PWM power to a coil.

[Guest]

Hi

The coil is powered through a power FET, 24V 1A.

What is the best practice when I want a magnetic coil to be powered 10 min.

The coil is powered with 24V and drain 1A this will make the coil real warm!

In old day I will pot full power to he coil for maybe 300ms and then continue the power through a resistor for some ohms, just making the coil in the right position..

* But in this days I will use PWM for it. But what freq to choice? And will it work?

Hints

[Ttelmah]

The 'simplest' technique, is just to have a resistor in series with the coil, selected to give the required 'hold' current, and have a capacitor across this to provide the momentary higher current for initial operation.
Alternative is PWM, and yes this will work fine. Key to remember when operating any coil, is the diode to catch the flyback energy during the 'off' cycles. This will be needed anyway (don't rely on the internal diode in the MOSFET, unless it is a type designed for this - the recovery time is generally poor, and the forward voltage will often be higher than the voltage across the FETwhen it is turned on, which can result in overall innefficiency, with the FET generating more heat from the diode, than from the actual 'on' state), but it's power rating will need to be higher if switching is more frequent. You also have to remember that this energy will feed back into your power rail.
Frequency is down to a whole 'raft' of parameters in your circuit. At lower frequencies, ripple on the supply rail will be worse. At higher frequencies, depending on how good the drive circuit feeding the FET is, and the performance of the flyback diode etc., switching losses wll start to rise. Higher frequencies will also increase the problems with RF interference. However with modern FETs, diodes, and drivers, frequencies that only a few years ago, would have been considered amazing, are now quite routine. As an example, I have a PIC, providing the basic control to a linear positional actuator, which is rated at just over 10KW, on a 100V supply, and the chopping frequency is 250KHz...

Best Wishes

[Guest]

Thanks for reply.

-<24v>-<coll>-<FET>-<gnd>-

First power on the coil I will set the FET on for about 500mS after that I only need power for holding in position. There I use PWM.

The PWM pulse to the FET, you suggest 250khz? Is it too fast?

[FvM]

The criterion for minimum PWM frequency is the coil inductance respectively the resulting current ripple at a certain PWM frequency. For larger solenoid valves or similar applications, a few kHz are most likely appropriate. More than 10 or 20 kHz don't have a purpose, I think.

[Ttelmah]

Also, how high you can go, will depend _massively_ on the drive being used to feed the MOSFET gate. You have to remember that the input capacitance of a power MOSFET, is quite large. As such, it takes a long time for a 'simple' drive (such as a logic output), to charge this capacitance, and discharge it 'through' the voltage region, where the MOSFET is behaving resistively. The more time you spend in this region, the more the FET will dissipate. Higher frequencies are _only_ 'better', if the drive is capable of getting the FET through this region quickly.
I'd say aim to be above audio (perhaps 20KHz), and even then you need to look carefully at how your drive works.

Best Wishes