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Half Bridge Theory

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LAN_403

M.A.D.

Fri Dec 09 2011, 09:00PM

Registered Member #4052 Joined: Thu Aug 11 2011, 04:43AM
Location: IN ,USA
Posts: 69

I give up. I have been searching for information on this forever to no avail.

What is it that actually limits the power throughput of a half bridge. It can not be inductance because then a DRSSTC would not work and the resistance of the primary circuit is miniscule at most. So, I am confused.

Inducktion

Fri Dec 09 2011, 09:26PM

Registered Member #3637 Joined: Fri Jan 21 2011, 11:07PM
Location: Buffalo, NY
Posts: 1068

Because it's resonant, the only real thing that limits current is the interrupter....It tells it to shut off after certain cycles, so that the primary current and voltage doesn't ring up too high, and then starts the process over again.

Dr. Dark Current

Fri Dec 09 2011, 09:51PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

If you ask about the half bridge converter itself, the components limit the maximum current you can draw from it. However if you are asking about the tank impedance of the DRSSTC, this depends on many factors.
If you had an ideal series LC circuit with no secondary in place, the current would ring up infinitely, it would have an infinite Q. However if you place a secondary coil into the circuit, the sparks start to load the circuit down, the Q of the circuit drops and the current stabilises at a finite value.
One of the factors that has a direct influence on primary impedance, is tuning. The further you tune the tank away from secondary resonance, the more current it draws and more power is transfered. However this works just to some extent. If you tune the primary too far away, the tank current rises so high that losses start to overcome the power transfer to the secondary and efficiency falls.

This was all assuming you have a primary current feedback. If you used a different form of feedback, the tuning would work differently.

M.A.D.

Sat Dec 10 2011, 04:19AM

Registered Member #4052 Joined: Thu Aug 11 2011, 04:43AM
Location: IN ,USA
Posts: 69

All right, thank you, that all makes sense.

However I still have a few questions.

What would happen to a SSTC if the interrupter was set at a high frequency, say 20KHz?

If the secondary limits the current draw, would that mean that higher coupling yeilds a lower power?

Also what limits the power drawn by a nonresonant circuit, like a flyback?

I thank you for your help, this is not for a specific problem, I am simply trying to understand the nature of a halfbridge.

teravolt

Sat Dec 10 2011, 05:24AM

Registered Member #195 Joined: Fri Feb 17 2006, 08:27PM
Location: Berkeley, ca.
Posts: 1111

M.A.D. wrote ...

All right, thank you, that all makes sense.

However I still have a few questions.

What would happen to a SSTC if the interrupter was set at a high frequency, say 20KHz?
:you would draw more power. power is aproduct of the time that it is ocilating

If the secondary limits the current draw, would that mean that higher coupling yeilds a lower power?
:more coulping is better and alows more power to be deliverd to the secondary and the spark
the traid off is flashover of your primary and secondary

Also what limits the power drawn by a nonresonant circuit, like a flyback? :couplimg

I thank you for your help, this is not for a specific problem, I am simply trying to understand the nature of a halfbridge.

Uspring

Sat Dec 10 2011, 04:41PM

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

M.A.D. wrote:

If the secondary limits the current draw, would that mean that higher coupling yeilds a lower power?

The primary tank looks much like a simple resistor when primary feedback and zero current switching is used. Its resistance is the sum of the loss resistance in the primary tank and an additional resistance coupled in through the secondary due to the arc loading. So that part of the input power going into the loss resistance heats up the primary and its caps etc., the part going into the coupled in resistance is responsible for the fireworks.

The latter part of the resistance depends on the tuning and is also proportional to the square of the coupling constant. If the coil is out of tune the coupled in resistance can become very low, much lower than the loss resistance. In this case currents are high but most of the energy is burned in the primary. If the coil is in tune this resistance can become high, which increases efficiency but also lowers current.

A higher k gets you a higher coupled in resistance, Whether this increases output power depends on where you are tuned. If you are way out of tune it will give you more power. If primary and secondary are tuned to similar frequencies it will give you less.

This all applies to the time after rampup of primary current, though. Initially most energy goes into the charging up of primary and secondary tanks.

M.A.D.

Mon Dec 12 2011, 08:36PM

Registered Member #4052 Joined: Thu Aug 11 2011, 04:43AM
Location: IN ,USA
Posts: 69

Thank you very much for the information.

How is the size of the bridge capacitors determined, because it seems almost random. and from what I have seen SSTCs have far smaller ones than flybacks do.

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