When a synchronous converter operates in high load condition,if the inductor current is always positive, the switching loss of the Synchronous mosfet is minor in comparison to the switching loss of the Control mosfet. This is because the Synchronous mosfet turns on and turns off in "soft switching" while the Controller mosfet turns on and turns off in "hard switching" (I have summarized a lot). But when a synchronous converter operates in low load condition and the inductor valley current become negative , Control mosfet and Synchronous mosfet turn on in "soft switching" and turn off in "hard switching". If I suppose that the 2 mosfets are matched, the switching loss for both devices, is the same? I'm sorry for my bad English, it's not my mother tongue, and it is not easy to express the concept that I want. Thank you all for the attention.
For purposes of this discussion there are three types of synchronous buck controllers/converters:
- Those that do not have a light load PFM, pulse skipping, burst mode or diode emulation mode. These will operate in CCM all the time, and when the load current doesn't exceed 1/2 the p-p ripple current the inductor current will go negative each cycle and you will have increased switching losses over one of the light load modes as Olin point out.
- Those that do have some type of light load operation.
- Those that have an input or programmable option for forced CCM (if for example you want to avoid the switching frequency changing, maybe to stay out of the audio band at certain times.) or give the option of a light load mode.
When inductor current is positive, the top switch turns off first, followed by a dead time. During the dead time if there's enough current to slew the switch node capacitance to ground (or a body-diode drop below ground), the lower switch will turn on with very little voltage across it. Therefore zero voltage switching or soft switching.
If you are always in CCM and the inductor current goes negative, say at no load, then as the OP points out when the bottom switch turns off with negative current, the top switch soft switches, and if the FETs were the same part number they would both have about the same switching loss. However, it's common to choose different FETs for the control and sync FETs to optimize conduction and switching losses depending on the conversion ratio, so there could be asymmetrical switching losses due to extra gate charge and transition losses on one FET vs. the other.
It's not clear what you are asking, but the inductor current never goes negative in a properly controlled switching power supply.
With synchronous rectification and in discontinuous mode, you do run the risk of leaving the synchronous switch on too long and thereby allowing reverse inductor current. So don't do that.
In discontinuous mode near the end of the synchronous on interval, the inductor current is low anyway since it goes to exactly zero at the end of the interval. The extra losses are therefore minor if you turn off the synchronous switch a little early. You get one diode drop additional voltage drop for the inductor current when it is low and about to be zero anyway, so really not much of a problem.
In general, shoot-thru and reverse current from leaving the synchronous switch on too long are worse than the extra diode drop for a short time, especially at the end of a discontinuous conduction pulse.