Is the V-I curve of a MOSFET switch really a straight line as we imagined? The RDSON is clearly a function of the current through the MOSFET. But with the device alternating between peaks and valleys, what current value do we use? We can do a "worst-case analysis" based on the highest RDSON (an instantaneous value) along the V-I curve. But is that value really "worst case", or is it even worse than "worst-case"?! Power supply guru Sanjaya Maniktala celebrates his ann
Performing an efficiency calculation for a power converter will certainly require knowledge of the Drain to Source ON-resistance (hereby called RDSON) of the switch. In doing so we may refer to the V-I characteristics of the said MOSFET. We will probably be thinking that all we need do is to find the slope - "V/I" - to get the RDSON.
But hold on just a minute! Is the V-I curve really a straight line as we imagined? If not, the RDSON is clearly a function of the current through the MOSFET. So what is the RDSON we need to take for our calculation? We can do a "worst-case analysis" based on the highest RDSON (slope) along the V-I curve: which would always be found to occur at the highest instantaneous value of current, i.e. the peak switching current. But is that value really "worst case", or is it even worse than "worst-case"?!
The current through the MOSFET is actually varying every cycle between two values, the peak and the trough. It is certainly not fixed at the peak value (at which we may be finding the "worst-case" slope). We are not interested in finding the worst-case instantaneous value of the RDSON, what we want is the worst-case value over the entire switching cycle. In a switching converter, the RDSON is actually varying smoothly between two values, just as the current is.
By a rather painful analysis, it can be shown that a very close fit to the exact integration-based calculation is obtained simply by (a) finding the RDSON at the extreme current values: the peak and trough, and (b) averaging these two values to get the effective RDSON over the entire cycle. Simple enough!
But hold on a minute longer. Look at the published V-I curve (the black part of the attached Figure ...this represents a typical integrated switcher device rated for 1.5A). The device is therefore supposed to function up to 100 degrees-C at 1.5A. But does the curve extend all the way? No at all! The 100 degrees-C is mysteriously truncated! No other information is available in the Datasheet. The only way out for us as designers is to try to extrapolate the V-I curve .see gray part.
Click to Enlarge
We now see that the curve intersects at a whopping drop of 17V at 1.5A at 100 degrees-C! Maybe that is what the vendor didn't want to circle out for us. But at least we can now find the effective RDSON.
However, we could erroneously take the average over the entire range to get
This estimate is way too optimistic. As mentioned previously, a more correct estimate is the RDS averaged over the RDS at the extreme values. At peak value
We also know that the RDSON- MIN is 10Ω from the datasheet since the RDSON is rather typically stated at only 1/10th the maximum current i.e. 10Ω at 150mA in this case.
So the correct effective RDSON is the average of the two
Now that we know the RDS is 50 percent higher than what was indicated to us, we also have a better idea of the conduction loss in the MOSFET.
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