HP E3620A power supply clone oscillation problem

[electros6]

Hi guys,
         I build a HP E3620A power supply clone and it works good expect one issue. I used two mosfets in parallel. when I connect one mosfet the circuit the circuit works good but when I connect the second mosfet I got severe oscillation in the output. I don't know what is the cause please guide me.
                                                                                                             regards
                                                                                                             N.Baskaran

[Kleinstein]

Having two MOSFETs in parallel, there is a slight change to get higher frequency oscillations (e.g. 100 MHz range) from parasitic inductance. Usually one should have resistors in the 10-50 Ohms range in series to the gates, rather close to the MOSFETs.

The second MOSFET also adds capacitance and thus slows down the output stage. This could be enough to make the regulator loop oscillate. So adjustment of the compensation to slower or less gain would help in the case. With a fast regulator circuit the tricky parts are parasitic inductance and  inductive coupling. So the circuit schematics is not enough also the layout and what type of caps is used can be important.

Using 2 MOSFETs parallel for linear operation is tricky: is might work with close matched FETs (same waver) of a type really made for linear operation or a very old type. However load sharing may not be very good: at high power the current will shift towards the hotter one.

[prasimix]

How much current you'd like to have and in which in/out voltage range when you wants two mosfets in parallel?