Suggestion for top BJT in an high voltage cascode CCS for a source follower

[zintolo]

Hi,

In a project I'm working on I need a source follower supplied with +40 and -140V, with a cascode of two BJT as CCS set at 6 mA.
The bottom BJT will be a 2N5551, while I'm open to suggestion for the top BJT.
Requirements are:

1. Being able to dissipate up to 900mW on peaks (it will swing up to +10V), 450mW as average (it will be set around -55V at idle), so I would say we need to exclude TO92;
2. Vce of at least 200V (I'd consider at least 230V for safety due to voltage fluctuations on the 180V we have as nominal);
3. Fit a classic TO92 pcb connection (1=b 2=c 3=e) because this is how the pcb has been done;
4. A good Hfe to improve the CCS impedance.

I've checked on mouser, and I've found some options:

1. KSC2752Y : https://www.mouser.it/datasheet/2/308/1/KSC2752_D-1810664.pdf
   TO126, Vce 300V, Hfe 40-80, pinout fits, 0.61 €/each;
2. TTC011B : https://www.mouser.it/datasheet/2/408/TTC011B_datasheet_en_20160209-1649840.pdf
   TO126, Vce 200V, Hfe 100-320, pinout fits, 0.64 €/each;
3. TTC008 : https://www.mouser.it/datasheet/2/408/TTC008_datasheet_en_20140403-1916100.pdf
   New PW-Mold2, Vce 200V, Hfe 80-250, pinout fits, 0.87 €/each;
4. TTD1409B : https://www.mouser.it/datasheet/2/408/TTD1409B_datasheet_en_20150806-1915948.pdf
   TO126, Vce 400V, Hfe 600, pinout fits, 1.62 €/each.

TTD1409B seems the best choice, do you have other suggestions?
Thanks in advance,

Kind Regards
Roberto

 

[MarcelvdG]

ZTX458 or ZTX558.

 

[zintolo]

Thanks @MarcelvdG ,

There's no information on thermal characteristics of those two NPN.
Is it safe to dissipate half Watt continuously and 1W peak in a TO92?

 

[wrenchone]

The ZTX parts are in the proprietary E-line package - this can comfortably dissipate around 1/2W continuous. It's also easy to glue a heat sink to the part using a metal-loaded epoxy like JB Weld or JB Kwik

 

[zintolo]

Thanks @wrenchone ,
are the ZTX suggested compared to the TTC and TTD I've found?

 

[MarcelvdG]

ZTX458 is an NPN transistor, ZTX558 its PNP equivalent. The last time I looked, the thermal specifications were in their datasheets.

https://www.diodes.com/assets/Datasheets/ZTX458.pdf

It's implicit as a P_tot and T_j spec: 1 W at 25 degrees Celsius, derate via a straight line to 0 W at 200 degrees Celsius.

 

[wrenchone]

The TO-126 transistors would be easier to heat sink if you find it necessary. The current TO-126 parts are also completely isolated, which makes heat sinking less problematic, though a trifle less effective.

 

[Osvaldo de Banfield]

Search for video output transistors for tv and PC monitor. Several of them are in TO126 are cheap, easy to find and video quality.

 

[wrenchone]

The good HV video transistors might be a bit thin on the ground these days, given the transition to flat screens.

 

[Osvaldo de Banfield]

No, there are myriads of them as surplus. Googling "transistor for video output" gives me several hundreds of inputs.

 

[zintolo]

I've checked on mouser (where I'm buying other things) and both ZTX458 and ZTX558 are out of stock.
I've also tried to find on mouser some transistors for video output, but again no luck.

The TTC011B : https://www.mouser.it/datasheet/2/408/TTC011B_datasheet_en_20160209-1649840.pdf
TO126, Vce 200V, Hfe 100-320, pinout fits, 0.64 €/each;

after a light bending of the pins, seems the best option at the moment.
What do you think? Has anyone ever used it for this purpose?

Thanks in advance

 

[wrenchone]

Osvaldo -
I tried doing a Google search for video output transistors and got a fair amount of NTE rubbish and people trying to finance their retirement with outrageous prices per device.

Zintolo -
Electronic Goldmine (a good source if you are in the USA) has at least 2 devices in their Japanese transistor inventory that are rated for chroma output service. One is in the TO-92L package, and one is TO-126. Both should have ECB pinouts. The TO-92L is capable of around 1/2W dissipation, and the TO-126 device can easily be fitted with a small heat sink.

https://theelectronicgoldmine.com/collections/japanese-transistors

 

[Osvaldo de Banfield]

Try this: digchip.com parametric search, under discrete/transistors. Fill in the parameters desired.

 

[wrenchone]

Why would I want to do that - I'm not the one trying to build this thing...

 

[wrenchone]

For Zintolo...
https://theelectronicgoldmine.com/collections/japanese-transistors/products/g43315

https://theelectronicgoldmine.com/collections/japanese-transistors/products/g15020

 

[Osvaldo de Banfield]

Thank you for your gentile answer. 🖕

 

[pweaudiotech]

Put a 'Tophat' thermal sink radiator on a hefty generic TO-5'er ... Digi-Key search runs fairly quick.
Pull up the Texas TI or National 'Semi' Catalog in tandem browser window and cross-reference-search multitask style.
On Semi has stuff too that is off grid NOS specs. Fairchild is my pick for Hulk TO5's. Just jury-leg the EBC<-->BCE as these trannies have sizable spread.

 

[pweaudiotech]

Edit time ran out ... I meant to say replace the TO92 with a TO-5 also.
Easier to match than to hatch,
IMO plastic is allergic to Tesla.

 

[wrenchone]

TO-5s and associated heat sinks take a fair bit of real estate (you have the choice of the "sun crown" or a hi-rise solution that doesn't fit all that securely to the part in question). Plastic takes less room, and with a TO-126, you can get by with a smallish bit of aluminum sheet (or maybe even no heat sink at all). I'm not allergic to plastic...