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I was looking for the datasheet for a transistor I'd discovered during some reverse engineering, a DK52T. Unfortunately I can't find its datasheet, but I found one for the DK52D. See the image below; this is some kind of weird combination NPN-PNP transistor I've never seen. I thought it might be something called a Sziklai Pair, but it doesn't fit that configuration either. How is this thing supposed to work?

It would seem to me the moment the base of the NPN goes high, the collector goes low, which in turn pulls the PNP base low, whereby it goes into conduction and turns the NPN base off. Basically the moment the transistor is turned on it shuts itself down immediately, WTF?

Is more specific information on the DK52T available (which should be very much similar to the datasheet I've attached)?

Enter image description here

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These transistors are devices which are highly optimized for use in ultra-low-cost and high-volume consumer products- off-line fluorescent lamp ballasts.

The anti-saturation network (for fast switching) and free-wheeling diode are integrated onto one die, minimizing the component count and silicon area required (the diagram shows two parallel 'fingers' of the composite power transistor):

enter image description here

The lateral PNP transistor structure prevents saturation and results in less Vce(sat) on the device compared to a typical multi-diode Baker clamp configuration. It does this by shunting away base current from the NPN when Vce drops below a volt or so. Note that the lateral PNP structure has a Veb breakdown voltage of at least 700V so it's not like any discrete PNP transistor (closest perhaps to a high-voltage PNP transistor operated in inverse mode).

More in this 1990s-era application note from Motorola (now Onsemi). In many datasheets they avoid showing the detail and just show a network within the part:

enter image description here

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    \$\begingroup\$ I can't help but wonder why they couldn't just use a schottky diode baker clamp--are these just made with a process that can't do schottkys? \$\endgroup\$
    – Hearth
    Sep 24 at 1:35
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    \$\begingroup\$ @Hearth I think a 700V Schottky would have a too-high Vf- we seldom see Schottky diodes of >100V for that reason. Also, according to patent US4390890A (IBM), there is additional process complexity to incorporate Schottky junctions. \$\endgroup\$ Sep 24 at 1:58
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    \$\begingroup\$ Ah, I missed the voltage rating. Though, since I work for a manufacturer doing silicon carbide stuff, I see schottkys of well beyond that all the time--though you certainly won't get that combined with a silicon process, and you definitely won't get that cheaply. \$\endgroup\$
    – Hearth
    Sep 24 at 2:39
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    \$\begingroup\$ Of course, it would also defeat the purpose since SiC schottkys have a forward voltage of over a volt, anyway, too high for a single-diode baker clamp. \$\endgroup\$
    – Hearth
    Sep 24 at 2:41
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The transistor boasts "high switching speed" it gets this by having the lower transistor turn on and steal base current when the main transistor approaches saturation.

Thus it is a type of Baker Clamp

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    \$\begingroup\$ There is no dot so no junction of the two bases. \$\endgroup\$
    – winny
    Sep 23 at 15:03
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    \$\begingroup\$ Unless one's silicon and one's germanium, the PNP won't kick in until the collector voltage goes below the emitter voltage. Saturation at 0.2V won't trigger it. \$\endgroup\$ Sep 23 at 19:25
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    \$\begingroup\$ @CristobolPolychronopolis The PNP presumably has a heavily doped emitter, while the NPN must have a very lightly doped collector to have the large breakdown voltage. The difference in forward voltage is likely to be substantial. \$\endgroup\$
    – John Doty
    Sep 23 at 20:29
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That is a strange part. At first I thought it was an integrated Sziklai pair, a type of Darlington with a lower saturation voltage. But your part is different.

When the collector goes below the emitter, the first transistor will turn on and short the 2nd transistor's base to its emitter, completing its turn off much faster by sucking out its base charge.


In the 60s and 70s, when PNP power transistors were expensive and sucked, a Sziklai pair was a way to use an NPN power transistor for both the pull up and pull down sides of a complementary output stage, called a quasi-complementary stage: true Darlington pulling up, Sziklai pulling down.

I don't think I've ever seen a datasheet for an integrated device; all of my experience is with discrete component equivalents.

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Revised after some thoughts, "saturation clamping" makes more sense. The way how the PNP works in the circuitry (clamping) is effectively what Baker Clamp is for. Thus, the manufacture's drawing seems agreeable.

It would seem to me the moment the base of the NPN goes high, the collector goes low, which in turn pulls the PNP base low, whereby it goes into conduction and turns the NPN base off. Basically the moment the transistor is turned on it shuts itself down immediately,

The collector goes low, but only as much as Vbe - Vce on PNP base-emitter. Thus the PNP does not turn on.

The purpose of PNP is to prevent the NPN conducting when the device is reverse biased (reverse-active mode); Ve > Vc & Vb > Vc. This is the case when two of the devices are back to back connected (antiparallel/inverseparallel) for AC applications.
PNP splits NPN base current when the NPN gets saturated (thus the Vce decreases).

Baker clamp:

The Baker clamp limits the voltage difference between emitter and collector by diverting base current through the collector. This introduces a nonlinear negative feedback into a common-emitter stage (BJT switch), with the purpose to avoid saturation by decreasing the gain near the saturation point. While the transistor is in active mode and it is far away enough from the saturation point, the negative feedback is turned off and the gain is maximal; when the transistor approaches the saturation point, the negative feedback gradually turns on, and the gain quickly drops. To decrease the gain, the transistor acts as a shunt regulator with regard to its own base–emitter junction: it diverts a part of the base current to ground by connecting a voltage-stable element in parallel to the base–emitter junction.


Edited multiple times,

Example of two devices back to back, conceptual, ignore the components value:
[Q1, Q2, D1] = device #1, [Q3, Q4, D3] = device #2, 10V = drive signal,V1 = line, R3 = load
Device #1: Q1 is on, Q2 does not turn on, since Vbe = 0.5V
Device #2: Q3 does not turn on, since Q4 turns on and clamps Q3 Vbc = lower than reverse-active threshold.

schematic

simulate this circuit – Schematic created using CircuitLab

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    \$\begingroup\$ The NPN might not conduct when reverse biased, but the diode will. This part might not be ideal for the rectifier app, unless there's something I'm missing? \$\endgroup\$ Sep 23 at 16:17
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    \$\begingroup\$ @CristobolPolychronopolis, right, the device is not a rectifier. \$\endgroup\$
    – jay
    Sep 23 at 17:19
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    \$\begingroup\$ Can you provide any example datasheets containing this schematic? \$\endgroup\$ Sep 24 at 0:25
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    \$\begingroup\$ @MicroservicesOnDDD , the schematic is to illustrate two of OP's "strange" TR. Ignore the component values. I do not know any examples exist, sorry. If you have a specific applications, I may try to come up with a circuitry. \$\endgroup\$
    – jay
    Sep 24 at 1:23

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