h-bridge high side issolation

[Sceadwian]

Seeing as how all I have are N channel fets right now and the issolation would be good anyways, what are my options for driving the high side fet's of an h-bridge with some kind of optical issolator? Are the currents/voltages available from an opto issolator strong enough to drive a FET in a switching application? Can anyone suggest a class of device or other methods for issolation and driving of a high side fet? Preferably something that doesn't overly complicate a generic h-bridge circuit.

 

[Analog]

The highside MOSFETs need Vgs + Vl to trigger. You need something there on the backside of the opto to create that voltage.

 

[Sceadwian]

I understand that, that's the reason I started this thread =)
I was thinking of a photovoltaic generator like the one described bellow, in series between the source and the gate.
https://www.irf.com/product-info/datasheets/data/pvi5013r.pdf

Although I'm not sure how I would go about switching the FET off, the photo generator isn't going to provide enough voltage if there's a pulldown resistor on the gate strong enough to provide decent switching times.
It specifically mentions in the PDF that it's ideal for issolating FET's but there are no example circuits.
There are two notes in the PDF file that confuse me.

One is the (Maximum Voltage Differential Between Outputs) of 1 volt which I'm not sure how to interpret.
The other is Off-State clamping resistance.
Does this mean when the device is off it acts as a series resistance?
The device seems ideal as it should have no problem reaching the 4 volts required to turn on the IRF510, and goes as high as 8volts which is more than enough to drive the FET into it's lowest RDS/on state for the voltages I'm using. If I use them on the high side only and drive the lowside fets directly I should be able to attain high speed switching using just the Nfets?

 

[dknguyen]

Can't you use an opto-isolator to drive your driver IC? It's connected to a separate power supply anyways (higher than logic, but lower than motor).

Check out these app notes:
https://www.electro-tech-online.com/custompdfs/2006/12/an-937.pdf
https://www.electro-tech-online.com/custompdfs/2006/12/an-978.pdf

(I was actually only searching for the bottom app note, but found the top app note first and it seems to suite what you want better. It actually talks about photovoltaic drive somewhere).

 

[JTECH]

How about using a full bridge driver IC? These have 4 independent outputs and are designed to drive H-Bridge configurations.Try the EL7661CN. I'll see if I can find the data-sheet and come back to you.

 

[Styx]

isolated/floating Gate-drives are not that hard
they really only have 3 components

1) signal isolation
2) power isolation
3) gate-drive

signal isolation in its simplest form is just an opto (6n137 are good logic-output). pulse-transformers can be used but there is alot of extra gubbins needed

The power again in its simplest form can come from an isolated DCC converter. TRACO and Newport do some very nice, cheap and very reliable DCC converters that can take 5V in (or 9, 12,18,...) and provide a kv worth of isolation for a range of different isolated rails

The actual gate-drive consists of a gate-resistor (to stabise the FET,IGBT...) and some LOW-IMPEDANCE controlledpull-up/down mech (usually totem-polled low-power/fast FETS)

in fact #1 and #3 can be combined in opto-isolated IGBT drivers (all that is needed is a gate-resistor and isolated power)


Gate-drives are extreamly simple (but they can get quite complex once you get into DeSat protection...) for something as nessecary as this

 

[Papabravo]

Opto isolators tend to be very slow and the current transfer ratio degrades with age. Depending on your configuration either the turn on or the turn off will be governed by a passive device. You need an active device to switch a FET gate in both directions to avoid some really nasty problems. I don't think you want to pusue the opto isolator method.

 

[Styx]

Opto isolators tend to be very slow and the current transfer ratio degrades with age. Depending on your configuration either the turn on or the turn off will be governed by a passive device. You need an active device to switch a FET gate in both directions to avoid some really nasty problems. I don't think you want to pusue the opto isolator method.

The opto isn't being used to drive the gate directly, it is to be used for signal-isolation.
Opto's can be very fast (using them in my gate-drive with a switching freq of 20kHz and it passes through pulses as narrow as 2.5us as well)

 

[Sceadwian]

Actually no it is being used to drive the gate directly. The link I supplied above produces an issolated unrefrenced voltage directly using an internal Light emitting diode coupled to some kind of photo voltaic receiver. It would only be used to drive the high side FET's which don't need to be switched at high speeds as this is only for generic motor control, I can PWM the low side fet for motor speed control. Please note the original post, the issolation is only a byproduct of coming up with a cheap simple way of getting a floating voltage source to the FET gate with as little hassle as possible. Although I'm not sure if the chips I mentioned count as cheap as they're 6 bucks each.