Charging a capacitor using a mosfet - Page 1
Forum section: electronics / projects

author: Pentoad
author_rank: Regular Contributor
timestamp: December 18, 2019, 10:55:58 pm
content: I am using a DMP2035U-7 P channel mosfet as a switch in one of my designs. It has a pulsed drain current rating of 24 amps. On its drain is the entire thing I am powering which only consumes about 10 milliamps. The problem is that the mosfet is very close to the load which has a 0.1 microfarad ceramic capacitor across it. I am concerned about the inrush current to the capacitor reducing the lifespan of the mosfet as the capacitor is basically a short circuit for an instant. Will such a small capacitor cause a problem?

author: David Hess
author_rank: Super Contributor
timestamp: December 19, 2019, 01:20:24 am
content: Such a small capacitor will not damage the power MOSFET however ringing with the parasitic inductance of the circuit if turn on is fast may damage the load.

author: Etesla
author_rank: Regular Contributor
timestamp: December 19, 2019, 01:28:37 am
content: In general, mosfets are much more tolerant to larger than rated currents than they are to larger than rated voltages. High voltages cause damage through dielectric breakdown, which is an almost instantaneous. High currents, on the other hand, cause damage by heating things up*. If a high current doesn't happen for long enough to heat things up, it doesn't really do any damage. So in your case with a small capacitor, sure the current may be large, but the duration isn't long enough to heat things up enough to cause any damage. Overall, you have nothing to worry about if your layout isn't complete garbage.* (if you ignore the secondary effects large current spikes might have like ringing due to parasitics, which causes high voltages, kind of like what @David Hess was saying)

author: Circlotron
author_rank: Super Contributor
timestamp: December 19, 2019, 01:49:32 am
content: You could try putting perhaps 5-10K or more of resistance in series with, and close to, the gate lead. This will allow the gate-drain capacitance to slow down the switching transition by Miller effect and that may be enough to keep the current transient sufficiently low.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 19, 2019, 01:49:44 am
content: You can use a much smaller transistor too. (There's not much wrong with an oversized part here, if it's simply what you have on hand.)Limit inrush by using a large series gate resistor. This won't limit short-circuit current should such a condition occur, but it works in the average case for inrush.Also if you have the budget, consider using an integrated power switch, like TPS2045 and etc., to provide current limiting, fault protection, fault detection, etc. as needed.Tim

author: Pentoad
author_rank: Regular Contributor
timestamp: December 19, 2019, 08:48:33 am
content: I was wanting to find a much smaller transistor which would be cheaper but was really struggling to. The one I am using is only 9 pence from Farnell. This is the layout and as you can see, the mosfet is very close to the capacitor and the comparator which is being powered. The IC for power management is too large and I've sadly no space left on the board for it.

author: ogden
author_rank: Super Contributor
timestamp: December 19, 2019, 11:16:48 am
content: Add decoupling cap on supply side as well, follow gate resistor advise. To power few mA load I would just use MCU I/O pin (or few parallel) directly.