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A circuit question about PWM control of e-magnet strengths through MOSFETs

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doncarlosalbatros

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I need to PWM control two electromagnet groups where each group has four electromagnets. Emagnets have nominal 24V 0.33A current ratings. **broken link removed** is the only datasheet I could find. To supply all the magnets I want to use this 24V 4.2A single power supply. To model these, I drew the schematics as follows:

fUntitled.png

Above each RL series represents an electromagnet. There are total 8 magnets. 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 MOSFET gate hence the magnet strength. This MOSFET is a logic level MOSFET.

The electromagnets are supplied by a 24V 4.2A power supply. The uC(Arduino board in this case) and the **broken link removed**non-inverting driver will be supplied by a 9V adapter.

In simulation I obtain the following power plot(and the peak current is around 1.4A) for this MOSFET I plan to use:

zokaed2.png


Before I start soldering I would like to know your suggestions about the circuit for this application. I should for instance figure out if I need heat sink. I afraid these magnets might make ringing noise so if I increase PWM to 30kHz(to eliminate noise) the FET's power dissipation will be high. Currently the PWM freq. is 500Hz. At what PWM freq. these MOSFETs would require heatsink?

Is it fine to use TC427 with 9V supply and trigger the MOSFETs gates this way?

I would be glad to hear your opinions/suggestions.
 

Hi,

You will find many discussions, documents and application notes about this topic.
I'm not sure if we need another thread...I see nothing special here.

The equation of conduction loss is: Pc = I x I x R. Use 100% duty_cycle for worst case scenario.
For 500Hz the switching loss should be negligible.

If you require a heatsink or not depends on the allowable temperature rise.

Even if the diode gets no DC current...I'd say a 1N4148 is too small. Use a bigger one, but fast. A 1A DC rated should be more than enough.

With low PWM frequency you may come into DCM situation, where the dutyCycle_to_current ratio is very nonlinear.
With higher PWM frequency you ensure CCM...and the ratio becomes more linear.

Klaus
 

Hi,

You will find many discussions, documents and application notes about this topic.
I'm not sure if we need another thread...I see nothing special here.

The equation of conduction loss is: Pc = I x I x R. Use 100% duty_cycle for worst case scenario.
For 500Hz the switching loss should be negligible.

If you require a heatsink or not depends on the allowable temperature rise.

Even if the diode gets no DC current...I'd say a 1N4148 is too small. Use a bigger one, but fast. A 1A DC rated should be more than enough.

With low PWM frequency you may come into DCM situation, where the dutyCycle_to_current ratio is very nonlinear.
With higher PWM frequency you ensure CCM...and the ratio becomes more linear.

Klaus

I think the coil has much more inductance then in my sim. It could be up to 25mH. Im worried this might damage the emagnet which has 0.33A current rating
 

Hi,

I think the coil has much more inductance then in my sim. It could be up to 25mH. Im worried this might damage the emagnet which has 0.33A current rating

What do you mean by "this"?
The 25mH?

Klaus
 

Yes, obviously error of reasoning. The coil current is set by the resistance, not the inductance. Higher L slows down current rise time and prolongates the fly back pulse. And gives the opportunity to reduce the pwm frequency.
 

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