Which is the best PNP turn-OFF  transistor? - Page 1
Forum section: electronics / general

author: Faringdon
author_rank: Super Contributor
timestamp: June 03, 2022, 08:11:43 pm
content: 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)https://www.diodes.com/assets/Datasheets/ZXTP25020CFF.pdf2........ZXTP2008 (SOT89, hfe=100 at 1A,Ic=5.5A, fT=100MHz, ton=43ns, toff=230ns)https://www.diodes.com/assets/Datasheets/ZXTP2008Z.pdf3........FZT790A (SOT223, hfe = 200 at 1A, ton=30ns, toff=600ns, Ic=3A/6Apk, fT=100MHz)https://www.diodes.com/assets/Datasheets/FZT790A.pdf4.........2N2906 (TO18, hfe=20 at 500mA,tOFF = 100ns, ton=45ns, fT =200MHzhttps://www.mouser.com/datasheet/2/68/2n2906-49966.pdf5............FMMT718 (SOT23, hfe=150 at 2A, Ic=1.5A/6Apk, tOFF = 270ns, ton=68ns)https://www.mouser.com/datasheet/2/115/FMMT718-68167.pdf6.......ZTX749 (TO92, Ic=2A/6Apk, hfe=100 at 1A, ton=40ns, toff=450ns, fT 100MHz,)https://www.diodes.com/assets/Datasheets/ZTX749.pdf7.......PBSS4032PT (SOT23, Ic=2A, ton=70ns, toff=185ns,hfe=150 at 1A, fT=160MHz)https://assets.nexperia.com/documents/data-sheet/PBSS4032PT.pdf[PBSS4032PT is used in Infineon LLC App Note]8.........NSS20200L (SOT23, Ic=4A, tstorage=300ns, hfe=180 at 1Ahttps://www.onsemi.com/pdf/datasheet/nss20200l-d.pdf[recomended in onsemi LLC app note]---____---_____---__Which of the PNP 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)

author: Zero999
author_rank: Super Contributor
timestamp: June 03, 2022, 08:56:28 pm
content: What do you mean by PNP turn-off transistor?Are you asking which transistor will turn off the fastest? The one with the shortest storage time, which is also dependant on how far it's driven into saturation.How is it going to be used in the Half Bridge converter? Presumably it's a MOSFET or IGBT gate driver. Those transistors are all too small to handle that kind of current and voltage.

author: Faringdon
author_rank: Super Contributor
timestamp: June 03, 2022, 09:49:24 pm
content: Thanks, you can see the PNP turn off here.....Page 17 of this LLC App Note….https://www.infineon.com/dgdl/Infin...N.pdf?fileId=db3a30433acf32c9013ad11cddde01b6It uses the PBSS4032 PNP.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?

author: David Hess
author_rank: Super Contributor
timestamp: June 04, 2022, 02:41:49 pm
content: Quote from: Faringdon on June 03, 2022, 08:11:43 pmAll of those parts except the 2N2906 are ring emitter types which sustain higher current gain and frequency at high current, but I think even the 2N2906 would work. Also consider using the 2N4403 as a higher power and current and gain alternative to the 2N2906.QuoteRise time and time on are the most relevant. Storage time, fall time, and off time are irrelevant in this application where the transistor has plenty of time to recover.The largest limit will be power dissipation of the transistor so I would look for a higher power part, or at least avoid the lowest power parts.

author: Zero999
author_rank: Super Contributor
timestamp: June 04, 2022, 04:15:42 pm
content: The PNP transistors in that schematic are emitter followers and don't saturate, so it storage time shouldn't be a problem.

author: Faringdon
author_rank: Super Contributor
timestamp: June 04, 2022, 08:01:25 pm
content: QuoteThanks, AYK, the PNP must hold the fet off as soon as its base is taken low, so that the opposite fet in the leg turning on doesnt turn on the fet....Do you mean its stopped carrying current long before its "asked" to turn back off again, so storage time etc dont matter?

author: David Hess
author_rank: Super Contributor
timestamp: June 05, 2022, 02:16:10 pm
content: Quote from: Faringdon on June 04, 2022, 08:01:25 pmStorage time does not matter because the PNP transistor is never saturated. By the time the MOSFET is turned back on, the PNP transistor is basically already off so turn-off time does not matter either.I have seen 2N2906 class transistors used for exactly this application in old designs and there was never a problem. The only difference in your application is that you are driving bigger MOSFETs, so there is more capacitance and the PNP transistor needs to handle more power but it is not a lot more.

author: Faringdon
author_rank: Super Contributor
timestamp: June 06, 2022, 10:37:42 am
content: Do you prefer thru hole PNP turn off with e-line PNP right next to the fet?......or smd one further away on a daughter board?

author: Zero999
author_rank: Super Contributor
timestamp: June 06, 2022, 01:00:11 pm
content: Quote from: Faringdon on June 06, 2022, 10:37:42 amWhat's that got to do with anything? Through hole or surface mount will work. The latter has lower parasitics and doesn't need any holes to be drilled in he board. The gate driver should be close to the MOSFET to minimise the effects of inductance and capacitance in the wiring.

author: Faringdon
author_rank: Super Contributor
timestamp: June 06, 2022, 05:46:04 pm
content: Thanks, yes i agree it should be close to the mosfet...but then we find to do that we need to have little drive PCBs next to the fets if doing it with SMD........which is adding cost for us.....because we'd rather lump the gate driver stuff on a more general PCB that does other stuff aswell.......AYK, SMD can not be placed on a high power main board....only thru hole...so it really looks like a thru hole PNP is best for a pnp_turn_OFF.... but then we have lead inductance.....to be honest, i dont remember seeing any high power boards with a thru hole PNP turn off cct near the fet(s).