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Original question:

UCC28251: Queries about PSpice Average Model

UCC28251: RAMP/CS PIN max voltage

user4103700
Prodigy 90 points
Part Number: UCC28251

Hi Peter,

following the question of that half bridge power supply, the operation conditions is as below:

Switching frequency:100 kHZ
Input voltage: 200~800 Vdc
Output voltage: 24 Vdc
Output power: 180W Rated,216W Max,

XFMR turn ratio: 3.714,

Primary inductance:1000uH typical,

Input voltage feedward used, voltage mode,

Primary side control,

use diodes as secondary side rectifier,

Type III compensation network with TL431B and Zener.

Since I use the input voltage feedward function, I put 813kohm resistor and 470pf cap on the pin RAMP/CS, when the input voltage exceeds 400V, the voltage at pin RAMP/CS is clamped at 4.3V, the higher input voltage, the higher slew rate of RAMP/CS.

what is the consequence of clamping the voltage at 4.3V continuously? So far, I didn't see any failure related to this, since I have done the HALT test, thermal cycling and high temp acceleration test on so many samples.

if i still want to use this input feedward function, what is the recommended value of Rramp (considering 200V~800V input) to keep RAMP/CS voltage below 4.3V?

if i don't use input feedward function, what is the recommended value of Rramp and Cramp when connecting to Vref?

Best regards,

Chengjun

  • 0
    TI__Genius 13660 points

    Hello,

    The RAMP/CS pin is clamped internally to not exceed 4 V for internal device protection.  So the desired ramp amplitude should be below 4V.
    If it is used for input voltage feed-forward mode, the voltage feed-forward will not work after the voltage is clamped at 4V. 
    The Rcs you used here is too small, please calculate Rcs with the formulas (4) and (5) in section 8.3.7.1 "RAMP: Voltage Mode Control With Feed-Forward Operation". It should be larger than 2.2 M ohm.
    If don't use feed-forward mode,  the desired ramp amplitude should be below 3.3V, since REF is 3.3V, then use formulas (4) and (5) in section 8.3.7.1 to calculate Rcs.

    Thank you.
    Teng

  • 0 in reply to Teng Feng
    Prodigy 90 points

    Hi Teng,

    thanks for your quick reply.

    if the RAMP/CS pin voltage is clamped at 4V, only input voltage feed-forward will not work? will the PWM controller be damaged or not? The maximum current into  the RAMP/CS pin is below 1mA at the maximum input voltage, I think it will not damage the PWM controller. 

    how do you get 2.2M ohm value? i don't use the SR function in my design, so how do i define the desired RAMP peak voltage, 3V or 4V? 

    Best regards,

    Chengjun

  • 0 in reply to user4103700
    TI__Genius 13660 points

    Hi Chengjun,

    The recommended RAMP voltage is 0 ~ 2.3V. Then you can calculate  Rcs by the below formula, in which Vcharge is 800V, Fsw is 100 kHz, Vpk is 0~2.3V, Ccs is 470pF.

    Since the absolutely maximum rating voltage at RAMP pin is 4.3V, a voltage larger than 4.3V may damage the device.

    Thank you. 

    Teng