PMOS = Si7145DP
NMOS = SiR422DP
Choke = 7447709100
LTC4011 Datasheet
Vin = 24V
Vout = 12..15V (10S nimh batt)
PMOS:
Old Rds_ON = 18mR
New Rds_ON = 3.8mR
Old Rja = 40 C/W
New Rja = 20 C/W
NMOS:
Old Rds_ON = 10.5mR
New Rds_ON = 6.6mR
Old Rja = 40 C/W
New Rja = 25 C/W
Not completely right.So these new FETs should theoretically create less heat and be more efficient, right?
Hi,
Not completely right.
Your values are for "conducting loss" .... but additionally there is switching loss.
Instead of just looking at the values I'd try to estimate the dissipation power.
Then you will see:
* how much is conduction loss
* how much is switching loss
* whether the value is in the expected range.
For example you could recognize overly high current caused by coil saturation.
Klaus
Every MOSFET manufacturer as well as every MOSFET_driver_IC manufacturer should have good application notes.Is there a good equation or application note for calculating switching losses you could recommend?
where are the gate resistors ....?
A good initial selection can be made by multiplying the calculated minimum by 1.4 and rounding up or down to the nearest standard inductance value.
Of course it could also be a poor layout or something wonky with the controller. Oscilloscope traces would say for sure. I would definitely look into simulating the circuit with LTspice, if you haven't already. They have premade demo simulations for practically every chip they make.
Large fets, & higher voltage fets have slower internal diodes - which lead to switching loss - so choose fets with just enough volt rating and just big enough ( or low enough Rds-ON ) to do the job
If you damage them when soldering - you will have issues - this is more common than you think - as is static damage during handling - which also leads to headaches ...
- - - Updated - - -
p.s. put a schottky across the active fets ( lower ) too - this always helps ...
- - - Updated - - -
p.p.s. that Vds waveform should be square - either the fets or the IC are soft ... or both ...
Hi, I have worked on several LT ICs, and to me the problem you are encountering may be related, as Easy peasy already told, to the switching specs of your si7145dp MOSFET. Qg on this fet is 150nC at 5V approx, the previous fet you were using (FDD6637) had a Qg of 25nC.
On the LTC4011 datasheet, page 18, you will find a formula to calculate the power dissipation of the IC. If you calculate it with the value of your previous mosfets, you will find something around 0.9W.
With your "new" fets, you should find something like 2.7W. This IC heats up at 38°C/W, so 2.7W seems a lot. My guess is that the IC doesn't manage to give enough power to drive correctly your fets, and drive them too low, which make them heat a lot.
You should give it a try by only changing your PMOS to the former one.
You can also try to power the INTVdd pin with an external power supply (no more than 5V), I know it is possible on the LTC4020, I don't know for the LTC4011.
INTVDD (Pin 14):Internal 5V Regulator Output. This pin provides a means of bypassing the internal 5V regulator used to power the BGATE output driver. Typically, power should not be drawn from this pin by the application circuit. Refer to the Application Information section for additional details.
To me your options are:
- to use mosfets with lower Qg (probably at the expense of a greater Rdson, but the switching loss are probably predominant here)
- to use external power for INTVCC
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