LTC 3871 buck boost converter stability problems


i designed a bidirectional three phased buck boost converter with two LTC 3871 ICs. The lowside voltage is 2.5 V to 3.7 V and the highside voltage is 5 V to 15 V. The max. lowside current should be about 35 A per Phase. I used DCR-sensing with an inductor DCR of 0.88 mOhm.

I only tested the buck mode so far, but there are some problems. It works properly in the area from 6 V - 8.5 V to 3.6 V with 30 A, but as i increase the highside voltage over 8.5 V, the maximum current on the lowside is about 5 A until it cancels its function.

I think, the feedback loop is not the problem, because i also tried it with a 220 nF capacitor as compensation network, but the problem is still there.

I did the modelling in LTspice and also in LTpowerCAD II and there it works without any problems.

If you need any further information, feel free to ask me. I really appreciate your help.

Greetings, Michael