WEBENCH® Tools/UCC28070: features explanation

Hello Harijarani

The UCC28070 can be programmed for a switching frequency in the range from 30kHz to 300kHz - these are the limits given in the current datasheet at http://www.ti.com/lit/ds/symlink/ucc28070.pdf   - I don't know where you found the 95kHz to 330kHz limits but certainly it can be used at 50kHz.

Vsense is compared to a 3V reference by the internal voltage error amplifier (labelled VA in the block diagram on P13 of the dataheet). When the system is in regulation, Vsense will be controlled by the feedback loop to be equal to the reference - ie, it should be at 3V. If Vsense is less than this, then the feedback loop will command a wider duty cycle to try to increase the output voltage. If Vo does not increase enough to bring VSENSE pin up to 3V then the system will continue to increase the duty cycle - eventually it will run out of control range when it reaches Dmax. The other thing that might happen is that the system reaches the current limit point when the Ipeak comparators will limit the duty cycle.

SO:I'd suggest you try the following to debug the system

One nice feature of this device is that it is possible to run it from DC ! - if you have a DC source you can run some initial tests just to make sure that the basic power train is running - try it at 160Vdc input. this removes all the AC related variations and makes it easier to see what is happening. You should work towards having a correct Vout which doesn't change as Vin changes. First of all check that the system is not 'locked' at Dmax and that it is not running into current limit.

You can also disable one of the two phases - simply remove the gate drive resistor from the 'unwanted' phase and tie the MOSFET gate to its source (very important to do this to prevent the MOSFET gate charging through stray fields). If you do this of course you will have to halve the load current on the output.

Can you double check the current limit point set by the resistor at the PKLMT pin -

Double check the resistor values you are using in the VSENSE and VINAC potential dividers - note that the UCC28070 requires that you use the same RATIO for these two dividers.

Let me know how you get on - feel free to post some waveforms - the Vds waveform of the MOSFET is a good place to start

Regards
Colin

Dear colin,

thanks for your quick response. as per your suggestions am working on the Vsense pin of  controller chip to get 3V. I need clarification in the following aspects from you.

1. Is there any relation between soft start  pin and Vsense pin

2. My design should work between input frequency range of 200-690 Hz for constant output voltage .In design section how this frequency adjustment can be done. In parameter calculations where one can consider this frequency range.In UCC28070 datasheet no where discussed about this variable input frequency .

3. One more final question, to get constant output voltage what are the different testing points i need to check except Vsense. I have done calculations for boost inductor, capacitor and resistive load based on my calculations . while simulating with those values not getting constant voltage.please give me your suggestions and support me.

awaiting for your reply.

Hello Harijarani
1/ During soft start (SS), the voltage amplifier (VA in the block diagram) uses whichever of the voltage at the SS pin or the 3V internal voltage as its reference. The VA then controls the system duty cycle so that VSENSE matches this reference.

2/ The UCC28070 'doesn't care' what the input frequency is - at least to a good first approximation and this is why this topic isn't addressed in the datasheet.

3/ There are a number of things to think about here -
First: there is an inherent ripple voltage on the output at twice line frequency - this is inherent to the boost topology and is not a function of the controller. It's amplitude can be decreased by adding output capacitance - or by reducing output current - or by increasing the line frequency.
Second: the control loop may be unstable - have you run a bode plot on it ?
Third: the input EMI filter (if you are including it in the model) can be unstable with a given PFC stage -
Fourth: the control loop may be running out of dynamic range somewhere -
What does the instability look like? - frequency, amplitude, load condition etc.

Regards
Coliln

Hi,

to get the constant output voltage what are the different testing points i need to check except Vsense. I have done calculations for boost inductor, capacitor and resistive load . while simulating with those values not getting constant voltage.please give me your suggestions and support me.

Hello Harijarani

there are a lot of things that could be wrong.
What does the instability look like ? - frequency, amplitude, waveshape etc, etc.

Are the current sense signals connected correctly ?

I would really need to see your schematic and sim results to be able to help much more than that - please post them here or if you are concerned about confidentiality you can send them to me directly at colingillmor@ti. com

Regards
Colin