Minimum load circuit for switching power supply - Page 1
Forum section: electronics / projects

author: kreyszig
author_rank: Regular Contributor
timestamp: February 14, 2023, 05:28:56 pm
content: If I want a circuit that provides minimal load for a switching DC power supply whenever the actual load is too low, or if there is an over voltage situation for the switching power supply for whatever reason, would something like this be appropriate?Thanks!

author: pqass
author_rank: Super Contributor
timestamp: February 14, 2023, 06:18:01 pm
content: If it's a minimum load, you could use just a [fixed current] two transistor constant current sink (a cc source uses PNPs).https://www.google.ca/search?q=transistor+constant+current+sink&source=lnms&tbm=isch&sa=X&biw=1424&bih=974&dpr=0.9If you want to protect for over voltage, use a "crowbar".https://www.google.ca/search?q=crowbar+circuit&tbm=isch&sclient=img&bih=974&biw=1424FYI: The circuit you provided is a bit odd in that (a) it applies a minimum current (Vcc/Rmin which varies with voltage) after a minimum voltage is seen on Vcc. Assuming Vcc can start a 0V, (b) op amp/comparator and Vref power shouldn't come from the thing you're monitoring; should have its own supply. Also, (c) if U1 is a comparator, many are open collector outputs, thus can't drive the NMOS which if it isn't a logic-level FET, (d) will usually need 10V to turn on hard.

author: kreyszig
author_rank: Regular Contributor
timestamp: February 14, 2023, 06:37:42 pm
content: Quote from: pqass on February 14, 2023, 06:18:01 pmThanks. Sorry this is not exactly what I am trying to achieve. I only want the extra load to be active when the switching DC power supply output goes into over voltage (e.g 5.5 V vs 5 V target), most likely due to a load that is too low. Whenever over voltage occurs, I don't want to completely short the circuit, I just want to apply the extra load to help bringing down the voltage. The switching supply I am considering recommends a minimal load of 10% of the device's rated current. I would prefer avoiding to simply add a resistor in parallel that is constantly consuming this 10%. The nominal voltage of the switching supply is the same as my target voltage, so adding an LDO would not work well either.Another option I was considering for the minimum load only (would not cover the over voltage for all scenarios), would be to use something like this instead:https://electronics.stackexchange.com/a/329663

author: exe
author_rank: Unknown
timestamp: February 14, 2023, 06:54:32 pm
content: Is this a linear circuit clamping voltage, or opamp used just as comparator? If so, I'd use a comparator.For this circuit to work you'd need a rail-to-rail opamp that accepts large differential voltages (i.e., no diodes reducing common-mode voltage). I'd also add decoupling.

author: Zero999
author_rank: Super Contributor
timestamp: February 14, 2023, 06:58:33 pm
content: Quote from: kreyszig on February 14, 2023, 05:28:56 pmWhat is the minimum load requirement?Your circuit is far too complicated.The output should be in the same loop as the minimum load, so it's disabled, when a current is drawn. Here's an example which draws a minimum current of just over 100mA. If a the load current increases, it shuts off the load resistor. A Schottky diode is used to bypass the current sense resistor, to avoid excessive voltage drop at high currents. If you want more accuracy, use an op-amp and voltage reference.

author: kreyszig
author_rank: Regular Contributor
timestamp: February 14, 2023, 07:16:22 pm
content: Quote from: Zero999 on February 14, 2023, 06:58:33 pmI am not sure to understand why the minimum load should be in the same loop as the output, as it causes the voltage to drop when there is a load applied? By having it in a different loop and using the fact that an over voltage situation requires more load to be applied, I could still apply the minimal load and disable it when the voltage drops, no? I was aiming to avoid dropping the voltage for the load as the supply will already tend to go towards under voltage as the load increases. Thanks!

author: exe
author_rank: Unknown
timestamp: February 14, 2023, 07:17:45 pm
content: BTW, what's the model of SMPS? I want to see the datasheet and minimum load specification.

author: kreyszig
author_rank: Regular Contributor
timestamp: February 14, 2023, 07:23:47 pm
content: Quote from: exe on February 14, 2023, 07:17:45 pmF1205S-1WR3:https://www.mornsun-power.com/html/pdf/F1205S-1WR3.html

author: exe
author_rank: Unknown
timestamp: February 14, 2023, 07:55:31 pm
content: Ah, those. I think have a few. Afaik, they are not regulated, otherwise why they'd had so poor regulation and ripple?I'd add a shunt regulator to them. The easiest way is to use something like tl431 (with a bjt). Like this one:https://www.ti.com/lit/ds/symlink/tl431.pdf(Figure 10-7. High-Current Shunt Regulator). I'd set shunting voltage a bit higher than what's under the load to make sure it's only working when needed. No need for a big transistor, something for 200mA-1A should work. Though, again, I'm not sure if the circuit is gonna be stable with a capacitor, please check that (e.g., in a simulator or oscilloscope, esp. when transitioning between states).

author: kreyszig
author_rank: Regular Contributor
timestamp: February 14, 2023, 10:23:12 pm
content: Quote from: exe on February 14, 2023, 07:55:31 pmThanks! Interesting applications with this TL431! When I look at the shunt regulator for Figure 10-7, it looks like the voltage is regulated through the voltage drop in the inline resistor, right?I was looking at their Figures 10-10 and 10-8 and I was wondering if these could work better?

author: Zero999
author_rank: Super Contributor
timestamp: February 14, 2023, 10:25:01 pm
content: Quote from: kreyszig on February 14, 2023, 07:16:22 pmThat's true. My circuit was designed to avoid extra power consumption. Voltage drop was a lower priority. If an op-amp is used, with sense resistor can be a much lower value.If you're not bothered about it drawing a load continuously, then why not just use a resistor? It's much simpler.

author: kreyszig
author_rank: Regular Contributor
timestamp: February 14, 2023, 10:39:02 pm
content: Quote from: Zero999 on February 14, 2023, 10:25:01 pmI would need to draw 20 mA continuously with a shunt resistor. I was hoping for a circuit design with smaller current draw from the regulation part when there is a sufficient load, but also a negligible voltage drop from the input voltage for the load,

author: Zero999
author_rank: Super Contributor
timestamp: February 14, 2023, 10:54:12 pm
content: Quote from: kreyszig on February 14, 2023, 10:39:02 pmYou need to sense the current draw, which normally involves using a resistor to convert the current into a voltage, which forms one of the inputs to an error amplifier, be it a simple transistor, or an op-amp. This will drop a small voltage. Another option is to use a Hall effect sensor, but these don't work so well at low currents and can be prone to oscillation, so the circuit will be more difficult to stabilise.Why not just use a better DC:DC converter which doesn't require a minimum load current?Here are some examples from the website you linked to a couple of posts ago.https://www.mornsun-power.com/html/pdf/UWE_S-1WR3.htmlhttps://www.mornsun-power.com/html/pdf/WRB1505XS-1WR2.html

author: kreyszig
author_rank: Regular Contributor
timestamp: February 15, 2023, 12:57:47 am
content: Quote from: Zero999 on February 14, 2023, 10:54:12 pmI was intending to sense the voltage instead of the current draw, using high impedance to minimize losses.I don't think I necessarily need a fully regulated supply. What I need is:-DC isolation-Supply voltage contained between 4.5 V and 5.5 V with no out of control oscillations-Maximum efficiency under maximum load-Keep cost and PCB space requirement in checkWhen I look at the other options you provided, it looks like WE_S-1WR3 requires about 16 mA to achieve regulation, based on the drop in efficiency compared to F1205S-1WR3, which is similar to adding a shunt resistor to F1205S-1WR3.WRB1505XS-1WR2 has the same minimum 10 mA draw as F1205S-1WR3. Both devices cost about twice as much as the F1205S-1WR3.So if I could basically cap the voltage coming from F1205S-1WR3 at 5.5 V under any reasonable condition, with an added circuit that draws an amount of current small compared to 20 mA under maximum load, without introducing significant noise or oscillations when paired with the supply, I think I would achieve what I am trying to do.

author: bdunham7
author_rank: Super Contributor
timestamp: February 15, 2023, 01:20:33 am
content: I think you could just put a ~5.2V zener across it and be done.https://www.mouser.com/ProductDetail/Vishay-Semiconductors/TLZ5V1C-GS18?qs=6Fe8ASSgjNtFGmv9Qaei0Q%3D%3D

author: kreyszig
author_rank: Regular Contributor
timestamp: February 15, 2023, 01:43:21 am
content: Quote from: bdunham7 on February 15, 2023, 01:20:33 amYes this might be the case. 500 mW power dissipation is pretty good. Is there a way to have a good idea of the reverse leakage current 0.2 V under the zener voltage (@5.0 V) and 0.3 V above it (@ 5.5 V)? I am not sure to understand Figures 7 and 8 in the datasheet for this component, but regardless the resolution of the x-axis seems an order of magnitude too low? These values would give me a good idea of how the efficiency compares to using a shunt resistor at 5.0 V and how aggressively an effective load is applied at 5.5 V.... The ideal circuit for my application would draw negligible current at 5.0 V and around 20 mA at 5.5 V I guess.

author: bdunham7
author_rank: Super Contributor
timestamp: February 15, 2023, 05:23:28 am
content: Quote from: kreyszig on February 15, 2023, 01:43:21 amIf you wanted something that precise you'd need to characterize them yourself as the range for this device is 5.09 to 5.37V @ 20mA. Since your DC-DC converter reacts to a small load by reducing its voltage back in line, if you can live with some variation in the maximum the tolerance may not be an issue. Given the dynamic impedances they have listed, I'd expect the current to be down to 1mA or less by 4.9V, what it would conduct at exactly 5V would probably vary quite a bit by individual unit. The knee is going to be a little bit softer at this voltage, but I think not too soft for your application.

author: Kim Christensen
author_rank: Super Contributor
timestamp: February 15, 2023, 05:45:34 am
content: Is the voltage source feeding the F1205S-1WR3 well regulated? Because input voltage variations will effect the output voltage in addition to load changes.This might effect how you design the shunt regulator. ie: You'll have to design for the worst case scenario of max input voltage during minimum load.

author: exe
author_rank: Unknown
timestamp: February 15, 2023, 08:30:09 am
content: Quote from: kreyszig on February 14, 2023, 10:23:12 pmYes, by inline resistor, but you don't need it as as your dcdc converter will drop the voltage.Circuit 10-8: it's a crowbar circuit that short-circuits output at overvoltage condition until it's disconnected from the source. So, doesn't fit your purpose.Circuit 10-10: linear regulator, it has a drop of 0.8V or something. You don't have extra 0.8V to loose. Although, I think, it can be an option to use a cmos ldo regulator with a low dropout voltage to stabilize output. For example, LDCL015 has min dropout 50mV at 100mA current. But maximum recommended input voltage is 5.5V (absolute max 7V). There are tons of LDOs like that, so pick the one you can buy easily.BTW, why increased output voltage is a problem? What are you supplying?

author: kreyszig
author_rank: Regular Contributor
timestamp: February 15, 2023, 01:39:01 pm
content: Quote from: bdunham7 on February 15, 2023, 05:23:28 amIZT1 on their datasheet is exactly my target minimal load for 5.5V. Based on this it looks like TLZ5V6A would be the best match? It would be great if they gave the voltage range at IZT2...

author: kreyszig
author_rank: Regular Contributor
timestamp: February 15, 2023, 01:43:38 pm
content: Quote from: Kim Christensen on February 15, 2023, 05:45:34 amIf I understand correctly how these isolated switching supply work, the primary of the isolation transformer is regulated, but the voltage for the secondary is not, so it will tend to decrease as load increases due to the transformer leakage (please let me know if I am wrong there?). So due to the regulation of the primary, I would have expected the output to be relatively insensitive to a change of voltage for the feeding source?

author: kreyszig
author_rank: Regular Contributor
timestamp: February 15, 2023, 01:54:07 pm
content: Quote from: exe on February 15, 2023, 08:30:09 amOk yes I think I understand. Since I don't need the inline resistor, then yes it looks like a great and relatively simple circuit that would do exactly what I want. So basically for Vref set at 5.5V, if 20 mA is required to cap the voltage there, then the NPN transistor would be dissipating 110 mW.The device I am supplying is a ADM2463EBRWZ transceiver. It looks like the isolated side draws 8-9 mA at rest and up to 145 mA at 500 kbps. So at rest the device draws slightly under half the minimum load for the switching power supply.Edit: It looks like the TL431 in SOT-23-3 can draw 150 mA of current and dissipate 0.58W of power by itself when a 1 cm^2 pad is used for the cathode (https://media.digikey.com/pdf/Data%20Sheets/Nexperia/TL431_Family.pdfTable 6). With my design about 0.11W would be dissipated if the transceiver was not connected at all, so it looks like I would not even need an external transistor? And I will be using a 4-layer PCB due to another section of the circuit. I was not even going to use a VCC plane for the isolated transceiver section (not enough components connected to VCC and very short traces, I was going to have two ground planes), but I could always use a VCC plane with a bunch of vias in the TL431 cathode pad so this component would remain nice and cool...

author: kreyszig
author_rank: Regular Contributor
timestamp: February 15, 2023, 02:17:44 pm
content: So far I think I like the shunt regulator idea the best as it is flexible/configurable and I assume it must have the most aggressive change in dynamic impedance around the set point. The zener diode idea requires less components, but the resulting response must be more dependent on the particular unit. I am not sure if I should expect more stability from one circuit vs the other or if it should be similar with that respect.

author: Kim Christensen
author_rank: Super Contributor
timestamp: February 15, 2023, 04:40:26 pm
content: Quote from: kreyszig on February 15, 2023, 01:43:38 pmI think they expect the user to regulate the input voltage of the F1205S-1WR3 to within 10.8-13.2V. I doubt there is a regulator inside the module, because if they were going to do that, why not just regulate the output?Someone correct me if I'm wrong, but it looks like if the input varies by 1% then the output will vary by 1.2%. Hard to tell exactly because for the 1.2 spec under "Linear regulation" they don't give the "units" and write -- instead. All the output voltage & current curves are for a nominal input voltage (12V) so you can't go by these curves alone.See annotated snippets from datasheet:

author: kreyszig
author_rank: Regular Contributor
timestamp: February 15, 2023, 04:51:51 pm
content: Quote from: Kim Christensen on February 15, 2023, 04:40:26 pmHmm ok I see, I missed that, you must be right. I looked at so many switching supplies and I assumed this one worked similarly. So 10.8-13.2V is a +/-10% range that would translate into a +/-12% variation for the output. So a 5.5V output with a light load could become 6.16V for a 13.2V input, which is overshooting the upper limit for my transceiver by 12%. Based on Figure 1, I might not even be able to bring the voltage down to 5.5V under maximum load if the output is 6.16V at 10% load...As power input I am going to use a VCC output from another board with mostly unknown specs in order to power mine. All I know from the other board is that the nominal output voltage is probably 12.7V. For the unit I have the unloaded output is 12.5V. I know that I can draw at least 2W from that board. If I used an LDO to feed the switching power supply, the loss in efficiency would be significant.

author: kreyszig
author_rank: Regular Contributor
timestamp: February 15, 2023, 05:25:34 pm
content: So after all it looks like there is really no way to get to my target supply and efficiency with the power supply I had chosen. The other options from the same manufacturer are not really interesting based on efficiency. A better option appears to be this one: NCS1S2405SC (https://www.murata.com/products/productdata/8807030587422/kdc-ncs1.pdf?1636083019000). It is regulated and seems to have pretty much the same efficiency or better as the unregulated F1205S-1WR3... Over twice the price though. My guess is that it might have a better efficiency due to lower leakage of the transformer as it as 1 kV isolation vs 3 kV. In a different thread I was considering making my own 1 kV isolated "Fly-Buck" switching supply with regulation of the primary...Edit: MTC1S1205MC (https://www.murata.com/products/productdata/8807030358046/kdc-mtc1.pdf) has both a good efficiency and 3 kV isolation. Availability is not the best though.

author: exe
author_rank: Unknown
timestamp: February 16, 2023, 11:04:40 am
content: I'd put an LDO instead of tl431 because it will probably be more power efficient if most of the time the ADM2463EBRWZ is idling.As of F1205S-1WR3 vs NCS1S2405SC, if it's one-off device, just go with whatever option.You may also use F1203S-1WR3 which is 3.3V device. Chances are, with no load it will produce output within what's allowed by ADM2463EBRWZ.

author: kreyszig
author_rank: Regular Contributor
timestamp: February 16, 2023, 01:49:16 pm
content: Since the regulated options are not particularly appealing from a price or isolation standpoint after all, it looks like the previous idea I had of using a Fly-Buck design (https://www.eevblog.com/forum/projects/efficient-regulated-5v-and-isolated-reg-5v-supply-from-unreg-2w-12-7-v/25/) with a regulated primary would meet my goals in terms of good enough output regulation, efficiency under heavy load, and price point...

author: Zero999
author_rank: Super Contributor
timestamp: February 16, 2023, 07:52:19 pm
content: Quote from: kreyszig on February 15, 2023, 12:57:47 amWhat makes you think the modules I suggested require a minimum load current? It's true the load regulation is specified only 5% but the off-load voltage will definitely be less than an unregulated supply. The fact the input current is specified with no load current is a sign they don't need a minimum load.Yes, they're less efficient and cost more, but engineering is always a compromise. It depends on which in the list of requirements is the most important. If it's board space, then regulated modules are the best solution. If it's cost, then you'll be better off adding a shunt regulator such as the TL431 or a zener diode, or LDO (low-drop-out) regulator to the output. An LDO regulator should give the lower stand-by power consumption, compared to a shunt regulator, but the voltage will drop a bit under full load. A shunt regulator will result in a higher stand-by power consumption, compared to a LDO regulator, but the output voltage won't drop as much under full load.Quote from: kreyszig on February 16, 2023, 01:49:16 pmYou might be better off regulating the secondary side. A Royer oscillator with a transformer could be used for isolation with a buck regulator on the output.EDIT:Here's an example of the circuit I was describing above. It's designed for 120VAC input and is way overkill, so will need to be scaled down for your application.Source: page 12, figure 14 of the TC33167 data sheet.https://www.ic72.com/pdf_file/t/85248.pdf

author: kreyszig
author_rank: Regular Contributor
timestamp: February 16, 2023, 08:22:21 pm
content: Quote from: Zero999 on February 16, 2023, 07:52:19 pmYes sorry I was wrong about this.Quote from: Zero999 on February 16, 2023, 07:52:19 pmI guess my most important criteria are in decreasing order efficiency at full load followed by regulation while keeping the cost in check (i.e. not $30 for an 1W reg supply). 5.0 +/- 0.5 V is good enough on either side of the isolation.Quote from: Zero999 on February 16, 2023, 07:52:19 pmOk I will look into this thanks! Why would it be better than the fly-buck design? Note that I need a 5V non-isolated output in addition to the 5V isolated output for my circuit as a whole.

author: Zero999
author_rank: Super Contributor
timestamp: February 16, 2023, 09:42:17 pm
content: What's the load on the non-isolated rail? Is it always higher than the isolated side? If so, you could use a buck regulator, with another winding on the inductor, forming transformer to get an isolated supply.Here's an example showing a -5V negative rail, created using a buck regulator with a transformer. It's easy to modify it to give an isolated 5V supply. Connect the cathode of D2 and positive of C3 to the isolated power supply's +V, rather than 0V. The negative of C3 is the isolated supply's 0V. Look at it another way, OUT- would otherwise be isolated if the positive side wasn't connected to 0V.12V +-5V 150mA 1173.asc

author: kreyszig
author_rank: Regular Contributor
timestamp: February 16, 2023, 11:10:42 pm
content: Quote from: Zero999 on February 16, 2023, 09:42:17 pmThe design would be for 300 mA non-isolated and 200 mA isolated. Isn't the circuit you describe basically the same as a Fly-Buck? Thanks!

author: Zero999
author_rank: Super Contributor
timestamp: February 16, 2023, 11:27:41 pm
content: Quote from: kreyszig on February 16, 2023, 11:10:42 pmThen it should work, with a buck controller of your choice.I haven't heard of the term fly-buck before.

author: kreyszig
author_rank: Regular Contributor
timestamp: February 17, 2023, 01:56:27 am
content: Quote from: Zero999 on February 16, 2023, 11:27:41 pmThanks!It is a design that has been pushed by TI:https://www.ti.com/lit/an/snva674c/snva674c.pdfThey had a bunch of transformers made for it:https://www.coilcraft.com/getmedia/34a4aeef-62e3-4a89-82f8-8a588cd0f865/Doc1393_FlybuckCoupledInductors_TI.pdfThey have also some controllers they identified for this design, but none of them are in stock right now. I have found some by other manufacturers that should work as well, like this one for examplehttps://www.monolithicpower.com/en/documentview/productdocument/index/version/2/document_type/Datasheet/lang/en/sku/MPQ4576GQBE-AEC1/document_id/9741/. Their main feature is that PWM can be forced at low load so it works well if the current turns negative. Their efficiency should be very similar to the buck controllers for high load (which is what matters for my application), but not as good for lower load. Regulation seems to be good enough for my application.

author: Zero999
author_rank: Super Contributor
timestamp: February 18, 2023, 12:21:20 pm
content: Yes, that's what I was talking about. I came up with the idea independently though.If you really want cheap, then a buck regulator can be made with a couple of transistors, but the inductor will need to be larger, so it probably doesn't save you much and the regulation won't be so good.https://romanblack.com/smps/smps.htm

author: kreyszig
author_rank: Regular Contributor
timestamp: February 20, 2023, 01:48:03 am
content: Quote from: Zero999 on February 18, 2023, 12:21:20 pmThis is a really cool circuit, I like it! This is nice to see how this can be done with a variable frequency and a few components.

author: kreyszig
author_rank: Regular Contributor
timestamp: February 20, 2023, 01:48:44 pm
content: Quote from: Zero999 on February 18, 2023, 12:21:20 pmI have a question about using a PWM vs constant on time converter and the implication on the ripple feedback circuit. I posted it in the threadhttps://www.eevblog.com/forum/projects/efficient-regulated-5v-and-isolated-reg-5v-supply-from-unreg-2w-12-7-v/new/#newwhere I discussed the converter circuit itself instead of the minimum load circuit. Thanks!