I am giving more details about the problem . pls read patiently.

we are using p-MOSFET's ORing to do automatic switchover b/w @ 12V_External (comes from PCIE edge) and emergency battery.

The circuit is as follows. enter image description here We have testing with different slew rates on 12V_External (@ CIN - comes from PCIe edge). at slower slew rates(~ ms) this works as intended. But when we tried with 35us ramp-up rate on 12V. we are seeing ~11V spike on Battery (presently we used DC power supply for battery) during turn ON of 12V_External.

This spike is not seen with other slower ramp-up rates.

I will explain little more details on this.

When 12V_EXT power ON, VBATT ON --> 12V_PLOSS will be LOW & 12V_SLOSSN will be HIGH - power feeds from 12V_External

When Power off on 12V_External, @ 7.015V of 12V_External, 12V_PLOSS becomes HIGH (at this 12V_SLOSSN is still HIGH) so, no power is goes to 12V. when 12V_External reduces 6.715 12V_SLOSSN starts swicthing and becomes LOW Turns ON VBATT MOSFETs (M2) - which then powers the 12V.

Now, when power came back, when 12V_External reaches 6.715V 12V_SLOSSN becomes HIGH( at this 12V_PLOSS is HIGH) so, again no power goes to 12V. when 12V_external reaches 7.015 12V_PLOSS switches LOW now power comes from 12V_External to 12V.

Above is the regular scenario.

Now the problem is>>>>

When Power comes up 12V_External with faster slew rate, there is path from 12V_External to VBATT because of that VBATT is increasing --> as 12V_SLOSSN,12V_PLOSS thresholds are depend on Battery voltage , 12V_SLOSSN,12V_PLOSS are switching @ higher 12V_External voltage.

One more thing, when M1 OFF & M2A,M2B ON - there is a path from 12V_External to Battery through Body diode of M1. But Vf of body diode is 0.8 to 1.2V, Since 12V_SLOSSN siwtches @ 6.715V, Vf of Body diode won't reach as per design.

any help will be useful.

Edit :

any thoughts on this?

I doubt dv/dt false Turn ON on M1 - But not sure how to find it.

  • \$\begingroup\$ This is not the complete schematic to do with your power switching, so nothing definitive can be said. We're not psychics. However, I'm willing to but a small bet on you seeing troubles due to gate capacitances / turn-on/off-time of MOSFETs. You need to take these into consideration. \$\endgroup\$ – Asmyldof Apr 1 '16 at 5:46
  • \$\begingroup\$ Compactors are directly them. from captures i didn't see much issues. How the issue will be if compactors can't drive the gate. as i mentioned on above, b/w 6.715 & 7.015 V of Input transition both mosfets (M1 & M2) are OFF. No case of cross conduction through mosfets. \$\endgroup\$ – user19579 Apr 1 '16 at 10:56
  • \$\begingroup\$ any thoughts on this? I doubt dv/dt false Turn ON on M1 - But not sure how to find it. \$\endgroup\$ – user19579 Apr 4 '16 at 6:14

Those FETs have a gate capacitance of about 3 nF (28 nC to switch). How are your comparators driving them ? to switch in 1 us will require 30 mA of drive current.

Are your comparators (and references) accurate enough -- is there guaranteed non-overlap between the comparators ? Do you have logic to ensure that the 2nd FET is not turned on until the 1st is actually off ?

User LT4351 instead.


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