# A circuit question using multiple MOSFETs with PWM control

I need to PWM control two electromagnet groups where each group has four electromagnets. Emagnets have nominal 24V 0.33A current ratings. Here is the only datasheet I could find. To supply all the magnets I want to use this 24V 4A single power supply. To model these, I drew the schematics in my mind and simulated it as follows: (left-click to view/enlarge)

To model emagnets each drawing 0.33A current, I used 72Ohm and 1u inductor. I also added 1N4148 as a flywheel diode to each magnet.

The idea is two PWM outputs of a micro-controller like Arduino's will control(by two potentiometers as analog inputs of the uC) each two groups's PWM input hence the magnet current. Since the uC does not have 8 separate PWM outputs, each PWM is triggering four gates of the MOSFETs as shown in the diagram above.

In simulation I obtain the following power plot for this MOSFET I plan to use: I have the following questions as concerns before I start soldering:

1-) Is it fine as in my diagram to connect all the gates at a point and trigger four MOSFETs from a single input?

2-) Are the gate resistors values for R_g1, Rx_1, R_g2, Rx_2 correct/sufficient?

3-) It seems I wouldn't need heat sink by looking at average power dissipation. But I wonder if I should change the PWM frequancy. Currently it is 490Hz. Would it make sense to increase it to 30KHz for instance to prevent ringing sound or another issue?

edit:

Using a non-inverting driver TL427: edit2:

I reduced MOSFETs(one for each group) and diodes(I used 1N4007) Here is the power plot for one transistor in between 1.15ms and 1.2ms: • Nooooo! Don't parallel MOSFETs without individual gate resistors. It will be batch dependent and you may have failures in the field sporadically. Last time I missed this and the product passed all type tests, the failure rate at the customer was in the high tens of precent. Oct 18 '17 at 19:39
• Please see my edit2 I now use one MOSFET and one diode for each group. TL427 is powered by 9V supply. I would be glad if you have an input.
– GNZ
Oct 18 '17 at 20:01
• Much better! Is 1N4007 up for the job? Oct 18 '17 at 20:06
• Do you mean 1N4007 can be slow? I guess I should stick with 1N4148 since I might increase pwm freq. (?)
– GNZ
Oct 18 '17 at 20:14
• Yes. Might be fast enough for your application, but I still ask leading questions. Voltage rating, peak current rating and trr needs to be matched to your particular situation. Also, diodes are dirt cheap, even the fast ones. Oct 18 '17 at 20:16

1-) Is it fine as in my diagram to connect all the gates at a point and trigger four MOSFETs from a single input?

Technically yes you can wire MOSFETS in parallel like that and they will work. However, you need to remember that the gate of a mosfet is effectively a capacitor. As such, four in parallel is four times the capacitance. That means the switching time of the four gates will be roughly four times slower than using a single device.

2-) Are the gate resistors values for R_g1, Rx_1, R_g2, Rx_2 correct/sufficient?

They look fine, and typical, to me.

3-) It seems I wouldn't need heat sink by looking at average power dissipation. But I wonder if I should change the PWM frequancy. Currently it is 490Hz. Would it make sense to increase it to 30KHz for instance to prevent ringing sound or another issue?

Higher PWM, assuming you can switch the MOSFETS that fast, will cause more heating losses in the MOSFETS and reduce the efficiency of the circuit. At some frequency point you will need a heat sink. It will however reduce the audible effects. There is a compromise in there somewhere that you need to figure out. Either way you need a frequency high enough so that the ripple on the current through the inductors is fairly small.

• I can use heatsink anyway but I don't know why the ringing sounds happens. Is that do you think something to do with the PWM frequency being the harmonic of whatever natural resonance frequency of the magnet coil? I also dont want to use extra component like MOSFET driver IC. Finally you wrote "you need a frequency high enough so that the ripple on the current through the inductors is fairly small" I didnt understand this. Which ripple? How would it be an issue?
– GNZ
Oct 18 '17 at 16:33
• Do you think it is better to use bipolar transistor like TIP120 instead of MOSFET for this application?
– GNZ
Oct 18 '17 at 16:39
• @161776 The changing current in the inductor and magnet causes mechanical vibration. In your case at 490Hz. The magnets act like really bad speakers. Raising the switching frequency does two things. 1. It attenuates the amount of noise the mechanics can make due to their inherent inertia, and 2. raises the frequency beyond human hearing range. Note we go to like 50 because some animals can still here over 40. Oct 18 '17 at 16:41
• How about using a logic level MOSFET like this one: docs-europe.electrocomponents.com/webdocs/14b7/… with a lower on resistance, so I think I could increase the freq to 12kH or more safely ?
– GNZ
Oct 18 '17 at 16:44
• @161776 in general, if you use a bipolar transistor your base current required will mean the stage driving it needs to also be more powerful. However, .33A load current is not much so a decent bipolar would work ok in this application. Oct 18 '17 at 16:44