Power Loss and Efficiency Calculation of Resonant LLC Converter
Introduction
Resonant LLC resonant converters have attracted much attention in recent years due to the soft-switching characteristics over a wide load-line range, higher peak efficiency, and lower EMI footprint. A resonant LLC converter can also offer high power density as the resonant components can be incorporated into the magnetics, and higher switching frequency operation can be used due to negligible switching losses and low EMI noises. However, optimal design and power loss calculation of resonant LLC converter is a challenging task.
The Power Supply Design Suite (PSDS) in PSIM provides the design tool for fast analysis and optimization of resonant converters with accuracy and precision to address the above issues and design challenges. The Thermal Module in PSIM can help us to calculate power losses of semiconductor devices quickly with information from manufacturer device datasheets.
In this application note, we will first use PSDS to design the resonant LLC converter with ideal switching devices. Then we will replace the ideal devices with the Thermal Module devices based on the manufacturer’s datasheet to determine the total power loss, which includes the conduction and switching losses.
System Specifications
The selected resonant LLC converter in this application note has following specifications:
Vin_rated = 390V; Vin_min = 375V; Vin_max =405V
Vo_rated = 12V
Po_rated = 500W
f_res = 125 kHz
Resonant LLC Converter with Thermal Module Devices
In general, several steps are needed to design a resonant LLC converter and calculate power losses:
- Using PSDS to perform resonant tank parameter optimization with the selected quality factor, magnetics ratio, frequency range, and with ideal switching devices.
- Using the Thermal Module to add the selected MOSFET and diode devices into the Device Database.
- Running the simulation with Thermal Module devices to determine the power loss of the converter.
1. Designing Optimal Values of Resonant Components
In PSIM, go to Design Suites >> Power Supply Design Suite and select Full-bridge Resonant LLC. After files are unpacked, a template circuit will be displayed as shown below.
Figure 1: The unpacked full-bridge resonant LLC converter template from the Design Suite
The Parameter Panel on the left of the schematic screen allows the user to input the design specifications, launch the Steady State Solver Tool and the Design Curve Tool. The parameter file on the left of the circuit stores the calculated parameter values of the resonant LLC converter.
For this application, we have selected Q_rated as 0.4 and K_ind as 4 to achieve the optimal LLC converter design with a narrower frequency variation range for the given line and load conditions.
The resonant tank component values of Ls, Cs, Lm are calculated as:
Ls = (Q_rated*Ro_rated_pri)/(2*pi*f_res) = 154.92 uH
Cs = 1/(2*pi*f_res*Q_rated*Ro_rated_pri) = 10.463836 nF
Lm = K_ind*Ls = 619.72 uH
where Ro_rated_pri is the rated value of output load resistance referred to the primary side.
The required relative frequency variation factor (K_rel_freq) can be determined as 0.94 to 1.06 based on the gain requirements. However, considering the overloading factor, the voltage drop across the output diodes, the resistance of the transformer, and the dead-time of the switches, the K_rel_freq is selected as 0.80 to 1.1.
2. Adding MOSFET and Diode into the Device Database
The Thermal Module provides a quick way to estimate the conduction and switching losses of semiconductor devices (diodes, IGBT, MOSFET, SiC, and GaN), and the core and winding losses of inductors.
A Device Database Editor is provided for users to add new thermal devices to the database.
We will demonstrate how to add a new MOSFET or diode device to the database using the Device Database Editor. A similar process is also described in the PSIM tutorial document Tutorial – IGBT and MOSFET Loss Calculation in the Thermal Module.pdf.
2.1 Adding MOSFET to the Device Database
We have selected Infineon’s MOSFET device IPP60R099CP (650V, 31A) for the primary side switches due to its very low Rds(on) and ultra-low gate charge. The device file “IPP60R099CP.dev” can be found in the “device\Infineon\MOSFET” folder in PSIM.
For illustration purposes, we will follow the procedures below to add this device into the device database file “IPP60R099CP.dev”:
1. In PSIM, go to Utilities >> Device Database Editor to launch the Editor.
2. Select Device >> New MOSFET. Choose <New device file>. Create the device file “IPP60R099CP.dev” under the folder “Device/Infineon/MOSFET” as shown below:
3. Enter the basic information such as the part number, package, and maximum ratings from the datasheet as shown below:
4. Enter the electrical characteristics information (shown below in left) based on the datasheet (shown below in the right).
5. Enter the electrical characteristics for the antiparallel diode as shown below:
6. Enter the thermal characteristics as shown below:
Next, we will use the Curve Capture Tool to import the electrical characteristics curves of the antiparallel diode from the datasheet.
First, click on the Edit button of the “Vd vs. IF” characteristics. A window for the Curve Capture Tool will be displayed.
Click on Add Curve, use the Graph Wizard button at the upper left corner to capture the curve at one operating temperature with the procedures listed below:
1. Display the graph of “Vd vs. IF” characteristics from the datasheet on the screen. Click on the Print Screen key (PrtSc) to copy the screen to the clipboard.
2. Click on the forward green arrow of the Graph Wizard. The image in the clipboard will be copied into the dialog window as shown below:
3. Position the image properly within the window so that the complete graph is in full view. Click on the forward arrow. Define the border of the graph by left-clicking on the graph’s origin (lower-left corner), and then move the cursor to the opposite corner (upper right corner) and left-click. Right-click to zoom in for easier cursor placement. After this, a blue frame will be superimposed on top of the original graph frame as shown below:
4. Click on the forward arrow. Check if the x-axis and y-axis definitions are correct. By default, the x-axis is IF, and the y-axis is Vd. But if the x-axis is Vd, and the y-axis is IF in the datasheet. We need to check the “swap X/Y axes” box to match the datasheet. Enter X0=0, Xmax=2, Y0=0, Ymax=100. Input the junction temperature Tj=25 for the 25oC curve. The dialog window is shown below:
5. Click on the forward arrow. Starting from the origin, left click on top of the 25oC curve to capture the data point. Right-click to zoom in for easier cursor placement. As you click along the curve, a red curve will be drawn, indicating the data points captured. The dialog window is shown below.
6. Click on the forward arrow, and the capture process will be completed. The dialog window of captured curve for Tj=25oC is shown below.
7. Click on the ‘Add Curve’ button to add a new curve. Repeat the above steps to capture a curve for the other operating/junction temperature.
Figure 2 below shows the device in the PcdEditor after this MOSFET model has been added into the device file “IPP60R099CP.dev”. The electrical characteristics curves of the antiparallel diode at different junction temperatures (25oC and 150oC) are shown in Figure 3.
Figure 2: Device Editor with MOSFET IPP60R099CP Figure 3: IF vs. Vd curves for MOSFET IPP60R099CP
2.2 Adding Diode into the Device Database
We have selected STMicroelectronics’ Schottky barrier rectifier STPS40L45C (45V, 20A) for the secondary side diodes.
The device file “STPS40L45C.dev” can be found in the “device\ST\diode” folder in PSIM.
The procedures to add the thermal information of STPS40L45C into the device database are similar to those described in 2.1. Figure 4 below shows the thermal information in the Device Editor after this diode model has been added into the device file “STPS40L45C.dev”.
The electrical characteristics curves of the antiparallel diode at different junction temperatures (25oC, 75oC, and 125oC) are imported into the device file “STPS40L45C.dev” shown in Figure 5.
Figure 4: Device Editor with diode STPS40L45C Figure 5: IF vs. Vd plots for diode STPS40L45C
We can now select the thermal models of newly added MOSFET and Diode.
In PSIM, select Elements >> Power >> Thermal Module >> MOSFET (database).
Place the discrete MOSFET element on the schematic. Double click on the MOSFET element to open the property dialog window. Click on the Browser button next to the “Device” input field and choose the device “IPP60R099CP” as shown in Figure 6.
Figure 6: The MOSFET “Device” input field and path to choose the device “IPP60R099CP”
Similarly, select Elements >> Power >> Thermal Module >> Diode (database).
Place the discrete diode element on the schematic. Double click on the diode element to open the property dialog window. Click on the Browser button next to the “Device” input field and choose the device “STPS40L45C” as shown in Figure 7.
Figure 7: The diode “Device” input field and path to choose the device “STPS40L45C”
Figure 8 shows the revised resonant LLC converter schematic with IPP60R099CP and STPS40L45C thermal devices.
The schematic file “Resonant LLC – power loss calculation.psimsch” can be found in the “simu” subfolder of this application note folder.
Figure 8: Resonant LLC converter schematic with MOSFET IPP60R099CP and diode STPS40L45C
3. Calculating Power Losses
Simulate the resonant LLC power circuit with the obtained values of Ls, Cs, Lm, Q_rated, and K_ind from PSDS with the added thermal devices.
Figure 9 shows the simulation waveforms as well as the conduction and switching losses of MOSFET Q1 and diode D1 at the rated output voltage (12V) and minimum input voltage (375V).
Figure 9: Simulation waveforms at the minimum input voltage Vin_min
The breakdown values of losses in MOSFET Q1 and diode D1 are also described below.
The following loss results are obtained for primary MOSFET (Q1) from the PSIM simulation when the MOSFET junction temperature is 125.52oC.
- Transistor Conduction Loss (W): 9.986
- Transistor Switching Loss (W): 0.613
- Anti-parallel Diode Conduction Loss (W): 0.089
- Anti-parallel Diode Switching Loss (W): 0
- Transistor Total Loss (W): 10.689
The following loss results are obtained for the secondary diode (D1) from the PSIM simulation when the diode junction temperature is 124.34oC.
- Diode Conduction Loss (W): 16.871
- Diode Switching Loss (W): 0
- Diode Total Loss (W): 16.871
The switching loss of the diode is zero because there is ZCS turn-on and ZCS turn-off, and there is no reverse conduction of the diode in this circuit.
The total power loss of each MOSFET device is close to 10W and total power loss of each diode device is close to 16W and the maximum junction temperatures of both MOSFET and diode devices are 150oC from the datasheet. If 20% of the maximum junction temperature is used as the design margin and 40oC of ambient temperature is considered, we can determine the heatsink thermal resistance (R_case_ambient_1) for MOSFET to be roughly 7.5oC/W and the heatsink thermal resistance (R_case_ambient_2) for secondary side diode to be roughly 3.5oC/W as shown in Figure 8.
After switch losses are calculated, we can calculate the efficiency of the converter. Figure 10 shows the efficiency curves of the converter at three different values of input line voltages (Vin_min, Vin_rated, and Vin_max) with variation in load current from 25% to 100%.
Figure 10: Efficiency vs. the load current percentage at different input voltages
Conclusion
This tutorial shows that a resonant LLC converter can be quickly designed using the Power Supply Design Suite. Furthermore, with the Thermal Module in PSIM, users can select specific switch devices and calculate the losses and the junction temperatures of the devices as well as the efficiency of the whole converter easily.