SG3525 push-pull converter mosfet heating problem - Page 1
Forum section: electronics / projects

author: sbdada
author_rank: Contributor
timestamp: February 26, 2018, 10:01:47 am
content: Hello,I'm trying to make this push-pull converter which initiated to convert 12VDC to around 200VDC rated for 200W. Everything works fine for around 100W to 150W or less, but for more, the problem begins. The mosfets starts to heat up. With a 200W load (a 200W incandescent lamp) within 5-10 minutes, heatsink become too hot to touch. I've no temp meter to measure the temperature though.Here's my diagram and real circuit.I've seen that other people are using this kind of design with only 1+1 Mosfet and a small heatsink and running 200W-300W Load without any problem. But here I've failed.First, I've used 1+1 Mosfet, single secondary coil with a bridge rectifier as illustrated in this diagram, mosfet heated up as well as the transformer and diode. Then I've added another pair of mosfet but the result was same. After that, I've changed the secondary winding from single coil to a center tap coil and changed the bridge rectifier to 2 diode configuration full wave rectifier. I've also added another pair of diode in parallel with the previous pair to increase the current handling capacity. Now the transformer and diodes are ok (little bit warm but not hot) but the mosfet still heated up.Later, I've added a cooling fan to blow air towards the heatsink, and it's good now. Mosfets are little bit warm, but not that much hot. But without the cooling fan it heated up again. I've loaded it for about 3 hours, and it runs without any problem.Here's my circuit/components details:Timing resistor RT = 22K || 68K ~~ 16.6KOperating Frequency = around 42.5KhzR5 = 10K || 22K ~~ 6KInput Voltage = 12V (nominal) 100Ah Dip Cycle batteryOutput Voltage = 197V-197.5V without Load, 196V-197V with LoadLoad = 200W Incandescent Light BulbInput Power = arround 182WOutput Power = arround 137W(voltage and current are measured with Mastech MS2115B and UT-203 Multi-meter and then power is calculated. It's clear that the efficiency is very low. there is a waste of power in the circuit)Transformer Core: EI 40 (Salvaged from a Computer PSU)Primary winding: 24SWG X 4 wire in parallel, 4 Turns, center tapped, by-fillerSecondary winding: 24SWG X 1 wire, 73 Turns, Center tapped, by-fillerI did various modification in both circuit's parts value and transformer to reduce the heat, but none of those solved my problem. Now I'm here for a help. What is wrong in my design? How can I solve it?Please help

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 26, 2018, 11:11:50 am
content: Well, yeah, it's a transformer coupled charge pump.Add a filter choke, and it becomes an inductor switched PWM supply. Feedback will suddenly do its job!It'll still be a shitty voltage mode control, but that can also be fixed in a few steps.To implement an average current mode control, you need only:- Current shunt resistor inserted in the output filter inductor current return path- SG3525 internal error amp wired for current regulation- Additional external op-amp (e.g. TL431) to perform voltage regulation.Then you can delete the circuitry for the shutdown hack, and have a safe, reliable design, more or less!Impossible to say what impact primary side leakage inductance has. Can you produce a windup drawing for the transformer?Tim

author: sbdada
author_rank: Contributor
timestamp: February 26, 2018, 01:11:06 pm
content: Do I really need a current mode control? I didn't see any circuit before that use a current mode control with SG3525. And yes, I've a very little knowledge about that.Anyway, here's the drawing, let me know if it is enough or not.First I winded the primary, total 8 wires in parallel. it takes only one layer. then brought out 4+8+4 terminal wires as i draw here. then 4/5 layers of insulation tape. then secondary in the same way. this time only 2 wires in parallel. It takes 5/6 layers, then another 2/3 layers of tape

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 26, 2018, 03:23:22 pm
content: Quote from: sbdada on February 26, 2018, 01:11:06 pmIt's a very old chip, back in those days it was enough that a switching supply simply work at all, without taking so much space / weight / power / money as a linear supply.IMHO, any supply that isn't current mode, is either suicide, or hacked enough to be roughly equivalent to a current mode control anyway, in which case you can save a lot of trouble by doing it right from the start.QuoteHmm, so primary is bundled together with itself, that's good. This affects the peak voltage measured at the MOSFET drain, and the required value of the R+C dampers connected to them.Roughly speaking: your primary wire length seems to be about 60cm, I would guess its leakage inductance to be under 0.2uH. This is a reasonable figure.I think switching speed will be limited by gate resistance and driver output to about 300ns, which limits how low you can get switching losses, but also prevents voltage overshoot on the drains. As long as you can run switching frequency low enough, overall switching losses can be kept down.The problem in question is this:The transformer leakage inductance, combined with drain capacitance, defines a resonant frequency, and impedance.If the gate drive is fast, that resonance will be excited, leading to high peak voltages.The drain time constant is,t_d = pi*sqrt(LL * Coss) / 2Gate drive risetime should be more than double this, to avoid voltage overshoot. If lower, you will need dampers (R+Cs) or other snubbing methods (e.g., clamp diodes) to deal with overshoot.So that's alright.The next problem concerns the leakage inductance between primary and secondary.For this, the distance between primary and secondary matters. For ~10m of secondary in a single block, expect over 20uH (secondary referred) leakage.The effect of leakage here, is to cause voltage overshoot at the rectifier, once a filter inductor is installed. An R+C across the rectifier DC side may be necessary, or a clamp snubber, to limit this.Some related examples here:https://www.seventransistorlabs.com/Images/Tubescope_Supply2.pngThis isn't really a clear illustration of any concepts I've covered so far, because the rectifiers are mixed (FWB and FWCT types), the filters are choke-input but the one uses a two-winding choke which may be confusing; the current monitoring is primary side, peak mode, uncompensated, and strapped in parallel with the voltage error amp -- if nothing else, a possible example of such a "hack" I mentioned earlier.Structurally, this is the perfect example of a current-mode circuit, but it is probably still not a good example for present discussion:https://www.seventransistorlabs.com/Images/Flashlight2_Schematic.pngFirst, understand that the circuits along the top generate PWM, based on the control voltage PWMV. This is equivalent to the circuitry in the SG3525, between the COMP and output pins. IC3B is the error amplifier. IBATT is the current shunt signal, which is exactly in series with the inductor (the output section is here:https://www.seventransistorlabs.com/Images/Flashlight2Sch.png). INSP is the inductor current setpoint: a voltage proportional to the desired input current.At this point, the circuit is a transconductance amplifier, with a voltage input and a current output.Additionally, IC3A is the outer loop error amplifier. In this case, ILED is sensed, regulating output current; but just as well, the output voltage can be sensed instead, and this is what D1 + R24 do if the LEDs were to become open circuit.(Indeed, I have hacked one of these boards for use as an inverting Cuk DC-DC converter, with regulated output voltage, and controlled current limiting. It worked first time, and very nicely at that!)You have 90% of this circuit, in an equivalent form. Fundamentally, you only need to add an inductor and resistor in series with the output rectifier, and an op-amp between the voltage feedback divider and the SG3525 input.Tim

author: sbdada
author_rank: Contributor
timestamp: February 26, 2018, 04:25:08 pm
content: Ok, I've just done a quick test. I've added an inductor after the rectifier and before the capacitor and then tested. Didn't used any snubber on rectifier yet. unfortunately within 10 minutes i've shorted 2 diodes. it heated up too fast. However, I've measured the drain waveform with an scope. First image is without inductor, and the second one is with inductor.In earlier testing, when i was using single coil and bridge rectifier on secondary, the drain voltage looks a little bit better with the same primary snubber (10 Ohm + 1nF) here it is:Anyway, I know that SG3525 is an old tech, but in my country, most of the time it is still the only choice. 95% of the converter or inverter still use SG3525 or SG3524. A very few of them are made with TL494 or MCU. Forgot about the vintage MC34063? That one also still popular here. So, choice is limitedAccording to your suggestion, I'll do some test tomorrow and will let you know. Thank you Mr. Tim

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 27, 2018, 01:50:37 am
content: Well, you need to use a few more than 1 microhenry...This is a great reference for showing the rough overall behavior of an SMPS:http://schmidt-walter-schaltnetzteile.de/smps_e/hgw_smps_e.htmlGo to half bridge PP and enter the values shown:Press Calculate and see the graphs.Note it fills in a recommended value of 470uH for the filter choke, automatically. This is usually on the large side, but a good starting point. I've halved it to 220uH. Less would probably not be desirable, as the output ripple will be higher and the control will be less stable even at only half load. More is not desirable as the inductor gets much larger, and the control must respond slower, raising the output impedance.Incidentally, note you have little headroom to increase the output voltage, so do not expect good voltage regulation at lower input voltages. At a nominal 13.8V battery, it should be okay.Tim

author: sbdada
author_rank: Contributor
timestamp: March 04, 2018, 04:53:57 pm
content: Hello Mr. Tim, good news. The circuit is running much better now. I did some modification and suddenly everything changed. I replaced the output diode UF4007 to UF5408, decreased the operating frequency to 32Khz, decreased gate resistors to 18 ohm and increased the voltage to 216V. Center tap output, 2 Diode full wave configuration, no inductor, no snubber on diode. I don't know why, but everything is pretty stable now. Diodes are warm, but not hot. Mosfets are hot but not that much. I've run it for about 6 hours without any cooling fan, and it still alive. With a cooling everything is supper cool. I don't have that much knowledge to analyse why or how this is working but thanks god, this is workingAnd yes, Thank you very much Mr. Tim