VK Logger Ham Radio Forum Archive

Hi All

I wonder if anyone out there has played around with the Mitsubishi MGF0906B L Band Gas FET? I originally pickup a surplus 2 GHz amp with the output stage blown up....it used 2 x MGF0906B as the output PA. I sourced replacement devices and after thoroughly checking for reasons that the Amp blew up...of which I found none....confidently installed the new devices. Poof, they both went the same way.....Drain to Source short. Prior to putting in the replacement devices I adjusted the bias so they'd be cut-off........

Are these devices hypersensitive to static discharge problems????

Any feedback welcomed.

Hi.
Iv had a play with these... You cant bias them off like a normal fet. The cut off bias is -4v or something weird like that.
Have you had a good look at the part PDF ? I know that if you put ht on with 0 v on the gate they will draw current and go poof if you dont current limit the psu.
Mark.

Hi Mark

Thanks for that....Yes with zero bias....Poof!!

I biased them for -4 Volts prior to installing them and still ....Poof!

Just wondering if my handling techniques are causing damage to the devices even before I activate them? I didn't use full blown ant-static techniques but even so...I did not physically touch them and made sure I minimised such a problem.

It's got me a bit stumped

Sounds like you are doing everything write.. Im a little rough with the handling but ill grab the flange first and then the gate so I don’t spike it. With power on the pcb I get -3.26 on the gate with 9.8v on the drain which results in an idle current of under 500ma.
If your still not sure bring the supply volts up slowly and don’t exceed its rating, this will stop it from going poof.
My - bias is from a onboard chip so its not there before I apply the DC, as a result there is a bump in supply current until the - bias rail stabilises.
Are the devices new or s/h that don’t work??
Let me know if i can help any further.

Mark.

Hi Arie.

I don't have an amp with 0906s, but I do have a single device 0907 amp (10 watts out for 1 watt in), built from a kit that David VK5KK produced back in 1998. I did get it working, but it wasn't used much before I replaced it with one of Pyro Joe's 30 watt jobbies.

But back to the point... David underlines the fact that the FET will die if the positive supply is connected to the drain before the negative bias is applied to the gate (for the 0907 the bias is about -3 v). Shorting the gate to ground will also destroy the FET. The devices are bolted down, I take it? The install notes I have stipulate setting up both positive and negative rails, then depower, before installing the device; check for shorts (particularly to ground) and then apply power with terminations on input and output. If all is ok, then apply half level drive and snowflake if necessary before trying full drive.

I don't think you're going to "feel the love" with 0v bias...

There's no mention in the documentation that the devices are hyper-sensitive; I think I used standard handling techniques (anti-static wristband) and managed not to kill it.

73,
Barry
VK3BJM

Hi Mark

Many thanks for the reply....Yes there is more too it. The MGF0906B devices that were originally in the amplifier had suffer the usual catastrophic Drain to Source short. I checked the bias supply which is generated by an ICL7661 neg. voltage generator and all good. I figured they'd be a delay between the negative rail reaching full volts and the application of the the Drain voltage, so I half had it in my head that they'd be a Drain current spike at switch ON. So all in all I figured despite this that since it was a proven (theoretically) product then it had been catered for.....there's a 1.5 ohm Drain resistor in series with the supply.

Now and this is probably the part were we'll all go ...."there yeah go!" The replacement chips came from China....are yes, the best that China can produce.

So I suppose I'm trying to eliminate all possibilities of a repeat performance if I now decide to source genuine replacement devices at a much higher price tag. Especially the ESD component as I've had experiences in the past with Hyper sensitive ESD components.

The replacement devices didn't last a second in circuit once power was supplied, so at this stage I'm leaning towards "quality Chinese replacements " as the source of the condition and not what I have done.....But I needed to check.

It pretty well sounds like your very familiar with the devices Mark and by your description of how you handle them (pretty much parallels with mine), I got a BC108s in a fancy package?

Hi Barry ...just spent a few minutes complying a reply and the internet drops out

Anyway...thanks for the information. Judging by what both you and Mark have described I bought BC108's in a fancy package. That's OK, I took a chance and it didn't come off......I just wanted to make sure that if I did get replacement chips at a considerably higher price they weren't going to do a repeat performance (well hopefully!!)

I had to get past the idea that .....blown FET... short between Drain and Source as original fault may be I am doing something that contributed to the replacement devices repeating the same condition.

Both of you have confirmed that I don't believe I have....

Oh well back to the drawing board.....

Thanks again

Hi,
Any chance of seeing the board.

I have seen one surplus PCB which needs an external regulator to provide a locked out supply to the drains of the FETS so the gate voltage can be generated, then the drain volts are allowed.

The supply sequencing may be another board

Kevin

Hi Kevin

Thanks for the info..... The brick itself is pretty simple, it runs off a single DC supply, the bias generator runs off that supply. The only inherit deficiency I can see is that it will take a finite amount of time for the bias to reach full rail voltage compared to the applied DC to the FETs Drain. They've compensated for that by using a 10 Watt, 1.5 ohm resistor in series with the Drain. The amp is specified for 12 Volt operation ....the FET's specified nominal V DS is 10 Volts (hence also the reason for the Drain resistors)

I did originally fire up the amp. with the replacement devices installed using a current limited supply....but still Poof! So I'm comfortable now that I just purchased faulty replacement devices and wasn't any fundamental issue I was missing .....I think? I'll take the plunge and get the genuine article and be even more careful!

Thanks Kevin

73s

Hi,
Hmm. No interlock... Thats interesting

With the FETs not biased properly, they will look like S/C and the 1.5 ohm will limit the current to 8A, with no current limiting, it is possible that an initial current spike may take the device out (rated at 3A maximum) even before the PSU current limiting takes hold.
Another factor often overlooked is that the Vd-g can be exceeded. Having a high current P channel FET in the +ve supply, which is held off until the negative bias comes up is very common. Only a few components are needed.

Another failure mode is switch on Power Supply spike where for an instant the Drain voltage exceeds the Device breakdown. Been caught with that

Good luck


Kevin

OK Kevin

You have raised some very interesting points there.........

I'll do some more engineering of the circuit before I'm tempted to install any more replacement devices. As it is a commercial amplifier but that doesn't mean to say the it has inherit problems and they contributed to the failed unit.

Thanks again Kevin.....

73's Arie