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LDmicro Forum - PIC driving a high frequency load

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PIC driving a high frequency load (by Evan Raftery)
What is the best method to drive a high frequency (40kHz) load at 140V @ 2.5A.?

Do logic level mosfets still require a mosfet driver to fire the mosfet from a pin on the PIC?
Fri Oct 23 2009, 23:42:13
(no subject) (by Jonathan Westhues)
What's the application?

It's often possible to drive a logic-level MOSFET directly from an I/O pin, perhaps with a resistor in series to limit the current coupled back through the device capacitances. (That resistor will also slow down the rise time, of course; and logic-level FETs already have larger gate charges than the equivalent part with higher threshold voltage).

But your voltage is relatively high. It may be necessary to use a more robust gate driver to tolerate that higher coupled current. A typical gate driver would also produce faster rise times on the gate voltage, which would decrease the power dissipated in the FET.

It may also be advisable to use some type of isolation, like an opto-coupler or a gate driver transformer. At these voltages and powers, it is in general quite easy to design a circuit that will cause a fire or electric shock; do not try to build a circuit that you do not fully understand.
Sat Oct 24 2009, 00:26:56
(no subject) (by Evan Raftery)
The application is a ultrasonic cleaning module. The technique would of been to modulate the frequency sinusoidaly between 30-60kHz. Am trying to clean inside a stainless tube that plates with ceramic buildups. I know the transducers are rated 50W. There will be a 6 x array of them. Have been told they rate as high as 1000V but know some are rated 140V max.

Is the gate driver driven directly of the PIC pin? The trouble is matching the driver for the mosfet.

I test all equipment in a metal cabinet and the mosfet would be bonded to a aluminium heatsink bonded to the pipework (water cooled) and then polyurethane encapsulated around the mosfet.
Do not fully understand the circuit at present but is why am doing homework first.
Sat Oct 24 2009, 02:57:10
(no subject) (by Jonathan Westhues)
Depending on the circuit configuration, you may want a high-side (pulling up) switch as well as a low-side (pulling down) switch. The low-side switch is generally easy, since you need a gate voltage around 10 V above ground. For the high side switch, you need a voltage around 10 V below the 140 V rail (if you're using a p-FET), or 10 V above that rail (for an n-FET).

That level shift--to provide a gate voltage up near the high voltage rail, and maybe to generate a gate voltage higher than that rail--is usually hard. Typical ways to do that include little pulse transformers, or gate drive ICs.

The most obvious way to drive that piezo would presumably be a full bridge ("H-bridge"), alternately applying +V and -V to the device. This allows you to apply a peak-to-peak swing of 2V with a supply voltage of only V. This requires two high-side switches, and two low-side switches.

A typical gate drive IC accepts logic levels with some reasonable threshold. We could consider the FAN7380, for example:

http://www.fairchildsemi.com/ds/FA/FAN7380.pdf

It accepts two digital inputs, for the high- and low-side switches, with a logic threshold between 0.8 and 2.5 V. So a PIC running at 3.3 or 5 V can drive that with no trouble.

Note that the FAN7380 is designed to work with two n-FETs; so that high-side switch requires a gate drive higher than Vb. So the IC uses a switched capacitor circuit, with Cboot and Dboot, to generate a voltage Vb + (Vcc - 0.6 V). That seems like a nuisance, but p-FETs are very rarely used in power electronics; since the electron mobility in silicon is greater than the hole mobility, a p-FET with the same dimensions as an n-FET will always have worse Rdson.

Another way to drive it might be with a resonant LC circuit: the transducer itself supplies some or all of the C, and you add sufficient inductance for the desired resonance frequency. Circuits of that form could be built with only a low-side switch, in which case you would need only a low-side gate drive IC. That also produces a relatively sinusoidal waveform. But the amplitude at resonance is rather sensitive to the component values, so it's not as clean as square wave (or "modified sine wave", +V, 0, -V, 0, ...) excitation with a full- or half-bridge.
Sat Oct 24 2009, 03:56:08
(no subject) (by Evan Raftery)
Thankyou, thankyou very much. I will try that & post results on what transpires.
Sat Oct 24 2009, 18:08:27
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