Big capacitors on amp PS, before or after a fuse?? - Page 1
Forum section: electronics / projects

author: Chris Wilson
author_rank: Super Contributor
timestamp: October 13, 2017, 11:02:04 pm
content: I have built a few of these LF (136khz) RF amps and if a FET goes bang the fuse often doesn't blow. I have had a thought that better protection may be offered by having the PS fuse *AFTER* the big 4700uF electrolytic to stop its stored energy being available after an event. Comments please

author: NiHaoMike
author_rank: Super Contributor
timestamp: October 14, 2017, 12:44:35 am
content: The MOSFETs always blow trying to protect the fuse. A fast reacting fault protection circuit would be a better bet.

author: retrolefty
author_rank: Super Contributor
timestamp: October 14, 2017, 01:04:14 am
content: QuoteSo what happens if the cap shorts to ground after moving to upstream of the fuse.In my opinion nothing should be upstream of the fuse except the voltage source. A fuse isa safety device but only if utilized properly in one's circuit design.

author: TimNJ
author_rank: Super Contributor
timestamp: October 14, 2017, 01:43:53 am
content: Is 25A the correct rating? (I'm not sure, I'm just asking.)Perhaps the fuse would blow if you dropped to a lower current rating. You could use some sort of NTC inrush limiter to keep the current low at startup.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 14, 2017, 02:25:07 am
content: As has been noted before, your 136kHz amp is less an amplifier and more a switching supply. Protecting MOSFETs on the microsecond scale is not only typical, but an operational control method for most switching supplies today.The cynical read of this is, "your circuit is bad and you should feel bad",but the constructive one is: you can add current sense resistors, comparators and a protection latch that turns off the transistors when an overcurrent event occurs. Just toss in a few more flip-flops.Heh, 74F, old fashioned. 74HC is more than fast enough here! Cap coupling to the gate driver is really weird, and hard on the gate driver's ESD diodes. (I mean, are they not already compatible logic level signals?) Would recommend dropping that.On a related note, the gate driver would be better utilized -- and the output transformer simplified, an important gain -- with the transistors swapped around for half-bridge operation, instead of push-pull. Current limiting can then be done slightly differently, perhaps with a current transformer.Shouldn't need any twisted pair either, if everything's on PCB the gate driver can be right up by the transistors, no worries.Tim

author: DrGeoff
author_rank: Frequent Contributor
timestamp: October 14, 2017, 02:57:27 am
content: Large caps should have a bleed resistor across them so that they aren't sitting there fully charged when you get your fingers and screwdrivers in there.

author: NiHaoMike
author_rank: Super Contributor
timestamp: October 14, 2017, 04:24:52 am
content: Quote from: DrGeoff on October 14, 2017, 02:57:27 amA very bad assumption to make. Apart from the resistor failing open, in today's world of minimizing standby power usage, it is very common to find PSUs that do not have bleed resistors. Always test and discharge before doing work.

author: TimNJ
author_rank: Super Contributor
timestamp: October 14, 2017, 04:44:55 am
content: Quote from: NiHaoMike on October 14, 2017, 04:24:52 amI do not think he was saying to assume equipment has bleeder resistors, just that it hypothetically could save your ass just incase you do something stupid.

author: bson
author_rank: Unknown
timestamp: October 15, 2017, 09:03:45 pm
content: The primary purpose of fuses is to prevent fire.What happens if a capacitor fails short?

author: Zero999
author_rank: Super Contributor
timestamp: October 15, 2017, 09:44:32 pm
content: Quote from: NiHaoMike on October 14, 2017, 04:24:52 amThe power supply voltage is only 50V, so shock due to a capacitor remaining charged isn't an issue.I believe there's a requirement for the voltage on capacitors in domestic equipment to drop below a certain voltage, after a certain period of time, after the power has been removed but I don't have the information to hand.

author: Chris Wilson
author_rank: Super Contributor
timestamp: October 16, 2017, 12:18:14 pm
content: Quote from: T3sl4co1l on October 14, 2017, 02:25:07 amThanks tim, you mention several points, please let me respond in order.I have tried using amps with inbuilt protection against excess PA over current, and tried adding it to my own. But they all seem to have a glitch in so far as the amp is powered on all the time and the signal from the driver just commences at the start of a TX. Two issues ensue, one is a high current surge on start up. set the over current level to admit this and it always seems too high to protect the FET's if anything happens to the output side like a shorted aerial.The damned FET's just seem to have such a low pain threshold that I gave up on electrickery other than a basic quick blow fuse. The other issues is the very high RF level seems to play havoc with complex control strategies.I queried the cap coupling and it's there in case whatever driver is used stops with an output high. they are DC blocks to stop both FET's being able to be turned on simultaneously, apparently. It seems more common for designers of these LF amps to have caps between the gates of the PA FET's and the driver chip though, the designers earlier version did this.Sadly re-designing to make this a half bridge is well beyond me at the moment, but i do not doubt it could be improved!Thanks to everyone for the replies.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 16, 2017, 07:59:01 pm
content: Quote from: Chris Wilson on October 16, 2017, 12:18:14 pmDid you ever stop to think, hmm, maybe they're interrupting startup for a reason?There are many levels of protection and control. If you add a latching AND gate immediately after the toggle f/f, you can have the toggle outputs turn the transistors on and off, except that, when the per-transistor protection fires, it resets that transistor. The transistor then stays latched off until the toggle turns it on again. A momentary upset (like transformer saturation, the most likely startup problem) doesn't crash the system. Now, this wouldn't catch a continuous fault, but then you could add a pulse detector, that watches for how frequent the fault triggers are; when they become too frequent, the whole thing is shut down.Or you could drop the discrete logic altogether and harness a chip for control and protection. Even the basic TL494 has a valuable feature: it contains a free-running oscillator, that can be locked to your 272kHz reference just the same, and its output duty cycle is adjustable. You can use the dead time function to implement soft starting. You can route current protection through this as well (but it's not latching, so that still needs additional hardware). Others may be out there with even better options (and integrated gate drive, and..).If you're getting noise and nuisance trips, that's a more basic problem. That's fundamentals: that's grounding, short switching loops, common mode filtering, that sort of thing. You need to fix that anyway, otherwise you'll be spewing who knows what in the HF bands!When I designed an industrial inverter some years back (5kW per module, up to 400kHz), I put in desat protection (a comparator watches the drain voltage, and fires a fault signal if it exceeds what Rds(on) would give). At first, this gave us incessant nuisance trips: at the switching edge, a flurry of false pulses would be transmitted. (Or not even false, simply because sometimes the transistors must turn on with high Vds!) We added blanking around the switching edges, so that events within 1us of an edge were ignored. If a fault remained after that, the system was shut off.This simple method saveddozensof transistors during development. I don't think we accumulated more than a handful of transistors during the 2 1/2 years I was there. And that includes the big industrial IGBT modules we moved on to, later in development!This system is so robust, you can short out the transistors, during startup, while it's running, and it simply ticks and turns off. No damage.In contrast, our predecessors, on that project, had left a small box brimming with accidents and failed experiments. They didn't have a clue how to design these things, or even how to work with them safely. When you don't give yourself the tools to succeed, you can't get anywhere.Tim