555 PWM circuit and using N mosfet to drive electromagnet coil(load).

Thread Starter

priyaloganathan

Joined Sep 12, 2016
10
Hello All,

I'm trying to convert direct dc to pulsed dc. Here goes the method I used so far for conversion.
First I used 555 timer in Astable mode to generate PWM pulses.(0-100% duty cycle).
These pulses are used to drive the MOSFET's gate. (SiHP6N40D), the output of the mosfet is used to drive electromagnet coil(load).

I got few problems with above circuit:
1) the mosfet output is inverted as compared to 555 output.
2) if I supply 10V to mosfet drain the max vtg read is 12V (CRO measurement). this happens for every increment of voltage eg 11V means 13V it reads)
3) after connecting load I see high voltage spikes at the rising edge of mosfet output, this also causes spikes in 555 timer output.
4) to solve problem(3) I placed a electrolytic capacitor across drain and source, the spikes are removed but there is change in the mosfet output waveform i.e the low state seems to be discharging of capacitor.
5) the mosfet is rated for 6A current handling capacity. the mosfet is too much heated with supply of 3A of current even though heat sink is provided.

Pls help me out . (sorry for my english)
Attaching the circuit for reference.
20160912_104536438_1024x495.jpg

Regards,
Priya
 

Thread Starter

priyaloganathan

Joined Sep 12, 2016
10
Also I found that only at low duty cycles its getting hot, while at higher duty cycles MOSFET is not heated a bit. What might be the possible reasons?
 

electrophile

Joined Aug 30, 2013
126
A p-channel mosfet driven in conjunction with a NPN transistor might be a better solution. That way your load will always be grounded and the mosfet will control power to it. This will also solve your inverted output problem. Possibly something like this. This was for a low voltage setup and you'll need to choose a higher Vbe transistor and a higher current sustaining mosfet.
 
Last edited:

AlbertHall

Joined Jun 4, 2014
9,642
4) to solve problem(3) I placed a electrolytic capacitor across drain and source, the spikes are removed but there is change in the mosfet output waveform i.e the low state seems to be discharging of capacitor.
This is not the way to do it and may contribute to the MOSFET overheating. Instead, connect a diode across the electromagnet to catch the back-emf pulse.
 

Thread Starter

priyaloganathan

Joined Sep 12, 2016
10
I connected a diode MUR860G across electromagnet to catch the back emf pulse. is it something to do with gate source capacitance that leads to heating of the MOSFET
 

AlbertHall

Joined Jun 4, 2014
9,642
is it something to do with gate source capacitance that leads to heating of the MOSFET
No. The mosfet should be either fully on or fully off and the transitions between the two should be as fast as possible. When the mosfet is off there is no current and no pwer is dissipated in the mosfet. When the mosfet is fully on the drain source voltage will be very low and so the power will be very low. While the mosfet is only partly switched on there will be both voltge and current and so more power to heat the mosfet.

Even if the fully on or fully off conditions are met there is still some power in the on condition (the voltage will not be zero) and how hot the mosfet gets depends on the RDSon (the on resistance) of the mosfet and heatsinking of the mosfet.

What current does the electromagnet use, which mosfet are you using, and is it attached to a heatsink?

P.S. At what frequency are you switching?
 

AlbertHall

Joined Jun 4, 2014
9,642
From the datasheet for that MOSFET, RDSon is 1Ω at Vgs=10V and Id=3A which is close to your usage I think. That is a dissipation of 9W in the MOSFET so it will need significant heatsinking. You might do well to search for a lower on resistance MOSFET to get the heat down.
 

kubeek

Joined Sep 20, 2005
5,718
Why are you using a 450V mosfet to switch 120V? Use a lower rated part to get better Rdson, like for example ipb107n20n3g
 

ScottWang

Joined Aug 23, 2012
6,882
A p-channel mosfet driven in conjunction with a NPN transistor might be a better solution. That way your load will always be grounded and the mosfet will control power to it. This will also solve your inverted output problem. Possibly something like this. This was for a low voltage setup and you'll need to choose a higher Vbe transistor and a higher current sustaining mosfet.
No current limits resistor to protect the Vb of Q1?
 

Thread Starter

priyaloganathan

Joined Sep 12, 2016
10
I tried npn transistor(2N2222A) in conjuction with N channel MOSFET to solve inverted problem. but it didnt worked. Pls suggest how can I resolve.
 

Thread Starter

priyaloganathan

Joined Sep 12, 2016
10
There is some problem in my 555 timer circuit.
If I place 10uF capacitor between 2 and 6, waveform seems to be distorted. but if 100nF is placed it seems to be exact square wave. what could be the reason?

Attaching the screenshot of waveforms. circuit simulated in proteus 8 demo s/w.100n.PNG 10u.PNG
 

Thread Starter

priyaloganathan

Joined Sep 12, 2016
10
Im magnetizing em coil with pulsating supply. I use an AC supply to demagnetize the same coil. How can I integrate both the supplies such that there should be switching between them which should not affect the pulsating supply?
 
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