ST UM2975 User manual

Introduction
The STDES-VRECTFD reference design represents a complete solution for high-power, three-phase active front end (AFE)
rectifier applications based on the three-level Vienna topology.
This reference design topology is mostly used for DC fast charging applications related to industrial and electric vehicles.
It features full digital control. The embedded STM32G474RET3 mixed-signal high-performance microcontroller provides the
full control of the power factor (PF), the DC voltage, and the auxiliary task to manage the protections and the soft start-up
procedure.
The high-bandwidth continuous conduction mode (CCM) current regulation allows the maximum power quality in terms of total
harmonic distortion (THD) and power factor (PF).
Figure 1. DC charging station
How to use the 15 kW three-level three-phase Vienna rectifier with digital control
for power factor correction
UM2975
User manual
UM2975 - Rev 1 - January 2022
For further information contact your local STMicroelectronics sales office.
www.st.com

Figure 2. STDES-VRECTFD reference design - power board
Fully assembled board developed for
performance evaluation only,
not available for sale
Figure 3. STDES-VRECTFD reference design - control board
Fully assembled board developed for
performance evaluation only,
not available for sale
The high switching frequency of the SiC MOSFETs (70 kHz) and the multilevel structure allow an efficiency of almost 99% as
well as the optimization of passive power components in terms of size and cost.
The high efficiency rectifier is designed for several end applications such as electric vehicle (EV), industrial battery chargers,
and industrial equipment, which requires a very high PF and low THD.
The STDES-VRECTFD is a fully assembled kit developed for performance evaluation only, not available for sale.
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1Getting started
1.1 Safety information
Caution: This reference design is intended for demonstration purposes only and is not for domestic or industrial
installations.
Danger: The high-voltage levels used to operate the reference design can cause serious injury,
electrical shock, and even death.
This reference design is intended for use by experienced power electronics professionals
who understand the necessary precautions against potential dangers and risks while
operating this board, even when it is not powered. The qualified personnel must be familiar
with the installation, use, and maintenance of power electrical systems. During operation, do
not touch the board as some of its components could reach a very high temperature.
1.2 Block diagram
Figure 4. STDES-VRECTFD block diagram
1.3 Features
• Three-phase, three-level AC-DC power converter
–Nominal rate for DC voltage: 800 VDC
– Nominal rate for AC voltage: 400 VAC at 50 Hz
– Maximum power: 15 kW
– Power factor: >0.99
– Inrush current control and soft start-up
– THD lower than 5% at nominal operation
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• Power section based on SiC MOSFETs and SiC diodes:
–High frequency operation (70 kHz)
– High efficiency: >98.6%
– Passive element weight and size reduction
• Control section based on the STM32G474RET3 microcontroller:
– Control and monitoring interfaces: SWD–UART, I²C and DACs
– 64-pin digital power connector
– LED status as UI
– Four integrated high-performance op-amps for additional features
1.4 Main characteristics
Table 1. Main characteristics
Description Symbol Min. Typ. Max. Unit Comments
Three-phase input voltage VAC 208 480 VACLL
AC line frequency Hz 47 63 Hz
Maximum output power POUTmax
15
7kW
VAC = 230 VRMS IAC = 21 VRMS
VAC = 110 VRMS IAC = 21 VRMS
Output voltage VDC 800 V
Power factor PF >0.99 - From 20% of load
Total harmonic distortion THDi <5 % From 20% of load
Switching frequency fsw 70 kHz
Table 2. Protection characteristics
Description Symbol Min. Typ. Max. Unit
HVDC overvoltage protection VDCovp 900 V
HVCAP overvoltage protection VCAPovp 500 V
AC overcurrent protection IACovp 30 A
1.5 Reference design description
1.5.1 Power board
The figure below shows the power board of the STDES-VRECTFD reference design.
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Figure 5. STDES-VRECTFD reference design - power board
The following figure shows the main sections of the power board.
Figure 6. STDES-VRECTFD power board sections
1.5.2 Power stage
1.5.2.1 Boost inductor
Boost inductors represent the energy storage elements that allow the PFC operation of the converter. This is
obtained by controlling the inductor current and using a proper conduction pattern in the power device section.
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Continuous conduction mode (CCM) performs the PFC operation of this reference design. The inductance is
related to several parameters: the desired current ripple, the available converter voltage levels, the switching
frequency, and the rated DC-AC operation voltages.
Table 3. Boost inductor parameters
Parameter Symbol Value Unit
DC voltage V0800 V
Switching frequency fsw 70 kHz
Rated AC voltage VAC 230 VRMS
Max. ripple current ΔiLppmax 10 %
Boost inductance VAC 470-6 H
1.5.2.2 Passive current limiter NTC
Figure 7. Three and four-wire connections
Figure 8. Focus on the STDES-VRECTFD NTC
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Figure 9. STDES-VRECTFD NTC specifications
1.5.2.3 Sensing
1.5.2.3.1 AC current
An isolated sensor measures the AC input current. It represents the boost inductor current to be controlled for the
proper operation of the power converter. Hall sensors are taken into consideration. A conditioning circuit allows
obtaining the correct value for the ADCs. The circuit shown below is replicated for each phase.
Figure 10. AC current sensing block diagram
1.5.2.3.2 AC voltage
The three-phase AC voltages are obtained using a two-stage sensor circuit. The first part represents an isolated
op-amp that allows measuring the HV through a voltage divider with an isolation barrier.
Isol-Op-AMP output is limited in volts and is scaled with a second stage of op-amps with a proper gain and bias.
This circuit allows measuring an AC voltage referenced by a virtual or grid neutral point. The circuit is replicated
for each phase.
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Figure 11. AC voltage sensing block diagram
1.5.2.3.3 DC current
An isolated sensor measures the DC output current. Hall sensors are taken into consideration. A conditioning
circuit allows obtaining the correct value for the ADCs.
Figure 12. DC current sensing block diagram
1.5.2.3.4 DC voltage
The DC voltages are obtained using two-stage sensing. The total DC bus voltage is split exploiting two voltage
dividers. Both voltages are needed to obtain the monitoring of each capacitor to avoid overvoltage, offering
independent DC voltages for the control.
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Figure 13. DC voltage sensing block diagram
1.5.3 Control board
The figure below shows the control board of the STDES-VRECTFD reference design.
Figure 14. STDES-VRECTFD reference design - control board
The following figure shows the main sections of the control board.
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Figure 15. STDES-VRECTFD control board sections
Figure 16. STDES-VRECTFD MCU pin assignment
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1.6 Power factor correction (PFC) benefits
The figure below highlights the PFC benefits in terms of rest factor and power factor.
Figure 17. PFC benefits
1.7 Converter operation
The figure below shows the current paths of the Vienna topology. To simplify the scheme, we considered the
single phase representation.
Figure 18. Switching paths of the Vienna topology
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2How to use the STDES-VRECTFD reference design
2.1 System setup
To use the STDES-VRECTFD, you need:
•a programmable AC emulator or a programmable AC source;
• a DC electronic load;
• a power analyzer;
• a digital oscilloscope.
You can test the STDES-VRECTFD up to 15 kW at 230 VAC RMS and 6 kW at 110 VAC RMS in a frequency
between 47 and 63 Hz.
2.2 How to connect the reference design
To operate the reference design power converter properly, consider the operating limits shown below.
Table 4. Operation condition limits
Description Value Unit
Three-phase input voltage range 208-400 VAC
Line frequency range 47-63 Hz
Maximum output power at 230 VAC 15 kW
Voltage limit of the bulk capacitors 500 V
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Step 1. Connect the power board as shown in the figure below.
The figure below shows the three-phase connection sequence (A-B-C). The neutral connection is
optional. The polarity influences the DC load connection.
Figure 19. STDES-VRECTFD connection
Step 2. Connect an external fan to manage the thermal dissipation.
The auxiliary power supply can be externally provided.
2.3 MCU programming and debugging
You can program and debug the microcontroller unit (MCU) through different tools.
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Step 1. Use ST-LINK/V2 and a 20- to 10-pin JTAG adapter to connect the platform to the PC.
Figure 20. ST-LINK/V2 and adapter
Figure 21. ST-LINK/V2 connected to the control board
Step 2. Select the main.c file in the project/Application/User path.
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Step 3. Click on the [Download and debug] button to start programming and debugging.
Figure 22. IAR EWARM program procedure
Step 4. Click on the [Run] button to start the code execution.
Figure 23. IAR EWARM debug procedure
2.3.1 Power supply section
The power supply needs two different input voltages. An embedded SMPS, based on the VIPER26 family,
provides self-powering from the DC-link.
As shown in the figure below, you can select either an internal or external supply voltage. Specific LEDs allow
identifying the selected configuration.
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Figure 24. Example of power supply configuration
2.3.2 Driver section
The driving section configuration is related to the power switch technology. In this case, we considered the SiC
power MOSFETs. The figure below shows this specific configuration.
Figure 25. Example of driver configuration
2.4 Preliminary test procedure
2.4.1 AC sensing
To verify the proper operation of the AC sensing (Figure 26), analyze the test points for voltages (Figure 27) and
currents (Figure 28).
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Figure 26. AC sensing section
Figure 27. AC voltage sensing test procedure
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Figure 28. AC grid current sensing test procedure
2.4.2 DC sensing
To verify the proper operation of the DC sensing, analyze the test points for voltages (Figure 29) and currents
(Figure 30).
Figure 29. DC voltage sensing test procedure
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Figure 30. DC current sensing test procedure
2.4.3 AC connection
Check that the STDES-VRECTFD AC main connection is in line with the figure below.
A three-phase sequence (ABC) is mandatory for the proper operation of the power converter.
Figure 31. AC connection and sequence
2.4.4 DC connection
The figure below shows the output DC connection. Ensure to apply the correct polarity.
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Figure 32. DC side load connection
2.5 Startup procedure
After connection and debugging initialization, you can perform the startup procedure.
An embedded finite state machine handles this procedure. It consists of different states that perform a preliminary
check of the converter and the precharge of the capacitor. Then the burst operation boosts the output voltage.
The STDES-VRECTFD reference design allows managing the complete startup procedure.
However, a controlled step-by-step procedure is available. It is useful during the preliminary test or after
hardware/sw modification.
Figure 33. Startup procedure
2.5.1 Controlled startup procedure
The AC power supply is slowly increased to verify the procedure step step-by-step as shown below.
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