Totem poles and teapots
Our industry is sprinkled with obscure terminology which old hands happily live with, but which sometimes puzzles newcomers. What’s a First Nation monumental carving got to do with transistor arrangements? Why are four diodes called a ‘bridge’ and what’s the point of a half-bridge? Sounds as useful as a chocolate teapot. Totem poles and (lack of) rectifier bridges, though, are the ingredients for the current best arrangement to convert AC mains to a constant DC bus voltage with power factor correction (PFC) in applications such as EV on-board chargers.
Evolution of PFC stages
Switched-mode AC-DC converters rectify mains voltage then chop the resultant high voltage DC at high frequency for conversion through a transformer down to lower voltages. Current draw from the mains, though, is far from sinusoidal, as bulk DC smoothing capacitors get ‘topped up’ with charge only at the peak of the mains voltage. The result is a poor mains ‘power factor’. Series 50/60 Hz inductors are a simple fix but are big and lossy, so an active switching circuit has been a standard approach which takes rectified mains voltage and boosts it to a fixed DC while controlling line current to be sinusoidal (Figure 1, upper). At high power this can get lossy; you can see that there are three diodes in the current path, D1, D4, D5 or D2, D3, D5 depending on mains polarity, each dropping voltage and dissipating power.
MOSFETs can replace diodes
If we replace D5 with a similar switch to Q1, the circuit can simplify so that Q1 and Q2 act as boost switch and synchronous rectifier, swapping their function on alternate mains polarities (Figure 1, middle). Now there is only one diode and the RDS(ON) of a switch in series with the current, reducing conduction losses considerably. You can even take it a step further and replace D1 and D2 with synchronous switches for extra efficiency gain. (Figure 1, lower).
Evolution of PFC circuits
Body diodes can conduct
There’s a catch though: Q1 and Q2 must always have a ‘dead time’ when neither conducts so that you don’t get catastrophic ‘shoot-through’ currents. During this dead time, the inherent body diode of the MOSFET acting as a rectifier, Q1 or Q2, conducts the full output current. When the device is reverse-biased in the next phase of the switching cycle, a large ‘reverse recovery’ current flows, causing power dissipation and EMI problems, canceling efficiency gains. High voltage MOSFETs can have particularly poor body diode reverse recovery characteristics so, for that reason, the bridgeless totem pole circuit has had limited use at higher powers. This assumes continuous conduction mode (CCM). Critical and discontinuous modes would not force body diode conduction but would be unsuitable at high power due to the excessive peak currents involved.
Is WBG the solution?
Things have changed now with the appearance of wide bandgap switches. SiC MOSFETs promise low channel conduction losses, high speed and a fast body diode. However, the forward voltage of the diode can be 2.5 - 3 V giving it high conduction losses. Stored energy EOSS in device capacitance is also typically twice the value of an equivalent Si-MOSFET, giving extra switching loss. Enhancement-mode GaN devices are a contender as they have no body diode but have nearly twice the normalized die area to ON-resistance RDSA than SiC MOSFETs and have no avalanche or short circuit rating, making their reliability in real applications a concern. Both SiC MOSFETs and E-GaN devices have critical gate drive voltages for reliable and efficient operation.
SiC cascodes - the practical WBG solution
There is a way to leverage the advantages of wide bandgap, though, by using SiC cascodes. These are a combination of high voltage SiC JFET with a high performance co-packaged, low voltage Si-MOSFET. They are normally-OFF devices with avalanche and short circuit ratings. Low switching losses are a feature with extremely low input, output and Miller capacitances and EOSS, stemming from the small die size – SiC cascodes have a normalized die size with ON-resistance about 3-4X better than E-GaN, SiC MOSFETs or 10X better than Si superjunction MOSFETs.
The Si-MOSFET in the SiC cascode introduces a body diode but, being a low voltage type, the diode can be extremely fast, giving low reverse recovery current and loss. Figure 2 compares the recovery characteristics of a 650 V rated UnitedSiC UJC06505T and a 650 V IPP65R045C7 Silicon superjunction MOSFET showing the dramatic difference, about 60 times less recovered charge.
Reverse recovery characteristics – Si cascode v. Si-MOSFET
Gate drive for the SiC cascode is uncritical with operating levels of typically 0 - 12 V and absolute maximums of +/-25 V.
So, do SiC cascodes deliver in practical circuits? A target would be to achieve the 80PLUS Titanium efficiency standard requiring a complete converter to exceed 96% at high line and half load. If the mains conversion stage reaches a challenging but realistic 97.5%, the PFC stage would need to exceed 98.5%. A demo board from UnitedSiC rated at 1.5 kW, using UJC06505K SiC cascodes running at 100 kHz, meets the brief with some margin (Figure 3).
EV chargers could be bi-directional
An enticing possibility with the totem pole PFC stage is to make it bi-directional so power could be returned to the grid from a vehicle as needed. If you look again at Figure 1 (lower) and imagine the DC bus is the power source and the AC line connection is the load, with appropriate drive to the MOSFETs, a half-bridge inverter materializes. A nice bonus.
Old name - new name
SiC cascodes are a natural choice for a highly efficient and rugged totem pole PFC stage. It’s another odd name for newcomers to recognize from the days of vacuum tubes but it’s here to stay.