How to Measure Eon and Eoff in LTspice: Complete MOSFET Switching Loss Guide

In power electronics, MOSFET switching loss is one of the most important parameters for converter efficiency, thermal design, and device selection. Whether you are designing a buck converter, boost converter, inverter, EV charger, motor drive, or GaN-based high-frequency converter, you must know how much energy is lost during MOSFET turn-on and turn-off.

These switching energy losses are commonly called Eon and Eoff.

LTspice is a free SPICE-based circuit simulator from Analog Devices. It supports schematic capture, transient simulation, device models, and waveform analysis, making it useful for circuit-level switching-loss estimation in power electronics. Analog Devices also provides LTspice demo circuits and device macromodels for many products.

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What are Eon and Eoff?

Eon is the energy lost during MOSFET turn-on. It occurs when drain-source voltage falls and drain current rises at the same time.

Eoff is the energy lost during MOSFET turn-off. It occurs when drain current falls and drain-source voltage rises at the same time.

In a real MOSFET, voltage and current transitions are not instantaneous. During the transition interval, voltage and current overlap, and this overlap produces switching energy loss.

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Basic Switching Energy Formula

Switching energy is calculated by integrating instantaneous power during the switching interval:

E = ∫ v_DS(t) × i_D(t) dt

For turn-on:

E_on = ∫ v_DS(t) × i_D(t) dt

For turn-off:

E_off = ∫ v_DS(t) × i_D(t) dt

In LTspice, this can be done manually from waveforms or automatically using .meas commands.

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Why Measure Eon and Eoff in LTspice?

Datasheet values are useful, but they are measured under specific test conditions. Your actual design may have different:

• DC bus voltage
• Load current
• Gate resistance
• Gate driver voltage
• PCB parasitic inductance
• Dead time
• Device temperature
• Switching frequency

LTspice allows you to estimate switching energy closer to your actual circuit conditions before hardware testing.

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Recommended Test Circuit: Double Pulse Test

The most common method for measuring Eon and Eoff is the Double Pulse Test (DPT).

A typical LTspice double pulse test circuit includes:

• DC voltage source
• MOSFET under test
• Freewheeling diode or synchronous MOSFET
• Inductive load
• Gate driver pulse source
• Current measurement point
• Drain-source voltage measurement point

The inductive load is used because most practical power converters operate with inductive current.

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How Double Pulse Test Works

First Pulse

The first pulse turns ON the MOSFET and builds current in the inductor.

i_L = (V_DC / L) × t

By adjusting the first pulse width, you can set the desired test current.

Off Time

After the first pulse ends, the MOSFET turns OFF. The inductor current continues through the freewheeling path.

Second Pulse

The second pulse turns ON the MOSFET again. This switching event is usually used to measure turn-on energy, reverse recovery effect, current overshoot, and ringing.

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Signals Required for Eon and Eoff Measurement

You need two main waveforms:

• VDS: Drain-source voltage of MOSFET
• ID: Drain current through MOSFET

The instantaneous switching power is:

P_sw(t) = V_DS(t) × I_D(t)

Energy is the time integration of this power.

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Step-by-Step Method to Measure Eon and Eoff in LTspice

Step 1: Build the Switching Circuit

Create a MOSFET switching circuit or double pulse test circuit in LTspice.

Use a realistic MOSFET model whenever possible. Manufacturer models from companies such as Analog Devices, Infineon, Wolfspeed, ROHM, STMicroelectronics, Texas Instruments, and EPC are commonly used for practical simulation.

Step 2: Run Transient Simulation

Add a transient simulation command:

.tran 0 20u 0 1n

The last value sets the maximum timestep. A small timestep is important because switching events occur in nanoseconds.

Step 3: Plot Drain-Source Voltage

Click the drain node and source node to plot:

V(drain,source)

Step 4: Plot Drain Current

Click on the MOSFET current or sense resistor current.

If current direction is negative, use a negative sign:

-I(M1)

Step 5: Plot Instantaneous Power

In the waveform viewer, add this expression:

V(drain,source)*Idrain

Example:

V(d,s)*I(M1)

Depending on current direction, you may need:

-V(d,s)*I(M1)

The correct power waveform should be positive during switching loss intervals.

Step 6: Integrate Power During Turn-On

Eon is obtained by integrating power during the turn-on interval.

Step 7: Integrate Power During Turn-Off

Eoff is obtained by integrating power during the turn-off interval.

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Manual Measurement Method in LTspice

You can manually measure Eon and Eoff from the waveform viewer:

1. Run transient simulation.
2. Plot VDS, ID, and instantaneous power.
3. Zoom into the turn-on transition.
4. Identify start and end time of turn-on.
5. Use waveform integration or cursor-based estimation.
6. Repeat the process for turn-off.

This method is useful for beginners because it helps visually understand voltage-current overlap.

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Automatic Measurement Using .meas Command

For accurate and repeatable results, use LTspice .meas commands.

First define instantaneous power using a behavioral expression.

; Instantaneous MOSFET switching power
; Use negative sign if LTspice current direction gives negative power

.meas tran Eon INTEG (V(d,s)*I(M1)) FROM 5.02u TO 5.08u
.meas tran Eoff INTEG (V(d,s)*I(M1)) FROM 10.02u TO 10.08u

If your current direction is opposite:

.meas tran Eon INTEG (-V(d,s)*I(M1)) FROM 5.02u TO 5.08u
.meas tran Eoff INTEG (-V(d,s)*I(M1)) FROM 10.02u TO 10.08u

After simulation, open the SPICE error log to view measurement results.

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How to Choose Integration Limits

Choosing the correct FROM and TO time values is very important.

For Eon, select the interval where:

• Gate voltage rises above threshold
• Drain current begins to rise
• Drain-source voltage falls
• Voltage-current overlap exists

For Eoff, select the interval where:

• Gate voltage falls
• Drain-source voltage rises
• Drain current falls
• Voltage-current overlap exists

Do not integrate long steady-state conduction regions, otherwise conduction loss will be included in switching energy.

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Example .meas Setup for MOSFET Switching Energy

Assume the MOSFET is named M1, drain node is d, and source node is s.

.tran 0 20u 0 1n

.meas tran Eon  INTEG (-V(d,s)*I(M1)) FROM 5.00u TO 5.10u
.meas tran Eoff INTEG (-V(d,s)*I(M1)) FROM 8.00u TO 8.10u

.meas tran Psw PARAM (Eon+Eoff)*100k

Here:

• Eon is turn-on energy
• Eoff is turn-off energy
• 100k is switching frequency
• Psw is total switching power loss

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Switching Loss Calculation from Eon and Eoff

Once Eon and Eoff are measured, switching power loss is:

P_sw = f_s × (E_on + E_off)

Example:

• Eon = 20 µJ
• Eoff = 15 µJ
• fs = 100 kHz

Then:

Psw = 100000 × (20µJ + 15µJ) = 3.5 W

So the MOSFET switching loss is approximately 3.5 W.

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Important LTspice Tips for Accurate Eon and Eoff

• Use realistic manufacturer MOSFET models.
• Use a small maximum timestep during switching transitions.
• Include gate resistance.
• Include parasitic inductance in power loop.
• Use correct current direction.
• Do not integrate conduction-loss region.
• Check VDS and ID waveforms carefully.
• Compare simulation with datasheet conditions.

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Common Mistakes Beginners Make

• Using ideal switches instead of MOSFET models
• Forgetting source inductance and loop parasitics
• Using too large a timestep
• Measuring current in the wrong direction
• Including conduction loss in switching energy
• Using unrealistic gate drive voltage
• Ignoring reverse recovery loss
• Not checking waveform ringing and overshoot

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Why Results May Differ from Datasheet

LTspice results may differ from datasheet Eon and Eoff values because datasheets use specific test conditions:

• Specific gate resistance
• Specific DC bus voltage
• Specific load current
• Specific temperature
• Specific gate driver
• Specific layout parasitics

If your simulation setup is different, your Eon and Eoff values will also be different.

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Measuring Eon and Eoff for GaN and SiC Devices

GaN Devices

GaN devices switch very fast and require extremely small simulation timesteps. PCB parasitics and gate loop inductance strongly affect ringing and overshoot.

SiC MOSFETs

SiC MOSFETs are used in high-voltage applications. Their switching energy is sensitive to gate resistance, drain voltage, and temperature.

For both GaN and SiC simulations, always use manufacturer models and realistic parasitic values.

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Best Practice: Compare Three Methods

For serious engineering design, compare:

1. Datasheet Eon and Eoff values
2. LTspice simulation results
3. Experimental double pulse test results

LTspice is very useful for design estimation, but final validation should be done through hardware testing.

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Frequently Asked Questions (FAQs)

What is Eon in MOSFET?

Eon is the energy lost during MOSFET turn-on due to voltage-current overlap.

What is Eoff in MOSFET?

Eoff is the energy lost during MOSFET turn-off when current falls and voltage rises at the same time.

Can LTspice measure switching loss?

Yes. LTspice can estimate switching loss by integrating MOSFET instantaneous power during switching transitions.

Which command is used to measure energy in LTspice?

The .meas tran INTEG command is commonly used to integrate power over a time interval.

Why is my Eon or Eoff negative in LTspice?

This usually happens because the current direction is opposite to your assumed direction. Use a negative sign in the power expression.

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Key Takeaways

• Eon and Eoff are switching energy losses during MOSFET turn-on and turn-off.
• LTspice can measure Eon and Eoff using voltage-current power integration.
• The main expression is VDS × ID.
• The .meas tran INTEG command automates energy calculation.
• Correct integration limits are essential for accurate results.
• GaN and SiC devices require careful timestep and parasitic modeling.

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Conclusion

Measuring Eon and Eoff in LTspice is an essential skill for power electronics engineers. It helps estimate MOSFET switching losses, compare devices, optimize gate resistance, improve PCB layout, and predict converter efficiency before hardware testing.

The basic method is simple: plot MOSFET drain-source voltage, plot drain current, multiply them to obtain instantaneous power, and integrate the power waveform during the turn-on and turn-off intervals. For repeatable results, use LTspice .meas commands with carefully selected time limits.

Although LTspice simulation cannot fully replace laboratory double pulse testing, it is a powerful tool for early-stage design, device comparison, and power loss estimation in modern MOSFET, GaN, and SiC converter applications.