new test with TPS40055

thanks Peter,

Now i'll do the change of the capacitor

For the Rt resistor I choose 150k+10k+10k to get rafly 300kHz

so by the equation Rkff=(20-3.5)*(58.14*170+1340) kOhm=185kOhm now on the board i have 150k+47k+47k=244k is it too high?

Rhys=Rkff x (Vpd-3.5)(0.2*(Vinmin-3.5)=185*(8-3.5)/0.2*(20-3.5)=832.5/0.2*16.5=252k  now i have 244k

Massimo,

No, I don't think 244k is too high, though Rhys might be too low.

The KFF pin is current driven.  With Rt = 170kOhms it takes approximately 80uA of current into the KFF pin to trigger UVLO and once on the current has to drop to about 72uA to shut off and your Rhys is driving about 18uA of extra current into the KFF pin, adding about 4.4V of hysteresis.  If you don't need this much additional hysteresis, you could consider removing or increasing Rhys.

Peter,

No way to make it overcrossing 5.9Amp

well I increase the cap of Ilim to 470pF no improvement

so i take out the Rhys but all just as before

What I do not understand is why in no load condition the upper fet is conducting so long,

I usually use PWM fixed frequency or variable frequency ZCS or ZVS and the load is setting my regulator to

the correct duty cycle or working frequency, Why here not.

Is that any chance to disable the safeties in this controller.

I think that my power stage is enuogh powerfull 110Amp pk fet at 55Volt may i improve current by reducing voltage

i mean 30V fet with 150Amps

regards

Massimo,

The TPS40055 controller is a fully synchronous, fixed frequency BUCK converter, even at zero load, the synchronous rectifier is driven fully complementary to the main switching FET so it's duty cycle is nearly the same at no load and full load.  Fro 24V to 14V, this is about 60%.

You can disable current limit by removing the resistor between VIN and ILIM.

I did some graphical comparisons between your No-load operation and 7A operation above.

1) At 7A load current the switch node pulse width is approximately 150ns longer than at no-load.  This is the TPS40055 controller's error amplifier loop compensating for the losses in the Power Supply.  This would represent 6.5-7W of loss being compensated.

2) The SW voltage at the of the switching cycle is 1V - 1.2V lower at 7A load current than at No-load.  If the Input voltage is actually 24V, the voltage drop to the switching node appears to be 1.5 - 1.7V.  The switch node dropping so low would be consistant with the supply still entering current limit.

You may want to check the DC voltage at the drain of the high-side FET at No-Load, 3A and 6A plus the AC ripple on the drain voltage at the same 3 load current.  These numbers are suggesting that you're experiencing a lot more drop from your 24V source to your power supply than expected.

Hi Peter,

well,  Yesterday I disable the UVLO and feed forward using the equation on SLUA310

I put a 13k resistor in between pin 1 and 3, and no RkFF but the problem was still the same.

By reading Your last post i first look at the 24Vdc, the supply drops of about 0.7 volt when 6Amp.

So I disable the current check, the system is now giving 14Vdc with 10 Amp without any problem

I made measurement on the voltage drop and in no load I get 24Vdc

with 6 Amp rafly 23.2

at 10 Amp the drop is about 1 volt

now i was checking with a differential probe the voltage on the upper mosfet in between drain and source,

I think that's good,in  comparison with SW node I get only some ringing at turn off and on

So the two questions are:

is there any problem to use the device with the Ilim disable? (i think no)

I look at the mos I use now the IRF3205 well are no logic level mosfet so can be also that the 8 volt gate

given by the device is not enough to full saturate the mos, do You think that could be usefull to try to use

the IRL3803 that is a 30 Vdc device 140 Amp but is also LL device

what I do with the Rkff should i try to restore the olds circuit?

is there any problem to use the device with the Ilim disable? (i think no)

Aside from the lack of power-stage protection due to a shorted output or excessive load current, no.  The IC will perform all other functions independant of current limit.  For the purpose of IC pin protection, I would recommend populating the RILIM resistor position with a resistor between 330 - 510kOhms to disable the function.  This prevents the ILIM pin from floating and being subject to noise injection.

I look at the mos I use now the IRF3205 well are no logic level mosfet so can be also that the 8 volt gate

given by the device is not enough to full saturate the mos, do You think that could be usefull to try to use

the IRL3803 that is a 30 Vdc device 140 Amp but is also LL device

If we look at figure 2 from the IRF3205 datasheet

 We see that even with a junction temperature of 175C the MOSFET's 8V Vgs saturation current is over 200A with a forward current of 60A representing a 1V forward drop.  Even at these elevated temperates, the forward drop across the FET at 10A should be less than 200mV with a gate drive as low as 5V.  Using the TPS40055 controller, which provides 8-10V of gate drive voltage, the IRF3205 should be fully saturated.

The IRL3803 does offer a lower Rdson at 8V verse the IRF3205, however it does it at significant expense in switching speed.  The IRL3803 MOSFET has 50% more switching charge (100nC verse 70nC from maximum datasheet specifications) and considerably higher total gate charge at 8V gate drive (140nC verse 78nC from typical Vgs verse Qg curves) It is unlikely that the improvement in Rdson will overcome the significant increase in driver current, switching and gate drive losses.

Peter,

Thanks for Your Help

regards

Massimo