Technology / Technical guide
Semiconductor Conduction & Switching Loss Mechanisms
A power semiconductor can dissipate energy while conducting and while changing state. Those mechanisms respond differently to current, voltage, temperature and event frequency. A low on-state parameter alone cannot determine the complete converter loss.
Separate time in a state from energy during a transition.
Loss is a waveform-dependent quantity; a device label or one typical parameter is not a complete operating-point model.
01 / Conduction
Read the voltage drop while current flows.
Instantaneous device power is voltage across the device multiplied by current through it under the stated sign convention.
Instantaneous device power is voltage across the device multiplied by current through it under the stated sign convention. A conducting switch has a nonzero drop, so energy is dissipated even between switching events.
For a simplified fully enhanced MOSFET with approximately constant on-resistance, conduction loss can be represented by Irms² Rds(on) over the defined period. That resistance depends on applicable device conditions; it is not a universal material constant.
02 / Different on-states
Avoid one resistance model for every device.
An IGBT’s on-state is commonly represented with a voltage offset and slope over an applicable range, rather than only one MOSFET-like on-resistance.
An IGBT’s on-state is commonly represented with a voltage offset and slope over an applicable range, rather than only one MOSFET-like on-resistance. Diodes and other participating paths add their own drops.
Infineon’s loss note separates these mechanisms using specified device models. The physical comparison concerns waveform and on-state behaviour, not proof that one device type always produces lower losses.
03 / Transition energy
Integrate voltage-current overlap in time.
During a real transition, device voltage and current need not change instantaneously.
During a real transition, device voltage and current need not change instantaneously. Their overlap contributes energy E = ∫v(t)i(t)dt over a defined event. Charging effects, recovery and the surrounding circuit can also influence the event.
A shorter visible transition is not by itself a complete energy measurement. The voltage-current waveform, interval definition and included contributions determine what a reported turn-on or turn-off energy means.
04 / Average power
Count the applicable events per unit time.
If the same turn-on and turn-off energies recur once per cycle under unchanged conditions, a simple estimate is Psw ≈ fsw(Eon + Eoff).
If the same turn-on and turn-off energies recur once per cycle under unchanged conditions, a simple estimate is Psw ≈ fsw(Eon + Eoff). Infineon describes this energy-frequency relationship for its example devices.
In a hypothetical arithmetic case, a combined 1 mJ per cycle repeated at 10,000 cycles/s gives 10 W. This is not a MARSTON loss or a frequency recommendation; changing the waveform conditions can change the energy per cycle.
05 / Thermal interaction
Recognise that losses and temperature interact.
Losses produce heat, while temperature can change conduction and switching behaviour.
Losses produce heat, while temperature can change conduction and switching behaviour. A value quoted at one condition should not be silently reused at another. The waveform’s current duty also matters alongside its peak.
Power loss, junction temperature and cabinet ambient temperature describe different quantities. The SOA guide explains why duration and thermal conditions matter beyond a single power value.
06 / Scope boundary
Keep the device model separate from whole-converter efficiency.
A converter also contains magnetic, capacitive, interconnection and auxiliary losses.
A converter also contains magnetic, capacitive, interconnection and auxiliary losses. Summing selected semiconductor terms alone does not necessarily provide total input-to-output efficiency. The included boundary must stay visible.
No MARSTON semiconductor type, measured loss, switching energy or thermal parameter is assigned here. The page explains mechanisms without device substitution, gate-drive changes or operational adjustment.
Continue the review with MOSFET, IGBT & SiC: Device Types and Material Platforms.
Continue the review with Power Device Safe Operating Area: Conditions and Duration.
Continue the review with PWM Switching Frequency, Carrier & Fundamental.
Continue the switching-device review with high-side gate drive and bootstrap supplies and Miller capacitance and gate charge. For additional commutation topics, review parasitic inductance and switching overshoot and diode reverse recovery.
General background: Infineon: conduction models and transition-energy loss relationships. These references explain industry concepts; they are separate from the applicable MARSTON product and project documents.
Key concepts and conditions.
Read each quantity with its reference, units and stated conditions. A general relation does not establish a model-specific value.
| Concept / quantity | Meaning / relation | Conditions / limits |
|---|---|---|
| Instantaneous power | v(t)i(t) in W | Stated device voltage and current signs. |
| MOSFET on-state | Irms² Rds(on) | Simplified fully enhanced model and conditions. |
| Transition energy | ∫v(t)i(t)dt in J | Defined event and included contributions. |
| Switching loss | fsw(Eon + Eoff) | Repeated equal events under unchanged conditions. |
| Thermal state | Junction and environment separately | Loss and temperature interact. |
Engineering questions.
Can low on-resistance establish minimum total loss?
No. Transitions, other paths and operating conditions also matter.
Is an IGBT represented only by Rds(on)?
No. Its on-state behaviour uses a different applicable model.
Does doubling fsw always exactly double switching loss?
Only in the simplified estimate with unchanged event energies.
Are 1 mJ and 10 W product measurements?
No. They are hypothetical arithmetic values.
Is cabinet ambient temperature junction temperature?
No. Heat-flow conditions separate those quantities.
Does the page provide gate-drive or device-replacement guidance?
No. It explains loss mechanisms only.
Technical enquiry
Discuss the technical question.
A power semiconductor can dissipate energy while conducting and while changing state. Share the model and applicable operating or design information for an engineering discussion.