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Posted on 29 June 2019

1st SiC JFET Easy1B Module

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Very low switching losses enable ultra low loss power electronic designs

Reducing losses is one of the most challenging trends in power electronics. Wide band gap devices like SiC switches are known to show best performance in terms of lowering conduction and switching losses. It is one decade ago, that Infineon introduced the SiC Schottky barrier diode to the market. Now the SiC JFET is matured and convinces with low power loss.

By Marc Buschkühle and Daniel Domes, Infineon Technologies AG, Warstein

 

In combination with all advantages of Infineon’s Easy1B PressFIT housing concept, a highly efficient, reliable, robust and ease of use solution can be shown.

SiC power devices

The integration of SiC power devices into a power module is not a completely new task. Some years ago, the loss saving potential of SiC Schottky barrier diodes was demonstrated reducing the turn-on energy of IGBTs and the recovery losses of the diodes, respectively. Since the SiC JFET is available as a product very soon, the full SiC loss saving potential can be leveraged by getting rid of IGBTs tail current losses as well. Target applications for the new SiC JFET module will be the efficiency sensitive field of renewable energy and UPS systems.

Simple Direct Driven JFET Topology

For the new 1200V/30A SiC JFET module a half bridge topology was chosen to suit most customer needs. This enables a fast time to market and drives efficiency up to a new level.

In Figure 1 the module schematic, the Easy1B module itself and an exemplary chip arrangement can be seen. One switch consists of three single SiC JFET chips with maximum on-state resistance at 25°C junction temperature of 100mΩ each.

Direct Driven JFET, a) Module schematic, b) 1200V/30A Easy1B module FF30R12W1J1_B11, c) Exemplary chip arrangement

Two parallel low voltage p-channel MOSFETs with just 3-4mΩ each are connected in series to the JFET group. The basic approach is the so-called ”Direct Driven JFET”.

The main idea behind this, is to permanently turn on the low voltage MOSFET in normal operation. Then, the JFET is controlled by means of its own gate drive stage performing the desired switching caused by the logic control signal. Compared to a conventional Cascode circuit, the Direct Driven JFET approach allows lowest dynamic losses combined with a good controllability of the JFET switching transients.

The p-channel MOSFET was chosen because it allows the lowest stray inductance in the JFET gate drive path and makes an integration of the whole driver circuitry in one single gate drive IC most simple.

Regarding the last mentioned aspect, Infineon is currently developing an IC solution, which will allow the customer to operate the Direct Driven JFET circuit like a conventional normally-off switch. Furthermore, all the safety aspects will be managed inside the IC without the need of taking care from customer side.

Optimized Module design

Since a family of silicon carbide diodes is already in high volume production used in Easy modules ( Figure 2 ), as a logical consequence the Infineon silicon carbide JFET devices will be introduced in conjunction with the well known compact Easy1B housing.

Highly flexible Easy module Family with up to 165 possible Pin positions

The Infineon Easy module concept was commercialized many years ago and has proven flexibility, quality and reliability in daily use in numerous high volume industrial applications. Furthermore the success story of the EasyB module continues as it will be used for automotive applications as well (www.infineon.com/autoeasy).

For simplified handling and highest quality of interconnections, the modules are equipped with PressFIT contacts. The mounting effort can be reduced enormously compared to standard soldering alternatives and production costs can be decreased.

Due to the flexible module Pin-Grid of 89 possible Pin positions an optimized layout with lowest stray inductances is done for the SiCJFET Easy1B module.

This is one fundamental basic in handling of high speed switching devices like SiC JFET. With simulations and tests within the lab an optimized layout down to 10-15 nH is achieved.

Low ohmic Static behavior

The output characteristic of the 1200V/30A SiC JFET module can be seen in Figure 3. Regardless whether the module is operated in forward or reverse direction, for the gate-source-voltage vGS=0 the JFET behaves pure ohmic. If the JFET is turned off in reverse conduction mode (e.g. vGS=-19V), the intrinsic body diode can take over the load current. This is a very important fact, since there is no need for an additional freewheeling diode which would result in additional module area and costs.

Low ohmic Static behavior of the 1200V 30A SiC JFET module

Another advantage is given by using the JFET channel even if the load current flows in reverse direction. In contrast to standard diodes the low ohmic static behavior can be seen as well.

Lowest switching losses

The switching waveforms of the JFET module can be seen in Figure 4. Compared to the Direct Driven JFET topology with n-channel MOSFET, the p-channel MOSFET solution introduces no additional stray inductances to the JFET’s gate drive circuit resulting in very fast turn-on and turn-off responses.

Switching waveforms of the Direct Driven JFET module at TJ=125°C, VDC=600V, iD=30A and RG,ext=0

The switching waveforms show the current commutation between lowside JFET group and highside JFET body diodes. With respect to the recovery current in the turn on waveform, current only changes while the Drain-Source-voltage drops. Therefore, the main part of the reverse recovery current is of capacitive nature. The commutation against the JFET body diode can be regarded as quite similar compared to low loss Schottky diode commutation.

In Figure 5 the low switching losses of the Direct Driven JFET module are depicted. Due to the absence of diode tail current, no recovery- losses occur.

Dynamic losses of the half bridge JFET module at TJ=125°C and RG,ext=0

Efficiency exceeds 99%

To demonstrate the loss-saving-potential of the SiC JFET module, a total loss calculation for a 22kVA three-phase two-level inverter was done. As a challenger for the JFET-module, a 1200V/25A High- Speed3-IGBT equipped with two 1200V/7.5A SiC Schottky barrier diodes was chosen. The turn-on and turn-off energies for switching the HighSpeed3-IGBT at 600V, 30A and TJ=125°C are measured to be 916µJ (JFET 490µJ) and 1480µJ (JFET 117µJ), respectively.

Like shown in Figure 6, the excellent JFET properties enable loss reduction down to 35% of those of fastest IGBT technology combined with SiC-diodes.

Calculated total inverter losses and efficiency direct driven JFET moducle

Up to approximately 35kHz switching frequency, the JFET inverter’s efficiency exceeds 99%.

Apart from the efficiency point of view, the JFET module is ideally suited for power electronic designs with high switching frequencies targeting the decrease of passives volume for achieving high power densities and lowered overall system costs.

Summary

A new normally-on SiC JFET based Easy1B Half bridge module was shown. The Direct Driven JFET topology makes use of series connected p-channel MOSFETs. Combined with an adequate gate drive circuit, the Direct Driven JFET approach can be regarded as practically normally-off. The very low switching losses of the JFET module enable ultra low loss power electronic designs. Benchmarked on fastest IGBT technology combined with SiC Schottky barrier diodes, the SiC JFET module can decrease overall inverter losses down to 35% by providing efficiencies of more than 99% for switching frequencies up to 35kHz.

 

 

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