BJT+FET in one package? - Page 1
Forum section: electronics / projects

author: stmdude
author_rank: Frequent Contributor
timestamp: September 04, 2017, 03:41:26 pm
content: So, I need to high-side switch a 5V/1A load from a 3.0V MCU, with a reasonably low RDSon (because, heat).Now, my first inclination is to slap an P-Channel FET on it and call it day, but the pull-up resistor from 5V to the gate is way out out-of-spec for the MCUs pin.So, I'm guessing I need to go with a NPN + P-Channel in a kind of darlington configuration, or a N-Channel FET + P-Channel FET in a similar arrangement. No big deal, except for the size and cost (we'll be making a few of these).Now, this doesn't seem like an uncommon problem to have. Are there chips that have both transistors needed in one package? Or, even better, an array of these..Any ideas?

author: Zero999
author_rank: Super Contributor
timestamp: September 04, 2017, 03:52:00 pm
content: Quote from: stmdude on September 04, 2017, 03:41:26 pmIt sounds like you trying to describe an IGBT but I'm not sure how that will help you.How much current does the load need? It sounds like you just need a logic level P-MOSFET.

author: stmdude
author_rank: Frequent Contributor
timestamp: September 04, 2017, 04:45:45 pm
content: Quote from: Hero999 on September 04, 2017, 03:52:00 pmIt needs 1A continous (worst case) at 5V. If I use a logic-level P-MOSFET, the pull-up to 5V (pullup between G and S to make sure it turns on fully) is outside of the specs for the MCU.I think the IGBT was the type of chip I was thinking of, but having a BJT switching the load doesn't seem optimal (high "RDSon"). I thought it was two FETs or a BJT driving a FET. I guess my memory isn't what it was.

author: stmdude
author_rank: Frequent Contributor
timestamp: September 04, 2017, 04:48:55 pm
content: Quote from: blueskull on September 04, 2017, 04:37:14 pmThat looks awfully close to what I need. Better yet, it revealed a good search on Digikey "TRANS PNP P-CH", which yielded a few alternatives. Excellent! Thanks a bunch!

author: wraper
author_rank: Unknown
timestamp: September 04, 2017, 04:50:43 pm
content: Quote from: Hero999 on September 04, 2017, 03:52:00 pmhe is describing exactly opposite to IGBT.

author: Zero999
author_rank: Super Contributor
timestamp: September 04, 2017, 05:08:58 pm
content: Quote from: wraper on September 04, 2017, 04:50:43 pmYes, I see now. He wants a bipolar transistor to control a MOSFET.Quote from: stmdude on September 04, 2017, 04:45:45 pmOh, I missed the 3V MCU part.Quote from: stmdude on September 04, 2017, 04:45:45 pmThat part has an N-channel MOSFET and PNP BJT. You'd have to use the BJT to switch the load. The MOSFET could be used to switch the BJT's base current.

author: stmdude
author_rank: Frequent Contributor
timestamp: September 04, 2017, 05:13:26 pm
content: Quote from: Hero999 on September 04, 2017, 05:08:58 pmYep. I think I'll go with the kind of chips suggested by blueskull. They should give me exactly what I need. Thanks for the refresher about IGBTs though!

author: Leo Bodnar
author_rank: Frequent Contributor
timestamp: September 04, 2017, 05:36:09 pm
content: These are about £1 eachhttp://rohmfs.rohm.com/en/products/databook/datasheet/ic/power/power_switch/bd2041afj-e.pdfLeo

author: ovnr
author_rank: Frequent Contributor
timestamp: September 04, 2017, 06:00:50 pm
content: Just buy a load switch.This one is $0.33, does 1.2 amps, and solves all your issues. And there are tons of others available.

author: aandrew
author_rank: Frequent Contributor
timestamp: September 04, 2017, 06:37:19 pm
content: Perhaps I'm missing something.I've driven this exact kind of scenario many times using combo P-N FET devices (something like NTZD3155C); you drive the N-FET with your MCU, and it in turn pulls down the P-FET gate.Is there a compelling reason to use a BJT+P-FET over this?

author: Cerebus
author_rank: Super Contributor
timestamp: September 04, 2017, 08:27:16 pm
content: Quote from: stmdude on September 04, 2017, 04:45:45 pmEh? A pullup between gate and source on a P channel device is going to turn itoffand that pullup shouldn't have to be so low that it'll present a significant load to an MCU unless you're asking it to switch quickly compared to the gate charge.Did you just mis-speak, or have I missed something here, or are you confused about how to achieve what you're aiming for?

author: Cerebus
author_rank: Super Contributor
timestamp: September 04, 2017, 09:21:08 pm
content: Quote from: evb149 on September 04, 2017, 08:41:20 pmAh, 3V - that explains whatImissed. Still doesn't account for the off/on dichotomy, which I hope is just mis-speaking.

author: jbb
author_rank: Super Contributor
timestamp: September 05, 2017, 12:15:23 am
content: I would prefer using N-Ch MOS as a driver because has basically zero gate current when on. An NPN BJT will need a bit of base current (100 uA?) which could be relevant for power consumption.Also, some 3V3 processors like STM32, LPC etc. have 5V tolerant IO pins which might be suitable for direct drive (open drain + pull-up resistor). But read the data sheet carefully...

author: Cerebus
author_rank: Super Contributor
timestamp: September 05, 2017, 01:32:31 am
content: Quote from: jbb on September 05, 2017, 12:15:23 amThat estimate is way off base. A typical small signal NPN BJT is going to have a current gain ofat least50, so a static base current of 100 uA would produce a collector current of at least 5 mA, but we're not looking at switching 5 mA, we're looking at switching enough to drag a, say 47k, pullup down from 5V to, say 0.3V, so we're talking about a collector current of 100 uA and a static base current of 2 uA (i.e. 50 times smaller than your estimate) for a grand total of 102 uA consumption versus 100 uA for a MOSFET.No sleight intended, but errors of that magnitude suggest that you might be well served by going back and studying some of the basics again - these are figures you really ought to be able to pull out of the air before you can reasonably make a decision as to what's the preferable component there.MOSFET input capacitance will be about 3x what the BJT would - 8 pF and 22 pF respectively for the MMBT3904 small signal BJT and 2N7000 small signal MOSFET.It's left as an exercise for the student to calculate what effect the capacitance would have on dynamic current consumption and switching speed.Whether the BJT or MOSFET wins on overall power consumption is going to depend on the answer.

author: Zero999
author_rank: Super Contributor
timestamp: September 05, 2017, 07:20:52 pm
content: What's wrong with the usual NPN+PNP BJT solution?Fair enough, the BC327 isn't good for an amp but it can be replaced with another transistor which is. The ZXTP25012EZ would be perfect. Change R1 to 47R, to give around 50mA base current and you're sorted.https://www.diodes.com/assets/Datasheets/ZXTP25012EZ.pdf