current sensing scheme in DC-DC converter

Dear all,

I known that there are some current sensing schemes in DC-DC converters, such as Sense Resistor, RDS Sensing, SenseFETs. But to minimum the current limit propogation deay for the peak current check, which one is the best method?

Thx.

it not only depond on the the three kinds as you list, but also depond on your total schematic.

They are all very fast to make a comparison. I think your consideration would take into account whether blanking is needed.

For example, if sensing a low-side NMOS using Rds-On, the sense voltage starts high before NMOS turns on. This seems like a false OCP, so the signal must be blanked for some time (50-100nS) after the NMOS gate goes high. This limits your response time to a minimum of 50-100nS.

In contrast, sense resistor only senses the actual current through NMOS, so it's blanking time is much shorter or it may need no blanking at all - at each on time it's voltage starts at 0!. But sensing across a resistor contributes some IR loss.

You can insert a sense resistor in only a small portion of the mosfet to minimize IR losses, or use a sensefet that mirrors a portion of the output current. Both of these can become nonlinear quickly as the ratio of Switch to sense device becomes large so accuracy of current limit is degraded.