Synchronous rectification has been found to be very effective in reducing conduction losses of buck and boost converters and secondary side rectifiers. However, implementation of the synchronous rectification is far from being simple. The control signals need to be precise, and in the noisy environment of a switch mode power supply, false triggering is hard to avoid. This is particularly true when the operation is in the DCM mode when resonant induced oscillations prevail.
In recent years, a new breed of control IC have been popping up: “ideal rectifier” controllers. These are in fact very similar to the synchronous rectifiers (replacing the conducting diode by a MOSFET) but unlike previous solutions, they are autonomous. That is, they do not require a control signal as the classical synchronous rectifiers do. A key issue in this case is of course the question of response time and in particular the delay from ‘on’ to ‘off’ state. It would be nice to see data on the response of these devices in a form similar to the plots of diode reverse recovery. I have not seen such data.
It would extremely interesting to read testimonial of actual use of such “ideal rectifiers” in practical circuits.
For a primer on the “Ideal rectifier” see
Sam,
You are correct in your observations. Implimenting accurate SR control is extremely challenging - you find that the timing requirements are very much influenced by the most minute amount of Source inductance (even a few pF) and the Gate capacitance, Miller capacitance, and Gate threshold all have a huge influence. It might seem as if it should not be too difficult to get the timing tuned correctly - and it isn't for a single converter, however when you want a design that is going into mass production you realize the device to device variance makes it very challenging to get design that works consistently well.
You also find the converter topology has quite an influence on the sensitivity to SR timing accuracy - the popular LLC is probably the most challenging topology to add SR to.
One perspective I think is really relevant in this discussion is the need to re-evaluate the control if you are going to employee SR. If you consider how most SR devicesd are controlled the aim is to emulate a "Dumb Diode" I.E. turn the gate on when there is Source to Drain current flow and turn the gate off when this current drops to zero.
The alternative perspective that I think is good "food for thought" can be best illustrated by considering adding SR to the LLC. As I mentioned above this is a very challenging topology to add SR to.
If you consider the power train topology of the LLC with SR then you realize that you now have a resonant tank in between two bridges of MOSFETs - conventional control would result in modulating the frequency of the input bridge and implimenting "Dumb Diode" control of the output bridge - but why this control strategy?
Clearly the power train topology is that of a Dual Active Bridge - albeit a Series Resonant Dual Active Bridge.
If you recognize that the power train topology is that for a series resonant Dual Active Bridge and control it accordingly then you find there are many advantages.
The first advantage that you find is that the timing accuracy of the SR device's is controlled by the voltage feedback control loop and hence the efficiency performance is not so influenced by the circuit parasitic variance.
The next advantage that you find is that you can now use a phase shift between the two active bridges to control the circulating tank energy. What this means is that you no longer need an LLC tank as you can achieve all the control needed with a simple LC tank.
The other advantages come from now using an LC tank in place of the LLC tank. The first is the huge reduction in the magnitude of the circulating tank currents - this is especially true if the converter requires a wide voltage control range. It is possible to reduce the tank and device currents by a factor of 3 at full load and much more at light load.
Reducing the circulating currents dramatically reduces the body diode currents at the end of the ZVS commutation. This reduces the body diode recombination losses and eliminates the need to use a MOSFET with a fast body diode. These standard MOSFETs are both lower cost and have lower condition losses.
Since most LLC converter designs achieve the parallel inductance by gapping the transformer changing to an LC tank eliminates the need for this gap. In turn this eliminates the frindging flux losses generated in the transformer windings.
Finally you find that the series resonant LC dual active topology is totally symmetric and hence fully bi-directional.
So in summary if you are willing to re-evaluate the control (assuming you have decided to go with SR) then there are many additional advantages that can be achieved with the design.
Hi Michael, Thanks for the excellent input. One disadvantage of the simple LC as opposed to LLC is that you have a limited gain range. In some application this could be a handicap.
Sam,
Yes you have limited gain with a series resonant LC if you use conventional control.
My suggestion is to control it as a series resonant LC dual active bridge - here you have more degrees of control freedom. Specifically you can now create a phase shift between the input and output gate drives. Modulating this phase shift allows direct control of energy in the tank (independent of the output load). This extra control variable give you any amount if gain and unlike the LLC that unnecessarily circulates the tank energy (IE at full load you don't actually want the additional magnetizing inductance current adding to the load current flowing through the Rds causing additional losses) with the series resonant LC dual active bridge converter you only circulate sufficient tank current to achieve the control (and ZVS) requirements.
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