synchronous rectification vs diode?

synchronous rectification or diode?

Can someone explain both in details?
thanks

What ??
CCM - continuous current mode
freewheel diode - is just a switching diode
https://en.wikipedia.org/wiki/Buck–boost_converter

sorry, I meant synchronous rectification or using diode

Usually synchronous is more efficient. Lets say the current is 1 amp. The diode will dissipate about .6 watts (diode drop X current). If a low resistance FET is used (say 10 milli ohms) it will only loose 10 mw.
(I^2 R).

D1 is a diode. The forward voltage is 0.7 volts at low current and maybe 1 volt a high current.
Q1+D2 makes a synchronous rectifier. In most power supplies we know when the diode should be on. During this "on" time we turn on Q1.
Q1 passes this current with a lower voltage across the part. Maybe 0.3 volts.

I have never seen synchronous rectification done in high voltage applications.
It is common in low voltage power supplies. Example: Some computers run at 1.8 volts. If there is 0.7 to 1.0 volts of loss in a diode then too much power is wasted in the diodes.

Thanks for the reply!

Is this converter(LTC3780 from linear techn) using synchronous rectification?

janice said:

Thanks for the reply!

Is this converter(LTC3780 from linear techn) using synchronous rectification?

Click to expand...

Yes it is

Hi,

Synchronous rectification is usually used in lower voltage applications because the efficiency with a diode that drops even 0.5v is comparable to the output voltage. In higher voltage supplies the diode voltage drop is much less significant because the ratio of the diode drop to the output voltage is very small.
For example, for a 5v output supply the approximate efficiency loss is 0.5/5 which is 0.1 or 10 percent, which is a lot keeping in mind this is just because of the diode alone.
For a 50v output supply the approximate efficiency loss is 0.5/50 which is 0.01 or 1 percent, which is much less.
With a MOSFET for the 5v supply instead of the diode if we can get the voltage drop down to 0.1 volts then the approximate efficiency loss is 0.1/5=0.02 which is only 2 percent.

MrAl said:

...................
Synchronous rectification is usually used in lower voltage applications because the efficiency with a diode that drops even 0.5v is comparable to the output voltage. In higher voltage supplies the diode voltage drop is much less significant because the ratio of the diode drop to the output voltage is very small.
For example, for a 5v output supply the approximate efficiency loss is 0.5/5 which is 0.1 or 10 percent, which is a lot keeping in mind this is just because of the diode alone.
......................................

Click to expand...

You neglected to include the effect of the duty-cycle which reduces the efficiency loss from the diode. Since the diode is only conducting during the off time of the PWM signal for a buck converter, the loss in efficiency will be inversely proportional to duty-cycle as largely determined by the input to output voltage ratio.

Hi,

So then my example would be for a rather large duty cycle. The main point being that the diode drop usually looses much more than the synchro transistor switch.

MrAl said:

Hi,

So then my example would be for a rather large duty cycle. The main point being that the diode drop usually looses much more than the synchro transistor switch.

Click to expand...

That is certainly true.

crutschow said:

That is certainly true.

Click to expand...

Hi Carl,

He he, well thanks for pointing that out though because with low duty cycle (large input low output) we wont see as much as a difference either. So maybe a bit of correction on my part is in order, where the difference between diode and synchronous is smaller with large input and low output as well as with higher voltages at least in most cases.