New Intelligent Power Modules Pit Soft Recovery Against Radiated Emissions
A new generation of IPMs from ROHM Semiconductor claims "best-in-class," but can it deliver when compared to other IPMs in the industry?
Achieving low-loss conversion has always been the prominent merit of power converters. However, conducted and radiated emissions are the arch-nemesis that designers must combat, both for system interoperability and getting their designs to market.
Recently, ROHM has released a family of new Intelligent Power Module (IPM) inverters, the 600V IGBT BM6437x series, which claims to have best-in-class low noise characteristics and a reduction in losses compared to previous generation technology.
The top-side pad exposed DIP-IPM. Image used courtesy of ROHM Semiconductor
Before digging into the specifics of this new family, it is necessary to briefly talk about what an IPM is, how IGBTs fit in, and how these devices achieve lower noise characteristics.
What Are Intelligent Power Modules (IPMs)?
Despite the name's implications, there is no ‘intelligence’ to these devices. Instead, it is the intelligent integration of feature-rich capabilities that lend this power inverter class its name.
In addition to their inverter function, the BM6437x series IPMs have under-voltage (UV) and short circuit current protection (SCP), as well as a highly accurate (±2℃) temperature sensing capability in one integrated dual-inline package.
An example of an optoisolated application circuit schematic for the BM64x series Image used courtesy of ROHM Semiconductor
The gate drivers are IGBT-based (insulated-gate bipolar transistors), offering high-voltage and high-current drive capacity, improving switching speed, and reducing high-power applications loss.
By using IGBT technology, ROHM's new IPM series can gain the benefits that come with it, including low noise characteristics.
Achieving Low Noise Characteristics with IPM Circuits
Considering the importance of electromagnetic compliance standards in getting a design to market, reducing aggregate sources of noise (both conducted and radiated) is critical.
ROHM’s claims their new IPM family has better than 6dB improvement in radiated emissions when compared to competitors in the same product class.
How do they achieve this?
According to ROHM, it is through the use of the ‘soft’ recovery characteristics of the integrated fast recovery diodes (FRD) and the optimization of the IGBTs.
A chart of the radiated emissions for the new BM6437x series. Image used courtesy of ROHM Semiconductor
When a diode enters the reverse conduction region, there is a period called the recovery time after the forward conduction cycle. Total recovery time (Trr) is a crucial parameter to the operating performance of a diode.
The critical consideration inside this recovery time is the di/dt rate. A high di/dt will result in additional noise (EMI issues). ROHM’s new series is said to have a ‘soft’ recovery, meaning a lower di/dt.
The BM6437x series has lower losses than the previous generation and favorable noise parameters. However, how does it stack up with the rest of the industry?
Industry Component Round-up: Selecting an IPM
An overview of the electrical characteristics of Mitsubishi’s PSS20S93E6-AG, STMicroelectronics's STGIB15CH60TS-E, and the ROHM’s new BM64375S might help to narrow down the selection process for your next inverter application.
All three IPMs are 600V/20A IGBT inverters, with UV & SCP, and over-temperature protection.
For switching losses, both the BM64375S and the PSS20S93E6-AG have similar Tc(on) cross-over timing (0.35 µs and 0.5 µs at 20A, respectively). Meanwhile, the STGIB15CH60TS-E datasheet indicates a Tc(on) response of 0.16 µs at 15A.
Considering Trr and its relation to Tc(on), the BM64375S has the best reverse recovery time at 100 ns (compared to ~300 ns for the other two models).
A switching characteristics timing diagram for the BM64375S. Image used courtesy of ROHM Semiconductor
Outside of the datasheet, there are radiated emissions profiles to consider. Mitsubishi's version 7 Super Mini DIPIPM also indicates an improvement in noise reduction over their previous generation technology.
Mitsubishi’s Super Mini IPM modules generational noise improvement. Image used courtesy of Mitsubishi
All things being equal (note: test setups, evaluation boards, and EMI chambers are never precisely equal), the BM64375S shows a qualitatively lower radiated emissions profile from 30 MHz to 100 MHz over the Super Mini series 7 IPM.
Based on some of the electrical characteristics covered today and the inclusion of the new "best-in-class" noise characteristics, the ROHM BM6437x series appears to be a competitive IGBT-based IPM for home appliances, small capacity motors, and industrial applications.
Featured image (screenshot) used courtesy of ROHM Semiconductor