The SpeedFit user interface consists of seven tabs to guide you towards an end goal of simulated current and voltage waveforms and performance data. This data can be used to quickly quantify the advantage of Wolfspeed devices in the application of interest. This manual walks you through the steps for using this tool. Each section of this user manual describes one of the seven tabs of SpeedFit.

Features of SpeedFit 2.0:

• It can quickly compare different Wolfspeed SiC devices.
• It can assist in determining the number of devices required to be paralleled. However, these results are not based on circuit or physics models but switching and conduction loss curves from the device datasheet.
• SpeedFit estimates semiconductor losses and junction temperature.
• It can help determine the required heatsink performance, assuming ideal sharing between paralleled devices.
• It has the capability of gathering inductor or transformer waveforms to simplify magnetic specifications.
• It helps evaluate different topology options for a given system specification.

Assumptions or Limitations of SpeedFit 2.0:

• All waveforms are ideal. Ringing and overshoots are not considered.
• Thermal simulation results are based on switching and conduction loss curves from the datasheets of the devices. They are not based on circuit or physics models.
• SpeedFit only evaluates the semiconductor losses; it does not calculate the total system efficiency.
• It assumes the ideal sharing of heatsink between different paralleled devices.
• It does not predict EMI or other circuit issues.

Note: For multi-stage converters, a separate simulation needs to be performed for each stage.

Figure 1: Application Tab

1. Input Voltage, Vin
2. Output Voltage, Vout
3. Rated Power, Pout
4. Turns-ratio (if the topology needs this specification), n:1
5. Device type: MOSFETs(discretes) / Modules

Figure 1: Parameter specifications for device selection

For each topology, the range of acceptable parameter values is shown under the textbox. It is visible when you click on the box to edit the entry as shown in Figure 2 below.

Figure 2: Parameter range is displayed when you click on the textbox to edit the value.

The parameters listed above are used to evaluate the blocking voltage and current rating for the devices to be used. Suitable devices available are auto-filtered based on blocking voltage and current rating requirement for the selected topology. You can select the power devices for their converter. By default, the tool provides a set of recommended device options based on power input, but a complete list of MOSFETs and Modules drops down if you enable the Show all button at the bottom of each list. The parts that are available for simulation have a radio button in the first column. If you enable the Allow Paralleling option, the tool will recommend devices that qualify for the current rating of your converter if they can be paralleled. These options can be seen in the bottom of Figure 3.

Figure 3: Device list

If Allow paralleling is enabled, the list is color-coded based on the number of parallel devices required, yellow color represents lower number of paralleled devices, whereas red list represents that high number of devices are to be paralleled for those parts. This can also be seen in Figure 3.

For DC/DC applications, the user can select both MOSFETs and Diodes. However, only MOSFETs or Modules can be selected for AC/DC and DC/AC applications. External Schottky diodes can also be added in parallel to discrete MOSFETs for AC/DC and DC/AC applications to reduce switching losses. Modules, by default, have Schottky diodes built in. Once you select a particular device, you can choose the number of devices to be paralleled to make up a single functional switch. The user also has the flexibility to change the default turn-on gate resistance (Rg,on) and turn-off gate resistance (Rg,off) for all MOSFET and Module devices. The gate resistance controls the switching speed of the device and has a significant impact on switching losses.

After devices are selected, a summary with images of the selected package is shown in the same tab. The datasheet for the selected device can be downloaded by clicking the download button.

Figure 4: Post-selection of devices: summary and additional device specifications

If you'd like to change the device selected, click on the Change device button, this will open up the list of devices again. Additional anti-parallel Schottky diode can be added as well through the option annotated in the figure above. You can also specify the number of paralleled diodes per device.

Please note: Power losses in the passive components are not considered in SpeedFit simulations, they are considered ideal components.

Figure 1: Circuit tab: enter topology specifications

Similar to the [Devices] tab, the acceptable range of each parameter is shown below the textbox when you click on it to edit the value.

Figure 2: Parameter range [example: 0.01 .. 20] is shown when editing the value in the textbox

Figure 1: Specify TIM for each device based on defined range

Next, you have three options for the thermal system simulation; either have a varying heatsink temperature throughout the simulation or fix it to a specific value. You can also define a custom heatsink using a Foster or Cauer network.

Figure 2: Heatsink Configuration showing two different heatsink configurations

Figure 2 shows the comparison between the two configurations for the heatsink temperature. Each switch position in the topology will use the junction-to-case thermal impedance, Rth,JC inherent to the device selected, which is included automatically. In either configuration, you are required to enter the thermal interface resistance, Rth,ch. Rth,ch is the thermal impedance of the interface between each switch position in the topology and the heatsink regardless of whether a single discrete MOSFET, a half-bridge, or a six-pack module is used.

For a fixed heatsink temperature, you can define the fixed heatsink temperature, Th. While using the variable temperature heatsink, you are required to specify the the thermal impedance (Rth,ha) and time constant (τha) of the heatsink, along with the ambient temperature (Tamb).

An additional heat source to a common heatsink can also be coupled, such as an auxiliary converter. This can be specified as an additional power loss, Padd in Watts. 

Figure 2: Thermal Tab

For the custom heatsink, a Foster or Cauer chain can be defined by the user. The order of chain can be modified by clicking on + or – buttons. 

Figure 3: Custom heatsink table

As you edit the thermal chain of the custom heatsink, you can see the updated thermal impedance plot at the bottom of the page. You can select the option of plotting this curve in the log scale.

Figure 4: Thermal impedance plot for custom heatsink

The circuit model displays the topology, heatsink configuration, and key parameter values specified by the user.

A simulation is executed by clicking the <Simulate> button below the circuit schematic and data tables. The SpeedFit performs a steady-state simulation of the system and displays several cycles of the input/output voltage and current waveforms for the equilibrium condition.

The scopes can be maximized or minimized from the top-right buttons provided on each scope. Each scope has a set of data cursors that can be enabled by clicking on the <Cursors> button at the top. The cursors allow the user to perform measurements of time, voltage, and current values.

Figure 1: Scope Options

The System overview table displays the input voltage, rated output power, switching frequency, and efficiency for the system. The Device overview table displays the part numbers and number of parallel devices for each device selected. The Temperatures table provides the maximum junction temperature (Tj) for each device, along with the maximum heatsink temperature and defined ambient temperature. The Losses Overview table summarizes the total switching, conduction, and combined losses for all the circuit’s MOSFETs, modules, and diodes. The total converter losses, or Combined Losses are displayed, which is the sum of these individual totals. The scopes also show junction temperatures of the MOSFETs, modules, and diodes. Losses Overview section summarizes the switching, conduction losses and combined losses in all devices, and also per switch position. 

Losses Breakdown section further quantifies the different kinds of switching and conduction losses. Please note, losses breakdown values are total semiconductor losses for the converter and not per switch position (in Watts). The different kinds of losses are:

1. Switching Losses are quantified as :

• Turn-on Loss : switching loss during turn-on transition of the device
• Turn-off Loss : switching loss during turn-off transition of the device

2. Conduction losses are of three types : 

• Forward conduction : power loss during first-quadrant conduction of the devices
• Reverse conduction : loss during third-quadrant conduction of the devices
• Body-diode conduction : loss when only body-diodes conducts during the dead-time 

Switching loss = Turn-on Loss + Turn-off Loss

Conduction loss = Forward conduction loss + reverse conduction loss + body-diode conduction loss

Combined Losses = Switching losses + Conduction Losses

Note: No Magnetic component losses are included in SpeedFit. The efficiency number is based on the losses of the semiconductor devices in the topology.

You can save the simulated waveforms and results by clicking the <Hold result> button to the right of the <Simulate> button. A Traces list appears above the scope displays, where each simulation trace is listed. You can compare the results in subsequent simulations. To modify any part of the system setup at the Simulation tab, you can navigate directly to the desired tab at the top of the tool or use the Back and Next buttons.

Note: While you can compare all combinations of configurations with the resultant data and waveforms presented, changing the circuit type only shows the active schematic. Comparing simulations using different converter types and displaying all historical traces is not recommended as the resultant waveforms may require different display scales.

Figure 2: Hold Result options

You can hold a simulation trace by clicking the green <+> button and remove it by clicking the red <-> button. You can rename each trace’s name by clicking on the respective text field. Traces can also be displayed and hidden by using the checkboxes to the left of the trace name. This allows the user to compare the current and other traces’ results using the same table and measurement of the scope (Figure 3). New simulation traces always appear at the top of the Results History list and each data table field.

Figure 3: Simulation Tab

Figure 1: Summary page with print and export data options