Home brew output s/cct protection for isolated offline SMPS - Page 1
Forum section: electronics / general

author: ocset
author_rank: Super Contributor
timestamp: June 09, 2019, 07:47:50 pm
content: Many’s the time when you have to do an isolated offline SMPS of say 100-200W and the PWM controller that you use doesn’t contain in-built output short circuit protection (eg LTC3705).As such, you have to implement it yourself with external circuitry. This post is basically seeking the simplest way to do this?As is known, some offline ACDC controllers have a feature whereby if for any period longer than ~300ms (or so), the controller is not in regulation, then it assumes there is an output overload or output short circuit, and it then shuts down for 2 seconds then restarts…(hiccup mode)…this is good, because it prevents high power being dissipated in the output overload or output short circuit. It also prevents damage to the converter.Anyway, very often we have to make such output overload protection circuitry ourselves....because the control chip doesnt have it inbuilt.Would you agree, that the simplest way to do this is to have a comparator on the output which signals through an opto to the primary side whenever the vout is less than 1v below the vout regulation level….then have a micro on the primary side to read this comparator output…and switch off the primary side controller as per the comparator signal?

author: dmills
author_rank: Super Contributor
timestamp: June 09, 2019, 09:04:21 pm
content: Or just, you know, pick the right controller chip to start with?Or monitor the primary side current level, or the mosfet voltage drop when in conduction, or... Lots of ways to skin this just depends on what your parameters are.If your controller has the usual sort of startup supply with run via an aux winding, diode, resistor, cap sort of arrangement then you can organise the hickup timing by just discharging the aux supply cap.You are a power supply guy, you KNOW this.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 10, 2019, 02:43:58 am
content: The LTC3705 looks weird, but the '06 is the one with current limiting and error amp. I don't see anything overtly wrong here.It's a really weird system though, and I don't think I would use it in a design myself.The irony here, I think, is the ABB/GE quarter-brick DC-DC modules use a PIC micro on the primary side. Haven't probed what it does, it's just there, a random PIC, what the hell?https://www.digikey.com/product-detail/en/abb-embedded-power/EHHD010A0B41Z/555-1341-ND/4840105(can actually make out the Microchip logo in the image)So, whatever it is they're doing with it, they saw fit to put one in.Tim

author: David Hess
author_rank: Super Contributor
timestamp: June 10, 2019, 02:46:28 am
content: The LTC3705 by itself has built in over-current protection which trips the fault detection and then restarts. What it does not have is current limiting.

author: MagicSmoker
author_rank: Super Contributor
timestamp: June 10, 2019, 08:41:03 pm
content: Quote from: treez on June 09, 2019, 07:47:50 pmNo, I wouldn't agree. How do you power the secondary side comparator in the event of an output short?If the converter is forward derived (forward, bridge, etc.) then output overcurrent protection will be automatically provided whenever primary side peak current limiting is used.If the converter is boost/flyback derived and operating in DCM then things are only a little more complicated, as limiting peak switch current does not effectively limit output current (it limits input power to the transformer instead). However, output voltage is inversely proportional to load current once the primary side peak current limit is reached, and since all windings track each other closely in a DCM flyback, the output voltage of a bias winding will also be inversely proportional to load current once the main secondary is overloaded. Just hang another diode feeding a *small* capacitor on the bias winding that is likely present to supply the primary-side circuit after start-up and you'll have a good proxy for output current on the main secondary.If the converter is a flyback operating in CCM then things are more complicated still, but that is all I have to say for the moment.

author: ocset
author_rank: Super Contributor
timestamp: June 10, 2019, 09:05:08 pm
content: QuoteThanks..ie, making it so that the absence of a signal means its gone short.I am not sure if you were referring to this but the problems i see for flyback bias coil in s/cct is that due to leakage L, it often doesnt collapse fully in output short circuit.Also, i am not sure if you were speaking of this either but shorting the output of an offline flyback results in high staircasing of primary current due to the lack of secondary voltage to discharge the secondary coil, also the LEB time on the primary controller means there's unfortunately an on time of the primary which allows the unfortunate staircasing, hence the need for "timeout" overload protection (s/cct protection).

author: MagicSmoker
author_rank: Super Contributor
timestamp: June 10, 2019, 09:56:21 pm
content: Quote from: treez on June 10, 2019, 09:05:08 pmNot quite, as this results in a classic "chicken or the egg" kind of problem at startup.Quote from: treez on June 10, 2019, 09:05:08 pmWell, you yourself didn't specify whether the "isolated offline smps" was a flyback or forward type, hence I tried to cover both in my reply.That said, overcurrent protection using the bias winding voltage as a proxy for main winding overcurrent isn't perfect, it's just cheap and expedient, which tend to be unspoken design criteria for an offline 100-200W PSU these days. If you expect to need to deal with prolonged hard shorts on the main output of a DCM flyback then you will either need to make the primary side current limit circuit capable of operating from the meager current of the mains-derived startup circuit, or else use a separate power supply to either operate it or a conventional current sense circuit on the secondary. The separate rectifier on the bias winding trick is more geared to dealing with overloads and "soft" shorts, and even then it can really only address the latter by putting the controller IC into hiccup mode.Quote from: treez on June 10, 2019, 09:05:08 pmStaircasing of the primary current is always a problem in peak current mode control and/or when leading edge blanking is used on the switch current sense signal. Paradoxically, this problem is worse when synchronous rectification is used (and sometimes even with Schottkys) because the forward drop of the rectifier is so low. Again, if you expect to have to deal with a hard short on a flyback (which I define as demanding more than 10x nominal load current) then you might very well need to use alternative means of supplying the overload protection circuit. There is another technique that can be used which is kind of kludgy and dirty: oversize the rectifier on the main secondary and intentionally give that winding a higher than ideal leakage inductance; the latter will limit fault current while the former will better tolerate it. I suspect a lot of commercial flybacks under 300W or so use these techniques, whether intentionally or not, to improve their robustness.

author: ocset
author_rank: Super Contributor
timestamp: June 11, 2019, 05:27:57 am
content: QuoteThaks, agreed, ..then we would use a micro in the primary to sort it out....ie handle the startup didgereedoo bit.