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.
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A fourth generation IGBT co-packaged with an ultrafast, soft-recovery, anti-parallel diode is optimized for minimum saturation voltage and low operating frequencies (CE(on) is 1.4 V at 10 A. Tailored for extremely low forward-voltage drop and reverse leakage current, the co-packaged diodes across the low-side IGBTs are optimized to minimize losses during freewheeling and reverse recovery. The switching techniques in this design offer a number of advantages:
• Achieving high efficiency by permitting high-side and lowside IGBTs to be optimized separately.
• No freewheeling on the highside, co-packaged, soft-recovery diodes, eliminating unnecessary switching losses.
• Switching low-side IGBTs at a low frequency of 60 Hz, as conduction loss dominates these IGBTs.
• No cross conduction, since switching is done on a diagonal device pair at any time (Q1 and Q4 or Q2 and Q3). Hence, there’s no possibility of bus shoot-through, because the IGBTs on the same leg of the bridge never switch in a complementary fashion.
• Co-packaged, ultrafast, soft-recovery diodes across the low-side IGBTs can be optimized to minimize losses during freewheeling and reverse recovery.
As a result, it eliminates the requirement for an isolated power supply for the high-side drive. This translates into improved efficiency for the inverter and parts count reduction for the overall system. The bootstrap capacitors for these drivers get refreshed every switching cycle when current freewheels on the low-side IGBT’s co-package diodes.
Because high-side Q1 and Q2 co-packaged diodes aren’t subjected to the freewheeling current and the low-side Q3 and Q4 diodes exhibit mostly conduction loss and very little switching loss, overall system losses are minimized and system efficiency is maximized. The cross-conduction possibility also is eliminated since the switching is implemented on a diagonal device pair only at any time (Q1 and Q4 or Q2 and Q3).
In addition, each of the output driver ICs features a high-pulsecurrent buffer stage that’s designed to minimize driver cross-conduction. Furthermore, because the system operates from a single dc bus supply, it eliminates the need for a negative dc bus. For the overall system, all of these factors translate into higher efficiency and lower parts count. Fewer components also mean less space and a smaller bill of materials.
In this inverter design, a +20-V supply is first applied to power the microprocessor and the housekeeping circuits. For the source code that’s implemented, the 8-bit PIC18F1320 microcontroller used in this inverter scheme generates the signals for the IGBT drivers that eventually generate the signals to drive the IGBTs accordingly.
Speaking of drivers, the low-side and highside IGBT drivers used in this design are fabricated in a proprietary, advanced highvoltage IC process (G5 HVIC) and latch immune CMOS technologies to operate to 600 V. They also incorporate high-voltage level-shifting and termination techniques that enable the drivers to generate the appropriate gate-drive signals from low-voltage input coming from the microcontroller. The logic input is compatible with standard CMOS or low-power Schottky transistor-transistor logic (LSTTL) output, down to 3.3- V logic.
Ultrafast diodes D1 and D2 provide the path to charge capacitors C2 and C3 and ensure that the high-side drivers are correctly powered. During the positive output half-cycle, the high-side IGBT Q1 is sine PWM modulated, while the low-side Q4 is kept on (Fig. 4). Similarly, during the negative output half-cycle, the high-side Q2 is sine PWM modulated while the low-side Q3 is kept on. This switching technique produces a 60-Hz ac sine wave across output capacitor C4, following the LC filter.
With the inverter designed for an output of 500 W, measured ac output power was 480.1 W with a power loss of 14.4 W. The ac output voltage at 60 Hz was 117.8 V with 4.074-A output current. Figure 5 illustrates the 60-Hz waveform for this 500-W output.
The setup’s measured efficiency was 97.09%. Using a similar setup, the inverter was next tailored for 200-W output, and the conversion efficiency was measured again. The ac power at the load was 214 W with a power loss of 6.0 W. The 60-Hz output voltage was 124.6 V at 1.721-A output current. Conversion efficiency measured at this power rating was 97.28%. A similar efficiency performance was observed even at the lower end of the output power, which is 100 W.
Figure 6 shows the inverter power loss measured for output power levels going from about 100 to 500 W. When inverter efficiency was measured across a similar output power range for the same dc input, high output efficiency of greater than 97% was maintained across a broad range of output power, even though the power loss increases as the output power goes higher (Fig. 7).
In conclusion, with the right combination of drivers and lowand high-side IGBTs, this solar-power inverter design delivers a consistently high power-conversion efficiency performance from about 100-W output to nearly 500 W. Because the efficiency is high, the low power dissipation didn’t present any thermal-management challenges. Consequently, the demo board on which the drivers and high voltage IGBTs were mounted operated without a fan up to 500 W.2
REFERENCES 1. Chou, Wibawa, “Choose Your IGBTs Correctly for Solar Inverter Applications,” Power Electronics Technology, August 2008, p. 20.2. “DC to AC Inverter IGBT Demo Board” Wibawa T. Chou, applications engineer, holds a BSEE and MSEE from Ohio State university, Columbus, Ohio.
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