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Which PNP for PNP turn-off circuit?

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Advanced Member level 5
Jun 13, 2021
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I was told by a Consultant level SMPS designer, (who was at the Marconi research place) many years ago, that the “Zetex” range of PNP BJT’s were excellent for use in PNP turn off circuits for high side FETs in LLC converters. This was, he said, because they have high hfe at high Ic.

Eg ZTX549 and FMMT718 and ZXTP2012A

ZXTP2012A datasheet:

ZTX549 datasheet:

FMMT718 datasheet:

..However, they have a slow turn on time of >270ns.

PNPs with much less gain at high Ic are significantly faster than this, eg 2N4403….

2N4403 datasheet:

When i opened up an offline 750W Half Bridge SMPS, it was using 2N2907's fpr PNP turn off.....
2N2907 datasheet

So which do you say is best for use in a PNP turn off circuit?
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Turn on time is near instantaneous for a BJT if you supply enough base current.

it is current that counts in a turn off ckt, usually the xtor has plenty of time to recover at near zero current before the ON pulse requires it to be off ...

ZTX749 is a good xtor for gate turn off
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For the Zetex 549 - it appears the switching times have been reversed in the data sheet, turn off time will always be longer for this sort of pnp

compare to the other 2.
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it is current that counts in a turn off ckt, usually the xtor has plenty of time to recover at near zero current before the ON pulse requires it to be off ...
Thanks, though is the following relevant to that at all...? an LLC converter, the PNP has only the LLC dead time to recover, which could be just 300ns

Which of the timing parameters is most relevant to the turn-off useage?
Storage time, Delay time, rise time, fall time, time to ON (tON), time to OFF (tOFF)

Which of the below PNP’s would be best as a PNP turn off txtor?
Its for use in a Half Bridge LLC of 2kW from 390Vin.
Also, which package would be most likely to be good for a PNP turn-off transistor?

1.......ZXTP25020 (SOT23, hfe=150 at 1A, Ic=4.5A, fT = 285MHz, storage time = 168ns, rise time = 49ns)

2........ZXTP2008 (SOT89, hfe=100 at 1A,Ic=5.5A, fT=100MHz, ton=43ns, toff=230ns)

3........FZT790A (SOT223, hfe = 200 at 1A, ton=30ns, toff=600ns, Ic=3A/6Apk, fT=100MHz)

4.........2N2906 (TO18, hfe=20 at 500mA,tOFF = 100ns, ton=45ns, fT =200MHz

5............FMMT718 (SOT23, hfe=150 at 2A, Ic=1.5A/6Apk, tOFF = 270ns, ton=68ns)

6.......ZTX749 (TO92, Ic=2A/6Apk, hfe=100 at 1A, ton=40ns, toff=450ns, fT 100MHz,)

7.......PBSS4032PT (SOT23, Ic=2A, ton=70ns, toff=185ns,hfe=150 at 1A, fT=160MHz)
[PBSS4032PT is used in Infineon LLC App Note]

8.........NSS20200L (SOT23, Ic=4A, tstorage=300ns, hfe=180 at 1A
[recomended in onsemi LLC app note]

Also, Page 17 of this LLC App Note….

…shows an LLC using an LM5100A Bootstrap high side driver IC…..this driver IC has a 3A pull down capability….and yet still they also use a PNP turn-OFF txtor. Why do they do that?......the PNP will simply slow up the turn-OFF in this case….due to its storage time and rise and fall time.
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Also, between dec 2021 and march 2022 i posted here about a 3v9 series zener getting used across the BE of a PNP turn off in a sucessful product (800W offline Half Bridge ).......this was presumably to speed up the turn off of the PNP (ie to facilitate it turning off faster) , but i wasnt sure?
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Also…. Concerning “PNP turn OFF”…

One of the main reasons for it is to prevent spurious turn-ON of a FET when the opposite FET in the leg turns ON….but this could never be a problem in the LLC…since there is no Miller discharge of a FET when it turns ON, because its Vds is already zero at the turn ON point….so no chance of such spurious switch ON…so why are Infineon using PNP turn off in their LLC app note?

You can expect ZVS operation in regular LLC operation but not necessarily during startup and other transient operation states. It's not enough to prevent shoot-through during "fair weather" conditions. Thus we use essentially the same gate drivers for LLC and hard switching bridges.
Do you think that this gate driver needs a few Ohms at R9 to reduce pulse current in the BE junction of PNP Q1?
Jpeg and LTspice attached


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