Using Low EOFF SiC Cascodes in Soft Switching LLC and PSFB Circuits
This article looks at soft-switch converters such as LLC and PSFB types and the low switching energy loss EOFF.
SiC cascodes can provide an array of benefits. One benefit that is very relevant to soft-switched converters such as LLC and PSFB types is very low switching energy loss EOFF. When the last percentage points of efficiency are being extracted, a low value for EOFF really helps. This article looks at the topologies and their losses.
Explaining the "Dark Side"
Cosmologists say that 73 percent of the universe is comprised of ‘dark energy’ and they have no clue what it is, except that it is somehow fuelling an accelerating expansion of the universe. Junior power design engineers sometimes think they know the answer – it’s that seemingly inexplicable extra loss they get from switching transistors at high power. All those microjoules multiplied by the number of MOSFETs and IGBTs in the world add up to…well, not enough actually.
Shedding Light on Switching Losses
Engineers have shed light on switching losses with a better understanding of the effects realizing that ‘hard switching’ in power converters leads to an inevitable overlap between voltage and current on transitions that give momentarily high dissipation. The solution has been ‘soft-switching’ converters that try to transition at zero voltage or current, the latest incarnations being LLC and phase shifted full bridge (PSFB) types. The comparison can be seen in Figure 1.
Figure 1. LLC (left) and PSFB (right) converters
These topologies are ‘resonant’ converters that utilize the fact that currents in inductors cannot change abruptly. Therefore the transitions of voltage and current can be separated so they don’t overlap and cause dissipation. This can be done quite easily for turn-on of power switches so that current only flows after voltage has dropped to zero. This is ‘zero voltage switching’ or ZVS. For turn-off, ZVS cannot exist and the only hope is ‘Zero Current Switching’ or ZCS. This is complex to achieve and the complexity normally outweighs the benefits so the turn-off transition is out of necessity typically a ‘hard switch’.
SiC Makes LLC and PSFB Converters Viable
One of the reasons that LLC and PSFB converters have now become mainstream is that the older technologies of switches such as IGBTs and Si-MOSFETs produced way too much dissipation with a hard turn-off. IGBTs particularly were problematic with their long ‘current tails’. However, devices such as fast MOSFETs, and the wide band-gap technologies of silicon carbide (SiC) and gallium nitride (GaN) have made the topologies viable with turn-off transition speeds that give minimum voltage/current overlap and consequent dissipation.
As efficiency targets are so competitive that mere fractions of a percent are considered worth chasing, devices that minimize the turn-off losses are being developed. To compare switches, a value EOFF is defined which is the energy dissipated at turn-off, a combination of current/voltage overlap causing dissipation in the switch channel and the energy required to charge the switch output capacitance COSS. Strictly, energy in COSS is not actually ‘lost’ as it returns to the bulk capacitor but does cause charge/discharge currents which add to conduction losses.
SiC Cascodes
A device that gives minimal overall EOFF is the silicon carbide cascode. In fact, it scores best against IGBTs, Si-MOSFETs and SiC MOSFETs for a range of parameters that affect efficiency (Figure 2). This high performance is due to the switching speed and very low value of COSS, a result of the relatively small die size for SiC cascodes. A significant effect also is that the already-poor IGBT switching losses increase strongly with temperature whereas the cascode and MOSFETs are nearly independent.
Figure 2. A comparison of SiC cascodes against other technologies.
The net effect of using the SiC cascode is that you can trade turn-off losses with operating frequency for an optimum system solution (power dissipated is EOFF x frequency). Also, faster turn-off helps maintain the minimum deadtime that a resonant converter needs to maintain ZVS up to higher frequencies. The SiC cascode also has a very fast equivalent body diode that helps with efficiency as it needs to conduct during resonant switching.
The dramatic switching speed of the SiC cascode sometimes has to be tamed for EMC reasons but this is easy to do with an increase in gate resistor value at the expense of EOFF. Figure 3 shows how different values of RG affect EOFF for a UnitedSiC UJC1206K device. Alternatively, an R-C snubber may be employed if high values of RG cause unacceptably long delay times. If desired, separate gate resistor values for turn-on and off can be implemented simply with an extra diode (Figure 4).
Figure 3. SiC cascode RG controls EOFF
Figure 4. A diode enables the control of a SiC cascode on and off times separately.
The output capacitance of the switches in resonant converters forms part of the resonant tank circuit. For a chosen resonant frequency, a high capacitance forces a lower inductance which is not always desirable. For example, in LLC converters this would result in high circulating magnetizing current which doesn’t contribute to the transferred power and just generates conduction losses, hurting efficiency. The low value of COSS in SiC cascodes really helps here. If the circuit needs the capacitance to be higher, it can simply be added as a discrete component – it can’t be taken away.
No More Lost Energy
SiC cascodes are a great solution for all switching circuit topologies with their speed, fast body diode, high-temperature operation, low RDSON, and ruggedness. You can add to that their low EOFF for the perfect match to the latest high-efficiency LLC and PSFB conversion topologies.
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.
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