Designing SMPS With TL494 - Page 1
Forum section: electronics / projects

author: psydaddy
author_rank: Contributor
timestamp: December 19, 2024, 12:24:35 am
content: Hi,I'm designing a SMPS with TL494. After reading the guide (attached) i came across this (page 28):"Initially, capacitor C2 forces the dead-time control input to follow the 5-V regulator, whichdisables the outputs (100% dead time). As the capacitor charges through R6, the output pulsewidth slowly increases until the control loop takes command. With a resistor ratio of1:10 for R6and R7, the voltage at pin 4 after start-up is 0.1 × 5 V, or 0.5 V."Where is R7 on the schematic or what they meant by this?All i need to do is dead-time + soft start.On page 23, figure 30, it shows a Soft-Start Circuit with a diode. I'm not sure if i should add a diode. I believe that adding a capacitor from REF to DTC is enough but the 1:10 ratio doesn't make sense to me. I'm not sure what it means.Thanks

author: Benta
author_rank: Super Contributor
timestamp: December 19, 2024, 01:02:41 am
content: My very first SMPS design was with the TL494 in the late 80s.Lots of traps, I urge you to find something more modern. It was designed when SMPS was totally exotic. Don't even think about going over 40 kHz with it.BTW: when you list page numbers, it's a good idea to include the document in question.

author: Andy Chee
author_rank: Super Contributor
timestamp: December 19, 2024, 03:56:08 am
content: Quote from: psydaddy on December 19, 2024, 12:24:35 amWhat input voltage? What output voltage/current?

author: psydaddy
author_rank: Contributor
timestamp: December 19, 2024, 09:27:16 am
content: Quote from: Benta on December 19, 2024, 01:02:41 amThanks.I'm fine working with other ICs. I chose this one because I thought there was a lot of online support, but I'm getting stuck, so I believe the support isn't as extensive as I expected.My plan is to design a half-bridge topology with a 24V and 1000W output. Do you have any recommendations for different ICs that are more modern?I found a few different ICs, but the datasheet isn't very well structured. Also the configuration of the oscillator also defines the dead time. I have to consider the R and the C to create the correct sawtooth waves that also specifies the dead time and frequency. The calculations are very confusing; I would prefer to have the frequency and dead time configured separately by hardware.Quote from: Andy Chee on December 19, 2024, 03:56:08 amThanksInput: 325V (50hz)Output 24V 1000W

author: peter-h
author_rank: Super Contributor
timestamp: December 19, 2024, 03:54:28 pm
content: This is an offline (mains input) switcher. The TL494 is rubbish for that application. You need a modern offline switching controller chip.Offline switchers need very fast features e.g. pulse by pulse current limiting.I used the TL494 in early 1980s for a primitive stepdown switcher (12V DV to 5V DC) and for that it was fine.

author: Benta
author_rank: Super Contributor
timestamp: December 19, 2024, 07:24:25 pm
content: For 1 kW, half-bridge can run into problems. I'd go for a full-bridge design at that power level.

author: psydaddy
author_rank: Contributor
timestamp: December 19, 2024, 11:43:18 pm
content: Quote from: peter-h on December 19, 2024, 03:54:28 pmWhich ones do you recommend?Quote from: Benta on December 19, 2024, 07:24:25 pmWhich problems can i get myself into?

author: Benta
author_rank: Super Contributor
timestamp: December 20, 2024, 12:10:48 am
content: Quote from: psydaddy on December 19, 2024, 11:43:18 pmThe main issue is the balancing/splitting caps, which tend to get very large and have to carry a lot of high-frequency ripple current. Mind you, it's 25 years since I last did a similar design, and caps are probably better today. Keep it in mind, though.

author: xavier60
author_rank: Super Contributor
timestamp: December 20, 2024, 12:53:07 am
content: Also an old part, the SG3525A still works well enough up to 50KHz after which its minimum dead time starts wasting too much duty cycle.Unlike the TL494, the SG3525A has double pulse suppression that prevents error amplifier noise from causing unwanted toggling of the steering flip-flop.It also has pulse-by-pulse shutdown, which I implement via its soft-start pin.

author: psydaddy
author_rank: Contributor
timestamp: December 20, 2024, 08:04:20 pm
content: Thank you. Any recommendations for the Frequency i should use? I believe i don't need to go all the way up to 300Khz.Is that a thumb rule to choose the frequency?

author: xavier60
author_rank: Super Contributor
timestamp: December 20, 2024, 08:37:21 pm
content: I usually end up close to 50KHz with ETD49 core which is likely a bit oversized for 600W projects, but it makes the winding volume utilization less critical. I don't fully understand what I'm doing with the magnetics. Main thing is to avoid primary saturation.

author: psydaddy
author_rank: Contributor
timestamp: December 20, 2024, 09:03:55 pm
content: Thanks, mate! I was considering using 300 kHz, but I'm concerned that my MOSFETs might not switch off in time, potentially causing a short circuit.Do you typically use MOSFETs or IGBTs? In your knowledge, is there any particular reason I should consider opting for IGBTs?

author: xavier60
author_rank: Super Contributor
timestamp: December 20, 2024, 09:09:58 pm
content: I have only used MOSFETs for now.

author: psydaddy
author_rank: Contributor
timestamp: December 20, 2024, 09:24:18 pm
content: By the way, do you know what I need to consider when calculating the capacitor that is in series with the primary winding?Is the purpose of that capacitor solely to create maximum impedance based on the primary current, Preventing the inductor from fully saturating in the event of a failure?

author: xavier60
author_rank: Super Contributor
timestamp: December 20, 2024, 09:51:22 pm
content: No, just for coupling. In my last project, I think I used two 2.2uF in series across the HVDC and connected the cold end of the primary to the center junction. I'll try to confirm the values.

author: Benta
author_rank: Super Contributor
timestamp: December 20, 2024, 09:53:59 pm
content: Quote from: psydaddy on December 20, 2024, 09:24:18 pmThis is for both half- and full-bridge implementations.The series cap is there to prevent flux-walk in the transformer. This happens when the switching times/voltages in the power stage are not 100% equal (impossible to achieve). It will apply a DC bias to the primary to equalise that, which ensures that transformer flux is centered around zero at all times.This may result in small differences in the secondary peak voltages. Not a big issue.In a half-bridge, you can skip it, as the splitting caps will do it for you.

author: psydaddy
author_rank: Contributor
timestamp: December 20, 2024, 10:15:45 pm
content: Quote from: xavier60 on December 20, 2024, 09:51:22 pmOk, let me know the calculations or the trial and error process you made.Quote from: Benta on December 20, 2024, 09:53:59 pmThis has anything to do with ZVS or this is a different topic?Do you know the calculations i have to make in order to achieve that capacitor value?

author: Benta
author_rank: Super Contributor
timestamp: December 20, 2024, 10:50:20 pm
content: Quote from: psydaddy on December 20, 2024, 10:15:45 pmZVS is a different topic.The cap is simply there to prevent net DC current through the transformer primary, which would result in flux-walk in the core. It's in no way resonant or anything like that. It's just a DC blocker.Like I said, in a half-bridge it's mostly unnecessary, as the (large) splitting caps will do the same function.

author: psydaddy
author_rank: Contributor
timestamp: December 20, 2024, 11:10:33 pm
content: Got it. After reading about the topic i got the main idea. Basically the flux will pile up each cycle until it reaches the maximum saturation of the core, leading to problems.Can you recommend anything that i can look into about this topic (paper, book, video, anything)? I might end up building a full bridge, I'm considering more as a challenge.

author: xavier60
author_rank: Super Contributor
timestamp: December 20, 2024, 11:27:51 pm
content: Quote from: xavier60 on December 20, 2024, 09:51:22 pmThe coupling capacitors are actually 1uF each. Ripple at full load is 20Vpp, measured between the junction of the two capacitors and HVDC negative.Frequency is 47KHz.

author: psydaddy
author_rank: Contributor
timestamp: December 20, 2024, 11:47:05 pm
content: Plastic Film Capacitor? Same used as X capacitor?

author: Benta
author_rank: Super Contributor
timestamp: December 21, 2024, 12:01:15 am
content: Quote from: psydaddy on December 20, 2024, 11:10:33 pmYes, you got it.The best reference is actually ancient and unobtainium today, but I found a (bad) PDF. It's from Motorola back then:https://www.epanorama.net/sff/Power%20Electronics/Power_Supplies-Switching/Motorola%20Switchmode%20Power%20Supply%20Design%20Guide.pdfIgnore the fact the BJTs and not MOSFETs are used and focus on the sections about topologies (and magnetics).

author: psydaddy
author_rank: Contributor
timestamp: December 21, 2024, 12:12:29 am
content: Quote from: Benta on December 21, 2024, 12:01:15 amThanks

author: xavier60
author_rank: Super Contributor
timestamp: December 21, 2024, 01:09:29 am
content: Quote from: psydaddy on December 20, 2024, 11:47:05 pmI have lost track of what they are and came from. Searching, brings up CBB22 a lot.

author: Benta
author_rank: Super Contributor
timestamp: December 21, 2024, 02:01:47 am
content: A general comment:Asking about (or even choosing) a controller is the wrong starting point.The right question would be: "I need to generate 24 VDC at 40 A from 325 VDC. How do I do that best?"From there on you work backwards:Topology selectionPower stage and magneticsDrive and safety circuits.Controller IC.All of those interact, of course, but starting with selecting the controller IC is not the way to go.At least that's my experience.

author: psydaddy
author_rank: Contributor
timestamp: December 21, 2024, 10:40:40 am
content: I mentioned the TL494 controller because it can be configured for both half-bridge and full-bridge topologies.Here's my current thinking (please correct me if I'm mistaken):- I'm not able to go for a flyback: I don't think a flyback topology is suitable due to the overall power requirements.- Half-Bridge vs. Full-Bridge: While a half-bridge design is fine, a full-bridge seems easier to implement and might result in lower losses (though I’m not entirely sure—thoughts?).- Core Selection: I’ve already purchased the E55 core, operating under the assumption that "bigger is better." However, I’m wondering if I made a mistake. Do core losses affect efficiency? Or do they just increase costs unnecessarily?- Operating Frequency: Initially, I was aiming for the maximum frequency, but I think I have enough space on my bobbin to reduce the frequency and increase the number of windings. Would 50 kHz be a good target?- Powering the IC: For now, I’m considering a simpler approach, such as a capacitor dropper or a dedicated flyback circuit to power the IC. While not ideal, it feels like a good starting point for my learning. In the future, I’d like to add auto-oscillation to fully isolate the PWM controller from the primary side.- Gate Drive and MOSFETs: I’m planning to use a gate drive transformer with MOSFETs. Although I have some gate driver ICs, I want to keep the PWM controller isolated for future upgrades.- Output Rectification: I’m aiming for a single-rail configuration with a center tap.I already have the oscillating circuit working, but I’m planning to experiment with the SG3525A controller IC.

author: xavier60
author_rank: Super Contributor
timestamp: December 21, 2024, 12:46:40 pm
content: There should be no difference in configuration between half-bridge and full-bridge, only difference is that the driver transformer has 4 outputs instead of 2.I used a pair of complementary MOSFET's configured like CMOS logic inverters to drive my driver transformer.https://www.aosmd.com/res/datasheets/AO4606.pdf.At 12V, it wants to produce large shoot-through current spikes, so I put a low ohms resistor with ferrite bead in series with its 12V supply.

author: Benta
author_rank: Super Contributor
timestamp: December 21, 2024, 07:36:47 pm
content: A few comments.Quote from: psydaddy on December 21, 2024, 10:40:40 am1: the problem with flyback is, that the transformer core has to store the full output energy in its magnetic field. That makes the transformer very large and bulky. For half/full-bridge transformers, the core only has to support the magnetising field.2: core selection needs a lot of curve perusal and calculations. It's a very iterative process where you need to go back-and-forth many times. I can't say if your choice is good.3: 50 kHz seems like a good starting point for this size of converter. Get some experience, then you may be able to go higher.Concerning gate drive: I'd also explore the use of an isolated/floating shadow supply for the upper drivers and use gate-drive optocouplers.On the power stage, don't forget dead-time control between upper/lower switches.Cheers.

author: schmitt trigger
author_rank: Super Contributor
timestamp: December 21, 2024, 08:33:47 pm
content: Quote from: psydaddy on December 21, 2024, 10:40:40 amOf course core losses affect efficiency.Cores losses are rated in watts per unit of volume, in the MKS system that would be kW/m^3. And you scale accordingly to the actual volume of the core size and configuration you have selected.The core loss is a very nonlinear function of frequency, temperature and flux density, and varies significantly among the different core materials.I attach a scan of an old TDK ferrite core databook, as an example.As Benta mentioned, the correct core selection and optimization is a very iterative process, which requires you to make a first educated guess and to select a core from one of the several vendors. Then go ahead and make all of the required calculations based on the selected core characteristics and desired operating conditions.But if your converter is a one-off and not a commercial one, you are usually OK selecting a larger than necessary core. But at the very least you do require to calculate the maximum flux density, otherwise you'll be mysteriously destroying your output transistors.

author: Andy Chee
author_rank: Super Contributor
timestamp: December 22, 2024, 02:08:49 am
content: Speaking of high powered half bridge converters controlled by a TL494, this scary concoction was etched in my memory when I too was searching for inspiration.https://w5jgv.com/hv-ps1/Of course, the above design is 1500 volt output, compared to your 24 volts. But the transformer core sizes for 1kW power should be comparable.

author: psydaddy
author_rank: Contributor
timestamp: December 24, 2024, 09:06:54 pm
content: Thanks matesQuote from: schmitt trigger on December 21, 2024, 08:33:47 pmWhen the current is not total to the load, does it offers low efficiency?

author: Benta
author_rank: Super Contributor
timestamp: December 24, 2024, 09:51:28 pm
content: Quote from: psydaddy on December 24, 2024, 09:06:54 pmFor the transformer, core losses are pretty much constant and relate to the magnetising current. Main copper losses are proportional to output power.Concerning efficiency, that's an open question. Magnetising current is mainly reactive and doesn't cause a lot of heat except in the switches. Your input stage plays a bigger role. PFC stage? Or just rectifier/caps?

author: xavier60
author_rank: Super Contributor
timestamp: December 25, 2024, 12:56:49 pm
content: Quote from: psydaddy on December 24, 2024, 09:06:54 pmWhat material type are your cores?

author: psydaddy
author_rank: Contributor
timestamp: December 26, 2024, 06:21:59 pm
content: Quote from: Benta on December 24, 2024, 09:51:28 pmAt this point, no PFC. So i can only relay on the capacitors.Quote from: xavier60 on December 25, 2024, 12:56:49 pmPC40, at least it was said on the Aliexpress Listing

author: Hiemal
author_rank: Regular Contributor
timestamp: December 28, 2024, 12:06:07 am
content: Look at the LM5045 / LM5046 series of IC's. They're both made to drive full bridges, though one is a standard, PWM driven one, and the other is a phase shifted full bridge.For 1kW of output power, and a relatively high input voltage, it might be wise to try and use a phase shifted bridge; it relies on leakage inductance present in the transformer to commutate the switches during deadtime, giving you ZVS (or close to, ZVS).If you haven't ever designed a power supply before to begin with, it might also be wise to start with a lower power design to get yourself familiar with the basics of them.You could also potentially look into an active clamp forward converter, or a dual switch forward.Dual switch forwards are VERY commonly used in ATX power supplies and the like, since they're simpler and automatically take care of any leakage inductance by recycling that energy back into the supply rail. You only need one gate drive signal, so your controller can be something simple like the UC384X series of current mode controllers. They don't give ZVS however.Active clamp forwards are less common at higher voltages (I don't know why though). They require a D and 1-D gate drive, but give you ZVS switching. The auxiliary switch also only needs to carry the magnetizing current, so it can be a lot smaller than the main switch. Takes care of leakage inductance by using that energy to commutate the switches during dead time. Aka, ZVS.A flyback converter is usually limited to around 100-200W of output power; you can of course go higher than that, but not only does the transformer itself start to become prohibitively large, but your peak currents will also become more and more unwieldy the higher the output power you request.Either way, this isn't going to be a simple project; 1 kW of output power does require a bit of care to the layout of your design, and, please, for the love of god, use a controller that usesCURRENT MODEcontrol. They have inherent current limiting, and will help prevent the deaths of so many MOSFETs, on top of greatly simplifying the feedback loop (it removes the resonant double pole present between the output inductor and output capacitor). Voltage mode control pretty much HAS to use a Type III compensator, whereas most current mode designs only have to use Type II compensators. Older controllers, like the SG3525, TL494, etc, all use voltage mode control.For powering the IC, there's specific circuits you can use that effectively "bootstrap" the IC when you plug in power, basically giving it just enough juice to switch for a few cycles, and then using an auxiliary winding, giving itself power to self sustain. You can also do it using what you mentioned, with a specific auxiliary flyback controller, but that does add what is effectively an additional power stage. It does give you the benefit though that you can effectively current limit into a short without the controller browning out.There's a LOT of different topologies and they all have their own upsides and downsides, and it's up to the designer (aka, you) to figure out what the pros / cons are of each, in terms of complexity, cost, performance, and reliability. It ain't easy.

author: psydaddy
author_rank: Contributor
timestamp: December 30, 2024, 09:19:23 pm
content: Quote from: Hiemal on December 28, 2024, 12:06:07 amThank you for your help.I might end up designing a flyback if I get to lost into it.Btw, do you know how to calculate the inductor on the secondary side? What i should take into consideration to calculate it?

author: Hiemal
author_rank: Regular Contributor
timestamp: January 01, 2025, 03:43:51 am
content: Quote from: psydaddy on December 30, 2024, 09:19:23 pmTake a look at Ti's Power Stage designer. It'll help you figure out values for various power supply topologies.Basically you're trying to balance how much current ripple across your inductor you're okay with. Changes how much of the "onus" is on the output capacitor for current handling capability, and how "peaky" the current ramp is on your primary side.

author: Andy Chee
author_rank: Super Contributor
timestamp: January 02, 2025, 12:57:22 am
content: Another simple topology calculator for ballpark component values is thishttp://schmidt-walter-schaltnetzteile.de/smps_e/smps_e.html

author: psydaddy
author_rank: Contributor
timestamp: January 06, 2025, 10:07:17 pm
content: Quote from: Hiemal on January 01, 2025, 03:43:51 amThanks, i look it up but they only have flyback tupologies.Quote from: Andy Chee on January 02, 2025, 12:57:22 amThanks, great website. I'll take a deep look

author: Hiemal
author_rank: Regular Contributor
timestamp: January 08, 2025, 03:18:21 am
content: You didn't look in the right spot evidently.https://www.ti.com/tool/POWERSTAGE-DESIGNER