REGULATOR RECTIFIER using buck converter method - Page 1
Forum section: electronics / beginners

author: Aggymrizal
author_rank: Newbie
timestamp: March 05, 2025, 05:43:39 pm
content: hello everyone, please help me as a beginner. (Maybe I will copy the same question on several forum sites to add references)I am making a regulator rectifier (RR) for a motorbike that uses the buck converter method. which is generally used for motorbikes is the shunt & series/open method. I have experience making shunt & series type (which uses SCR), where the shunt type can run normally at all RPMs but the drawback is extreme heat on the heatsink. but for the series/open type it is cooler but it errors/overcharges at RPM 10K++. therefore I decided to make a series type using a MOSFET & buck converter system.The basic scheme is as in the picture. The power source comes from a permanent magnet generator (3 phase big bike / 1 phase small bike) then goes to the diode bridge. Driver uses TL494, frequency 10kHz, used 240uH &/ 1mH inductor, mosfet used IRFB4115 &/ IRFP4242. But still the problem is that the mosfet gets hot & eventually breaks down, even though in the first few minutes everything works fine when the motor RPM is low.My personal prediction is that the error lies in the voltage spike that occurs when the MOSFET is off, because the power source comes from a generator which is inductive in nature. I once measured using an avometer, the voltage from a 3 phase generator at 10K++ RPM can reach around 150Vac (voltage can vary according to RPM), & some information says the generator's max power is around 200-350W. Output power target around 14Vdc, 5-20Amps. The question is, why mosfet get hots & damaged?Please give me the best input so I can improve this project. thank you very much friend🙏

author: MrAl
author_rank: Super Contributor
timestamp: March 07, 2025, 01:24:58 am
content: Quote from: Aggymrizal on March 05, 2025, 05:43:39 pmHello,Are you talking about Q1 in your schematic?Mosfets are a little tricky when used in switchmode. The drive has to be right and the spikes have to be snubbed.To take care of the spikes across the drain and source, it is usual to use a snubber circuit.To take care of the drive requirements, it is usual to use an actual Mosfet driver chip. Two transistors like that can work but will not be as good as a Mosfet driver IC chip because the dedicated driver chip can supply the needed current to switch the Mosfet fast enough. The rise and fall times contribute to the total power dissipation and that can be a lot with a not-so-good driver circuit. Typical driver chips can put out 1 amp or more, which can drive the Mosfet much better.One of the other problems in your schematic looks like that asymmetrical drive diode, the 1N4007. You should not use a regular rectifier diode in the drive circuit because it's too slow. It would be more usual to use a Schottky diode which can switch much faster. Remember rectifier diodes are used in 50Hz to 400Hz circuits normally, but not in higher frequency switch mode power supplies except of course as line frequency rectifier diodes, but not in the transistor switching circuits.So to start, here is a list of things to check out...1. Change the rectifier diode to a Schottky diode of the proper rating, if you really need asymmetrical drive.2. Change the drive circuit transistors to a dedicated Mosfet driver chip.3. Add a snubber circuit across drain and source, and keep the wiring as short as possible. The snubber is for when the Mosfet turns off.4. If needed you can also add an external higher current Schottky across drain and source. That takes over the reverse function of the internal diode and so moves some of the heat out of the Mosfet itself. You might try this last.5. Look for inductor saturation, which can be a huge problem. If the inductor is not rated for the full current this would cause the Mosfet to burn out. You usually have to use a scope to see this so I hope you have one. If you don't have a scope that is a problem in itself because scopes are almost mandatory when working on switchmode power supplies in R&D.6. Make sure the drive voltage for the Mosfet matches the specs for the device. If the Mosfet does not get enough voltage to turn on, it will have higher resistance which will dissipate more power and reduce efficiency by a large factor.7. I see you are using an opto coupler in the drive circuit. These can be a problem in the drive circuit. They have to be fast for one thing, but it helps if you use a dedicated Mosfet driver chip after that which will clean up the rise and fall times. I can't quite make out the part number of the opto coupler or I would check it for you, so check out the specs carefully.

author: Aggymrizal
author_rank: Newbie
timestamp: March 07, 2025, 08:43:26 am
content: This is the answer I've been waiting for, thank you very much for your input. My mini oscilloscope just broke yesterday, I'm buying a new one.please suggest the frequency?what value do you think the inductor should be?& what rating (volt & current) of the mosfet should be used?or are the specs I use enough?& finally, do I need to add a capacitor after the 3-phase bridge diode output?

author: MrAl
author_rank: Super Contributor
timestamp: March 11, 2025, 04:41:21 am
content: Quote from: Aggymrizal on March 07, 2025, 08:43:26 amHello again,What do you mean by "suggest the frequency", don't you already know what you need there?If not, the frequency is related to the inductor value. If the MOSFET switching frequency is 20kHz for example, then 50uH minimum, and 100uH better. 1mH would be kind of large, and the thing is, the larger the value the lower the current it can handle in most cases, although you can find one that does higher current but it will be more expensive.It looks like your switch frequence is 10kHz so I would think 240uH would be good. It MUST be able to handle the full output current without saturating. This has to be checked at the peaks of the current.You do not have capacitors after the bridge rectifier? I'm not sure why you would want to try that. Usually there is enough capacitance to keep the DC voltage high enough all the time. That is, somewhat low ripple.Is it true that your input 3 phase can reach as high as 150vac RMS? That's pretty high for a 14vdc output power supply using a buck circuit.The MOSFET has to be able to handle the full voltage and then some, as well is the input current peaks. If you had a 200vdc buss, then you need at least 250v MOSFETs. Luckily the input current should not be that high for the input/output DC voltage ratio you seem to be working with here. If you had 200v input and 14v at 20 amps output, then the input current average would be less than 2 amps, though the peak could be higher. I would think a 5 amp rating would be good but this is something you have to test also. As you increase the output load, you check to make sure the MOSFET is not getting a current that is too high, and that ties in with the inductor saturation also. The inductor cannot be allowed to saturate even for an instant or else in the input current can go really high.You have to think about the input and output power too. If you have 200 watts in you cannot get 14v and 20 amps out because that would be 280 watts. If you had 200 watts input and 50 percent efficiency, you would only be able to get 100 watts out. With 75 percent efficiency you could get maybe 150 watts out, which is roughly 11 amps at 14 volts. You have to think about that.When you use the scope you have to make sure the probe does not allow the input of the scope to be overdriven or else the scope can be damaged. You have to check the max input of the scope and how much it gets with say a 10x probe. If your scope only allows say 5v input max and you use a 10x probe, then the max you can measure is still rather low like 50 volts max. You have to check that before you do any measurements.