Ultra-Fast 1200 V IGBTs Reduce Switching and Conduction Losses
A new family of 1200V IGBTs has been optimized for lowest switching losses and smoothest turn-off in higher frequency and motor control applications. This paper will explain new approaches such as the Solderable Front Metal (SFM) technology, which greatly extends power cycling capability, while dual-sided cooling further reduces power dissipation to provide a highly efficient solution.
By utilizing field stop thin wafer technology, a new generation of 1200V IGBTs are capable of achieving typical voltage drop (VCE(ON)) as low as 1.7V and typical fall time of less than 100 nsec at their nominal current ratings. The devices also exhibit a positive VCE(ON) temperature coefficient, which allows for easy paralleling. Another benefit of thin wafer technology is the reduction in junction-to-case thermal resistance (Rθ(j-c))and improvement in the transient thermal response of the IGBT.
Three types of devices are available:
- U or Ultrafast without short circuit rating
- K10 with 10 usec short circuit withstand capability
- SFM for Solderable Front Metal requiring no bond-wires for assembly
Instead of bond wires, interconnect of the SFM IGBT utilize copper straps for increase current handling capability, lower voltage drop and increase reliability. This technology also enables double-sided cooling similar to IR’s DirectFET® packaging allowing the heat generated to be dissipated from both the top and bottom of IGBT.
The U type IGBT without short circuit rating has an advantage of lower Vceon compared to its 10 µsec short circuit (K10) counterpart. At the same rated current, the U type IGBT will give 300mV less voltage drop compared to K10 IGBT. Fig. 1 shows current and VCE(ON) trade off between U and K10 IGBTs.
The U type is prefered for applications where the IGBTs will not see a direct short circuit due to the presence of an output inductor. This is the case for most UPS, renewable energy or welding applications. For motor drive application where the winding of the motor can short to ground or to another phase winding, K10 IGBTs are required. The 1200V K10 trench IGBTs are guaranteed to withstand short circuit for 10 µsec at TJ = 150oC with 600V across the IGBT. With the latest fast analog and digital controllers, this guaranteed short circuit time is more than adequate for the controller to detect and shutdown the gate to the IGBTs in less than 10 µsec in order to interrupt the short circuit current.
Design Flexibility
The 1200V trench IGBTs are available in discrete packaging (TO-247 and Super TO-247) and in wafer forms for engineers designing industrial power modules. In discrete packaging, IGBTs with and without copack diodes are available. The parts without diodes are introduced to give design engineers flexibility to choose their own diodes either for cost or performance reasons. An example of an application that does not require a copack diode is power factor correction or a boost converter. Here, the function of the diode across the IGBT is only as protection in case there is current flowing from emitter to collector of the IGBT. A small external diode rectifier will be sufficient, as opposed to a high performance copack diode that saves the cost of the overall system.
1200V trench IGBT discrete devices are switched at four times the nominal current with 960VDC across the device at final test in the factory. This ensures each IGBT is screened for possible defects. The highest current rating in discrete package is a 75A device that fits in super TO-247 (IRG7PSH73K10). This provides an alternative for design engineers to lower the cost of a system by switching from industrial power module to discrete devices. Another advantage with the availability of higher current rating discrete devices is paralleling devices becomes unnecessary.
Switching Characteristics
Fig. 2 shows a typical turn off event of the 1200V/20A,IRG7PH35UD, ultra-fast trench IGBT at junction temperature of 150 oC.
The crossover between voltage and current during turn off transition contributes to switching loss and needs to be minimized by making the fall time and tail current as small as possible. This generation IGBT is able to achieve a balance loss between conduction and switching at 20 kHz by utilizing both IR’s trench gate and thin wafer technologies. Another important feature is the low rate of current change (di/dt) which results in lower voltage spike at turn off. It can be seen on Fig. 2 that with a bus voltage of 600Vdc, the voltage overshoot across the IGBT is only slightly more than 700V, which is much lower than the breakdown capability of the device. Also, there is lack of voltage ringing at turn off which will contribute to overall system EMI reduction.
The diodes in the copack U type IGBT are 1200V fast recovery low QRR. Having a fast recovery diode is important in minimizing turn-on transition loss of the IGBT. The turn-on current on the IGBT is the combination of load current and reverse recovery current of diode copack on the complementary leg of the inverter. The shorter the turn-on transition, the lower IGBT turn-on loss will be. The turn-on transition of IRG7PH35UD at 600Vdc, 20A and at junction temperature of 150 oC is presented in Fig. 3.
Although the trade-off of low QRR diode is in its higher voltage drop (VF), the benefit of having low QRR still outweighs it. On most high frequency inverters, the copack diode will have to take the same IGBT peak current during the freewheeling period when the IGBT is turned off. However, the duty cycle is typically opposite to that of the IGBT. Fig. 4 shows a typical IRG7PH35UD copack diode recovery waveform at 600V, 20A at TJ = 150 oC. Here, it can be seen, the recovery of the diode is very fast and the recovery current decays to zero and transfers to the IGBT in about 100 nsec.
Design Implementation
Half-bridge topology is the basic building block of any power converter. 1200V IGBTs are typically used for such converters. For solar or UPS inverters, the half-bridge inverter with output inductor and capacitor form the final output. Twenty kilohertz is typically the switching frequency of these inverters. The bus voltage of the inverter must be lower than the breakdown voltage of the IGBTs and is typically ±400V dc. Therefore, 1200V IGBTs are ideal choice with enough voltage margin to handle over-voltage during transient conditions. Fig. 5 shows the basic schematic and IGBT current waveform of a sine wave half- bridge inverter
For a half-bridge inverter the losses will be contributed by both the IGBT and Diode copack.
IGBT Power Losses =
Diode Power Losses =
The above equations calculate conduction and switching energy losses of the IGBT and diode at each switching cycle. By taking the sum of the energy losses over one cycle (T), the power losses of the IGBT and diode can be obtained.
VCE, VF, EON, EOFF and ERR with respect to current are parameters that can be obtained from 1200V IGBT datasheets. More precise values can be obtained by measuring these parameters on the actual circuit implementation once the system is built. However, in most cases, this takes a lot of resources and therefore datasheet parameters are good starting point to estimate losses in the inverter.
Using this approach, a half- bridge inverter with the following specifications is to be designed: VDC = ±400VDC, VOUT= 230VRMS, FSW = 20 kHz. IRG7PH42UD is chosen to illustrate how the previous power loss equations can be implemented on a simple spreadsheet program. Fig. 6 shows the result of the calculation with output current varying from 5 Arms to 20 ARMS.
The result shows that 1200V U IGBT has balanced conduction and switching losses which help in designing optimized system at 20 kHz. It can also be observed that the contribution of diode losses is not insignificant. Low voltage drop (VF) and low reverse recovery energy (ERR) of the copack diode are important parameters in the design of a high frequency inverter.
Recently released Generation 7, 1200V field stop trench IGBTs offer balanced conduction and switching losses by reduction in Vce(on), fall time and tail current. Copackaged with low QRR diodes, the package part version will reduce power dissipation further. This generation is available as both discrete versions in plastic packages and in wafer forms for engineers designing industrial power modules.