TDPV1000E0C1
Application Note:
TDPV1000E0C1 Single-Phase Inverter Evaluation Board
1.
Introduction
The TDPV1000E0C1 inverter kit from Transphorm provides an easy way to evaluate the
performance advantages of GaN power transistors in various inverter applications, such as solar
and UPS. The kit provides the main features of a single-phase inverter in a proven, functional
configuration, operating at or above 100kHz. At the core of the inverter are four GaN transistors
configured as a full bridge. These are tightly coupled to gate-drive circuits on a board which also
includes flexible microcontroller options and convenient communication connection to a PC.
The switch-mode power signals are filtered to provide a pure sinusoidal output.
Fig. 1. Single-Phase Inverter Evaluation Board
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TDPV1000E0C1
The control portion of the circuit is designed around the popular C2000
TM*
family of
microcontrollers from Texas Instruments.
information directly from TI.
Source code is available along with related support
In addition to this general resource, however, Transphorm
provides original firmware which comes loaded in flash on the microcontroller. The source
code, configured as a complete project, is also provided on the USB memory stick which comes
with the kit.
This project is a convenient starting point for further developments.
The
microcontroller itself resides on a small, removable control card, supplied by TI, so that different
C2000 devices may be used if desired.
provided on the USB memory stick.
*C2000™ is a trademark of Texas Instruments Incorporated.
The schematic for the TDPV1000E0C1 circuit board is
Kit Contents
The kit comprises
A TDPV1000E0C1 single-phase inverter assembly
A Texas Instruments F28035 controlCARD
A 12V power supply with universal AC adaptors
Related media (documentation and software) on a USB memory stick
Cable for, high-voltage DC input
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TDPV1000E0C1
Warning
While this kit provides the main features of an inverter, it is not intended to be a finished
product. Our hope is that this will be a tool which allows you to quickly explore ideas which can
be incorporated in your own inverter design.
which should be kept in mind:
Along with this explanation go a few warnings
To keep the design simple and to provide ready access to signals of interest, high-voltages are
present on exposed nodes. It is up to you to provide adequate safeguards against accidental
contact, or use by unqualified personnel, in accordance with your own lab standards.
There is no short-circuit or over-current protection provided at the output. Current-sense devices
are connected to the AC outputs, and may be used for over-current protection, but it should not
be assumed that the firmware, as delivered, includes such a feature.
2. TDPV1000E0C1 Input/output Specifications:
•
Input: 0-400Vdc:
•
Output: Vdc /
2
Vrms at 50/60Hz
*
, up to 1000VA;
•
PWM Frequency: 100kHz to 200 kHz
**
•
Auxiliary Supply (Vgg): 12Vdc.
*
The output frequency may be changed in the software. As delivered it is 60Hz.
The switching frequency may be changed in the software. As delivered it is 100kHz.
**
3. Circuit Description
Overview
Refer to Figure 2 for a block diagram of the inverter circuit. A detailed schematic is also
provided in pdf format on the USB stick which comes with the kit.
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TDPV1000E0C1
The TDPV1000E0I inverter is a simple full-bridge inverter. Two GaN half bridges are driven
with pulse-width modulated command signals to create the sinusoidally varying output. The
output filter largely removes the switching frequency, leaving the 50/60Hz fundamental sinusoid.
The high-frequency (100kHz+) PWM signals are generated by the TI microcontroller and
connected directly to high-speed, high-voltage gate drivers.
A connection for external
communication to the microcontroller is provided by an isolated USB interface. Except for the
high-voltage supply for the power stage, all required voltages for the control circuitry are derived
from one 12V input.
Fig .2. Circuit block diagram
The inverter takes advantage of diode-free operation*, in which the freewheeling current is
carried by the GaN HEMTs themselves, without the need of additional freewheeling diodes.
*US patent 7,965,126 B2
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TDPV1000E0C1
For minimum conduction loss, the gates of the transistors are enhanced while they carry the
freewheeling current.
The high and low-side Vgs waveforms are therefore pairs of non-
overlapping pulses, as illustrated in Figure 3.
Figure 3: non-overlapping gate-drive pulse. A is a deadtime set in the firmware
Gate Drivers
High-voltage integrated drivers supply the gate-drive signals for the high and low-side power
transistors. These are 600V high-and-low-side drivers (Silicon Labs Si8230 family), specifically
chosen for high-speed operation without automatic deadtime insertion. The deadtime between
turn-off of one transistor in a half bridge and turn-on of its mate is set in the firmware.
Output Filter
A simple LCL filter on the output (L3, L4, C37, and C54-57) attenuates the switching frequency,
producing a clean sinusoidal waveform for output connections at terminals J4 and J5. The filter
inductors and capacitors used on the demo board were chosen to provide an optimal combination
of benefits: low loss, good attenuation of the switching frequency, and small size. Consult the
schematic and/or bill of materials to verify values, but in general the cutoff frequency will be
around 5-10kHz, to accommodate 100kHz switching. The inductors have powder cores with
relatively low permeability (60-90) and soft saturation characteristics. The inductors and/or
capacitors can be changed to evaluate different filter designs.
Current sensing
Hall sensors U8 and U10 provide linear current feedback to the microcontroller. These signals
could be used to control output power flow, and/or to protect against short circuits.
The
firmware provided with the kit, however, does not actually make use of this feedback. Note that
these are placed at an intermediate point of the output filter Refer to the bill of materials to
confirm the sensor part numbers, but typical would be the Allegro ACS712-20A sensor, which
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