small inverter driver chip

If I get nA (and sometimes pA) leakages, almost none of my boards work, so clean is a given. But I don't know how predictable the frequency would be if it depended on device capacitance for the "c". That's too much risk to zap one 0603 cap, I guess.

Buying the SO-8 dual mosfet driver reduces parts count more! Circuit "design" is, more and more, a matter of just buying the right chip.

I did find my 1310 nm single-mode laser, $36 in small qtys. These guys seem very nice...

John

Yup, got it, thanks.

A circuit like yours is shown here (p19) but with two NFETs instead: . Without really thinking it through, it looks like you'd need to be careful with the gate drive of the upper FET though.

In your design, an input voltage spike could kill the PFET. Do you clamp the PFET gate to V+ with a zener?

Clifford Heath.

exactly

not really. as long as the bottom FET is well driven, it doesnt matter too much what the top FET does. lousy gatedrive simply limits maximum duty cycle, which you can compensate for with turns ratio.

the P-FET gate is clamped to (+Vdc + Vf) max by the N-FETs commutating diode, and -Vf min by the N-FETs D-S diode.

Yes if +Vdc is too high you can break the FET, but if you are going to use +12V/+15V then the FET rated Vgmax will be +/-30V anyway, and as JL showed you really have to work at it to overvoltage a FET gate. The +Vdc supply rail ought to have some reasonable amount of capacitance, which bounds the spike energy/clamps the spike voltage - if you have 100uF on a 15Vdc bus and dont want it to go above 30V, then dE = 50uF*(30V^2 -

Cheers Terry

:)

I'm too stingy to buy those cute little single-gate things, but you use plenty of them. I like it.

that'd be a no-no for me. I sit these things right on top of nasty, nasty stuff. I tend to use pullups no bigger than 10k, and capacitively load every input as much as it will tolerate. Along with solid 0V planes, etc.

Hi John,

do you have a link to the transformer datasheet?

Cheers Terry

We're using the SMJ-140, but they have lots of other ratios...

These are made in India, so take about 4 weeks to get. Cost is about $1 at 1K pieces. They are very nice, no quality problems so far. They have a nice high inductance, low dcr, low leakage inductance, so are good for tiny inverters in the low watts range. And there are other sources for the same parts, just in case. See pic in abse.

I'm thinking the L is sorta high for most flyback apps.

I never though that ISDN made much sense, but the transformers are great.

John

John, I think that you need some power supply help, just how much power is required on the +30V and -5V rails? That ISDN transformer is large enough for maybe 30 watts! The primary inductance is so large that it cannot be used as an inductor in a switcher. Idc is 5mA and 2R0 windings, so what is that....150mW? 30mH and 100pF is self resonant at 91KHz so higher frequency is out of the question. Winding AC resistance is probably >20R0. Best bet, short out the primary windings and use the leakage inductance in a boost switcher. Let's have those power requirements and redesign with low cost inductors. I bet Terry must of gasp when he saw that transformer spec. When I see 30mH, I am thinking of >100 volts. Lots of turns for wasted inductance. Harry

For the design I am currently working on, I have gone and re-arranged the relevant DCM flyback equations, and now choose Bmax and solve for Fsmps given Psmsp. Then I crank up Psmps to include necessary tolerances, and voila.

but 30mH is a bit high to do much of anything with for a flyback, unless the voltage is very high and the frequency is very low.

I'd use that puppy as a crude dc-dc, but slap a PCMC loop around it (which is essentially free) to ensure startup is nicely controlled. Or use naff all output C, such that dTj is acceptable for your switching transistor(s).

Cheers Terry

John,

you may want to look at the MC34151 (MC33151) dual MOSFET driver. One of them can oscillate while the other could drive your coil. There was an application note 15 years ago on something similar (just voltage doubler, you will need only the oscillator of it), if you cannot locate it let me know and I'll dig my implementation of it for you. I guess with this biasing voltage for a PIN diode you are doing something which I might see as competitor stuff, but then my help is only minute and aged enough :-).

Dimiter

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John Lark> Hi,

In article , John Larkin wrote: [....]

Just a thought:

If the transformer is made into a series tuned circuit it could be driven single ended and still make as high of a voltage. This assumes you get a "Q" of at leat 2.

If the drive comes from a PWM controller chip, you could get regulation by moving the duty cycle away from 50-50. There are quite a few controller chips that have a 50-50 upper limit on the duty cycle. You may be able to make something that regulates this way.

He didnt gasp, he banged a few numkbers into an equation and went "Hmm,

Alas the voltage rating isnt quite enough for me. I'll stick to kool-mu toroids, so I can get my 6kV isolation.

Cheers Terry

Yes, I recall asking for help at the start of this thread. I need an oscillator, a flipflop, and dual 10-volt totem-pole driver, and I was wondering if I missed something cute out there. Apparently not.

Maybe if you water cooled it.

Funny, somebody else already made that comment. Me.

If you're hung up on "flyback", yes. But there *are* other topologies.

We already use these transformers in two products...

with a couple more in the works. They work great. The classic push-pull square-wave inverter is quiet, efficient, and has decent regulation and an inherently low output impedance, even run open-loop, as these are.

$1 is low cost here. And we have reels of these xfmrs in stock.

Whoops, the timer just dinged. My custard bread pudding is done.

John

What is this thing people have for flybacks? Nasty, noisy, badly regulated things!

John

Not really, if you know how to make them. I have no problem powering

are all flyback convertors. Obviously I am not recommending you to do it, if you have not done it yet. I am only setting the record straight, flyback convertors can be used and have been used in very low noise circuits.

Dimiter

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John Lark> >

I like them for several reasons. With a diagonal half-bridge I get fairly good open-loop regulation from no load to full load, with fairly good efficiency, and can do that across an evil isolation barrier (think

Secondly, if I use a sufficiently high voltage secondary diode, I can stand off a worst-case fault (full dc bus volts across gate drive) without snotting the primary circuitry.

and of course I can operate continuously into a dead short, no worries. easy to detect also.

I'm designing a 250W unity power factor universal input flyback at the moment, along with a 20W flyback that runs from 100Vdc to 1400Vdc.

yeah they can be noisy, but only if you dont know what you are doing. not so good for low voltage high power, unless you crank up the frequency enough to use ceramic caps.

Cheers Terry

SG3525

Half-bridge on the drive side? That's not a flyback.

John

Of course I've done it, and posted HV flyback designs to this NG before your time. The push-pull square-wave converter has a nice smooth input current flow, close to 100% output conduction (hardly needs filtering), is quiet, and has load regulation so and in my app needs no feedback to the driver. In a multiple-channel config (like the pic I posted to abse) you only need a single simple complememtary square-wave generator as the common gate drive for all channels, and each channel only needs a pair of 2N7002's (6 cents total) to drive a ct'd primary... and no channel-channel sync problems! Adding series gate resistors softens up the edges - nice trapezoids - and greatly reduces spikes, somethning that's not as nicely doable in a flyback.

Really, lots of people seem to have an unreasonable affection for flybacks.

John

Yikes, I'd clean forgotten about that one... haven't used that in, well, a long time. I can probably tolerate the small voltage loss in the bipolar output stage. Thanks.

John

What could you do with these?