current sensing scheme in DC-DC converter

[jery_xin]

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.

 

[sunking]

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

 

[electronrancher]

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.