555 timer mosfet driver for HF transformer (question about voltage) - Page 1
Forum section: electronics / projects

author: pulse
author_rank: Contributor
timestamp: October 29, 2015, 02:33:13 pm
content: Hi, I'm new on this forum, so hi again everybodyToday I've tried to make simple induction heater with 555 square wave generator that would drive IRF540N mosfet, through which the voltage vould be applied to coil of induction heater (which is 3 turns of 1.5mm2 wire and about 15mm in length). But unfortunately I had no success. I was using a frequency of about 70-80kHz, which somehow became 500kHz when connected to coil and the whole thing didn't want to work, so I decided to use high frequency (HF) transformer (like the ones that are used in computer PSUs) instead of coil and after some work on 555 circuit I've managed to make it work (at about 10kHz). I know that the frequency is too low, but it works. I'm using my LM383T power supply at only 1.2V (lowest possible) on the high power side of this circuit (MOSFET side) and I get about 10VAC RMS out of transformer which is nothing unusual, but the output waveform seems to be very nasty (it messes with my multimeter, showing different icons on its display and making them flicker), so there are probably some HV spikes? Because (here comes the interesting part) when I add bridge rectifier to the output of transformer, the rectified voltage becomes 10-15 times higher (about 100-150V) and I'm wondering why that happens. I didn't yet tried it, but if I'd connect a load to rectifier, I guess that the voltage would drop instantly...Any ideas about that? Oh, and please excuse me for my not so good englishBest regards, pulse

author: necessaryevil
author_rank: Regular Contributor
timestamp: October 29, 2015, 02:56:03 pm
content: Don't worry about your english, it's fine, but do you have a circuit diagram? Anyway I'm glad to see someone playing around with switch mode power supplies, it's an interesting topic!And yes, I guess you are right about your multimeter and voltage spikes. It's best if you hook up an oscilloscope if you have one.

author: mij59
author_rank: Frequent Contributor
timestamp: October 29, 2015, 03:15:29 pm
content: The MOSFET oscillates at 500 kHz due to parasitic inductance and capacitances, the output is probably modulated with the signal from the 555 timer.

author: pulse
author_rank: Contributor
timestamp: October 29, 2015, 03:32:15 pm
content: Quote from: necessaryevil on October 29, 2015, 02:56:03 pmWow, thanks for your fast and very kind responseYes I have oscilloscope, but it's currently not connected, since I have too small room to fit everything in (it's old and very big scope). I might need to connect it, however.Yes I have a circuit diagram, I'll upload a picture of it. Thanks!

author: pulse
author_rank: Contributor
timestamp: October 29, 2015, 03:35:03 pm
content: Quote from: mij59 on October 29, 2015, 03:15:29 pmOh, thanks! I didn't know thatBut I'm still not sure why my induction heater doesn't work :-/

author: mij59
author_rank: Frequent Contributor
timestamp: October 29, 2015, 04:14:13 pm
content: Quote from: pulse on October 29, 2015, 03:35:03 pmYour circuit will not work, you need some kind of push pull configuration to drive the transformer.When the MOSFET is switched off the transformer generates a very high voltage on the drain, the inductance of the transformer and parasitic capacitances generate a dampened oscillation.

author: necessaryevil
author_rank: Regular Contributor
timestamp: October 30, 2015, 09:10:48 am
content: Yep. A HF transformer is not just a transformer for higher frequency, but also has a high inductance. The benefit of this is that the output can be regulated by changing the duty of the square wave. But the output voltage isn't simply a function of the input voltage and the winding ratio.What mij59 means (I think) is that the voltage across the primary widing also increases. The mosfet will only turn on if the gate voltage is ca 5-10 volts higher than thedrainsource voltage. Since the drain is connected to the primary of the transformer, the increasing across the primary will cause the mosfet to turn off.I do not have enough knowledge to help you further with the development of your circuit. But maybe you can look for an existing circuit based on an TL494. The TL494 is a cheap and very common IC which is dedicated for switching power supplies, it is not a coincidence that the TL494 appears in Daves last video (EEVblog #811), a lot of the cheaper computer power supplies use it too. You can search for TL494 on google or look for app notes.

author: pulse
author_rank: Contributor
timestamp: October 30, 2015, 09:49:21 am
content: Quote from: mij59 on October 29, 2015, 04:14:13 pmOk, but what about adding a diode across the coil in opposite direction, so that it would take care of HV spikes?Thanks!

author: pulse
author_rank: Contributor
timestamp: October 30, 2015, 09:58:29 am
content: Quote from: necessaryevil on October 30, 2015, 09:10:48 amThanks for mentioning the TL494But I don't understand why the MOSFET wouldn't turn on when the voltage on gate isn't 5-10V higher than the one on the drain... IRF540N has the voltage rating of 100V, but that would mean I'd need to supply at least 105-110V to the gate of IRF540N? Of course that would destroy it and that's exactly why I don't understand that.Thanks

author: necessaryevil
author_rank: Regular Contributor
timestamp: October 30, 2015, 10:06:41 am
content: It is not likely that the circuit is going to do what you want by adding diodes. The output voltage will depend on the load and the duty (the on/off ratio of the mosfet). Place the mosfet on the low side of the transformer, put a resistive load on the secundaries and modify the circuity around the 555 so you can adjust the duty. This might give you a chance, but the best is just to look up some schematics.

author: mij59
author_rank: Frequent Contributor
timestamp: October 30, 2015, 11:59:06 am
content: Ok, but what about adding a diode across the coil in opposite direction, so that it would take care of HV spikes?Thanks!Yes this will prevent the high voltage spikes, and also dumps all the energy in the diode and transformer.

author: mij59
author_rank: Frequent Contributor
timestamp: October 30, 2015, 12:27:58 pm
content: Quote from: necessaryevil on October 30, 2015, 09:10:48 amIn this application the mosfet is used as a switch.The resistance between drain and source is a function of the voltage between gate and source (Vgs).In this case if Vgs is zero the mosfet is switched off, the resistance between drain and source is very high.If Vgs is 12V the drain source resistance is very low, for the IFR540 this is about 44 mOhm.The absolute maximum rating Vgs for the IRF540 is 20V, go higher and the mosfet will be toast.

author: alsetalokin4017
author_rank: Super Contributor
timestamp: October 30, 2015, 12:54:18 pm
content: Quote from: necessaryevil on October 30, 2015, 09:10:48 amNo, the mosfet GATE needs to be driven 5-10 volts higher than the SOURCE voltage for the mosfet to turn on. And the mosfet is already in the Low-side Switch configuration.The 555 timer may not be able to provide enough current from the Pin 3 output to fill the mosfet's gate capacitance quickly enough for proper switching. A driver circuit (2-transistor totempole perhaps) could be placed in between the 555 and the mosfet Gate.If the 555 is turning the mosfet fully ON at the frequency in use and no driver stage is used, it may be beneficial to put a pulldown resistor (100k or higher) between Gate and SOURCE pins of the mosfet to assure that the mosfet can turn off cleanly when the 555 Pin 3 signal is low. And the 1k Gate resistor is too much, try 100R or even 10R here.A fast, high current reverse biased diode across the load coil (that is, from Drain to Source of the mosfet) will help to protect the mosfet from the inductive spike when the coil is turned OFF.In general though, I think the circuit isn't really appropriate for inductive heating. A self-oscillating ZVS circuit will probably work a lot better.

author: alsetalokin4017
author_rank: Super Contributor
timestamp: October 30, 2015, 12:56:29 pm
content: Quote from: pulse on October 30, 2015, 09:58:29 amThe reason why you don't understand that is because it's wrong.

author: alsetalokin4017
author_rank: Super Contributor
timestamp: October 30, 2015, 02:55:08 pm
content: @pulse:OK, I've breadboarded your circuit. Using 12 V from SLA battery for the 555 timer/GATE supply and 2.5 V from my bench PSU for the load supply through the mosfet. I couldn't find an IRF540 mosfet so I'm using an IRF530n instead. For a load I'm using a random transformer from an old CRT monitor (or TV) chassis. I also went to 100R for the gate resistor, it works better. I've put an NE-2 neon on the secondary of the transformer as a final load. (It also works at 1.2 V but with a dimmer neon. Waveforms about the same.)Yes, there are spikes which are probably confusing your DMM. No, this circuit probably isn't going to work as an induction heater with a 3-turn work coil and a higher voltage load supply. Yes, it will probably work fine as a mosfet-killer.

author: necessaryevil
author_rank: Regular Contributor
timestamp: November 01, 2015, 10:00:40 am
content: QuoteOooops. I meant source of course. Fixed it.