Achieving more efficient power conversion using SiC cascodes

By Zhongda Li, NPI Manager at UnitedSiC

Anyone who’s pulled a warm phone charger from a wall socket will sense that power conversion can be inefficient. Joules that were supposed to be transferred to the phone’s battery instead end up warming the atmosphere. Anyone who has a drawer full of such chargers for long-defunct mobile phones, tablets and other gadgets will also know that we are increasingly reliant on power conversion in our lives. It’s the combination of these two factors that makes it worthwhile for semiconductor companies and circuit designers to keeping try to boost the efficiency of power converters – even if each individual improvement appears small, the cumulative effect can be large.

One of the biggest sources of conversion losses comes from the switching circuits used in power converters, especially if they include MOSFETs or IGBTs. Seasoned designers know that the ‘hard switching’ that happens in many power converters leads to an inevitable overlap between voltage and current on transitions, which causes momentary periods of high power dissipation.

This issue is being addressed through the development of ‘soft-switching’ converters that try to transition at zero voltage or current. The latest versions of this approach are the LLC and phase-shifted full bridge (PSFB) circuit topologies shown in Figure 1.

Figure 1: LLC (left) and PSFB (right) converters

 

These ‘resonant’ converters take advantage of the fact that the current in an inductor cannot change abruptly since it is coupled to the magnetic field it induces. This makes it possible to separate voltage and current transitions so they don’t overlap and cause unnecessary power dissipation. This is quite easy to achieve when turning power switches On, using an approach known as zero voltage switching (ZVS). Unfortunately the approach doesn’t work when turning switches Off, and so designers have turned their attention to achieving zero current switching (ZCS). Implementing ZCS is so complex that doing so can outweigh its benefits, which means that turn-off transitions are often made using a ‘hard switch’.

 

Making LLC and PSFB converters viable

One reason for the increasing popularity of LLC and PSFB converters is that IGBTs and Si MOSFETs suffer too much dissipation when turned off hard, with IGBTs having a particular issue with their long ‘current tails’. Devices such as fast MOSFETs, and the wide band-gap devices made possible by materials such as silicon carbide (SiC) and gallium nitride (GaN), offer much faster turn-off transition speeds that minimize the overlap of voltage and current overlap and thus dissipation.  This has made LLC and PSFB topologies a practical alternative to traditional topologies.

Stringent efficiency targets for all sorts of equipment also make it commercially worthwhile to attain fractional percentage gains, and so semiconductor companies are developing devices that minimize turn-off losses. The key performance comparator here is the energy dissipated at turn-off, designated EOFF. This is made up of a combination of the current/voltage overlap causing dissipation in the switch channel and the energy required to charge the switch’s output capacitance (COSS). The energy stored in COSS is not lost, because it returns to the bulk capacitor, but its related charge and discharge currents add to conduction losses.

 

SiC cascodes enable optimum conversion performance

SiC cascodes offer minimal overall EOFF, with better performance on a range of parameters that affect the efficiency than IGBTs, Si MOSFETs and SiC MOSFETs (see Figure 2). This performance advantage is due to the device’s switching speed and very low COSS, a function of its relatively small die size. The performance of SiC cascodes is also nearly independent of temperature, unlike IGBTs whose switching losses increase strongly with rising temperature.

Figure 2: Comparison of a SiC cascode with other technologies

 

In switching circuits, the overall power dissipation is proportional to the operating frequency multiplied by the energy dissipated at turn-off, EOFF. Using SiC cascodes, designers can trade off these losses with the operating frequency to achieve the best system solution. Faster turn-off helps maintain the minimum deadtime that a resonant converter needs to maintain ZVS up to higher operating frequencies. And the SiC cascode has a very fast equivalent body diode that helps with efficiency, as it needs to conduct during resonant switching.

You rarely get something for nothing in circuit design, and in the case of SiC cascodes their high switching speeds may need to be moderated to avoid creating electromagnetic interference. This can be done by increasing the gate resistor value, at the cost of a knock-on effect on EOFF. Figure 3 shows how different values of RG affect EOFF for a UnitedSiC UJC1206K device. If high values of gate resistance cause unacceptably long delay times, designers can use an R-C snubber circuit. It is also possible to implement different gate resistor values for turn-on and turn-off by adding an extra diode (Figure 4).

Figure 3: The relationship between the gate resistance of a SiC cascode and EOFF

Figure 4: Adding a diode enables separate control of SiC cascode on and off times

 

The output capacitances of the switches in resonant converters form part of its resonant tank circuit. At a chosen resonant frequency, the presence of a high capacitance demands the use of a low inductance, which can be undesirable. Why is this? It’s because in LLC converters this would result in a high circulating magnetizing current, which wouldn’t contribute to power transfer but would generate conduction losses that reduce overall conversion efficiency. SiC cascodes have low values of COSS, which means that designers can take advantage of this lower capacitance if they want to, but add discrete capacitances if necessary – something which is much easier to deal with than trying to mitigate the effect of a high intrinsic COSS.

 

Convert to cascode conversion

The high switching speed, fast body diode, high temperature operation, low RDS(ON) and ruggedness of SiC cascodes make them a great solution for all switching circuit topologies. Their low EOFF also makes them a good match for the latest high-efficiency LLC and PSFB conversion topologies. As the struggle to eke out every fractional efficiency gain continues, now may be the time to consider converting to cascode conversion.

References:

UnitedSiC application note AN0014 – March 2017: 650V Cascode in LLC Second Stage Power Conversion for Servers

UnitedSiC application note AN0013 – May 2016: USCi Cascode in High Voltage Phase Shift Full Bridge

 

Computer-Aided Design and Optimization of High-Efficiency LLC Series Resonant Converter, IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 27, NO. 7, JULY 2012, Ruiyang Yu et al.

 

SiC JFET Cascode Loss Dependency on the MOSFET Output Capacitance and Performance Comparison with Trench IGBTs, University of Denmark, Pittini et al.

Author: Zhongda Li