PMP40766: Doubt in CC Loop Controller

Arun Thawait

Part Number: PMP40766

Dear sir ,

We have designed a charger of specs 69.2V,4A (72.8 V OVP)

When I am connecting a load resistance of 14 ohms , 17.5 ohms,28 ohms the output voltage comes out to be < 43 V (output voltage below which charging current is CC/10) while the charging current is 0.4A and the output of CC loop controller (2.67) comes out to be less than output of CV loop controller (9.6V) as expected . But when I am connecting the battery terminals having initial voltage (53V) the charging current comes out to be 3.5 A instead of 4A also output of both CC and CV loop controller comes out to be high (9.6V) also I have set IREF to be 1V but IOUT_FB at inverting terminal (6) of TL103 comes out to be 0.911 V instead of 1V (When CC=0.4 A IREF=0.1V= IOUT_FB ) .

(1) Why reference is not meeting the feedback value when battery load is connected ?

(2) is it possible that any parameter is limiting the CC Current to reach 4A because CC Controller output is high so it has to reach 4A to meet IREF=IOUT_FB ?

over 3 years ago

0 San Chen over 3 years ago

TI__Prodigy 580 points

Hi Arun,

Let's solve the issues one by one, so the OVP issue has been solved by changing the R1? Have you enter your power stage parameter in the design tool to see if the values are right with VCR, ISNS, LL setting? Then we move to this issue.

+1 Arun Thawait over 3 years ago in reply to San Chen

Prodigy 155 points

No sir there even after chaging R1 there is no impact on output voltage .Once I made changes in R3 R4 and C4 .the range of output voltage oscillation becomes 57.2-67.2. Since output voltage is not hitting the ovp point the bulk pin voltage has to be maintained at greater than 3 and less than 4 for for converter switching but in this case also bulk pin voltage is varying from 2-3 volts .

By selecting R1 =4.5 k we ensure that the ratio between BMTH and BMTL is 0.6 .is it possible that we get oscillation in output due to this in No load case because FB replica is oscillate between these upper and lower threshold value to maintain stability at no load / light load ??

Yes sir I have taken parameters values from design tool.

Regards

Arun

0 San Chen over 3 years ago in reply to Arun Thawait

TI__Prodigy 580 points

When control loop is limited at 1/10CC, LLC will work at burst mode, so the average current will be lower than 1/10CC.

and when you test with 53V battery, the charging current is 3.5A that close to 0.911V reference, so do you happen to short the R43 that the reference changed to 0.937V? I can't see any thing else to make the IREF leave 1V.

0 Arun Thawait over 3 years ago in reply to San Chen

Prodigy 155 points

No sir , I have checked I REF comes out to be exactly 1.002 V while I out_Fb comes out to be 0.911V .But that is not the case when CC =0.4 A .for this condition both comes out to be 0.1 V

0 San Chen over 3 years ago in reply to Arun Thawait

TI__Prodigy 580 points

Hi Arun, Yes I see feedback correct on 1/10CC.

when IREF is 1V and Iout_FB is only 0.911V (do you measure Pin1 of U3? ), I can't think out any cause, is there possible the D8 and D9 in inverse direction? Or you tune R17 smaller or larger to see any change.

0 Arun Thawait over 3 years ago in reply to San Chen

Prodigy 155 points

Dear sir ,

I have attached the figure of

(1) Vds ,Vgs of lower MOSFET and anode to cathode voltage of secondary diode .

(2) Vgs of lower MOSFET when there is no load .

Under Normal condition of LLC converter operation the duty cycle of switches is approximately 50% but as you see from Vgs figure we are getting random pulses across Vgs .Also the voltage across secondary diode is having oscillations . In this case my CC controller is having IREF=I_FB=1V.

what could be the possible reason for this type of behavior of converter .

My output voltage is 53.3 volt in that case .

VDS lower MOSFET when CC=3.6A