I have a nice 4-winding ISDN transformer I want to use to generate some odd supply voltages from a +10 basic supply. I need a biggish voltage to bias a PIN photodiode, and some -5 to run some analog stuff. The transformer is 1:1:2:2, so there are possibilities.
I want to drive both ends of one untapped winding, so what would work would be an oscillator, a flipflop, and a pair of rail-rail drivers, like maybe a full-bridge thingie or a dual mosfet gate driver. Current will be low, 100 mA maybe.
So, anybody know of a single, tiny chip that might do all this? (Flyback wouldn't work well with this xfmr.)
Commercial circuits would at least be designed for centre tapped transformers at these low powers.
However you could use 1 chip and an external bridge.
Make a H bridge circuit with PNP/NPN totem poles. Take a a TL494/594 which is a PWM controller with two uncomitted output transistors. Cross connect the base's of the H bridge via a resistor and an output transistor, that is the base of each PNP transistor is connected to a resistor and then to the collector of one of the 494's output transistors. The emitter of the output transistor is connected to the base of the NPN transistor on the opposite arm of the bridge.
The TL494 does have deadtime control......you are going to need it as this circuit is not fast and you will quicklt destroy small bipolars with a minimum of shootthrough ;-)
BTW, it would appear that your problem is getting different voltages rather than achieving high efficiency. Have you considered using one of those little audio amplifier chips of the sort they use in e.g. portable radios? Most of these can easily be made to self oscilate at the top of the audio scale by means of a simple feedback network. Sine wave solutions can achieve low noise without resorting to hi spec capacitors.
You could try Baxandall's class-D oscillator - if you connect the 1:1 windings as centred-tapped pair, the 10V supply gives you 31.4V peak-to-peak across each of the 2:2 windings, 62.8V if you put them both in series. You'd need an additional inductor to put between the cenetre-tap and the +10V supply, and two transistors (or MOS-FETs) to make the system oscillate. No IC's at all.
Baxandall invented the class-D oscillator to drive high voltage inverters - it sounds very much what you want.
I've referred to it here before - check Linear Technology's application notes AN-45, AN-51, AN-61, AN-65. Jim Williams does describes the inverter as a Royer inverter, but he's being a bit too inclusive.
I often design isolated power transfer circuitry and have tried audio amps. Mostly disappointing. Many of these do not like to go close to the rails, lose steam up there and occasionally become quite unorthodox (foldback, pumping etc.). And they got rather hot. In most cases I ended up using cheap motor drivers. Digikey has tons of them.
The key here is efficiency, the OP appears to have quite low power requirements and therefore would not be needing to drive them to thier limits if efficiency is not a problem. He mentioned 10V/100mA, that's
1W total output power (or 100W super real cool music power), so even at
50% efficiency it aint going to get that hot unless it is really tiny ;-)
Having said all that, your solution of using motor drivers does make more sense unless he wants low noise (negative supplies suggests that he is doing precision analog stuff).
That's a good point. With an audio amp you could actually regulate the input level into the transformer if you wanted to. If that loop makes sure it stays far enough off the rails you'd be enjoying almost the whole PSRR of that audio amp in noise isolation. There could be a thunderstorm going on on that 10V rail and this supply would remain blissfully unaware of it.
If it had to be rock bottom in cost I'd probably do this one as a blocking oscillator, in discretes.
Why not use a pair of complementary FETs, in a diagonal half-bridge arrangement. Drive the low-side N-FET as per usual, and the N-FET drain can provide the gate drive for the high-side P-FET. I do that all the time, its pretty darned cheap. All one needs to do is ensure the FETs allowable gate voltage range is sufficient for the Vcc used ;)
its a diagonal half-bridge, so the primary inductance is in the way (P-FET - top LHS switch, N-FET = bottom RHS switch, the opposite diagonals are diode)
ergo no pr0oblems at all with shoot-thru, and the P-FET gatedrive can be fairly slow and it still works fine.
I have made hundreds of thousands of these, ranging from 50kHz to 2MHz.
I think you're saying that you generate +ve and -ve supplies out of one inductor just by deciding which FET to turn off first... have I got that straight? Cute idea anyhow.
Low power MOSFET designs can get away with shoot-througth ;-) MOSFETS can handle large peak currents without the secondary breakdown problems that plagued bipolars. Add to that the fast switching speed and all you actually get is a brief spike that is limited only by circuit impedance. But as such crude design techniques are often associated with lo grade caps on the supply, then the current in the spike will not blow the MOSFET.
BTW, I am not recommending the technique, just pointing out why you can do it :-)
I'm glad you are not promoting it. The current spike would be noisy and hard to filter. It would be uncontrolled and would change with temperature, supply voltage, circuit timing, and normal part variations.
Maybe ok for cheap toy products, but it's usually a good idea to avoid generating spikes like this in instrumentation designs. Or in designs you intend to show in sed:) Regards,
The two-switch topology works nicely at high power and voltage (with different gate drive, obviously) because the stress on the transistors is low. See Billings Chapters 10/11. Pressman calls it "double-ended forward converter" topology, also called the "two transistor (or switch) forward converter" or "asymmetric forward converter".
The turns ratios may not match with the voltages John requires (although starting with a 10V supply they might, depending on what 'biggish' translates to, and if it could be series with the 10V input, which could get it into the 20V+ range using a 1 & 2 windings in series and series with the input, and still leaving a 2 winding for -5 or maybe -7V with max duty cycle ~0.4). Or perhaps -3 will be enough for the negative supply and both 2 windings can be used for the bias voltage.
Since deadtime isn't an issue, maybe an SOT-23 PIC10F, an SMT dual complementary MOSFET and a dual Shottky (BAT54?)
Best regards, Spehro Pefhany
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If that's the same thing Terry proposed, why even bother switching the top end of the load? Why not simply tie it to VCC along with a diode clamp?
There still may be a problem with a dc component. The ISDN transformer may not handle it very well when asked to deliver power.
Gawd, now I've heard everything. A programmable timer:) Will have to study that some more.
My approach would be a 74AC74 oscillator, divide-by-two and deadtime generator, 74AC00 output driver and gate, two npn's and two pnp's as the H- bridge, and 5 diodes, 6 resistors and two caps (plus bypass as needed). Lots more parts, but it would work from 5V and give the full H bridge as John asked.
74AC74 oscillator? I made an oscillator from a flipflop once, but it took a lot of parts and broke some rules.
I can do it (and probably will) with a sot-23 schmitt/rc oscillator, an sc-70 flipflop, and an so-8 dual mosfet gate driver (a little one, rated 4 amps per side), total 5 parts and about 1 cm square, plus a couple of bypasses somewhere. Oh, there's +5 available for other reasons.
If I got brave, I'd make the osc with just the schmitt gate and a resistor, no cap.
Maxim makes an oscillator-flipflop-driver, but the outputs are open-drain, and that won't work with my transformer.
What I want is to derive some regulated -5 for opamps and something like +20 to +30 to bias the photodiode. The pin photodiodes get faster as the backbias increases, partly because the capacitance goes down but mainly because the increased field sweeps carriers out faster.
So, 10 volt diff drive into a "1" winding. The other "1" winding gets bridge rectified to -8, then lin reg'd to -5. The two "2" windings are seriesed and diode rectified to +40ish, then zenered down to +28 for the pin bias, something like that.
I have quantities of the IDSN transformer, so I'd like to use it here. It's cute, about a cm cube, surface mount, under a dollar each. What I don't have is a lot of space.
Nice, ...your 50% limit is because of the diode from N-FET drain to +Vdc. Remove cathode from +Vdc and tie to a storage cap to ground. This output forms a boost circuit. Set duty to 75% and get OPs 30V boosted output. Windings on inductor/transformer will generate the other voltages needed. POC! Harry
Err.. But a simple boost configuration from 10V to 30V beats all. Set duty to 67%. Use the two N=2 winding in parallel for the primary boost inductor. Use the two N=1 winding in parallel, half wave rectify for -5VDC. Send bottle of Tequila to Harry for this design effort.
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