Replacement for FCX718 - urgent help needed!

[ChildOfVision]

Hi!
URGENT help needed!
Does anybody know for adequate replacement for FCX718 transistor?
I intend to use it/them in battery-charger application with DS2711. I do not intend to use the currents greater than 1A (continuous) but if it can handle 3A even better  . Even Maxim's adviser doesn't help me much  . It should be a high hfe, low saturation voltage transistor, since DS2711 pins can sink only 20mA of current. So if somebody knows for such a transistor, that can handle up to 1-3Amp of continuos current @ base current of 20mA, please let me know! And: it must be available on futurlec.com because it is my only source of electronic parts (eventually eBay)!

Sorry for my uuugly English!
TIA!

[alm]

high hFE and low saturation voltage are useless specs, you'd have to give actual numbers. I also had to track down the datasheet (was posting a link so hard?) to figure out that FCX718 is a PNP transistor. Using a parametric search engine and figuring which are sold by futurlec.com seems like a nice homework exercise for you. At Digikey searching for a PNP transistor with Ic >= 1A, hFE >= 50 and Vce(sat) < 1V (is that 'low'?) returned 1100 in stock items.

[ChildOfVision]

Sorry, I totally forgot the datasheet (I hope that you thought on FCX datasheet!)!!!

Digikey is just a dream to me!
The man from Maxim said it was only FZT953 "looks close" to FCX.

Using a parametric search engine and figuring which are sold by futurlec.com seems like a nice homework exercise for you.

Last 10 days, I downloaded every fucking datasheet for EVERY of the PNP transistor available from futurlec.com, but with no avail. I just hoped, that I missed something...

I can't believe that such a transistor(s) doesn't exist (except in Digikey).
So I am still awaiting good news from somebody!

Btw. I gave "actual numbers": transistor must give (i.e sink) 1A when (-)20 mA/~1V is applied to its base!

[amspire]

Futurlec just doesn't stock a large range of transistors, and 2.5A PNP transistors in a SOT89 case are not common.

Either you buy a genuine FCX718 from DigiKey

http://parts.digikey.com/1/parts/1004930-transistor-pnp-20v-2500ma-sot89-fcx718ta.html

or you substitute the transistor for one in a larger case from Futurlec like the FTZ749 or FTZ751. 

Richard.

[ChildOfVision]

Thank you amspire!

As I said Digikey is just a very distant dream to me  !
If I read datasheets for FZT749 or FZT751 correctly, these transistors require 100mA on the base for 1A, and only under pulsed-current condition!
FZT953, which Maxim's advisor "recommends", also have similar characteristic.
If you look at datasheet for DS2711 (I don't expect that!) you will notice that one can use optocouplers for 2 transistors that supply current to batteries, and that is great - I can use virtually any PNP transistor. These two transistors are essentially just switches.
But I can't figure out how to use such a trick for first transistor that regulates current, and it is driven by "Current sense feedback" pin of DS2711, and that pin also can sink only 20 mA?

I attached datasheets for all, so if someone is bored...

[IanB]

If I read datasheets for FZT749 or FZT751 correctly, these transistors require 100mA on the base for 1A, and only under pulsed-current condition!

Why do you think that? I am reading the FTZ749 datasheet attached to your post and I don't see that. It looks to me as if 20 mA base current will produce 1 A of collector current with VCE less than 0.2 V. And that condition appears to be well within the DC safe operating area. What am I missing?

[amspire]

I think you are right. The FTZ951/953 look better - they will manage the gain of 100 at 2A. They could be better then the FCX718 as the gain of the FTZ951 is only spec'ed at 1V. At 2V, it would be higher.

[ChildOfVision]

Huh, that's promising  !
I am not a beginner, but I am not some expert neither! I admit that I don't understand that datasheets fully! I found that "100mA for 1A" data under "Electrical Characteristics/Collector-Emitter Saturation Voltage", where I find: "I_C=-1A, I_B=-100mA*".  Can you please explain to me how did you derive your data (I_B=20mA, I_C=1A)? That would help me for future explorations of datasheets!

[ChildOfVision]

Thank you amspire!

If you and IanB agree that FTZ953 are even better than FCX I will buy them!
But, please, explain me a little more how did you find it in the datasheets!

[IanB]

On the FTZ749 datasheet you can look at the graph VCE(sat) v IC. There are two curves there representing IC/IB=100 and IC/IB=10. Use the 100 curve and take a collector current of 1 A. That indicates that with a base current of 10 mA (=1 A / 100), then VCE(sat) will be ~0.2 V. It follows that with a base current of 20 mA (IC/IB=50) the saturation voltage will be even lower.

[ChildOfVision]

Thank you, I think I understand now ! That table on the first page just confuse me!

[amspire]

Is this for a one-off design, or for a commercial design? The reason why I am asking is that for a one off design, you can adjust the current limit to match the gain of the transistors. If it is for a commercial design, then you have to use worse case specs and with only a 20mA drive current, the transistors are near the thermal limits if you look at the full 2A through a single transistor.

So if you look at fig 5 of the data sheet for the DS2711, they set the current for 2A total. If you are charging one battery, then the 2A is going through one transistor.  With a 20mA base current, the original FCX718 looks as if it will have a typical V_ce of about 1V (2W power), and definitely less then 2V. If a transistor had a 1.4V V_ce at 2 A, that would be 2.8Watts power which is way above the specs, even at 25C ambient.  So I would assume that for a commercial design using the original FCX718, you would have to wind the current down to a lot less then 2A.

The FTZ951 has a worse case (minimum) gain of a 100 at 2A, 1V V_ce. So the worse case power dissipation at 2A is about 2W. The transistor is spec'ed at 3W with a total of 4 square inches of copper  for heat dissipation at 25 degrees, so we are getting closer to a reliable design.  The FTZ953 is not specifically spec'ed at 2A, and with a higher voltage rating, it will probably have a higher V_ce then the FTZ951, so I would not use it.

Richard

[ChildOfVision]

Is this for a one-off design, or for a commercial design?

Well, I have two DS chips, and I intend to build 2 chargers (in one box) for 4 NiMH batteries in parallel. I do not intend to charge each battery with more than 1A. Say, some 600mA is fair enough. I must admit that I don't fully understand your theory - I will study that more detailed! I think that you didn't understand well how DS2711 charge batteries: I will not build that charger(s) by schematic in Fig.5 - that schematic represents "switching-mode" charger. I will build LINEAR mode charger like in attachment (sorry for poor quality of PDF, I got it from Maxim's advisor). But the point is the same: even if the charge current is set to 2A, only one battery is charging for half second, then the first battery is disconnected and second battery is charging for half a second and so on. (Sorry, I am aware of very bad English in last few sentences!).

Anyway, since I have to order parts from futurlec soon (for some reason), if I understood you well, you suggest me to use FTZ951, not FTZ953 (as Maxim recommend)?
From attached schematic, I assume that the biggest load is at first (far left) transistor: according to my requirements (~600mA per battery, 1A at the most) - it must regulate current (with FB signal from chip) of 600 mA all the time; other two transistors are active 1/2 seconds and then rest for 1/2 seconds (about 50% duty-cycle) in alternate way.

Btw. thank you very much for your effort!

[amspire]

That switching between the batteries explains a lot.  It makes much more sense now. I was wondering why there was only one current sense resistor.

The 951 is slightly better then the 953 but there is not a lot between them.  I think with the lower V_ce and higher power dissipation that it will be superior to the FCX718.

Richard

[IanB]

I just read through the data sheet for the DS2711. That's a very nice IC. The algorithm looks good and it appears to handle correctly some of the things that the otherwise admirable Maha C9000 gets wrong. It's almost tempting to construct a home build charger out of it since there are few if any commercial chargers that do as good a job.