PNP BJT Transister - Inverter  - Page 1
Forum section: electronics / beginners

author: Docara
author_rank: Regular Contributor
timestamp: December 14, 2021, 10:37:08 am
content: Hello everyone,I really struggle with transistors.I am need to drive an active high Automotive High Side switch with an open drain (1-wire 28Vmax, 25mA max) Switch.I realise I would need a PNP transistor and have come up with the following (ideal) circuit from a circuit simulation web site (link below) which gives a nominal 2mA current when conducting.https://www.falstad.com/circuit/I don't fully understand the datasheet terms and picking a suitable tranny, all the ones I find have a VBC of 5V - I need one a lot higher and the fact there are negative voltage around has pushed me over the edge.I would be great full for some help in choosing a suitable SMD device and feedback on the circuitAdditionally, would a FET of some sort be better for low power applications - if so how what could be a suitable circuit and values.ThanksEdited to add a closed switch image to show values

author: Ian.M
author_rank: Super Contributor
timestamp: December 14, 2021, 11:01:52 am
content: Quote from: Docara on December 14, 2021, 10:37:08 amPart numbers and datasheet links please.

author: Docara
author_rank: Regular Contributor
timestamp: December 14, 2021, 11:25:36 am
content: Hi IanHere is the part number of the automotive switch ( there are others but they are all from the same family and depends on what I can get - supply issues obvs)BTS5020_1EKAhttps://www.infineon.com/cms/en/product/power/smart-low-side-high-side-switches/high-side-switches/profet-plus-12v-automotive-smart-high-side-switch/bts5020-1eka/Its not to the datasheet directly but a quick rundown of parameters with a link to the datasheet below itI've been trying to find a switch on current value but couldn't so have assumed it it purely voltage deviceDS2413Phttps://www.maximintegrated.com/en/products/interface/controllers-expanders/DS2413.htmlAgain its a quick reference page with an application circuit and there's a link to the actual datasheet

author: Ian.M
author_rank: Super Contributor
timestamp: December 14, 2021, 11:44:51 am
content: The BTS5020_1EKA has logic level control inputs that can be driven from 3.3V or 5V logic. Applying 14V to its control inputs would destroy it.A 2K2 pullup to logic Vcc on the DS2413P output(s) to get a logic level signal + a 4K7 series resistor between each DS2413P output and the BTS5020_1EKA control input its driving should be sufficient. Any inversion required can be handled in your application firmware. The 4K7 resistor is for protection of the DS2413P during fault conditions - see 'Figure 52 Application Diagram with BTS5020-1EKA' in its datasheet.

author: Terry Bites
author_rank: Super Contributor
timestamp: December 14, 2021, 01:06:17 pm
content: A Mosfet can be used but make sure its Vds is more than double your battery volts.

author: Docara
author_rank: Regular Contributor
timestamp: December 16, 2021, 06:37:27 pm
content: Hi IanThank you for the reply, however I'm afraid I've dropped a big hairy one - and, embarrassingly, you've just found it for me!!!Ages ago during original idea phase, I had accumulated quite a list of reasonable devices sorted for suitable for current and uC/DS2413 switching - anyway due to supply issues I checked to see which was available nowish and deleted the ones that will be available next century however I've deleted the wrong - list I've deleted the Vin = Vsupply list not the 3.3-5V logic one, and I can't find the deleted fileSo, apart from my cock up posting here, I'm not going to be using logic level devices, so my original question still stands.Thanks again

author: Ian.M
author_rank: Super Contributor
timestamp: December 16, 2021, 07:16:43 pm
content: So what exactly *are* you going to be using? If supply chain issues preclude specifying a particular part, please specify limiting values and an example part that meets them.Note that a DS2413P doesn't have sufficient output voltage rating for reliability in an automotive application if its used to directly level shift up to the +Vbatt supply!If however you can guarantee the supply transients *NEVER* exceed +28V or -0.7V, you can drive a power P-MOSFET direct from the DS2413P, with a 10K pullup to the +14V rail, and either a 1K series gate resistor for MOSFETs with Vgs_max >+/-28V, or a resistor in series with the DS2413P output to form a potential divider to limit the gate drive swing for MOSFETs with a lower Vgs_max rating.If you cant guarantee the +14V rail is free of objectionable extreme transients, you'll need to either cascode the DS2413P output with a transistor with adequate voltage rating, or use it to control an inverting driver with adequate voltage rating, and possibly diode clamp its output as well. A 'jellybean' NPN with Vceo >120V and a 2K2 pullup from its base to the 5V logic supply should do the job, either connected in cascode, with its emitter to the DS2413, or in grounded emitter, with its base pulled down by the DS2413 to turn it off. Note that both configurations limit the voltage 'seen' by the DS2413 output to 5V or less, so are also suitable for direct drive from a 5V MCU pin. Its possible to also use a MOSFET cascode, but selection of the MOSFET is far more critical, and it only saves one resistor and a couple of mA from the logic supply.