Complementary ThermalTrak Power Transistors

On Semi introduced in 2006 a series of complementary power transistors, called ThermalTrak. These come in five-lead TO-264 packages and use the extra two pins for an integrated, isolated temperature-sensing diode. The transistors are reasonable parts, with 230 to 350V ratings, and with 30MHz fT. There are four sets of parts in the series, with pretty detailed datasheets, except, hah, little information about the diode. For example, what's the current ratio for the same "Vbe" drop?

There was an ON Semi press release in Jan 2006, and an EETimes article in Oct

2005.
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The article simply replicates a portion of ON Semi's Feb 2005 application note (both were written by Mark Busier): .

Most class-AB amplifier biasing circuits use a temperature-sensing transistor, so an adjustable Vbe ratio may be created for settable biasing.

In this note they use six output transistors (three each in parallel for push and pull), giving them six sensing diodes to wire in series. They substitute this set of diodes in place of the usual trimmed multiplied Vbe, to develop about 3.4 volts to drive two emitter-follower stages plus the output transistors. They have no provision at all for bias adjustment.

The driving transistors include one each MPSA06 and MPSA56, which are common TO-

92 parts, and which surely are not part of the heat-sink thermal-tracking system. The whole thing leaves one feeling like a great opportunity has somehow been missed.
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    - Win
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Winfield Hill
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** With complementary emitter-follower stages, this transistor must be mounted on the main heatsink along with driver an output devices.

Accurate compensation of the bias current setting can only be expected when all relevant BJT chip temps fall to match the heatsink during a silent time after period of high power output.

The goal of having on chip diodes is compensation tracks the chip temp rather than heatsink temp.

I am sure you well understand this, but you did NOT write it down.

A nice example of academic privilege, I assume.

** Why ever not? They need to be.

** The app note is basically an appalling POS, pure marketing hype.

FYI:

Sanken made 5 pin devices with on chip diodes that were power darlingtons.

Part numbers SAP15N/SAP15P plus SAP16N/SAP16P.

At least the diode temps tracked the chips temps of all the B-E junctions involved.

.... Phil

Reply to
Phil Allison

[Hey, I'm not an academic, I'm an instrument designer at a research lab, a different beast entirely. Before that I ran my 35-employee company for 17 years. But whatever.]

I disagree. Counting the driver transistors, there are six devices in series, biased from the roughly 3.4 volts. Two of the six have compensating diodes, so compensation for the others, 2/3 of the total, must be intrinsically via the heat sink; that's what they claim NOT to be doing.

Yes, and the design, if analyzed, looks pretty bad. For example, consider biasing of the VAS transistors. Can you tell from calculations what the currents are?

Plus the intrinsic pulldown current can't match the pullup current, without an input-stage unbalance.

Yes, and their current STD01 and 02 series.

Sigh, another example of possibilities squandered, IMHO. These are Darlington, so each should have two diodes, but they have one diode in the NPN and five diodes (?!) in the PNP, what's up with that? Once again we're forced to rely on temperature tracking through heat sink and insulators.

--
 Thanks, 
    - Win
Reply to
Winfield Hill

Well, of course you can turn off the output stage for a few microseconds once in awhile, and measure the V_BE drops with a PIC. ;)

I do that sometimes with the monitor photodiodes of diode lasers--if you forward bias them enough that the photocurrent isn't important, you have a nice small temperature sensor right their on the same stud as the laser. The loop bandwidth can be as much as a few hertz, which really helps reject external forcing.

Cheers

Phil Hobbs

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Dr Philip C D Hobbs 
Principal Consultant 
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Phil Hobbs

Or a flawed audiophool concept? Wonder how many they sold? Mouser shows them unavailable and obsolete. ...Jim Thompson

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| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
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Reply to
Jim Thompson

Fig 15 of the data sheet shows the I-V curves. Of something.

Yes, the dual emitter-follower drivers add a lot of uncertainty, and some tempco problems, to the circuit.

Why buy exotic sole-source 5-pin transistors, when mosfets are so cheap? Digikey wants $5 each for those NJL3281 things. It's apparently not monolithic, so the temperature sensing is likely mediocre.

One trick that I have used in this sort of amp is to use rather large emitter resistors with a few tenths of a volt across them. That bias is easy to stabilize. Then put a power diode across each emitter resistor, so large signal swings can blast through to the load.

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John Larkin         Highland Technology, Inc 
picosecond timing   laser drivers and controllers 
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John Larkin

** No you don't. We are saying the same thing.

** How academic of you, did you even read Sanken's app note?

The NPN device has one PN diode and the PNP has 5 *Schottkys*.

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At 2.5mA, the 5 diode string reads 1.45V drop.

I have a couple in my bins, used in repairs to "dB Technologies" amplifier modules fitted to their Opera series powered speakers.

Unlike the On Semi parts, the Sanken 5 pin devices work as advertised, in fact without the on-chip diodes complementary Darlington output stages have woeful bias stability.

... Phil

Reply to
Phil Allison

?? E.g., NJL3281, 2074 in stock, $3.69 ea qty 100, complement NJL1302, 1085 in stock, $3.69 ea qty 100.

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 Thanks, 
    - Win
Reply to
Winfield Hill

Thanks very, much, I didn't have that app note. Schottky! who would have thought.

It appears you have to go to the Japanese-language site to find it, and to see evidence of the part's availability.

--
 Thanks, 
    - Win
Reply to
Winfield Hill

The part I looked up...

...Jim Thompson

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| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
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Jim Thompson

** Typing " Sanken SAP15p pdf " into Google gets you many hits on the same app note.

The devices are no longer made, but you see small quantities advertised on Ebay etc.

... Phil

Reply to
Phil Allison

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