That heat doesn't just disappear — it concentrates in your power supply, your MOSFETs, your wiring, and your connectors. In enclosed installations — think marine hatches, RV slide-outs, or industrial enclosures — that heat has nowhere to go. Ignoring it leads to thermal shutdown, reduced component lifespan, and in worst cases, outright failure.

Power Supply Losses Are Bigger Than You Think

An AC-to-DC power supply operating at 85% efficiency sounds pretty good on paper. But run the math on a 120W output system and you'll find 21.18W of pure heat being generated continuously inside that PSU. That's equivalent to a small soldering iron running inside your control box. At higher loads — say a 24V system pulling 15A through a cheaper 80% efficient supply — you're looking at 90W of heat. That's not a minor inconvenience, that's a fire risk if your enclosure isn't ventilated properly.

MOSFET Heat in PWM Circuits — The Dual Threat

MOSFETs in PWM actuator controllers have 2 distinct heat sources, and most designers only think about 1 of them. Conduction loss is the obvious one — current flowing through RDS(on) creates I²R heating, just like any resistor. But switching loss is the sneaky one. Every time the MOSFET transitions between on and off states, the gate charge must be supplied and dissipated. At 20,000 switching cycles per second, those tiny per-cycle losses multiply into real watts.

At 20kHz, switching losses often dominate over conduction losses for most actuator applications. This surprises most designers. In our default MOSFET example, conduction loss is only 0.625W while switching loss hits 7.2W — more than 11 times higher. That ratio shifts depending on load current and switching frequency, but the lesson is clear: don't ignore switching losses.

"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

RDS(on) is the critical MOSFET spec for actuator drivers. Lower is always better for reducing conduction loss. But here's the tradeoff that catches people — lower RDS(on) MOSFETs typically have higher gate charge. Higher gate charge means higher switching losses. So you can't just blindly pick the lowest RDS(on) part and call it done. You need to balance both numbers for your specific switching frequency and load current. This calculator lets you explore that tradeoff directly.

When Is Heatsinking Not Optional?

Heatsinking is not optional for PWM actuator controllers above about 5A. The thermal resistance calculation tells you exactly what heatsink you need. If the calculator returns a value under 5 °C/W, a small TO-220 clip-on heatsink will handle it. Under 2 °C/W, you're looking at a proper finned aluminum extrusion. Under 1 °C/W... you probably need forced-air cooling or a larger MOSFET.

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

Consider a feedback actuator being PWM throttled at 50% duty at 20kHz — a common scenario when you're using position control to slow an actuator near its target. The MOSFET may be dissipating more heat than the actuator itself is consuming as useful work. That's counterintuitive but mathematically real. The actuator sees pulsed power at half duty, but the MOSFET eats switching losses on every single transition regardless of duty cycle. This is exactly why we built this calculator — so you can see these numbers before your board starts smoking.

What does an advanced MOSFET heat dissipation example look like?

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

You're designing a controller for a 24V linear actuator drawing 8A under load. The MOSFET you've selected has RDS(on) = 35 mΩ and Qg = 60 nC. You're running PWM at 25kHz with a 75% duty cycle for speed control. How much heat does the MOSFET dissipate?

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 is a very different scenario from the simple example. Here, conduction loss dominates at 1.68W versus only 0.036W switching loss. Why? The higher load current (8A vs 5A) drives I²R heating hard, and although we increased the switching frequency, the much lower RDS(on) of 35 mΩ combined with the higher gate charge of 60 nC still produces relatively low switching losses. The heatsink requirement of ~35 °C/W is extremely relaxed — even a bare TO-220 tab in open air would manage this. The lesson: at higher currents and lower RDS(on), conduction loss can overtake switching loss. Always run the numbers for your specific combination.

What are common mistakes when using this calculator?

  1. Ignoring switching loss. Many designers enter only the conduction-loss parameters and forget that at 20 kHz, Qg × V × f can dominate the thermal budget. Always run both loss terms.
  2. Using Qgs or Qgd instead of total Qg. Datasheets list multiple gate-charge values; the switching-loss formula requires the total gate charge.
  3. Assuming nameplate PSU efficiency equals real efficiency. 85% is a starting estimate, not a measurement. A no-name supply may run at 75–80%, and efficiency drops further at light or peak load.
  4. Blindly minimizing RDS(on). Lower RDS(on) MOSFETs typically carry higher Qg. Optimize both numbers together for your specific switching frequency and load.
  5. Skipping the heatsink because the calculator returned a "small" number. Anything under 5 °C/W still needs a real heatsink, not bare PCB copper.
  6. Forgetting ambient temperature. The formula assumes 25°C ambient. Inside a sun-exposed enclosure at 40°C, available thermal headroom drops from 60°C to 45°C and the required heatsink gets bigger.

How can you verify the calculator output is reasonable?

  1. Cross-check PSU loss by hand. Output power = V × I. Input power = Output ÷ (Efficiency ÷ 100). Heat = the difference. For a 12V × 10A × 85% system, heat must be near 21 W. If the calculator shows something far off, recheck the efficiency field.
  2. Cross-check BTU. 1 W ≈ 3.412 BTU/hr. If the BTU result is not roughly 3.4 × the watt result, something is wrong.
  3. Check the conduction-vs-switching ratio against your design conditions. At low current (< 5A) and 20 kHz+ switching, expect switching loss to dominate. At high current (> 8A) and lower switching frequency, expect conduction loss to dominate. A result that breaks this pattern suggests a wrong RDS(on), wrong Qg, or wrong frequency.
  4. Verify against datasheet thermal spec. The chosen MOSFET has a maximum power dissipation in free air and with a heatsink. If your calculated heat exceeds the free-air rating, you must add a heatsink — the calculator's °C/W output tells you which one.
  5. Bench-verify with a Kill-A-Watt meter. Measure input AC watts and DC output watts directly. The ratio is the real efficiency. Plug that real number back into the calculator rather than trusting the nameplate.

Frequently Asked Questions

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

85°C is a safe maximum junction temperature for most power MOSFETs — well below the 150°C or 175°C absolute max ratings, giving you healthy margin. 25°C ambient is standard room temperature. If your enclosure runs hotter — say 40°C in a sun-exposed cabinet — you'd reduce the numerator from 60 to 45, requiring a lower thermal resistance heatsink.

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

Both values are on page 1 of any MOSFET datasheet. RDS(on) is listed in milliohms (mΩ) at a specific gate voltage — usually 10V. Gate charge Qg is listed in nanocoulombs (nC). Make sure you use the total gate charge, not Qgs or Qgd alone. If you can't find the datasheet, search the part number on Digi-Key or Mouser — they link directly to it.

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

This calculator uses the gate-charge-based switching loss model (Qg × V × f), which captures the dominant gate driver loss. It does not model the VDS × ID crossover loss during hard switching transitions — that requires rise/fall time data and is typically smaller for low-voltage actuator applications. For a more conservative estimate, multiply the switching loss result by 1.5 to 2.

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

Yes, but you need to think about which MOSFETs are actually switching. In a typical H-bridge PWM scheme, 2 MOSFETs are switching at PWM frequency while 2 are held static. Run the calculator for 1 switching MOSFET and multiply by 2 for total switching losses. Then add conduction losses for all 4 MOSFETs (the static pair run at 100% duty). It's a bit more work but the formulas are the same.

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

Switching loss scales linearly with frequency — halve the frequency and you halve the switching losses. The catch is human hearing extends to roughly 20kHz. Drop below that and your actuator or MOSFET will physically buzz at the PWM frequency. That's why 20kHz is the standard for quiet operation. If noise doesn't matter in your application — say an industrial setting — you could run at 10kHz and cut switching losses in half.

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

If you have a modern switch-mode supply (the small, lightweight kind), assume 85% as a safe starting point. Cheap no-name supplies might be 75–80%. Premium 80 PLUS certified supplies hit 88–92%. Old linear transformer-based supplies are terrible — often 50–65%. If you want to measure directly, use a Kill-A-Watt meter on the AC side and a multimeter on the DC side, then divide output power by input power.

Does duty cycle affect switching loss? +

No — and this is a key insight. The gate-charge-based switching loss (Qg × V × f) is independent of duty cycle. The MOSFET switches on and off once per PWM period regardless of whether duty is 10% or 90%. Duty cycle only affects conduction loss, because it determines how long current flows through RDS(on) per cycle. This is exactly why switching losses can dominate at low duty cycles.

Heat dissipation is one of those things that's easy to ignore during the design phase and impossible to ignore once your system is running. Take a few minutes to run your numbers through this calculator before you build. If you're designing an actuator control system and need components that already account for thermal management — our actuator controllers and power supplies are designed with these losses in mind. Get the thermal budget right upfront and everything else falls into place.

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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