[Design View / Design Solution]
Build An Efficient 500-W Solar-Power Inverter Using IGBTs
Combining optimized low- and high-side high-voltage IGBTs in a full-bridge topology, a dc-ac inverter uses a solar module to efficiently generate 500 W at 120-V/60-Hz single-phase ac output from 200-V dc input generated by a photovoltaic panel.
With the global trend toward green power and affordable energy gaining momentum, applications such as home appliances, lighting, and power tools, as well as other industrial equipment and uninterruptible power systems (UPSs), are rapidly tapping the benefits of solar energy, converting the sun’s power to the desired alternating current (ac) or direct current (dc) at the required voltage.
To efficiently generate the desired output voltage and current for these applications, however, the power inverter needs the right combination of controller, driver, and output power devices. This dc-ac inverter design is optimized for a power output of 500 W with a single-phase sinusoidal waveform of 120 V and 60-Hz frequency. The design’s 200-V dc input can come from a dc-dc voltage converter connected to a solar array panel.
For this type of an application, a variety of advanced power devices such as metal-oxide semiconductor FETs (MOSFETs), bipolar junction transistors (BJTs), and insulated-gate bipolar transistors (IGBTs) is available. To achieve the best conversion efficiency and performance, though, choosing the right power transistors for this solar power inverter can be challenging and time consuming.
Over the years, though, studies have shown that an IGBT offers many advantages over other power device options. Some of these include higher current-handling capability, easy gate control using voltage instead of current, and the ability to co-pack an ultrafast recovery diode for faster turn-off.
The IGBT is a minority carrier device whose turn-off time is determined by how quickly the minority carriers recombine. Hence, with recent improvements in process technology and device structure, its switching characteristics have been significantly enhanced. In addition, it offers superior conduction characteristics and a wide safe operating area (SOA), and it’s very rugged. Based on these fundamental benefits, this power inverter uses IGBTs as the power switches of choice.
Because the topology employed for the power inverter is fullbridge, this solar inverter design uses four high-voltage IGBTs (Fig. 1). While transistors Q1 and Q2 are designated as high-side IGBTs, Q3 and Q4 are labeled as low-side power devices. To keep the total power losses low and power conversion efficiency high, this dc-dc inverter solution combines low- and high-side IGBTs to generate a single-phase ac pure sinusoidal waveform at 60 Hz. Another article written by this author discusses how to select the high-voltage IGBTs appropriately for this solar-power inverter application.1
SWITCHING IGBTs In essence, to keep the harmonics low and the power dissipation minimal, the inverter uses pulse-width modulation (PWM) for high-side IGBTs, while low-side power devices are commutated at 60 Hz. By using PWM frequency at or above 20 kHz with 50/60-Hz modulation for high-side IGBTs, the output inductors L1 and L2 are kept practically small to provide effective filtering of the harmonics. In addition, the audible noise from the inverter is minimized because the switching frequency is above the human hearing spectrum.
Examining a variety of switching techniques and IGBT blends, the best combination for attaining the lowest power losses and highest inverter performance is to use ultrafast trench IGBTs for high-side transistors and standard-speed planar devices for the low-side section (Fig. 2).
Compared to fast and standard-speed planar devices, the ultrafast trench IGBTs switching at 20 kHz provide the lowest total combination of conduction and switching power dissipation. Likewise, for low-side switching, standard-speed planar IGBTs at 60 Hz result in the lowest level of power dissipation.
When investigating the switching characteristics of a highvoltage (600 V) ultrafast trench IGBT, it becomes clear that these devices are optimized for switching at 20 kHz. Moreover, they offer minimum switching loss at these frequencies, including lower collector-to-emitter saturation voltage (VCE(on)) and total switching energy (ETS). This keeps the combined conduction and switching power losses to a minimum. As a result, ultrafast trench IGBTs, such as the IRGB4062DPBF, are selected as high-side power devices.
In fact, to further assist in keeping the switching losses low, the IRGB4062DPBF is co-packaged with an ultrafast soft-recovery diode. Another benefit of switching at 20 kHz for high-side transistors is that the output inductors are reasonably small, and filtering the harmonics is easy. In addition, these IGBTs don’t require short-circuit rating because the output inductors L1 and L2 limit the current di/dt when the inverter output is shorted, giving the controller enough time to react appropriately.
Furthermore, short-circuit-rated IGBTs offer higher VCE(on) and higher ETS than non-short-circuit-rated IGBTs of the same dimensions. Consequently, with higher VCE(on) and higher ETS, the short-circuit-rated IGBTs contribute higher power losses to lower the efficiency of the power inverter.
Besides lower conduction and switching losses and increased current density from the same package, ultrafast trench IGBTs offer a square reverse-bias operating area and a maximum junction temperature of 175°C, as well as the ability to withstand four times the rated current.
Unlike the high-side devices, conduction losses dominate the low-side IGBTs. Since the frequency of operation is only 60 Hz for low-side transistors, switching losses are insignificant for these devices. Standard-speed planar IGBTs are tailored for low frequencies and lower conduction losses.1 As a result, with the low-side devices switching at 60 Hz, the lowest level of power dissipation for these IGBTs is achieved using standard-speed planar IGBTs.
Since the switching loss for these devices is insignificant, it doesn’t affect the total dissipation for standard-speed planar IGBTs. Keeping that in mind, the standard-speed IGBT IRG4BC20SD is, therefore, the right choice for low-side power devices.
If 200V from a solar panel is possible, great. But something that could handle lower DC voltages and provide isolation is a toroidal transformer. The larger transformers (800VA and up) have efficiencies that exceed 95%. A 48V panel would not exceed voltage levels that require special protection for personnel, and can be used to provide a 25V AC or 30V AC output using the schematic. This is fed into the low voltage side of the transformer, giving an isolated 110VAC (and 220VAC if needed) output.
At these low input voltages, substituting a MOSFET for an IGBT is also an option.
Larry Pierson -November 05, 2009
Lose the unnecessary software stuff and I'll buy your parts.
Ed -September 03, 2009
I wanted to read this article, but the flashing crap on the screen is such a distraction that I didn't. Don't you know that this sort of stuff looks good to you, but readers find it obnoxious?
Anonymous -September 03, 2009
This is an igenious design. However, there is one small issue: grounding! Grounding isn't need for a portable system, but if this idea is going to be used as the basis for a grid tied system, it is going to be an issue. One side of the AC mains is grounded and one side of the solar panel should also be grounded. This tends to limit possible topologies.
JRT -September 02, 2009
Ok, but how does it scale if we want more like 100,000 Watts?
Tracy -September 02, 2009
SOLAR 200 VDC input? What solar technology supplies 200 VDC?
Anonymous -September 02, 2009
I am curious about the 200 VDC input that the article talks about, is that really the input bus voltage from the solar cells(there is a little about a DC-DC converter) or a typo?
Parkolay -August 25, 2009
Very nice design. Well done. Efficiency would further improve by tampering with the waveform itself. A good start is reading Don Lancaster's Magic Sinewave articles at Don's website. When you want to use the inverter to put power back into the grid you have to synchronise it with the grid sinus wave and switch off when the grid power is down. Also filtering to comply to conductive and radiated emissions is necessary. This will increase power loss.
Cuno -August 24, 2009
Very good design. The efficiency numbers are surprising at the mentioned wattage. Would the efficiency be the same if MOSFETs were used in place of IGBTs?
Santosh -August 20, 2009
As the previous comment said - very good article. However, not being a great programmer, it would sure help to have the source or compiled code available for the micro.
Any chance ? Thnx in advance, Bruce
Bruce L -August 20, 2009
NIL
WAHID -August 14, 2009
This looks like an excellent design, however, for anyone thinking about this as a 'do it yourself' project to connect PV to the GRID, this design would not generally be acceptable. Grid Tie inverters are designed and tested to IEEE 1547, IEEE 1547.1 and UL 1741 standards to provide protection to utility workers, emergency personnel and the general public. This design does not incorporate these features.
This should only be used for a stand alone system, and never paralleled to the local utility system.
Dave Bassett -August 13, 2009
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