TIP122 replacement part

So, an IC I'm working with recommends use of a TIP122 as an external Darlington for rapid variable current control of a solenoid - however, the TIP122 is only available in a TO-220 package, but I require a surface mount part for my board design.

I'm never really done anything with normal BJT transistors, only MOSFETs for PWM control, so I'm not too sure how to classify BJT's.

I've done a Farnell parametric search for a replacement to the TIP122 using the following parameters;

NPN
80V min collector emitter voltage
5A min collector current

...and consequently have found two devices in a D2Pak package, a package that I am already using, good. Looking at ear datasheets, these devices seem very similar to the TIP122, can anybody see any issues with these devices?

http://www.fairchildsemi.com/ds/FJ/FJB102.pdf

No issues.
You are looking for a replacement for the TIP122. But you should look at the solenoid, and see what you need.

What is "rapid variable current control" ?
A power mosfet is more rapid.
Is variable indicating PWM, or analog varying ?

Analog varying as opposed to PWM, yes. Peak and hold current operation of an injector, specifically. Whilst I suppose a FET could technically be used, operating in its linear region there'd be rather a lot of heat to deal with.

Many thanks for the input re replacement.

jtw11:
So, an IC I'm working with recommends use of a TIP122 as an external Darlington for rapid variable current control of a solenoid - however, the TIP122 is only available in a TO-220 package, but I require a surface mount part for my board design.

http://www.adafruit.com/datasheets/TIP120.pdf

I'm never really done anything with normal BJT transistors, only MOSFETs for PWM control, so I'm not too sure how to classify BJT's.

I've done a Farnell parametric search for a replacement to the TIP122 using the following parameters;

NPN
80V min collector emitter voltage
5A min collector current

...and consequently have found two devices in a D2Pak package, a package that I am already using, good. Looking at ear datasheets, these devices seem very similar to the TIP122, can anybody see any issues with these devices?

Intelligent Power and Sensing Technologies | onsemi
http://www.onsemi.com/pub_link/Collateral/BUB323Z-D.PDF

Is the TO-220 size your problem, or it is a through-hole problem?

I've SMT mounted TIP-120 transistors simply by removing the center lead and bending the base and emitter leads into little "feet" (same shape as an SMT TO-220) and soldered it right to the board.

(side view of what I did):

tab pins
| |
** **
**__|
|--_**

Hope you can make out that sketch! :slight_smile:

The only thing I'd be concerned about is power dissipation. The TO-220 package is usually used because the part can get hot and needs the area to dissipate heat.

AverageGuy:
The only thing I'd be concerned about is power dissipation. The TO-220 package is usually used because the part can get hot and needs the area to dissipate heat.

That's why I surface mounted mine - to a large pad of copper - a heatsink! :slight_smile:

It's a through hole issue far more than a space issue - the TIP122 would be my only through hole component other than an LMD18200 which is mounted in an area where through hole is not an issue.

It would however be nice to have a smaller footprint than an SMD TO 220 would give, but I will look at both options. I have already bent the legs out on a DIP8 package that isn't available as a SMD part, sovim happy to do modifications to existing packages.

On the subject of heat, how does one calculate heat dissipation in a transistor, the datasheets don't seem to quote any values of resistance like a FET datasheet would.

Darlingtons such as TIP122 are all but obsolete. Use a mosfet instead. Choose one with a low enough Rds(on) and it will run much cooler than a TIP121 would.

MOSFETS are used elsewhere in my design for switching applications, this is for a current varying application.

Here's the IC I'm using that the datasheet calls for a Darlington - http://www.ti.com/lit/ds/symlink/lm1949.pdf

I'd rather implement the circuit using a MOSFET as opposed to a Darlington, but I'm not sure this IC supports that?

It looks to me that the IC would work using a logic-level mosfet along with a resistor of about 470 ohms or 1K between the gate and ground to limit the gain. But if you want to play safe and use a darlington, how about http://www.farnell.com/datasheets/39975.pdf.

When you say work with a FET, I assume you're referring to operating the IC in the switching mode it supports? Or still analog varying? (There must be a better term than 'analog varying')

I'd rather not use the switching mode due to RFI if possible, given the proximity to other circuitry.

I'll have a play this week with a Darlington vs a FET and upload some scope shots etc when I'm done.

Under any circumstances the heat dissipated is exactly the same, Mosfet or BJT as the device is operating in it's linear range

So, an IC I'm working with recommends use of a TIP122 as an external Darlington for rapid variable current control of a solenoid - however, the TIP122 is only available in a TO-220 package, but I require a surface mount part for my board design.

The current isn't a "Variable" it is a minimum value required to hold the injector activated after it has been activated, the current for activation and the "Holding" current are two different fixed values. AS to the TIP122... it is a darlington in one package but it can easily be constructed from 2 discrete transistors. The LMD18200 is an interesting package. It's an 'H' Bridge motor driver IC capable of 6A peak current and you want it to drive 5A driver transistors (TIP122's) I think it's time to post the circuit too. I make the assumption between pull in and holding currents because you make no mention of monitoring or adjusting the solenoid current dynamically (No apparent feedback)
Choosing a transistor is trivial, making it work is quite another thing. For example the TIP122 has a 2V min Vcesat @ 3A Ic (page 2 "On Characteristics", Collector Emitter Saturation Voltage.
This is only the saturated voltage and one of the reasons why a mosfet wont work here. If the designer had wanted a low saturation switch it would have been easier to use a n NPN-PNP switch pair,
that the designer chose that transistor indicated there was a reason and although possible I doubt that the cost of the fet's was an issue at all.
Post the circuit or at least one injector driver completely from control input to solenoid coil. The other thing that hasn't been covered here is the solenoid itself, Its DC R, Inductance in both open and closed positions (the Values ARE Very different).

Bob

LMD18200.pdf (209 KB)

TIP`120-127.pdf (258 KB)

When you say work with a FET, I assume you're referring to operating the IC in the switching mode it supports. I'd rather not use the switching mode due to RFI.

I'll have a play this week with a Darlington vs a FET and upload some scope shots etc when I'm done.

The RFI is a function of the energy being switched and the time in which it is being switched.
Besides the usual diodes and snubbers the only really efficient method of controlling RFI and EMI is to control the rise time of the pulse.
The faster the pulse rises to it's final value the more electrical noise it will make in doing so.

Bob

The H bridge has nothing to do with this part of the board design whatsoever, I was just referring to it as being the other through hole component of the board I'm working on.

The circuit I'm using is Figure 1 from the LM1949 datasheet, the typical application & test diagram. Current sensing is performed by the LM1949 itself.

In terms of injector resistance, a range of 12-18 Ohm is what I'm working with - but as for the two different values of inductance, currently these are unknown to me - although, a typical high impedance injector is around the 6 to 8mH mark in its closed state.

When you say that the current is not variable, I don't think I follow you? Varying current is the whole point of peak and hold operation, where a large current is used to initially open the injector, whilst the lower hold current simply holds it open and along with this comes the benefit of faster open/close times.

Certainly the '1949 is not the newest of ICs, maybe that's the reason behind the choice of driving a BJT as opposed to a FET - I'm not too sure how common/developed MOSFETs were around the time the LM1949 s made. However, it's the only (certainly that I know of) peak and hold driver IC.

The RFI is a function of the energy being switched and the time in which it is being switched.
Besides the usual diodes and snubbers the only really efficient method of controlling RFI and EMI is to control the rise time of the pulse.
The faster the pulse rises to it's final value the more electrical noise it will make in doing so.

So really, dealing with the heat generated in the transistors is the solution here - or simply switch the injectors on or off completely using a very low on resistance FET.... But then that defeats the point of having a board able to switch high and low z injectors in both saturated and P&H fashions.

Then again, saturated drive isn't the biggest issue in the world - one just loses a little correlation between pulse width and fuel delivery.

jtw11:
When you say work with a FET, I assume you're referring to operating the IC in the switching mode it supports? Or still analog varying? (There must be a better term than 'analog varying')

No, I meant in the linear mode, same as the Darlington.

Ah, I see. In which case, Docedison's statement rings true again - there will still be plenty of heat to deal with. Maybe losing peak and hold capability is the way to go...

Swings and roundabouts :smiley:

Thanks for all input thus far