Reduce Transient Voltage from Step Up/Down Converter? - Page 1
Forum section: electronics / projects

author: PhysicsDude55
author_rank: Contributor
timestamp: March 13, 2020, 05:01:42 am
content: My project is creating a mobile power supply and soldering station from an 18V tool battery. I got a TS100 soldering iron (really cool little iron!), and was just going to feed direct ~18V from the battery to the iron.I then ran across these step up/down converters on Amazon. They will output 0-30V from a 5-30V input. So I can input my 18V tool battery and get any voltage output from 0 to 30V with the turn of a knob. Really cool! At work I often have to power up 12V and 24V devices, so being able to get both of those voltages from an 18V battery without doing something elaborate is really useful for me.However, the soldering iron is having a hard time running off the converter/power supply. It will constantly reset while heating up. When I hooked up the voltage output to my scope it showed that the voltage was dropping to half or less of the outputvoltage and then overshooting it every time the heating element turned on in the soldering iron, which is multiple times per second. I added a 1500 uF capacitor and it helped, but it still has a huge voltage transient.I was first using a 0-30V 0-4A 35W/50W peak converter, which the soldering iron is pushing the specs of that. So I ordered a larger and better (more expensive) unit that's rated for 5A 100W output (has cooling fan). The larger unit is just as bad, if not worse!Sorry for the crude cell phone pictures of the scope output. I have a picture of the power supply running the soldering iron with and without the 1500 uF capacitor. The scope math is displaying the Max Voltage (Ma), Minimum Voltage (Mi), and Vpp. The power supply is set at 22.0V, the upper and lower bounds of the graph on the oscilliscope is +28V and +8V. The spaced out spikes are the iron heating up, and then the massive amounts of voltage variation is when the iron is up to temperature, and I assume the heating element turns on/off like 10 times a second to maintain temperature. The iron is rated at 65W, or just below 3A draw at ~24VIs there an easy to solution to this? Add more output capacitance? I'm kind of uneasy about adding 10,000 uF or something crazy like that to the output, seems like that would basically wreck havoc with the constant current and current limiting output functionality from the power supply, although that's not the end of the world for my application.It seems like the more conventional voltage step down power supplies have much less output voltage transient than these step up/down units.

author: Zero999
author_rank: Super Contributor
timestamp: March 15, 2020, 10:36:10 pm
content: Did you try and ferrite beads?Please post some links to the DC:DC converters. They sound pretty crappy to me. Perhaps you could have a go at designing your own?And can't you just run the soldering iron straight from the battery? I Googled and it should be able to run straight from 18VDC.https://www.hobbyrc.co.uk/ts100-soldering-iron-b2-with-xt60-lead?gclid=Cj0KCQjwpLfzBRCRARIsAHuj6qVooFnaJz0Jk1c2F1GTe6LEnnw5mRl0KzM_Y234ILWxf_txpklmuwkaAsQkEALw_wcB

author: PhysicsDude55
author_rank: Contributor
timestamp: March 17, 2020, 03:07:13 pm
content: Thanks for your reply! The Ferrite bead is a great suggestion. I did not try that, and I have a bunch of ferrite beads. I will see if that helps.These voltage converters are not high quality. They are typical low tier Chinese products with instructions that are barely readable in English. Nevertheless, the fact that they can output 0-30V smoothly from an 18V input is pretty neat.I can supply the soldering iron with 18V directly from the battery, but I'm really looking for a more multipurpose solution so I can power 24V equipment at work without getting too much of a voltage spike or droop.I ended up adding 3500 uF of capacitance and added a .5 ohm 10 watt resistor and that helped quite a bit. There's only about a 3V transient on a 24V output, about half of what it was before.I got the converters off Amazon because I wanted the prime shipping. I'm sure they can be had cheaper direct from China, although its hard to search and find for this type of unit specifically.I have this one (smaller version), which is pretty limited:https://www.amazon.com/gp/product/B07Z4S5F34/ref=ppx_yo_dt_b_asin_title_o02_s01?ie=UTF8&psc=1And then bought this one (bigger version) which I like quite a bit. Its has a very similar interface to those little bench power supply units that are pretty popular:https://www.amazon.com/gp/product/B082TVFY7L/ref=ppx_yo_dt_b_asin_title_o00_s00?ie=UTF8&psc=1

author: ETITsynthesizer
author_rank: Contributor
timestamp: March 17, 2020, 08:44:08 pm
content: since the DC to DC converter has a servo that has the load as the input and the load has a circuit that detects changes in the input voltage, you have two circuits going into an arm wrestle. you need a constant load or a constant current or a constant voltage. you need some constants. can't have them reacting to each other. this is like the inverting opamp output to input. obviously it will oscillate.as a fun and possibly dangerous experiment, stick two 7805 outputs together with no load. see how much power they consume and how much heat they generate. this is a destructive test that will cost $1. have a fire extinguisher ready and don't walk away. this explains what happens when regulators stop playing nice with other regulators. maybe you can figure out why this happens and what can mitigate it.

author: TimNJ
author_rank: Super Contributor
timestamp: March 17, 2020, 09:04:55 pm
content: Do you have a schematic for the DC-DC converter you are using? I wonder what peak current the TS100 is drawing while it's on. If it's very high and very fast, the feedback loop of your DC-DC converter might not be able to react quick enough to keep the voltage stabilized. Try zooming in one one of these transient events and maybe we can see whether it's a poorly compensated control loop or something else.

author: ETITsynthesizer
author_rank: Contributor
timestamp: March 17, 2020, 09:27:04 pm
content: Quote from: TimNJ on March 17, 2020, 09:04:55 pmI would bet that this is it but I also think that the transients coming from the load are what triggers it to react late and overshoot. then it undershoots. then it overshoots right when it starts to level out because the load switched off. this causes it to overshoot and we start again. the load is a PWM circuit obviously. you need to find the PWM frequency and notch filter it out. or you need to phase shift the feedback circuit a little more so the overshoot matches the load switching on perfectly. in practice here these are almost the same thing actually just using different ways to describe it.

author: PhysicsDude55
author_rank: Contributor
timestamp: March 17, 2020, 11:36:00 pm
content: I'm away from home and thus away from my scope for the next weeks, but as far as I could tell once the iron got up to temperature, the time inbetween the heating element turning on and off varied depending on how often it needed to energize the element to maintain the set temperature.For what its worth also tested the power supply output by connecting an 8 ohm load to it (8 ohms @ 22V = 2.75A = 60W) which roughly matches what the 65W soldering iron draws. The power supply reacted the same way.. voltage droop and overshoot for ~100ms, and then stabalized.Ultimately with 3500 uF of capacitance, its good enough for my purposes. The soldering iron goes from room temperature to 300 C within 15 seconds.