Common base PNP where collector and emitter currents differ significantly? - Page 1
Forum section: electronics / projects

author: ocset
author_rank: Super Contributor
timestamp: April 23, 2018, 07:42:35 pm
content: ¬¬Hello,In the attached 6W Buckboost LED driver schematic, the high-side current sense is referred down to ground via the common base connected PNP (FMMT560Q).We are worried about situations in which the collector and emitter currents of the PNP would start to differ significantly. (eg, at low [eg sub 1mA] collector currents, or at high temperature, where emitter-base leakage current may make collector and emitter currents differ.)Can you confirm, in this schematic, that collector and emitter currents are always likely to remain close in magnitude?FMMT560Q datasheethttps://www.diodes.com/assets/Datasheets/FMMT560Q.pdfLTspice simulation also attached.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 23, 2018, 07:50:02 pm
content: More or less by definition, the ratio (alpha) gets bad around saturation.Tim

author: jmelson
author_rank: Super Contributor
timestamp: April 23, 2018, 09:19:31 pm
content: Somebody has been smoking too much <something>. That circuit is INSANE! two op-amps, a switching power supply controller, several assorted transistors, 4 voltage sources, and on and on, for 6 W?There are single chips that will do this very nicely, possibly with an additional resistor or two to set the parameters.Jon

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 24, 2018, 04:19:27 am
content: Quote from: jmelson on April 23, 2018, 09:19:31 pmYou must be new here, look at his other posts tooTim

author: ocset
author_rank: Super Contributor
timestamp: April 24, 2018, 04:53:30 am
content: QuoteThanks, i should have said, Vin is from 18v to 137vdc....where it must work at 6wBut it must also work at 1.2w when vin = 8-14vdc.And must work at 3.6w when vin = 14-18vdc.QuoteThanks, yes , i can see i have forgotten to add a base resistor, as in page 12, figure 3 of the following datasheet....http://www.analog.com/en/products/power-management/led-driver-ic/buck-boost-led-drivers/lt3756.html

author: BrianHG
author_rank: Super Contributor
timestamp: April 24, 2018, 07:15:28 am
content: Try just using a lower voltage LED string, IE leds with some in parallel instead of just all serial, with either 1 buck converter (lower efficiency at high voltages) or 2 buck converters, with the higher one switching straight on when the voltage gets too low, better efficiency at high voltage.

author: Zero999
author_rank: Super Contributor
timestamp: April 24, 2018, 08:04:01 am
content: Quote from: treez on April 24, 2018, 04:53:30 amYou should have stated the input voltage range in the first post.I've noticed, you've posted quite a few questions relating to an 18V to 137V LED driver. It's certainly possible. I've seen commercial switched mode power supplies which can work off this kind of voltage range. For example, the Rigol DS1054 oscilloscope, works from 36VDC to 400VDC, range of 1:111/9even though it's only designed for 100VAC to 250VAC. Perhaps you can get the schematic for the PSU in the Rigol DS1054 and work from that?

author: ocset
author_rank: Super Contributor
timestamp: April 24, 2018, 11:22:29 am
content: I mean, the PNP in the schematic of the top post cannot saturate because its Base-Collector junction cannot get forward biased in that schematic and setup.It seems weird though that such a simple method of referring a sense signal down from the high side is not more widely used? I mean, its just basically a cheap PNP transistor as shown in the top post. It makes you search for some hidden gremlin in this PNP method.

author: Zero999
author_rank: Super Contributor
timestamp: April 24, 2018, 01:36:12 pm
content: Quote from: treez on April 24, 2018, 11:22:29 amYes, the transistor can't saturate.This seems odd: I'd expect the beta to be higher at higher temperatures and lower currents, down to a point of course.

author: ocset
author_rank: Super Contributor
timestamp: April 24, 2018, 07:24:46 pm
content: Thanks yes, i wonder how low the collector current has to go before the Hfe starts getting really low?

author: jbb
author_rank: Super Contributor
timestamp: April 25, 2018, 01:21:06 am
content: Oh treez, you don't disappoint.At risk of threadjacking, how about using a flyback converter to drive the LEDs? It'll probably be a little less efficient, but you get some advantages:use of turns ratio to optimise the number of LEDsbuck-boost voltage gain characteristic to match the LEDs to the wide input voltagethe LED current shunt resistor can be referenced to 0Va tertiary winding can be added to make some bias voltage (remembering that the LED string voltage is sort of constant)you can get one flyback controller chip to do most of the work. I would suggest you consider some kind of over voltage protection circuit to prevent destruction if an LED goes open circuitassuming constant frequency and DCM operation, setting a peak primary current can give you a well-behaved input current limitAlternatively, there are some options for pampas in the sensing system:LT makes some differential amplifiers which have pretty good CMRR and are rated for 250V common mode inputLT and others also make current sense amplifiers which do the same thing as your U1 and Q2, but might be a bit smaller