On Fri, Jun 17, 2011 at 3:10 PM, Forrest Christian wro= te: > I have an application where I am high-side switching several 65V 1A > loads from a microcontroller, using P Channel mosfets. What is the P-MOS you are using? > For various > reasons I am stuck with the P channel mosfets as the output devices - > mostly because I need to slow the turnon and turnoff of the FET's to > round the turnon/turnoff waveform to avoid an inductive spike in nearby > wiring by the quick turnon or turnoff. For various reasons, I have to use integrated high side switch with integrated protection circuitry. :-) One of the reason is that I can not find a good P-MOSFET for the job. It is much easier to find an N-MOSFET for the low side switch. > In any case, I also need to short-circuit protect these devices. =A0 I > keep running into issues when trying to do this: > > 1) PTC's don't seem to be the right tool for this job. =A0 Yes, they seem > to work, but only 'mostly ok'. =A0I can go into more details if necessary= .. I trust you. But why not give more details so that the other people will learn from you as well. > 2) I would love to do this in software in the microcontroller, using a > high-side 'current sense amplifier', but I have yet to find a high side > current sense amplifier which is both reasonable in cost (dollar or less > @ QTY 1K), and works on both 3.3V on the ADC side, and 65V on the 'high > side current sense' side. Most likely you do not want to do this in the MCU. What if the MCU has some glitches? > 3) I swear I have seen a simple, elegant, current limiting circuit which > uses a MOSFET and a few external components - but alas no luck yet. There are all kind of protection circuitry but I am not so sure if there are good enough for not. If you consider short circuit protection, then what about other protection -- over temperature, over voltage, reverse polarity, etc? So for cheap designs, just use the P-MOS and probably a diode for inductive load. Then use a integrated high side switch for more demanding design. > Does anyone have any ideas or pointers for somewhere I haven't > looked yet? --=20 Xiaofan --=20 http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .