Half bridge DC-DC converter topology

On Mon, 9 Jul 2012 19:09:57 -0400, "P E Schoen" <paul@peschoen.com>
wrote:

>"legg"  wrote in message news:9dmlv7t8au4i0dp6kfp6vaue44a8dl8lj0@4ax.com...
>
>> On Mon, 9 Jul 2012 03:53:21 -0400, "P E Schoen" <paul@peschoen.com>
>> wrote:
>
>>>I tried the 50 nH in series with the battery with minimal observed change.
>
>> You should se a 20Vppk waveform on 'in' and primary current
>> exhibiting a dual slope. V'in' will show up on the driven end of L1.
>
>Well, V(in) is more like a distorted sine wave with peaks of 27V and valleys 
>of 20V. And it does form the upper waveshape of the driven end of L1, so I 
>can see where battery lead inductance is a major factor.
>
>I was a bit surprised at how much inductance a length of wire can have. I 
>used an inductance calculator and found that a 36" length of battery cable 
>0.25" diameter has 1025 nH of inductance.
>http://www.consultrsr.com/resources/eis/induct5.htm
>
>But when I used that in the simulation it actually seemed to improve the 
>operation. V(in) varies from 25.5 to 20.5 volts (looks like a rectified sine 
>wave) and the DC output actually seems to increase. The peaks seem to be 
>shifted to the center of the conduction cycle and the current in the primary 
>L1 looks almost like a sine wave.
>

It's possible that the disparity is due to viewing different time
frames. I was looking at around 1.5mS, where there are still surge
influences. Things settle down under static conditions if you wait for
15mS. If loads were only static....and the simulation was remotely
accurate.....
<snip>
>Sounds reasonable. Do you know of a calculator that can obtain these 
>figures? The following seem to be pretty good:
>http://www.bcae1.com/trnsfrmr.htm
>http://www.smps.us/magnetics.html
>http://www.epcos.com/web/generator/Web/Sections/DesignSupport/Tools/Ferrites/Page,locale=en.html
>
A pencil and paper are quicker, and more portable. Was off topic here,
originally thrown off by a reference to measuring 6 turns on LV
primary - you meant 3 turns, which made more sense, but by the time I
figured that out....

>>> So I'm hoping to get 5 to 10 volts per turn at 50 kHz, and with
>>> 12 V P-P square wave on the 3 turn primary I should get
>>> about 288 V P-P on the secondary.
>
>> With a 24V source, the drive voltage is approximately 24Vppk,
>> approximately 12Vpk. It's the peak voltage that is produced
>> on the full wave output rectifier, but only if the total input
>> voltage is reflected accurately.
>
>> Look at the output voltage of your simulation, before the
>> choke. You'll see that the voltage there only goes positive
>> about a third of the conversion period. The rest of the time,
>> the voltage is dominated by the leakage inductance of the
>> simulated transformer during phase reversal. This loss of
>> conduction period shows up as reduced filtered output
>> voltage, in spite of expectations due to turns ratio.
>
>> This is mostly the effect of leakage inductance occuring
>> in your model.
>
>> While the effect is real, the scale of this effect in your
>> simulation is masking realistic performance.
>
>With the higher battery lead inductance of 1025 nH, the voltage at the 
>output of the bridge only briefly drops to a minimum of 89V, has a peak of 
>413V, an average of 306V, and RMS of 315V. I get input power of 1.03 kW and 
>output of 939W for an efficiency of 91%.
>
>With an (unrealistic) lead inductance of 1 nH, the input power is 963 W and 
>output is 872 W, for 90.5% efficiency. It is interesting and useful to know 
>that realistic battery lead inductance may actually help the operation. I'm 
>still using the 0.99 coupling factor.
>
>Thanks for the tips. I can see that, at these power levels, everything needs 
>to be modeled as accurately as possible. But in the end I will still need to 
>build it and test it.

You know that Lp and Ls can swing simply due to micro-gap quality /
cleanliness of core butt joints etc... Leakage inductance doesn't. 
If you use the coupling coefficient to set leakage inductance, as the
simulator does, it WILL swing just as wildly as the Lp being used in
the simulation.

The relationship used by the simulation is 

Llk= (1-k)Lm , where 

Llk is the leakage inductance seen in one winding when the other
winding is shorted.
Lm is the magnetizing inductance of same single winding.
k is the coupling coefficient.

Note that leakage inductance effects in the secondary use Lm and Lk
measured on the secondary winding, with the primary being opened or
shorted, as required. While the two leakage terms are normally related
by the usual N^2 factor, low voltage structures' lead-out effects can
be signifigant, as they form part of the turn structure and are often
missing from the measurement.

One last thing, in a full wave rectifier fed by near 100% duty, stress
on the filter inductor is usually pretty small. Even so, in this
simulation the current zeros during the phase change. This means you
should probably check for core loss in the filter choke, before sizing
it.

RL