Inrush current during power up

[mkiepiela]

Due to large capacitance on P12V0_SW rail, a high inrush current flows when T3 current switch is being turning on. It can be a problem especially when more than one HVAMP is supplied from a single power supply. Some soft start circuit would be useful.

For already manufactured items (v1.1) we made modification of the current switch circuit. We used the gate-source capacitance of T3 to slow down the turning-on process. Thanks to that the R_ds of T3 is limiting the current charging capacitors on P12V0_SW rail during the starting up.

Original circuit:

Modified circuit:

The modification of PCB done:

• carrefully desoldering and picking up the 4th pin of T3 (gate),
• soldering the resistor between picked up gate pin and solder pad of R23.

We used the 2 MOhm resistors which makes the power up process much smoother but still keeps T3 in its safe operating area.

[gkasprow]

Thanks! Make sure you use an ESD environment. MOSFET gates are very fragile and even finger touch can kill one. Just equalize potentials before touching the circuit.

[gkasprow]

fixed in v1.2