Half Bridge Bipolar Transistor PSU – Base Drive Transformer Design Data? - Page 1
Forum section: electronics / projects

author: Chris56000
author_rank: Frequent Contributor
timestamp: January 25, 2018, 09:12:56 pm
content: HI!I've got in hot-water at work trying to repair a 200W Half–Bridge Bipolar PSU which is dead due to a faulty base drive transformer assembly!This transformer is wound on two TN14/9/5 Ferrite Ring cores colour– coded pink for 4A11 grade (initial AL= 700 approx)– the top core consists of a single T.I.W. wûinding that goes to the main control PCB, whilst the bottom core has a CT secondary of about 47T with three base driving windings over the top!Can anyone point me in the direction of design documents, schematics or literatures that goes into detail of how this type of transformer is designed and wound please? Also, I need an example showing typical numbers of turns (or the turns ratios) and a sketch (if possible) showing how to phase the windings – is the phasing (marked with dots in textbooks, etc) critical for the circuit to start and run?I can't find anything other than stupid fancy chip circuits for MOSFETs or winding details on main convertor power transformers, which is NOT what I'm looking for!There is also a 2u2 capacitor connected from the centre–tap of the rectified H.T., thence via an inverted–U shaped wire passing up thro' the base drive transformer, thence to the primary of the main power transformer, and finally to the mid–point of the output stage.Is this how the circuit is brought into operation? There is NO separate "Housekeeping" supply fitted to the unit – only four small diodes used as a bridge rec. feeding the SG3524 control chip from the top core winding!Chris Williams

author: chris_leyson
author_rank: Super Contributor
timestamp: January 25, 2018, 10:06:17 pm
content: Switched Mode Power Supply Handbook 3rd ed Billings and Morey. Chapter 16 Proportional Base Drive Circuits for Bipolar Transistors might give a few clues. A toroidal core and proportional base drive were used in the TEK 7000 series power supplies, self oscillating resonant converter half bridge.QuoteWhy three base drive windings ? Probably not saturating Royer or Jensen converter as that would use 3R1 grade.

author: Paul Price
author_rank: Super Contributor
timestamp: January 25, 2018, 10:12:20 pm
content: There is no free lunch. Some sort of aux. P/S must be used to startup.Most ATX PC P/S use a separate transformer that runs off the rectified AC mains. A resistor is used to trickle-charge a small (4.7uF approx) capacitor to a voltage that will reliably start a one-transistor blocking oscillator that powers the main control chip using a small startup transformer.It only takes a few cycles of this bootstrap circuit to start up the P/S and the an aux winding on the big main SMPS transformer takes over the drive power for the control IC circuitMost often the startup oscillator fails because the startup osc. resistor (usually around 100k-200k) that connects to the rectified HV AC opens up.Can you scribble a schematic and post a pic of the P/S PCB ?.

author: xavier60
author_rank: Super Contributor
timestamp: January 25, 2018, 10:24:05 pm
content: The driver transformer is likely to function the same as older PC power supplies did. The single turn loop will be a current feedback winding. Once one of the power transistors has been turned on, output transformer primary current flowing through the current winding keeps the transistor turned on. The drive is terminated by one of the transistors on the driver transformer's primary being turned on by the control IC. These drive control transistors actually alternately short circuit the drive transformer's primary windings.Theses older PC power supplies had no standby supply. They would self start due to a small DC bias applied to the bases of the power transistors which would cause brief self oscillation, then the control IC would take control.

author: xavier60
author_rank: Super Contributor
timestamp: January 25, 2018, 11:07:11 pm
content: Quote from: Chris56000 on January 25, 2018, 09:12:56 pmWhat has gone wrong with the drive transformer?

author: David Hess
author_rank: Super Contributor
timestamp: January 26, 2018, 04:05:40 am
content: Motorola published a lot of application notes discussing bipolar off-line power supply design in detail.What exactly failed? Did a hammer get dropped on the cores? If you know the turns and have the cores then they should not be difficult to rewind.

author: NiHaoMike
author_rank: Super Contributor
timestamp: January 26, 2018, 04:14:48 am
content: What voltages does it output? It might make more sense to replace the whole thing with a more efficient modern unit.

author: Chris56000
author_rank: Frequent Contributor
timestamp: January 26, 2018, 08:31:25 am
content: Hi!It's a. Custom–made OEM unit giving odd voltage outputs, +12v, -18V,, +33V and +7V! The unit is built on two separate pcbs, one being the mains one that carries this transformer and the other is the control PCB with all the main power transformer control chip etc., on it!The insulation had flaked off the original windings causing short–circuited turns by the wayI've got a copy of Keith Billing:s book but I couldn't find base transformer details in it!I'll post some photos later – I got only a very rough estimate of the base drive windings – about four turns each – the wire was only about 170mm long!Chris Williams

author: xavier60
author_rank: Super Contributor
timestamp: January 26, 2018, 08:46:27 am
content: Quote from: Chris56000 on January 26, 2018, 08:31:25 amTry to work out exactly how many turns are in the base windings. It is very important. With that few turns, they should have been easy to count.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 26, 2018, 10:58:09 pm
content: There are different designs used with BJTs, so an exact answer depends upon the circuit in question.This should serve as a useful example:The FT37-43 (actually a Fair-Rite 5943000201) ferrite bead is a small NiZn part, somewhere around, uh, Ae = 10mm^2 and Bmax = 0.3T I think. (Ed: actually more like 7 mm^2, and 0.2T due to remenance and unipolar drive.)That means, for a winding of 60 turns, we can get a maximum flux of (10 mm^2/t) * (0.3 uVs/mm^2) * (60t) = 180 uVs on the primary. From a 12V supply, that's a square pulse 15 us long (although in practice, both voltage and duration will be shorter due to the series 51 ohm supply resistor).When the primary switch turns on, the output transistor is forced on. With a 4:1 ratio, and about 1V Vbe(on), the primary actually only drops about 4V (not 12V, hence the 51 ohm resistor serves as current limiting, to about 150mA).When the output transistor turns on and load current is drawn, this current also flows through a tertiary winding on the drive transformer. The 1:5 ratio between load current and base winding gives a forced hFE = 5 condition. This is positive feedback, so the transistor remains switched on, even if the primary side transistor turns off.In a typical ATX computer supply (half bridge configuration), the base transformer is driven in shorting mode commutation with a pair of transistors, so that the base voltage can be shunted to zero, forcing the output transistors off.In this example, turn-off is provided by the drive transformer saturating. The transformer has some inductance still (it's not a super high permeability core), which acts to shunt more and more drive current (increasing the forced hFE) as time goes on. Eventually, two things can happen: 1., the transformer saturates, and its current draw goes way up, shunting base voltage off; or 2. forced hFE rises above the transistor's hFE(sat), the transistor comes out of saturation (while still carrying full load current -- this method costs more turn-off switching loss), load current commutates to the clamp diode (UF5404), and with no current in the tertiary winding, the transistor turns off. Because the inductance is still reasonably high and the transistor has no shortage of hFE under this load, the first case occurs here. (The second case is very common in blocking oscillators, like the "joule thief" circuit, where a non-saturating inductor is used.)Transformer core calculations are all about ratios, so it's pretty easy stuff. Follow the units (dimensional analysis).As for transformer winding design, where you place the wires matters -- the further apart any two wires / windings are, the higher the leakage inductance. Probably not a big issue for a small drive transformer, but this can be critical for large power transformers, especially at low impedances.Tim

author: Chris56000
author_rank: Frequent Contributor
timestamp: January 27, 2018, 04:24:28 pm
content: Hi!Some pictures of the PSU I've had aggro with!Chris Williams