UCC29002: UCC29002 transient response

Hi Chuck,

As you say, the bandwidth  of the UCC29002 needs to be set at about 1/10 th of the power module bandwidth.
This is obviously to prevent some of the modules from oscillating around the set point as they attempt to reach a steady state value.
Slowing each load sharing module down to 1/10 th of their bandwidth is a compromise that is required to achieve stability with equal current sharing.

The current sharing error is related to differences in the output impedances only if you do not use the UCC29002.
The UCC29002 compensates for differences in the output impedances and increases the output voltage of the higher impedance modules so as to take their output currents up to the level of the lowest impedance module ( the "master") and it needs to do this at low bandwidths.

The current sharing error between modules is due to pcb layout, current amplifier mismatch and current shunt value differences.
It is normal possible to get 1% accuracy at full load.
Accuracy drops at lower currents as the low voltage across the current shunt becomes difficult to measure accurately.

TI does not have a high speed current sharing controller and current sharing accuracy is measured in steady state.

Hope this helps.

Regards

John

Hello Chuck

The load share controllers have a feedback loop so that the DC currents share accurately between the modules being controlled. However, the modules being controlled also have their own feedback loops and the loop bandwidth of the current sharing controllers must be less than that of the modules in order to prevent the two control loops interfering with each other. This is the main reason for the 1/10th b/w restriction. There is of course some margin so that for example you may be able to bring the load share controller b/w up to about 1/5 that of the modules. However the main function of these load share controllers is to force accurate load sharing in the steady state condition because this is the condition where the thermal issues are worst and load sharing is of most value. They were never intended to deal with short term transient load changes.

So far as I am aware this is a fundamental limitation of this approach to load sharing.

If you want to load share between power stages and have fast response, then you need to use current mode control and connect the output of an error amplifier and daisy chain it across as the current demand signal into the controllers for each of the power modules. If this is what you want to do, and if you need more details than the sketchy outline given here then please let me know and I can send something more substantial.

Regards

Colin