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Three-Phase High-Power Converter Design and Analysis Workflow

This example shows how to design and analyze high-power converters.

High power converters are important building blocks for future electric mobility and microgrid solutions. To design an optimum converter with lower weight, loss, and cost, you must perform a detailed analysis of different converter parameters and scenarios. This example helps you simulate the steady state, the transient electrical, and the thermal characteristics of a three-phase two-level converter that uses IGBT devices.

Converter Design and Analysis General Workflow

This flowchart shows the workflow for the analysis of the characteristics of a converter. This analysis allows you to study the effects of each parameter to design an efficient converter.

High-Power Converter Design Workflow

This example follows the workflow given in the figure below,

Three-Phase Converter Insulated-Gate Bipolar Transistor (IGBT) Device Selection

This example is based on a 15 kW converter operating with three phase grid with frequency of 60Hz and line voltage of 208 V. Based on this, the rms current will be 15e3/(sqrt(3)*208) = 41.6 amps which equates to 59 amps peak. The minimum DC bus voltage required is 2*208*sqrt(2/3) = 340 volts, and hence a standard 400V DC bus is chosen.

Given the expected currents and voltage, an IGBT that can take >450Vdc and 50A is needed. An IGBT device with a 50 A continuous current rating and 650 volts rating in a TO-247 package is chosen. The parasitic inductance of the bus bar that connects IGBT device with DC bus is estimated to be 30nH.

IGBT Parameter Extraction

IGBT devices are the most used semiconductor switches for high-power inverters thanks to their unique ability of better switching frequency, simpler gate driver, and available power rating. To model an IGBT with different fidelity levels specific to your particular application, choose between the N-Channel IGBT and IGBT (Ideal, Switching) blocks in the Simscape™ Electrical library. IGBT (Ideal, Switching) block being more suited to system-level simulations and the N-Channel IGBT more suited to detailed design, such as gate driver design, simulating peak switching voltage and current.

N-Channel IGBT Library Block

The N-Channel IGBT block provides a detailed model of an IGBT device using either non-linear equations or a 2D or 3D table-based model. It models the steady state and transient behavior of an IGBT. Most manufacturers datasheet contains parameters required for this block. You need to add an external diode with N-Channel block.

These are the characteristics curve that you can find in most manufacturer datasheets:

  1. $$ I_{ce} \; versus \; V_{ce} $$ at two different junction temperatures.

  2. Capacitance $$ (C_{ies},\; C_{oes},\; C_{res}) $$ as a function of $$ V_{ce} $$.

  3. Diode $$ I_{F} $$ as a function of $$ V_{F} $$ for two junction temperatures.

  4. Parameters for thermal model (Foster, Cauer, or simple $$R_{jc}\; and\; R_{ca} $$).

  5. Switching loss curves $$ E_{on} \; versus \; I_{ce} $$ and $$ E_{off} \; versus \; I_{ce} $$

From the datasheet characteristics curve, you can create the $$ I_{ce} \; versus \; V_{ce} $$ three dimensional matrix and capacitance arrays. To achieve approximate results, you can use non-linear IGBT equation models by using the parameters in the datasheet. Most datasheet provides the $$ I_{ce} \; versus \; V_{ce} $$ curves for $$V_{ce} $$ up to 5V, but the IGBT in switching mode goes through transition from 0 V to DC bus voltage. In such cases you should provide some estimated values at higher voltage ranges as shown in the figure below:

This will avoid extrapolation to non-physical current values. In converters an IGBT device is operating in a switching mode, so for good simulation speed and accuracy, ensure accurate and smooth $$I_{c}(V_{ge},\; V_{ce}, \;T_{j}) $$ values around the smaller collector-emitter voltages.

The plot below shows the digitized device capacitance characteristics.

External diode forward current current characteristics $$I_{F}(T_{j},\;V{F}) $$, obtained from the IGBT datasheet are shown below.

Simulated Switching Transient

The below figure shows the simulated N-Channel IGBT switching turn-on and turn-off characteristics.

IGBT (Ideal, Switching) Library Block

When used in large system level models, the detailed-N-Channel IGBT library block can slow down the simulation. For system-level harmonic or thermal simulations, you can use the IGBT (Ideal, Switching) block. This block provides instantaneous turn-on and turn-off. You can directly provide the switching loss matrix as an input. The thermal circuit computes the IGBT switching losses. This thermal circuit studies the temperature rise and the heat sink/cooling system performance of the converter.

N-Channel IGBT Device Model Subsystem

For better transient study, this example externally connects the lead parasitic inductance of the IGBT. The Subsystem reference keeps the IGBT device model universally accessible. Changes in a single block are applied across all models. Base workspace shows the IGBT & diode parameters.

IGBT (Ideal, Switching) Device Model Subsystem with Cooling Circuit

This example uses junction-to-case thermal resistance and device thermal mass to externally model thermal circuits to the IGBT (Ideal, Switching) block. A variable thermal resistance models the heat sink with forced cooling. The thermal resistances are fixed based on the cooling fluid flow rate across the heat sink fins. To model junction and case temperature rise, you need the IGBT and diode thermal mass parameters. Some datasheet provides Foster and Cauer thermal model parameters. Use the Cauer thermal model when connecting to external heat sink system.

Gate Driver Design

Simscape™ Electrical also helps you to design a gate driver circuit that you can use in the control of power electronics devices. This example models a three gate driver that you can use to switch semiconductor devices.

Ideal Gate Driver

An ideal gate driver models the charging gate resistance and the discharging gate resistance and provides flexibility to define the turn-on and turn-off gate voltages $$ V_{ge} $.

Gate Driver with Electromagnetic Isolation (Pulse Transformer)

This example models a simple gate driver circuit with electromagnetic isolation. This high frequency transformer-based gate driver only requires a single gate driver power supply isolated from the power circuit.

Open the 'ee_converter_design_igbt_switchingloss_testharness' SLX model and, in the masked selection subsystem on the right-hand top corner, choose opto-coupler gate driver to simulate a gate driver with the N-Channel IGBT library block.

Gate Driver with Opto-Coupler Isolation

This example also models a simple gate driver circuit with optical isolation. This opto-coupler-based gate driver is compact with lower losses. This requires two gate driver power supplies that are isolated from each other and the power circuit.

Open the 'ee_converter_design_igbt_switchingloss_testharness' SLX model and. in the masked selection subsystem on the right-hand top corner, choose opto-coupler gate driver to simulate a gate driver with N-Channel IGBT library block.

Test Harness for IGBT Steady State Characteristics

This model provides test harness to plot the steady state $$ (I_{ce}\; versus \; V_{ce}) $$ curve for different junction temperature $$(T_{j}) $$ and gate-emitter voltage $$(V_{ge}) $$.

IGBT Steady State Characteristics Test Harness Parameter Selection Mask

To define the simulation parameters, open the right-hand top corner parameters selection mask.

Test Harness for High Power Converter Switching Loss Estimation

This model plots the variation of switching loss characteristics for these IGBT simulation parameters:

  1. Collector-emitter current $$(I_{ce}) $$

  2. PWM switching frequency $$(f_{pwm}) $$

  3. Gate-emitter voltage $$(V_{ge}) $$

  4. Collector-emitter voltage $$(V_{ce}) $$

  5. Gate resistance $$(R_{gon}\; and\;R_{goff}) $$

  6. Junction temperature $$ (T_{j}) $$

This example plots the IGBT characteristics for both double pulse and sinusoidal PWM gate signals. The IGBT device under test is placed in the lower (IGBTL) leg of a converter. To determine the single turn-on and turn-off characteristics, this model uses a double pulse gate signal. To simulate the voltage $$(V_{ce}) $$ and current $$(I_{ce}) $$ stress on the IGBT-diode device combination in a typical two level converter, a sinusoidal PWM gate modulation signal is used along with a sinusoidal output collector current. Sinusoidal $$(I_{ce}) $$ load current with sinusoidal voltage modulation signal option enables you to simulate for different power factor, power frequency, and switching frequency. IGBT characteristics are plotted for both double pulse and sinusoidal PWM gate signals.

Switching Loss Estimation Test Harness Parameter Selection Mask

To define the simulation parameters and plot the switching loss characteristics, open the right-hand top corner parameters selection mask.

Estimation of IGBT (Ideal, Switching) Block Parameters from N-Channel IGBT

IGBT (Ideal, Switching) library blocks are better suited for simulating large system level model for harmonics and thermal analysis. Some datasheets directly provide switching loss $$E_{on}\;E_{off} $$ and $$V_{ce}(T_{j},I_{ce}) $$ parameters. These parameters are required for the chosen gate resistance $$(R_{gon}\; and \;R_{off}) $$, gate-emitter voltage $$(V_{ge}) $$ and bus bar parasitic inductance. It is achieved by simulating the test harness models with non-linear equation or 3D table based N-Channel IGBT library block.

This script 'ee_converter_design_igbt_ideal_parameter_estimation' provides a method to estimate these parameters for user defined $$(R_{gon},\;\;R_{off}\;V_{ce}) $$, and circuit parasitic.

Three-Phase High Power Converter with Cooling System

This model simulates IGBT and diode junction temperature rise. You can use this model to choose design parameters like required cooling system, PWM technique, switching frequency. Use parameter selection mask to select simulation parameters.

Parameter Selection Mask for Three-Phase High Power Converter

To define the simulation parameters, open the right-hand top corner parameters selection mask.

IV Characteristics Simulation Results

IV characteristics at full range This plot shows IGBT IV curve for $$V_{ce} $$ from 0 to $$V_{dc} $$

IV characteristics near Vce(sat) This plot shows IV curve near IGBT on-state $$V_{ce} $$ voltage from 0 to $$2V_{cesat} $$

Simulated Switching Loss Characteristics for Single Turn-On and Turn-Off

The IGBT switching loss values change according to the definition of the switching region and on the free wheeling diode. In this example, some of the diode reverse recovery is part of the turn-on switching region and any power loss (Vce*Ice) greater than lossFactor*max(Vce*Ice) is a switching loss. This example uses lossFactor of 0.02 (2% of max switching power). During switching, any power loss (Vce*Ice) greater than lossFactor*max(Vce*Ice) is considered as a switching loss.

Simulated loss for different Ice This plot shows the simulated loss for different Ice.

Simulated loss for different Vce This plot shows the simulated loss for different Vce.

Simulated loss for different Rg This plot shows the simulated loss for different Rg.

Simulated Switching Loss Characteristics for Sinusoidal Load Current and Modulation Signal

Simulated output for sinusoidal load current This plot shows the simulated output for the sinusoidal load current when $$ f_{pwm} = 3000Hz $$.

Simulated loss for different Ice This plot shows the simulated loss for different Ice when $$ f_{pwm} = 3000Hz $$.

Simulated loss for different PWM frequency This plot shows the simulated loss for different PWM frequencies.