Full Bridge MOSFET Driver for Helmholtz Coils 1

[Oxyozo]

I am designing a MOSFET full-bridge amplifier to "square wave" drive a pair of large diameter, air-core, Helmholtz magnetic coils, for a medical research project. I have been searching International Rectifier and Linear Technology data and I am not confident that I can select drivers and MOSFETS that are best for my application. I think the LT1160 driver might be a good choice - but I have no experience! Can anyone give comment on the LT1160; if it is the most suitable, or if there are other more suitable drivers at reasonable price?

Specification:
- TTL input (Square Wave)signal: from 1Hz to 300kHz.
- MOSFET High Voltage Rail = 30V to 50V (adjustable)
- Full Bridge, Class D Amplifier configuration.
- DC Resistance of Coil Pair = 16 Ohms

Thank you.

 

[sreid]

Usually with Helmholtz coils, you want to drive some specified, variable current through the coils. Is this what you need to do?

 

[Oxyozo]

It is not necessary to create perfectly uniform magnetic fields over the area. But, yes, I am planning to regulate the current for particular frequencies by selecting appropriate supply voltage. I need to minimise power dissipation within the electronics.

 

[VEBill]

Several points come to mind:

Are you looking for a Class D amplifier with an even-higher-than 300kHz switching speed? I don't think so. In that sense, it wouldn't really be Class D.

Your specs are almost within range of commercial off-the-shelf audio amplifiers (the 300kHz is the only thing that stands out). You might be able to purchase a Class-D amplifier, make a few simple modifications (remove the low pass filtering on the output, convert the switching signal to be the The Signal, etc.), and be done.

If you're pumping out (for example) 150 watts at frequencies above audio, then you'd better be inside a shielded room. Because these are radio frequencies and some of them are in use. You don't want the plain white van with the funny antennas pulling up to your lab.

 

[Oxyozo]

Thanks for the reminder to make sure we keep the RF under control. We are in what is supposed to be a shielded room, but we better make sure to check that it is effective.

Regarding audio amplifiers - we started by looking at audio amps, but they are generally limited at both ends; usually cutting off around 20Hz and 40kHz, while at the low end we need to run down to 1Hz.

 

[sreid]

You need to tell us what the end result is you need. At first you say you need a full bridge with a TTL input and later you say you were looking at audio amplifiers which, of course, have an analog input.

Also you appear to need a band width up to 300 kHz. The ability to drive current into the coil will be limited by the inductance of the coils which you describe as "large." What is the diameter and the number of turns per coil.

 

[sreid]

It appears that your experiment is going to be done inside a screen room. I would assume that this is the case because the experiment would be affected by external radio sources. If that's the case, you do not want a switching amplifier inside the screen room.

 

[VEBill]

I think that my point has been missed.

Inside a Class D audio amplifer is a switching frequency well above the audio range. Conceptually at least, one might be able to find a suitable 150 watt Class D audio amplifier, make the PS rails adjustable, replace the internal switching signal with a BNC input, bias the gates on, and you'd be done.

Also, as mentioned earlier, if you're making a square wave at 300kHz, then I don't think you need or want or even could use a Class D amplifier (where the switching frequency would have to be several times higher than 300 kHz). You don't need a linear amplifier if all you're doing is making square waves. It'll just be a push-pull amplifier driven to saturation and thus hopefully low power dissipation.

 

[Oxyozo]

Sorry for any confusion by my response to VE1BLL's suggestion to consider using audio amps. We actually did work with audio amps in the early days of this project when we were using sine frequencies. But this is history.

The spec is now for square wave TTL input.
The coils are 600mm diameter and number of turns is 150.
We want to have capability to drive the pair of coils either in series using one driver, or individually using separate drivers. We are needing to produce high dE/dt pulses at pre-selected target frequencies. The exact pulse shape and amplitude specs will depend on feedback we get from tests. For now we want to control the dc current through the coils with the supply line voltage and to have fastest possible rate of current switching.

I was looking at simply placing the coils as the load in a full-bridge MOSFET arrangement. Do you see any problems with this arrangement and any suggestions on components?The bigest issue that I am aware of is to avoid the possibility of shoot-through. Are there any other issues important to consider?

 

[VEBill]

Driving sharp-edge medium frequency square waves into inductors can be stymied by the inductance of the load.

 

[sreid]

Now that you have clairified things, the LT1160 (2 ea.) in a bridge should work OK. 100 V FETs will be the fastest switches you can get. The LT1160 spec shows that it has shoot through protection so that won't be a problem. The high side gate drives are powered from a bootstrap circuit so the high side driver won't stay on for along time. To recharge, the low side must be turned on.

The FET body diode is used as an inductive kick clamp so the FETs you choose must be rated for this. Most modern FETs are.

 

[ScottyUK]

At 300kHz a half bridge should feasible if your drive waveforms are symmetrical and there is no DC bias. That reduces your power circuit losses straight away.


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If we learn from our mistakes I'm getting a great education!

 

[Oxyozo]

Thanks "sreid" for the review of the LT1160. Could you comment on the Harris Driver HIP4081 as a possible alternative to the LT1160?

I am favoring using the HUF75343 MOSFETs as switches because of their really low on-Resistance of 0.009 Ohm (this would really keep the power dissipation to a minimum!) They also seem pretty fast, but their 55V rating is lower than I would like - because of possible inductive kickbacks and the fact that I am wanting to run with line voltages up to 50V.

Would good design practice dictate that I go with a 100V device and compromise on the super low on-resistance? Any suggestions?

 

[sreid]

As far as I can see, the Harris driver does not have any under voltage protection for the high side drivers. If if doesn't, that's a real problem with a bootstarp power supply. I like the way the LT device does the shoot through protection.

I like to see at least a 25% voltage margin for power devices. Rds ON may not be of absolute importance to you. But if the FETs start "Zenering" from inductive kicks, they'll warm up fast.

 

[Oxyozo]

Thanks for all your comments ... they are helpful and very much appreciated. It is such a good feeling when help is given so generously. Thanks again!