[SOLVED] Transistor for shunting high voltage pulsed RF signal to GND

[Robert T]

I have a TIP50 shunting to ground at the end of a LF signal pulse. The purpose is to ground the antenna to remove the "ring down".
This has been working well until the other day when the TIP50 failed (died?) - I assume that even though the TIP50 was switching to ground after the signal was switched off, the voltage is still too high even though for such a short period of time.
TIP50 is rated at 450V.

The signal on the antenna is 900V peak to peak.
The signal is switched to 0V in 120usec with the TIP50.

I tried a BU505 which is rated at 1500V however it does not switch as the TIP50 did. Takes the antenna 1.5msec to ring down.
I have tried decreasing the value of the base resistor with the BU505but this has no effect.

Is anyone able to advise me please as to a solution for this issue. Maybe a more suitable transistor?

Thanks, Robert

 

[betwixt]

I can't advise on any particular type but this sounds like an application where a MOSFET would be better suited than a bipolar transistor. When turned on it would look more like a resistor to ground than the diode a bipolar transistor would present so it might also reduce harmonic radiation. Perhaps searching for high voltage MOSFETs used in older TV horizonatal output stages might turn up something useful. There are several rated at more than 1KV and several Amps but be careful as most have an internal diode between drain and source pins. Even then, you might be able to combine N and P types in series so the diodes oppose each other.

Brian.

 

[chuckey]

Do you have a DC VCC?, because as you say, its a ring, so turning on the transistor, when the pulse is +ve will cause the transistor to conduct and absorb energy. The question is, what happens when the ring goes negative?, the collector base junction will be forward biassed and current will have to be supplied by the base circuit, through the base collector junction, to the negative ring. This will depress the base voltage to negative and reverse bias the base emitter junction, which will then punch through. I would include a high voltage diode in series with the collector to block the negative volts. It will increase the ringing time but will protect your transistor, as the negative volts will be held off the transistor. or you could use a P-N-P transistor as well to deal with the negative rings.
frank

 

[mtwieg]

Depending on how fast the switching needs to be and the frequency of the RF, you might consider using a PIN diode switch instead of a transistor.

 

[Robert T]

Would you be able to elaborate on combining N and P type MOSFET's so they oppose each other please Brian. I have a number of MOSFETs here which are rated at 1KV or above.

Robert

 

[chuckey]

If your FETs have the built in diode then they have to be put in series, now for any polarity you have the forward biased diode of one FET in series with the high voltage blocking of the other. When both FETs are turned on, you have a forward biassed diode plus the on resistance of a FET. I think the upper FET will have to have its drive transformer coupled because the voltage at the FETs junction will be at the line voltage in the when the FETs are off, due to those diodes.Otherwise the voltage rating of the gate to source voltage will be exceeded.
Frank

 

[Robert T]

Thanks Frank. I looked to find a P type FET but the voltages only go up to 400V.
What about using a IGBT? Insulated gate bipolar transistor - Don't know much about them. These are available in high voltage, and from what I understand, are protected from being able to be punched through.
Robert

 

[Robert T]

mtweig. - I looked at a pin diode but the frequency is too low from what I could find out. Thanks.