Looking for Adjustable 300V DC current limiter for smps debugging - Page 1
Forum section: electronics / projects

author: Miyuki
author_rank: Frequent Contributor
timestamp: February 14, 2017, 08:12:00 pm
content: Hi,I want to ask how do you solve problem when you need to test and measure smps ?Now I use common light bulb in series, it have nice indication and can prevent catastrophic failure.But is any cheap better solution ?I'm thinking about big linear regulator, just a high voltage transistor(s) on huge heat sink controlled by current sensing op amp.How do you solve this problem?

author: MagicSmoker
author_rank: Super Contributor
timestamp: February 14, 2017, 11:27:23 pm
content: An incandescent light bulb is by far the cheapest yet effective approach.Even a simple "two-transistor" current limiter will cost a lot more because you will actually have to run many transistors in parallel to handle the combination of high voltage and current which occurs during limiting (to stay within the FBSOA). For example, Fairchild FJL6920TU is an 800V/20A/200W NPN BJT that is optimized for horizontal deflection use, which is a "more linear" application than, say, a switchmode power supply, but if you look at the FBSOA curve it shows a max allowed collector current of around 0.15A when there is 300V across the C-E junction. So to reliably handle a mere 1A you will need 6-8 of them at a cost of 4USD each.But if you want to mess around with such a circuit anyway, attached is an LTSpice file for one with a ~1A trip point that experiences a 5A load after a short delay.

author: Smokey
author_rank: Super Contributor
timestamp: February 15, 2017, 12:05:08 am
content: How much current are you talking about here?

author: james_s
author_rank: Super Contributor
timestamp: February 15, 2017, 04:30:07 am
content: I've always used a light bulb. It helps to have an assortment of wattages. They can be wired in series for higher voltage or parallel for higher current.

author: David Hess
author_rank: Super Contributor
timestamp: February 16, 2017, 05:02:36 am
content: Are you sure you want a constant current supply? Switching regulators have negative input resistance so if the input source goes into current limiting, the combined system will latch off or at least the input voltage will drop until the switching regulator shuts off.If you want just a fast electronic circuit breaker, then the safe operating limits of a transistor switch on the input are significantly reduced.If a variable AC line voltage input is acceptable, then a variac is a simple solution. Combine this with a fast electronic AC circuit breaker to protect the variac without having to sacrifice fuses.

author: Miyuki
author_rank: Frequent Contributor
timestamp: February 16, 2017, 09:16:20 am
content: I dont need fast circuit breakerI need limit current and measure whats going onI was just interested if is something better than light bulb

author: MagicSmoker
author_rank: Super Contributor
timestamp: February 16, 2017, 11:20:21 am
content: Well, there's another issue that can render any kind of high speed current-limiting pointless: the energy stored in the input/bulk capacitors is usually more than sufficient to destroy the switch(es) if something causes it/them to stay on too long or cross-conduct or the like. The flyback converter has a natural advantage in this respect because a significant inductance (the primary winding) is in series with the switch...as long as the transformer core doesn't saturate, that is.However, in voltage-fed, buck-derived converters (forward, bridge, push-pull), the switches are directly connected to the input capacitor with a minimum of loop inductance, and this capacitor needs to be quite large, relatively speaking, to handle the high ratio of ripple current to average current. Limiting current *into* the input capacitor with a light bulb or circuit breaker, etc., won't prevent failure of the switches if they are erroneously turned on for too long or with insufficient dead-time (causing shoot-through) or pulse-by-pulse current limit is not employed or functional.

author: David Hess
author_rank: Super Contributor
timestamp: February 16, 2017, 01:05:43 pm
content: Quote from: Miyuki on February 16, 2017, 09:16:20 amWell, I do not think a standard adjustable current limit circuit is going to do what you want. As soon as the current limit is reached, the system will latch off or start oscillating which is likely to destroy something. The reason a light bulb works is that as the current drops, its resistance decreases with lower filament temperature. Light bulbs are not very suitable for SMPS work eitherFor SMPS work where I want to alter and monitor the input conditions, I use a variac and current meter.

author: Zero999
author_rank: Super Contributor
timestamp: February 16, 2017, 07:12:17 pm
content: Quote from: MagicSmoker on February 16, 2017, 11:20:21 amIf this current limiter is for DC use, then perhaps it could be connected, after the input reservoir capacitors?

author: Marco
author_rank: Super Contributor
timestamp: February 16, 2017, 07:47:54 pm
content: Quote from: Miyuki on February 14, 2017, 08:12:00 pmYour problem is that your problem is way too poorly defined ...It kinda sounds like you just need a resettable electronic fuse, not a linear regulator.