Isolation and High-side or Low-side Brushless Motor Driver

[lobabob]

I currently have a low side drive (MOSFET acting as a switch to ground).

Which are the situations you would use each in and is one better then the other?

Does anyone know any 5V inverters that are optoislated? This is my islation between the PWM signal and the MOSFET input (which is TTL). Otherwise I have to use two parts which is annoying...

On my motor driver there is an LC on the output to smooth out the PWM but Im getting some strange results. I presume this is becuase the positive supply of the motor is directly connected to VCC and the negative supply is connected to the MOSFET which is switched to ground. I have some protection diodes infront and behind the inductor to dissapate spikes.

 

[Oznog]

Well high side switches are a long story.
First off note you generally need a PMOS, which all things equal has slightly inferior properties to an NMOS.

I know of no 5v IC optoisolated integrated with digital logic in one component. In fact I've never seen a small optoisolator at all.

Note optos are generally slow and thus difficult to use with PWM.

LC to smooth the output? Motor outputs generally do not need smoothing. The motor itself is an inductor.

Show your schematic and we can give pointers.

 

[ericgibbs]

hi,

>> Does anyone know any 5V inverters that are optoislated? This is my islation between the PWM signal and the MOSFET input (which is TTL).

Do you mean 5V psu type invertor or a TTL signal invertor.??

You can buy small isolators, which transform 5Vdc to say 5Vdc,12Vdc or 15Vdc, rated at 20 thru 100mA.
These devices are small switchers which have over 1500Volts breakdown voltage levels.
Sizes about 1cm cube, cost approx £5GBP

If its just a opto-isolator for TTL there a numerous types to choose from.

Check this link: https://www.fairchildsemi.com/whats_new/opto.html

 

[Hero999]

I'm assuming the MOSFET is going to to be pulsed on an off. Just use a pulse transformer, simple, cheap and isolated.

 

[Sceadwian]

Possibly a good use for an old audio issolation transformer.