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NXP Power MOSFET spice models Quick introduction Phil Ellis April 2015 Introduction NXP power MOSFET spice “models” attempt to give an accurate representation of a typical device at 25°C for key static and dynamic characteristics. The “models” are actually SPICE subcircuits and contain the Berkeley SPICE level 3 semi-empirical .MOSFET model and the SPICE .DIODE model. The subcircuit contains additional SPICE components to represent device non linear capacitances and package parasitics. 2 Key points Power MOSFET models attempt to accurately simulate – – – – – Rdson considering Id, Vgs and also global .TEMP (default is 25°C) Transfer characteristic (Id vs Vgs characteristic) @ 25°C Diode forward characteristic @ 25°C Capacitances (Ciss, Coss, Crss) @ 25°C Gate charge characteristic (Qg vs Vgs considering Id, Vds) @25°C NXP also create RC thermal models (in Foster network format) to simulate the relationship between Tj and Tmb NXP also provides some LTspice VDMOS models although these don’t model package parasitics but can run faster if a small loss in accuracy can be tolerated The following limitations apply – We generally don’t have thermally corrected models (Tj affects Vgs threshold, Rdson, diode characteristics, transfer characteristic) – Qrr is poorly modelled by the standard spice diode model – The spice mosfet model is only valid at 25°C – Applies to a typical device (not worst case) 3 Gate charge characteristics Vgs vs Qg simulated compared to measured for a given sample Modelled using capacitance measurements 4 Transfer characteristics Transfer curve: simulated vs measured Output characteristics follow from this 5 Other characteristics modelled Rdson vs Vgs Diode forward characteristics Note that curves are from a single typical device measurement. Datasheet values may be from averages of large batches There can be significant variation between parts due to manufacturing variation, this is indicated in the datasheet. The devices used to make the datasheet can be different from the devices used to make the spice model, so the mosfet model might not exactly match the datasheet 6 Switching characteristics Example of real device switching vs spice simulation. Switch off simulation agrees with real circuit quite well but switch on is not so good due to the poor modelling of reverse recovery in the diode (SPICE program doesn’t account for this properly). There are modifications to the subcircuit to more accurately model this behaviour. Switching characteristics don’t change too much with temperature (increase by approx. 15% at 150 for power MOSFETs since the capacitances are dominated by gate oxide thickness however the depeletion (drift) region behaviour is temperature dependant. Vgsth is strongly temperature dependant and this will affect performance (Miller plateau voltage will influence gate drive current, this tracks Vgsth) 7 Switching: simulation compared to actual Actual: MOSFET switch off Salmon = High side Vds 10V/div Green= Low side Vds 10V/div Blue = Low side Id 20A/ div Horizontal 50ns/division Simulation Pink = High side Vds 10V/div Green= Low side Vds 10V/div Blue = Low side Id 20A/ div Horizontal 50ns/division Note Id inverted MOSFET Switch on is less accurate but still useful 8 Usage of NXP power MOSFET subcircuits Can be used to calculate switching and conduction loss in pwm applications, particularly if the load current varies such as in motor control. Temperature compensation must be applied in order to give best accuracy at other temperatures Particularly useful in determining optimum performance when trying to decide which device to use rather than determining the exact losses For ultimate accuracy, the simulation must be benchmarked against actual device operation SPICE simulation is easier to use and more accurate than using calculation tools in spreadsheets, maths programs etc. Easier to apply to any topology, can include circuit parasitics easily. 9 10 Subject / Department / Author - July 18, 2015