Switching Power supply schematic - Page 1
Forum section: electronics / beginners

author: redgear
author_rank: Frequent Contributor
timestamp: September 18, 2020, 05:57:49 am
content: I am using the HLK-5M05 in one of my projects. The datasheet show a typical circuit application like the one attached in the image. Do I need both varistor and fuse on the input side? I am not able to source a inductor with the specified ratings, what can I use as an alternative? I'm planning to add a TVS diode on the output so even if the voltage regulator fails I can protect the rest of the circuit, Any suggestions?

author: Gyro
author_rank: Super Contributor
timestamp: September 18, 2020, 11:11:12 am
content: Yes, you do need the fuse. It's there for protection if the VDR clamps a particularly large transient and / or fails short. In the US the next protective device in the chain is the breaker on the distribution board.The magic search term you need is 'common mode choke' rather than inductor. 10mH is one of the more common values.Yes, a TVS on the output would offer some protection, although the clamping voltage is significantly higher that the "holdoff" rating, depending on current (probably around 7V for a 5V device). Note that a TVS exposed to a constant overload (regulator failed) will cook and fail short. Depending on the level of protection you want, you can go right up to a series fuse and crowbar circuit. It might be practical to use a series mosfet and voltage sensing circuit to disconnect the load instead. It's a cost/benefit thing.

author: redgear
author_rank: Frequent Contributor
timestamp: September 18, 2020, 11:18:26 am
content: Quote from: Gyro on September 18, 2020, 11:11:12 amThanks a lot. I will read up on using a series mosfet.The recommended current rating of the inductor is 500mA. Will it be enough for a 1A supply? DC current is 0.7 times AC current right so shouldn't it be rated to ~1.45A?

author: Ice-Tea
author_rank: Super Contributor
timestamp: September 18, 2020, 11:21:48 am
content: Quote from: Gyro on September 18, 2020, 11:11:12 amOr when someone plugs it in a 230V socket

author: Gyro
author_rank: Super Contributor
timestamp: September 18, 2020, 11:29:37 am
content: Quote from: redgear on September 18, 2020, 11:18:26 amWhen you say 1A supply, do you mean the input current (can't be because of the 500mA fuse) or output? If so, at what output voltage?P.S. Sorry, I went back and looked at the datasheet section - it actually recommends a common mode choke current rating of70- 500mA with fairly non-critical value. Remember that the mains input current isn't going to be the same as the low voltage output current, it is the wattage that is important. It makes sense to rate the common mode choke at the same value as the fuse (although in practice, even with a lower rated one, the fuse will pop first).P.P.S. The capacitor (C1) should carry a class X (X2 is sufficient for the US) approval for across-the-mains use.

author: redgear
author_rank: Frequent Contributor
timestamp: September 18, 2020, 12:47:24 pm
content: Quote from: Gyro on September 18, 2020, 11:29:37 amThank You.I meant the output at 5v. Cool, I will pick a inductor with 500mA current rating.What ripple current will the 220uF electrolytic cap on the output side see?What do the number 40/100/21 on the X2 cap denote? I guess the first two represent the min and max temperatures. What is the third one for?

author: Gyro
author_rank: Super Contributor
timestamp: September 18, 2020, 06:24:18 pm
content: You're welcome.The ripple current will depend on several factors: The output impedance of the PSU, the amount of output voltage ripple of the PSU and the load current you are drawing. It may be that they just feel that there needs to be more bulk capacitance on the output than they can comfortably fit inside the PSU package for best noise performance, or may be required for stability of regulation. As the manufacturer doesn't seem to have made any specific recommendation for ESR then I suspect that it is the former and not critical.The markings you quote on the capacitor look to be manufacturer specific, family for instance, if you check the manufacturer's website, the datasheet ought to say. For the X2 classification, you should see a bunch of standard logos (VDE, UL, SA etc.).

author: TimNJ
author_rank: Super Contributor
timestamp: September 19, 2020, 03:31:15 am
content: Quote from: redgear on September 18, 2020, 12:47:24 pmThe 3rd number is the number of days the capacitor can maintain its specifications when exposed to 95% humidity @ 40C. Bit of an odd one.https://forum.digikey.com/t/climatic-category-on-film-capacitors/2952