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30 kHz / 3 Phase signal

30 kHz / 3 Phase signal

(OP)
Hi, I'm trying to generate 3 x 30kHz signal each phase seperated by 120 degrees. These 3 phase will be used to turn On/Off FETs that will power some high frequency transformers.

What I'm looking for is the lowest cost effective solution to generate the signals. I was thinking of using 3 Op-Amps but I'm a bit worried about the reliability/accuracy for a production product. I'm sure there are plenny of cheap PWM ICs that could do this more effectively, I just don't know them.

Here are my specs:
3 phase 30 kHz signals, seperated by 120 degrees
Accuracy of ±5%
Supply voltage 10V
Waveform: Square, Triangle or Sin, I just want the reference
Power Output: Very small as it will trigger FETs.

RE: 30 kHz / 3 Phase signal

Square wave has to be the easiest, this is an almost trivial task using standard TTL logic. A simple counter and some simple logic would be a good starting point. Sine is possible but a lot more complex and not worth the hassle if you can use a square wave.

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

RE: 30 kHz / 3 Phase signal

Re "Power Output: Very small as it will trigger FETs"

Some gates need lots of charge and that translates into a high load if you run at 30 kHz. It depends on your FETs.

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: 30 kHz / 3 Phase signal

What about the Linear Technology LTC6902 multiphase oscillator? I've used it for synchronizing switching regulators. It would make generating 3-phase outputs trivial, although you'd need some sort of gate driver if you need 10V output.

Glenn

RE: 30 kHz / 3 Phase signal

The LTC6902 has four phases, three phases are a bit more tricky to do with a counter and decoding.

How about a low end PIC microcontroller ?
One chip, low cost and simple.
And you could do neat things, like building in dead time.

RE: 30 kHz / 3 Phase signal

Warpspeed, the LTC6902 can be configured for 2, 3, or 4 phase operation, depending on the state of the PH pin. For 3-phase operation, you leave the PH pin floating.

However, I do agree that if you need things like deadtime, using an MCU would give much more flexibility.

Glenn

RE: 30 kHz / 3 Phase signal

I like ScotyyUK's Idea.  A three bit state counter:

Ph3 Ph2 Ph1

1   0   1
0   0   1
0   1   1
0   1   0
1   1   0
1   0   0

RE: 30 kHz / 3 Phase signal

(OP)
Wow thanks a lot guys, I think I'll go with a state counter as it's much cheaper then what the LTC6902 seems to be retailing at. I even thing I can make some kind of dead time with a state counter.

A uC would have been neat too but since it's a volume item I would have had to make a mask on the IC... which I try to avoid.

Anybody has recommendations on a cheap CMOS gate that could do the job?

BeerBaron

RE: 30 kHz / 3 Phase signal

It is going to require a clock, a counter, and whole bunch of logic gates or decoders to do this. Many chips, and a lot of board space. That is the problem.

A best way to build a state machine is to use either an EPROM or a programmable logic array chip, (or use a microcontroller).  All of which will require individual programming.

RE: 30 kHz / 3 Phase signal

BeerBaron, would you really need to mask program the MCU? Even on the high-volume consumer electronics that I've worked on, the price difference didn't make up for the convenience of a flash-based MCU. And it's nice to be able change code in case of an "oops" that's found later.

Glenn

RE: 30 kHz / 3 Phase signal

To include short dead times, the counter would have to be longer.  Then a CPLD from, say, Altera would be my choice.  Free design tools [really easy to use] and on board programming thru a serial link (two pins on the board, free programming software too).

http://www.altera.com/

RE: 30 kHz / 3 Phase signal

I couldn't get to sleep.  An 8 bit binary counter short cycled to 255 counts [0 to 254] is divisible by 3 [85 x 3] and has one illegal state [256].  Clock at 7.65 MHz [30 kHz x 255] and decode.  Allows increments of 130 ns "Dead Bands."

RE: 30 kHz / 3 Phase signal

Have a little pink star sreid. It won't help you sleep but at least your effort is rewarded!

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

RE: 30 kHz / 3 Phase signal

Hello.
TTL counter is the simplest way to go. Or just do PLD if you know how to program.
Yes, "dead" time is very important.

From TTL to 10V FET gate - use any low side FET driver from IXYS or Micrel. Do not use IXYS "4 series"!!!!
They have "5" now  - improved.
Depending how big is your FET - need to use "big" gate driver.
For 30kHz and average 50A FET - 2 A driver should be OK.
"dead time" - start in 100nS-200ns range.
"dead time" can be easy done with 1R, 1C, and diode.

This whole design is kind of strange- you will be driving 3 phase SQUARE(???) signals into one transformer? Or few transformers?

Are you sure you don't need 3 sine signals 120deg shifted? then PWM them with some higher frequency?

RE: 30 kHz / 3 Phase signal

Pulse transformers work just fine with square waves, particularly with a 50/50 duty cycle as this will be.

Saturated switching of the driver is also going to be a lot more efficient than some kind of linear power amplifier.

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