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All around questions for DRSSTC

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Jase53

Tue Jun 05 2012, 03:40PM

Registered Member #4495 Joined: Thu Apr 19 2012, 12:05PM
Location:
Posts: 5

Hey
I am building my first DRSSTC for a project so i am trying to understand the theory correct.

The Z surge decides how fast my coil rings up per half cycle. Can i calculate how much it rings up per half cycle so i know what pulse width maximum rating is so i don't burn of my DRSSTC ?

How can I calculate the Q factor of the DRSSTC ?

thanks for the help in advance.

E.TexasTesla

Tue Jun 05 2012, 04:36PM

Registered Member #4362 Joined: Sat Jan 21 2012, 03:44AM
Location: Texas
Posts: 98

Welcome. Im not sure what your trying to calculate but I would recomend a tried and tested DRSSTC such as Steve Wards work.
Build it first and correctly. Then you could extract measurments and experiment from a know good unit.

:)

Jase53

Tue Jun 05 2012, 07:45PM

Registered Member #4495 Joined: Thu Apr 19 2012, 12:05PM
Location:
Posts: 5

I have calculated my Z surge to about 8 ohm. what i have understood from wiki is that a lower Z will run higher peak currents and require fewer driving cycles, while a higher Z runs lower peak current, but for more cycles. the current it rises with per cycle.. can i calculate it ?

brandon3055

Tue Jun 05 2012, 10:34PM

Registered Member #4548 Joined: Mon Apr 23 2012, 03:52AM
Location: tasmania
Posts: 271

While on the topic of DRSSTC'S what other differences are there between SSTC's and DRSSTC's besides the tank cap eg what's different about the driver

Ben Solon

Tue Jun 05 2012, 11:54PM

Registered Member #3900 Joined: Thu May 19 2011, 08:28PM
Location:
Posts: 600

Destroyer of mosfets wrote ...

While on the topic of DRSSTC'S what other differences are there between SSTC's and DRSSTC's besides the tank cap eg what's different about the driver

that is really the fundamental difference. but for the driver circuitry, now you want your primary resonating on its own, so primary current monitoring via ct's are required. this means that your primary will always resonate whether or not the secondary does as well.

if you tried to match the secondary's ~fres, the primary would be hard switched and the igbt's would die, you would lose the low impedance and the lc circuit works against you.

Goodchild

Wed Jun 06 2012, 05:31AM

Registered Member #2292 Joined: Fri Aug 14 2009, 05:33PM
Location: The Wild West AKA Arizona
Posts: 795

Jase53 wrote ...

Hey
I am building my first DRSSTC for a project so i am trying to understand the theory correct.

The Z surge decides how fast my coil rings up per half cycle. Can i calculate how much it rings up per half cycle so i know what pulse width maximum rating is so i don't burn of my DRSSTC ?

How can I calculate the Q factor of the DRSSTC ?

thanks for the help in advance.

To calculate the Z surge of the tank you can do it a couple of different ways. Z = sqrt(L/C) you can also just find the reactance of the L or the C at the resonant frequency of the primary LC. Both of these will come out to the same number.

You don't really need to calculate the max current ring up but you can if you want using ohms law, tank Z, and your drive voltage. Generally an over current circuit is implemented that burst by burst limits current so high PW are not a problem and are simply cut short at a preset max current value. Do try and keep your Z value between 8 and 20 ohms.

Primary Q can be calculated with Q = Z / ESR Although this value is usually not all that important in design because the ESR of the primary is so low it's negligible.

A more useful number is Secondary Q and it is calculated in the same manner as primary Q.

brandon3055

Wed Jun 06 2012, 06:06AM

Registered Member #4548 Joined: Mon Apr 23 2012, 03:52AM
Location: tasmania
Posts: 271

Thanks Ben123324 I didn't think there was much more then that

Uspring

Wed Jun 06 2012, 12:44PM

Registered Member #3988 Joined: Thu Jul 07 2011, 03:25PM
Location:
Posts: 711

Jase53 wrote:

The Z surge decides how fast my coil rings up per half cycle. Can i calculate how much it rings up per half cycle so i know what pulse width maximum rating is so i don't burn of my DRSSTC ?

Say you drive your primary tank with alternatively +V and -V. Then your tank voltage will rise by 2V every time you switch, i.e. each half cycle, if you switch at zero current. The primary current is then given by your total voltage Vt and primary Z, i.e. Vt/Z.
Losses in the primary will limit the max current to the point, where the power burned up in the primary resistance is equal to the power input from the bus.
The secondary can also limit the primary current. This depends on how well the tanks are tuned and if there are ground strikes.

Jase53

Wed Jun 13 2012, 08:56PM

Registered Member #4495 Joined: Thu Apr 19 2012, 12:05PM
Location:
Posts: 5

Goodchild:

The primary ESR value for a resistor is low. shouldnt that make my Q factor for primary circuit really high.
say if i had a Z surge at 8 ohm and my ESR was 10m ohm it would give me a Q at 800 ??

zzz_julian_zzz

Mon Nov 12 2012, 02:23AM

Registered Member #3964 Joined: Thu Jun 23 2011, 03:23AM
Location: Valenzuela City
Posts: 332

@ Eric Goodchild,
What is the good Q for secondary?=) Thanks in advance!

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