>1.5A boost-buck converter confusion - Page 1
Forum section: electronics / projects

author: dragon-architect
author_rank: Contributor
timestamp: August 29, 2016, 07:14:19 pm
content: Hello all! It's my first time posting here, so let's see how this goes...I've been following along with Dave's power supply design series to design my own as a learning experience, and so far I've got most of everything down on the schematic. However, I've hit a road block with the tracking switch-mode pre-regulator, and it's mainly to do with the voltage and current specifications that I've been working with for my own design:Output Voltage: 0V-20VOutput Current: 0A-2AOn the primary rail that connects the output of the pre-regulator with the linearly regulated output, I have four voltage drops:I-in sensor shunt, 0.2ohms, 400mV drop @ 2ALT3083, 310mV dropout across the PNP transistorI-out sensor shunt, 0.2ohms, 400mV drop @ 2AQLOAD, load connect power MOSFET, 540mVDSdrop calculated from RDS=0.27ohms at VGS=10VWhich gives a total VDropof 1.65VThe current draw off of the rail, since I have a few voltage dividers that each draw 100-200uA and the constant current source set to 1mA, is only 1.5mA total, so it's of little consequence.So with all that said, I'm having to design a boost-buck DC-DC converter that meets these specifications:Input voltage: 12V (I will be using the 12V rail of an old 350W ATX power supply as my main source of power; I've been designing a separate interface board to make that one work since I can use it as a general power supply for anything I want to do)Output voltage: 2V-22V (barely within the 23V absolute max limit of the LT3083)Max output current: 2.1A (100mA overhead is more than enough to cover that 1.5mA from earlier)For the moment, I've been working with the MC34063 since it seems to have the most stuff available online, and I've been using this datasheet and application note from On Semiconductor to help me out as well:http://www.onsemi.com/pub_link/Collateral/MC34063A-D.PDF(circuit examples start on page 5)http://www.onsemi.com/pub_link/Collateral/AN920-D.PDF(boost-buck circuit example starts at page 18)This is where I run into the road block. The MC34063 has an absolute current limit across pins 1 and 2 of 1.5A, and I need it to switch for up to 2.1A. Thedatasheetfor the MC34063 has example circuits on how to use external transistors for current boosting applications, but only for the boost and the buck examples. There are no sample circuits of a boost-buck hybridat all. Theapplication note, by contrast, has circuit examples for a boost-buck hybrid, butnothingabout external current boosting transistors forany of the example circuits.The attached image below is my attempt at cobbling both together, but I'm neither certain if this is even remotely correct, nor certain of how to adapt the math to this combined circuit. Forgive the squashed look, I kinda cobbled it together a hurry in a cramped part of the schematic.So I have a few questions to ask, and then some:When calculating ton/toff, Lmin, and I'pk(switch)(from AN920/D), do I just need to use the saturation voltages of the external PNP and NPN transistors and totally ignore the saturation of the internal darlington inside the MC34063, or do I also have to factor that in as well?Would y'all recommend I use darlington transistors external to the MC34063, or could I get away with just using regular BJTs with a sufficient current rating (say, 3A-5A?)Are there switch mode controllers out there that are pin-compatible with the MC34063 that have a higher current rating that I haven't found yet that wouldmassivelysimplify this endeavour?Furthermore, on page 21 of AN920/D, for the boost-buck formulae, it states that the PNP transistor Q1 is driven to saturation with a forced gain of about 20. I'm honestly at a complete loss as to how the application note came to this number. Is it a function of RBand RBEas a voltage divider across the base of the PNP transistor, or something else entirely?Seriously, any help here would be hot.

author: void_error
author_rank: Frequent Contributor
timestamp: August 29, 2016, 07:37:40 pm
content: Quote from: dragon-architect on August 29, 2016, 07:14:19 pmThe MC34063 is old, cheap and inefficient. Cheap is good if you want to use it for only a few hundred mA. I've played around with a boost converter using the 34063 a few years ago and had some problems turning the external NPN off fast enough. The solution was to reduce the value of the resistor across the BE junction of the external NPN but that also reduced efficiency.I suggest you take a look at TI's convertershereandhere. Unless you need something in a DIP package you should be fine. A lot of them come with a spreadsheet which you can use to calculate the values of the external components which is quite handy.Another topology to step the input voltage up or down is SEPIC, but you'll either need two separate inductors or coupled inductors (1:1 transformer).Also, Linear Technology should have a few regulators that suit your needs, but a lot of if not all come in packages smaller than an SO-8.

author: dragon-architect
author_rank: Contributor
timestamp: August 29, 2016, 09:14:57 pm
content: A SEPIC is probably a little bit beyond me at the moment, I'm afraid. Though I would presume from what I have found thus far that a SEPIC just needs an external PWM controller for the MOSFET switch, correct? I'm not terribly put-off by surface mount packages so long as they're not any more compact than TSSOP. This circuit already has several TSSOP packages in it since it's bloody hard to find a good ADC or DAC in a PDIP.For clarification, switch-mode DC-DC converters is an entirely uncharted territory in electronics for me, so I am effectively flying completely blind.

author: void_error
author_rank: Frequent Contributor
timestamp: August 29, 2016, 10:11:26 pm
content: A lot of SEPIC converters seem to be using a flyback/boost controller, likethis one, there's a schematic for the SEPIC configuration on page 7 of the datasheet but it's too wimpy for what you need. Thought I'd mention it since I'm going to use a few of them. It can even be made to output a negative voltage.For buck-boost converters I'd look at something with external MOSFETs for more than about 1-2A.Looking at the LT3083 datasheet it says the absolute maximum input voltage for short-circuit / overload conditions is 18V which means it might blow up with a 22V input if the output gets shorted to ground (assuming you're using a D2PAK or TO-220 package).

author: dragon-architect
author_rank: Contributor
timestamp: August 31, 2016, 07:15:34 pm
content: Hhhh... I've been trying to wrap my head around SEPICs, but they're definitely a bit beyond me at the moment.What options would you recommend for a standard switch-mode design similar to the 34063? I'm kind-of eying theTL5001Asince it appears to have almost all the same guts as the 34063, just minus the internal transistor. But it's an automotive PWM controller and I would need to use external transistors as shown by the sample buck circuit on its datasheet. Perhaps I could use external MOSFETs with this as you have suggested? Or if you have any good guides that I can look up that have good examples of boost-buck configurations and the formulae associated with them?As far as packaging goes, I'm trying to avoid anything smaller than TSSOP since I'll have to hand-solder all of this (would prefer DIP or SOIC if it can be helped).Regarding the voltage limit on the LT3083, because I didn't see that until you pointed it out and I double-checked the datasheet, would it be easier for me to search around for a different LDO with a higher voltage limit or to implement some sort of short detection circuitry (such as a differential amp and comparator across the main output and ground that can detect a voltage drop of zero and cut everything off completely?)EDITI just real-quick sketched what I think the MOSFET-flywheel configuration would be for a boost-buck using the TL5001A, based on what I already kind-of know. Is this fairly close to what I'd need to do?TIDE

author: Zero999
author_rank: Super Contributor
timestamp: August 31, 2016, 10:01:17 pm
content: Quote from: dragon-architect on August 29, 2016, 07:14:19 pmYes it does, refer to figure 13a and 13b on page 8.http://www.onsemi.com/pub_link/Collateral/MC34063A-D.PDFI'd probably go with 13b and try moving the base resistor to the emitter of the internal transistor (pin 2), as it will switch much faster.Note that the output voltage is negative, rather than positive.

author: dragon-architect
author_rank: Contributor
timestamp: September 03, 2016, 02:39:04 am
content: I'm... not entirely certain how an inverter would help me in this case. Furthermore, the three circuits provided for an inverting configuration are all boost converter circuits based on what I can see in the datasheet. So this really just leaves me evenmorelost.The first image I've attached to this post is pretty much the extent of what I've been able to find online regarding the arrangement of a boost/buck circuit, where the bases/gates of the transistors are controlled by a PWM signal that is itself varied based on feedback of the voltage detected off of VCC.And going off of that, the second picture I've attached to this post is what I was able to figure out on my own using aTexas Instruments TL5001A(which requires external transistors to pump the flywheel) and aMaxim MAX333controlled by a comparator to do the necessary logic to switch around the transistors depending upon whether or not the circuit is running in boost or buck mode.I know it's horribly over-thought and over-complicated, but this is what I've been able to figure out from what y'all've given me thus far and from what I was able to find out online (which is to say, not much). I know I'm missing an obvious solution to this, but as it is I'm getting nowhere fast.EDITSo... I think I may have found a simpler solution? It's going to take me some time to read through the data sheet and figure out how the heck it works, but it looks like I may be able to use aLinear Technologies LTC3789. Anybody got any opinions on this chip?TIDE