----- Original Message ----- From: "PicDude" To: "Microcontroller discussion list - Public." Sent: Friday, December 24, 2004 5:11 PM Subject: Re: [EE] DC-DC Converter startup transients > On Friday 24 December 2004 03:42 am, Alan B. Pearce scribbled: >> Projects I have been involved in over times past destroyed FPGA arrays >> because of spikes like this, which occurred for exactly this reason. In >> the >> case I am familiar with, the FPGA was fed from a DC-DC converter as it >> needed to be isolated from other circuitry, and the uncontrolled spike >> before the feedback loop took effect did exactly what you describe. I did >> not realise the cause at the time, and only learnt about it later through >> a >> Design Ideas in EDN magazine where they showed using a zener as clamp on >> such a transient. > > > Exactly what I'm thinking, as there would be no way to "fix" the DC-DC > converter since the problem is inherent in the design. I'll like to > experiment with this, but have to way to see the problem in practice, so > that > I could see the fix. Did they just use a simple zener (say 5.1V) in > parallel > across the output? > > As an alternative, I'm wondering if this spike would be present still or > different if the converter is switched on via it's on/off or shutdown pit, > vs. being powered up initially? If it would reduce or eliminate the > problem, > perhaps the fix could be as simple as delaying startup with a simple > startup > reset circuit. Any ideas? > It's sometimes worth checking if an unregulated converter will do the job. The idea seems to have been reinvented recently and renamed "factorised power" but a fixed ratio convertor cannot overshoot, though an output filter may ring. Nonlinear compensation might be exploited to give a converter a faster response to overvoltage. A converter may be soft-started. Most designs I see seem to have a soft start. Note that soft start may be counterproductive if it allows a control loop to wind up in which case adding a ramp to the voltage reference may be more effective. A load device may withstand a considerable overvoltage when held idle, that would cause damage if running. Also normally safe devices may be vulnerable to a hot restart. -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist