Author: Andrei Velcescu, Applications engineer, Manchester
This interactive application note contains embedded Cloud based simulations to augment the text.
To open the embedded simulation, simply hover over the simulation image. Left click anywhere in the graphic area once the central play button changes in colour. This opens the schematic in the Cloud environment. See the interactive application note tutorial page for more details on how to use the simulations.
The precision electrothermal models come in two variants, downloadable LTspice ones and online VHDL-AMS.
Online on PartQuest Explore
The precision electrothermal models are available in VHLD-AMS format which is supported by PartQuest™ Explore (https://explore.partquest.com). Here, the models are available within the partner library and are ready to be used with no additional set-up. Further examples of models and simulations using PartQuest™ Explore can be found on Nexperia’s Interactive Application Notes (https://www.nexperia.com/applications/interactive-app-notes)
Figure 1. Nexperia Power MOSFET simulation models
Importing models in LTspice™
Download the precision electrothermal model ZIP file from Nexperia.com, e.g. BUK7xxxx-40H_LTspiceV3. The ZIP file contains the library file and symbol. Moreover, the file can be downloaded from the product page Documentation or Support tabs.
The precision electrothermal models have 5 pins, the 3 traditional electrical pins for Gate, Drain, and Source and two additional pins for interacting with the thermal properties of the MOSFET, see Fig. 2. These are namely the junction temperature pin, Tj and the mounting-base pin, Tmb. Tj can be probed and gives the junction temperature in °C. In addition, this pin can be connected to a fixed voltage source to set the junction temperature as a constant, this disables the self-heating behaviour of the model. Otherwise, it can be left open and monitored.
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Figure 2. MOSFET Electrothermal model temperature pins |
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Tmb represents the thermal connection of the device to the outside world e.g., the heat path from the mounting base of the MOSFET to a PCB or heatsink. This pin can also be connected to a fixed voltage source, this will set the ambient temperature. In addition, thermal models of PCBs can be connected to the MOSFET Tmb pin model to simulate the system thermal performance in Simulation 1 below.
Simulation 1. MOSFET self heating
In many half-bridge applications, the MOSFET may replace the free-wheeling diode to achieve synchronous rectification. The MOSFET’s RDSon produces a lower voltage drop Drain to Source than the diode’s VF so power loss is reduced making the application more efficient. However, MOSFETs conducting current through the channel in reverse, such as in synchronous rectification, will have lower threshold voltage. This is known as the body effect and is accounted for in the models derating with negative VDS and ID. The body effect is also sometimes referred to as third quadrant operation. Fig. 3 shows both the first and third quadrant of operation. The first quadrant is ID / VD as previously shown and in the third quadrant when the VGS = 0 is the diode curve. Fig. 3 also shows operating in the third quadrant with a positive VGS and the increased ID due to the Body Effect. See simulation 7 below.
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Figure 3. Full quadrant operation of a MOSFET showing ID / VD, VSDS and the Body Effect; simulation results |
Simulation 7.
Included in the precision electrothermal models are temperature dependant drain leakage current and breakdown voltage. In addition, the slope resistance of avalanche breakdown is modelled, see Simulation 8 below. Fig. 4 shows the simulation results plotted with ID on linear and logarithmic scales.
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Figure 4. Avalanche breakdown; ID as a function of VDS simulation results |
Simulation 8. Drain leakage (IDSS), breakdown voltage (BVDSS) and avalanche breakdown
Capacitances
The MOSFET’s internal parasitic capacitances and their voltage dependence is modelled. This is critical for accurate switching time simulations and gate charge modelling. The capacitance values have insignificant temperature dependence so in the model they are fixed with respect to temperature. The gate charge does have some temperature dependence; the Miller plateau voltage is dependent on the gain of the MOSFET which in turn is temperature dependant. See simulation 9 below.
Simulation 9. Capacitances
Gate charge
To determine the gate charge, use the relationship in Equation (1),
(Eq 1). 
the gate current IG is determined by the settings of Ig1 in the schematic set-up. See Simulation 10 below.
Simulation 10. Gate charge
QRR is not solely dependent on the MOSFET but on the test circuit too. Specifically, the stray inductances of the PCB are critical in determining the QRR behaviour. As such it is difficult to represent the QRR in a simulation with a great accuracy without knowing or characterising the test bench being compared to the simulation. In addition, there are two standard methods of measuring QRR that will again yield different results. The preferred method for testing QRR is the double pulse method which more closely aligns with application use cases, for example half-bridge topologies such as DC-to-DC convertors and motor drives. Further information about simulating the QRR in a double pulse set-up is detailed in Nexperia application note AN90011. Fig. 5 shows an example QRR waveform. See Simulation 11 below.
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Figure 5. Diode reverse recovery (QRR) simulation results |
Simulation 11. Double Pulse Circuit
| Page last updated 19 October 2023. |