Forums

isolated DC/DC converter

Started by John Larkin November 3, 2011
This

http://www.panoramio.com/photo/61564837

might work for powering a thing I'm doing. I can use a standard cheap
Coiltronix dual-coil inductor as the transformer. The complementary
emitter followers will have no shoot-through and can have controllably
slow switching edges, since they will just follow the base drive.

Anybody got ideas for the base driver device? Ideally it would be
self-oscillating, set with some R-C; have a moderate slew rate; swing
to the rails. I'm thinking roughly 150 KHz maybe, a few watts output.

Maybe an LM8261 opamp? I'd have to see if it winds up when it rails.
It might not.

Some sort of fet gate driver would be OK, but few go to 24 volts.

Something discrete maybe, like a 2N7002 to 24-, and maybe a
bootstrapped pullup?


John




Given base charge storage, if the driver rise/fall time is fast, the 
design could have shoot through.

On 3 Nov., 04:51, John Larkin
<jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
> This > > http://www.panoramio.com/photo/61564837 > > might work for powering a thing I'm doing. I can use a standard cheap > Coiltronix dual-coil inductor as the transformer. The complementary > emitter followers will have no shoot-through and can have controllably > slow switching edges, since they will just follow the base drive. > > Anybody got ideas for the base driver device? Ideally it would be > self-oscillating, set with some R-C; have a moderate slew rate; swing > to the rails. I'm thinking roughly 150 KHz maybe, a few watts output. > > Maybe an LM8261 opamp? I'd have to see if it winds up when it rails. > It might not. > > Some sort of fet gate driver would be OK, but few go to 24 volts. > > Something discrete maybe, like a 2N7002 to 24-, and maybe a > bootstrapped pullup? > > John
ir2153x and a dual fet instead? but then I guess you might as well pick a standard converter ic -Lasse
On Wed, 02 Nov 2011 23:45:15 -0700, miso <miso@sushi.com> wrote:

>Given base charge storage, if the driver rise/fall time is fast, the >design could have shoot through.
...and given magnetizing energy, emitters will be pulled above and below the rails. Shottkys can discourage this. The driver still may not be close enough to the rails to prevent base current and conduction in the switch, CE, to the opposite rails. Consider using a driver or controller intended for the function, with schottkys. RL
On 11/3/2011 11:51 AM, legg wrote:
> On Wed, 02 Nov 2011 23:45:15 -0700, miso<miso@sushi.com> wrote: > >> Given base charge storage, if the driver rise/fall time is fast, the >> design could have shoot through. > > ...and given magnetizing energy, emitters will be pulled above and > below the rails.
But only by one Vbe above or below.
On Thu, 03 Nov 2011 11:51:16 -0500, legg <legg@nospam.magma.ca> wrote:

>On Wed, 02 Nov 2011 23:45:15 -0700, miso <miso@sushi.com> wrote: > >>Given base charge storage, if the driver rise/fall time is fast, the >>design could have shoot through. > >...and given magnetizing energy, emitters will be pulled above and >below the rails. Shottkys can discourage this. The driver still may >not be close enough to the rails to prevent base current and >conduction in the switch, CE, to the opposite rails. > >Consider using a driver or controller intended for the function, with >schottkys. > >RL
I want to avoid custom magnetics, and I want it to be cheap, in case we build a lot of them. I'd like to trade off efficiency for switching noise, which this topology allows. There will be a 12-bit, 250 Ms/s ADC a couple of inches away. And besides, I'm a circuit designer. John
John Larkin wrote:
> This > > http://www.panoramio.com/photo/61564837 > > might work for powering a thing I'm doing. I can use a standard cheap > Coiltronix dual-coil inductor as the transformer. The complementary > emitter followers will have no shoot-through and can have controllably > slow switching edges, since they will just follow the base drive. > > Anybody got ideas for the base driver device? Ideally it would be > self-oscillating, set with some R-C; have a moderate slew rate; swing > to the rails. I'm thinking roughly 150 KHz maybe, a few watts output. > > Maybe an LM8261 opamp? I'd have to see if it winds up when it rails. > It might not. > > Some sort of fet gate driver would be OK, but few go to 24 volts. > > Something discrete maybe, like a 2N7002 to 24-, and maybe a > bootstrapped pullup? >
If 370kHz fixed is ok maybe these? http://www.aosmd.com/res/data_sheets/AOZ1210AI.pdf Around 50c in quantities. Problem is that your two transistors then become unemployed :-) -- Regards, Joerg http://www.analogconsultants.com/
On Thu, 3 Nov 2011 06:05:13 -0700 (PDT), "langwadt@fonz.dk"
<langwadt@fonz.dk> wrote:

>On 3 Nov., 04:51, John Larkin ><jjlar...@highNOTlandTHIStechnologyPART.com> wrote: >> This >> >> http://www.panoramio.com/photo/61564837 >> >> might work for powering a thing I'm doing. I can use a standard cheap >> Coiltronix dual-coil inductor as the transformer. The complementary >> emitter followers will have no shoot-through and can have controllably >> slow switching edges, since they will just follow the base drive. >> >> Anybody got ideas for the base driver device? Ideally it would be >> self-oscillating, set with some R-C; have a moderate slew rate; swing >> to the rails. I'm thinking roughly 150 KHz maybe, a few watts output. >> >> Maybe an LM8261 opamp? I'd have to see if it winds up when it rails. >> It might not. >> >> Some sort of fet gate driver would be OK, but few go to 24 volts. >> >> Something discrete maybe, like a 2N7002 to 24-, and maybe a >> bootstrapped pullup? >> >> John > >ir2153x and a dual fet instead? but then I guess you might as well >pick a standard converter ic > >-Lasse
I've done some sims of totem-pole fets, and it's a little tricky avoiding shoot-through. It can be done, but takes more parts. The complementary emitter follower has a lot of nice properties, like controllable edge rates and no snubbers. John
On 3 Nov., 20:03, John Larkin
<jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
> On Thu, 3 Nov 2011 06:05:13 -0700 (PDT), "langw...@fonz.dk" > > > > > > > > > > <langw...@fonz.dk> wrote: > >On 3 Nov., 04:51, John Larkin > ><jjlar...@highNOTlandTHIStechnologyPART.com> wrote: > >> This > > >>http://www.panoramio.com/photo/61564837 > > >> might work for powering a thing I'm doing. I can use a standard cheap > >> Coiltronix dual-coil inductor as the transformer. The complementary > >> emitter followers will have no shoot-through and can have controllably > >> slow switching edges, since they will just follow the base drive. > > >> Anybody got ideas for the base driver device? Ideally it would be > >> self-oscillating, set with some R-C; have a moderate slew rate; swing > >> to the rails. I'm thinking roughly 150 KHz maybe, a few watts output. > > >> Maybe an LM8261 opamp? I'd have to see if it winds up when it rails. > >> It might not. > > >> Some sort of fet gate driver would be OK, but few go to 24 volts. > > >> Something discrete maybe, like a 2N7002 to 24-, and maybe a > >> bootstrapped pullup? > > >> John > > >ir2153x and a dual fet instead? but then I guess you might as well > >pick a standard converter ic > > >-Lasse > > I've done some sims of totem-pole fets, and it's a little tricky > avoiding shoot-through. It can be done, but takes more parts. The > complementary emitter follower has a lot of nice properties, like > controllable edge rates and no snubbers. > > John
the ir2153x has build in dead time, somewhere between 0.6 and 1.2us depending on which one -Lasse
Joerg wrote:
> John Larkin wrote: >> This >> >> http://www.panoramio.com/photo/61564837 >> >> might work for powering a thing I'm doing. I can use a standard cheap >> Coiltronix dual-coil inductor as the transformer. The complementary >> emitter followers will have no shoot-through and can have controllably >> slow switching edges, since they will just follow the base drive. >> >> Anybody got ideas for the base driver device? Ideally it would be >> self-oscillating, set with some R-C; have a moderate slew rate; swing >> to the rails. I'm thinking roughly 150 KHz maybe, a few watts output. >> >> Maybe an LM8261 opamp? I'd have to see if it winds up when it rails. >> It might not. >> >> Some sort of fet gate driver would be OK, but few go to 24 volts. >> >> Something discrete maybe, like a 2N7002 to 24-, and maybe a >> bootstrapped pullup? >> > > If 370kHz fixed is ok maybe these? > > http://www.aosmd.com/res/data_sheets/AOZ1210AI.pdf > > Around 50c in quantities. Problem is that your two transistors then > become unemployed :-) >
P.S.: Note that the pull-down FET is wimpy, 10ohms. So if you decide to lose the external transistors make sure this is ok for the load. -- Regards, Joerg http://www.analogconsultants.com/