BJT Base drive transformer design - Page 1
Forum section: electronics / projects

author: BennVenn
author_rank: Regular Contributor
timestamp: June 16, 2015, 11:23:50 pm
content: Does anyone know of a design note regarding base drive transformers? There is a surplus of information on gate drive and the comparison between the two but I've yet to see any data on the requirements of simple base drive. Am I over thinking this? Do I just design for a current rating determined by the HFE with a voltage at the max Base-Emitter rating?Designs I've reversed all use an R-C-D network for what I assume is fast switching? And then there is the auxiliary winding on the transformer which some say drives the base harder with larger Collector-Emitter currents? True? or is it just part of the resonant start up circuit?

author: moffy
author_rank: Super Contributor
timestamp: June 16, 2015, 11:42:12 pm
content: The only place I've seen base drive of a BJT through a transformer is when the drive winding is part of the main transformer, resonant, and typically used for driving fluorescent tubes. What is your application?

author: BennVenn
author_rank: Regular Contributor
timestamp: June 16, 2015, 11:50:03 pm
content: Its pretty common in Half bridge SMPS's when BJT's are used. Just about all medium-high power ATX supplies use this topology.Seem's BJT's are a suited to this considering price, EMI etc...

author: Richard Head
author_rank: Frequent Contributor
timestamp: June 17, 2015, 09:48:38 am
content: BennVennA BJT base drive is actually a bit more involved than a MOSFET gate drive design. The reason is that you have to be careful not to exceed the RBSOA of the BJT. The HFE falls with collector current so you need to drive it harder as the current increases. By driving it hard at low current just increases the switching/storage time so should be avoided. A small transformer placed in the emmiter lead (configured as a CT) will increase the base drive in proportion to the emmiter current and yield better switching performance. It's called a proportional base drive.A resistor in series with the base normally has a small capacitor placed across it. This charges up under forward biased conditions and reverse biases the base when the drive is pulled to ground, speeding up the turn-off. For high speed high current switching you have to be really careful to not exceed the RBSOA and even then the long term reliability of the device is questionable. MOSFET's solved all (most) of these problems.

author: Richard Head
author_rank: Frequent Contributor
timestamp: June 17, 2015, 01:30:50 pm
content: Emitter not emmiter! Sorry

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 17, 2015, 02:07:46 pm
content: FYI, AFAIK: RBSOA is a recombination and avalanche phenomenon -- in essence, while the transistor is turned on, the C-B depletion region is shorted through by charge carriers. When turning off, the charge carriers clear the middle of the junction first, spreading outwards. Thus, the insulating depletion region starts conductive, then becomes an insulator with gradually more thickness. Which means: if you apply too much voltage, too soon, during turn-off, you get what looks like avalanche breakdown, but at a lower voltage than Vceo.The double downside is, avalanche creates more free charge carriers, which thus take more time to dissipate. So the current takes longer to decay to zero, even once the off-state voltage has been reached. All the while, switching losses get huge, and pretty soon, psst...BANG. Hence, you want to stay inside the SOA.Usual solution is to use a larger transistor (2-4 times rating), fast drive, and some sort of snubbing (namely dV/dt snubbing to assist RBSOA).BTW, the same phenomenon occurs for PN junction diodes as well. This isn't usually encountered because reverse recovery proceeds in an orderly fashion, most of the time; but circuits with sharp recovery diodes* and excessive loop inductance (regenerative snubbers come to mind) can force it, causing excessive diode dissipation, switching losses, EMI and so on.*Most diodes these days are "soft recovery", meaning they stop conducting somewhat gradually. They can still be excited in such a way that they recover abruptly. Diodes optimized for this purpose are called drift step recovery (sometimes Grekhov) diodes, and are capable of generating high voltage pulses (>100V into 50 ohms) with rise times of < 100ps.A recent example, I was testing a quasi-resonant snubber on a flyback converter (namely, to absorb the spike due to transformer leakage inductance). Using a lossy powdered iron choke, the performance was mediocre, but functional. Using a high-Q coil improved performance somewhat, but cooked the diode -- without coil losses to dissipate recovery energy, the diode took it all, probably in part due to dynamic avalanche (the above effect) and regular avalanche.Tim

author: BennVenn
author_rank: Regular Contributor
timestamp: June 17, 2015, 10:40:11 pm
content: Thanks Richard and Tim,That explains the extra winding on the drive transformer, drive the base harder with increased emitter current and not so hard when not required. It would also double as a start up oscillator by driving the core into saturation then reversing the flux.I've noticed the transistors are typically rated at 12A even though worst case current draw would rarely exceed 4A, and larger supplies stack 4 in parallel. Paralleling would reduce the emitter current and allow a lower base drive current - and faster turn off?Is there a linear relationship between base current required at the higher emitter currents? Or do I go off the Hfe/I graphs and design around that.Or should I abandon the BJT idea and stick with MOSFETs?We're moving into a rural property later this year. There is an existing 12v 1000Ah Lead acid installation which is charged via 400watts of solar. This is usually OK becuase the place was used as a weekender and the pannels top the cells up in a few days. There are a few large generators but all provide 240v which is great for welding, machining etc but not so good for topping up the batteries.I'll be rebuilding the whole setup to run at 48v with custom inverters and chargers. I've already built an array of 48v-340v converters each tested at 800watts continuous. Power distribution will be at this 340dc and pure sine converters will provide the standard 240ac to the house.Right now I'm looking at building a 48v 100a charger, most likely split into 500w-1000w modules to make full use of the generators (also considering rectifying the output and sending it to the 340dc bus). I enjoy designing and building so the extra $$$'s spent in my opinion is worth it. If it all goes to custard I'll modify a few 24v truck alternators to bulk charge the batteries.I'm happy designing and winding transformers, calculating snubbers, PWM feedback etc... Just interested why most half-bridge designs use BJT's and not MOSFETs. If it is just cost, I'll go with the MOSFETs

author: Richard Head
author_rank: Frequent Contributor
timestamp: June 18, 2015, 06:16:16 am
content: BennVennI should also mention that BJT's suffer from second breakdown. It's basically a very high local current density on the die that eventually stuffs the device. MOSFETs don't have this problem at all.In fact, MOSFETs have basically replaced BJTs in all but the most cost sensitive equipment. A larger die area is required for a MOSFET than a BJT (all things being equal) and that makes them more expensive.Another advantage of MOSFETs is that they have an inherent anti-parallel diode that directs the magnetising/leakage energy back to the bus. Very useful for bridge converters.Also, under short circuit conditions the tardyness of BJTs due to storage time means that you can have huge current overshoots because they don't turn off when you tell them to!

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 18, 2015, 06:21:43 am
content: Quote from: Richard Head on June 18, 2015, 06:16:16 am1. Second breakdown is only a slow phenomenon. It depends on thermal time constants (>> 1ms). BJTs and IGBTs have full SOA for time scales around switching (<< 10us).2. MOSFETs do it too.Tim

author: NiHaoMike
author_rank: Super Contributor
timestamp: June 18, 2015, 06:23:57 am
content: Quote from: BennVenn on June 16, 2015, 11:50:03 pmI haven't seen high voltage bipolars in any decent PC PSU made in the last 10 years. Most use MOSFETs, with a few using fast IGBTs. Next up looks to be HEMTs.

author: BennVenn
author_rank: Regular Contributor
timestamp: June 19, 2015, 01:48:15 am
content: I've built of a few circuits to play around with ZVS and standard PWM driven half bridge. The ZVS was more efficient overall with the only heat generated being in the core itself (N87, ETD49 wound to 0.15T ). The PWM half bridge was switched with IRFP450's (These were free from a friend) and the gate driver alone consumed 200ma@60khz. I'll work more with the ZVSI've been reading app notes on quasi-resonant ZVS SMPS's and it seems output is regulated by raising the switching frequency above resonance. In the case of a Compact Fluoro Ballast which uses ZVS, the frequency is set by a base drive toroid and the resonant LC circuit has no great effect on switching frequency. Am I to tune the output transformer/capacitor to resonance with the driver? Or keep its resonant frequency above switching frequency?Is pulse by pulse regulation a bad idea with a ZVS SMPS?I'll ultimately go with an off the shelf Quasi-resonant ZVS IC, I'd just like to see some of the effects of a miss-tuned system on my scope before I get stuck into some high power stuff. Being able to spot and correct fault conditions on a low power system seems invaluable.

author: Richard Head
author_rank: Frequent Contributor
timestamp: June 19, 2015, 06:16:26 am
content: BennVennThe quasi resonant topologies (such as phase shifted bridge) use resonant transitions and PWM. Fully resonant topologies control the output power by varying the switching frequency.The LLC topology operates above resonance and moves toward resonance to increase output power. If correctly designed it should never quite reach the resonant point at full power and low line.If it's allowed to operate below resonance then the switch transitions cease to be resonant and become lossy.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 19, 2015, 09:41:54 am
content: If you want a flat out oscillator, you can make a ZVS or ZCS.CFLs and Royer oscillators (the traditional MOSFET "ZVS") are examples of this.They always run at whatever power level they do. The load match determines power output.CFLs start with a very heavy load (the low resistance of cold filaments), then transition into a nominal load (the plasma voltage drop, damping the onboard series resonant LC circuit). The very high voltages and currents experienced during startup are very stressful on the transistors and resonant capacitor (which inevitably is too small, so it self-heals some on every startup, and eventually wears through, leading to failure, if nothing else burns out first). As the capacitor value shrinks with wear, resonant frequency rises, and power output and efficiency drop.I'd call it a wonder of modern industrial process control, that you're essentially equally likely to see a CFL fail due to any of the main causes: filament open (oscillator doesn't even start up, no resonant current flow), resonant cap failure (self-heals one too many times, burns up), transistor failure (usually aggravated by increasing switching losses and high operating temperature), or good old fashioned electrolytic expiration (high operating temperature).The Royer also has no control method, usually being varied by supply voltage (which obviously doesn't work well at low voltages).Attempting to build a digital Royer is futile, because the negative resistance analog startup behavior is necessary to get a correct frequency lock (assuming the load network is simple, anyway). As soon as you start looking into fast risetime gate drivers, you suddenly have to adopt the entire PLL-VCO-driver-feedback system, which is typical of most high-Q resonant drivers (induction heaters, the better Tesla coil drivers, etc.).Tim