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Registered Member #46007
Joined: Wed Apr 30 2014, 08:02PM
Location: Walsall, UK
Posts: 36
Hi Everyone, I am new here and looking to build a half bridge DRSSTC using a dual IGBT brick from Littlelfuse Link.
I have only some electronics skills so will be adapting schematics for my needs. I have looked at a couple of half bridge designs namely the OneTesla schematic Link and Kaizer DRSSTC 2 Link.
I notice that both these designs use a different approach for the two capacitors that tail the primary coil to the supply rail and ground (C4 & C7 on Kaizer). The Kaizer uses two 1uf MKP's and a separate 820uf of bus capacitor whereas the OneTesla looks like it utilises the bus capacitor in this role as two series 1000uf caps. From what I can see in the half bridge application these caps must form part of the primary circuit - can anyone please explain? Also is there a benefit to either of these approaches and is there any correlation to IGBT selection / current and how this is handled e.g. cap value of the MKP's?
I'm in the UK using 230V so would be looking at the moment to use a non-doubled voltage - just FW rectified line as in these.
In a half bridge the primary is capacitively coupled to the bus via a capacitive divider. This blocks DC current and allows the circuit to complete when current flows in either direction across the primary.
My understanding is that the capacitive divider caps determine the impedance of the half bridge while the bus cap is the energy storage for the half bridge. That is so say you can enlarge the divider caps to the point that they also function to store energy and eliminate the bus cap, but if you size them too small you won't be utilizing much of the bus cap because the bridge will be high impedance. To that end I use ~4000uF caps for the divider and leave out a bus cap in my designs. Any wiring between bus cap and bridge imcreases impedance so by eliminating it you gain performance. Just remember effective bus capacitance is halved since the two caps are in series.
Registered Member #834
Joined: Tue Jun 12 2007, 10:57PM
Location: Brazil
Posts: 644
The most usual form is the first. The second requires a higher-voltage bus capacitor and two additional ones. It avoids the high-frequency current through the bus capacitor, however. These capacitors are just for energy storage, and have little function in the primary circuit. The capacitors in the capacitive divider must have low series resistance and inductance, because the primary current passes through them.
Registered Member #46007
Joined: Wed Apr 30 2014, 08:02PM
Location: Walsall, UK
Posts: 36
Hi all, Thanks for the replies. That makes sense and those capacitors would also be required to conduct the full peak current of the primary circuit. If lower value separate MKP capacitors are used here then the value would be somewhat dependant on the operating frequency of the coil to keep the reactance lower? I'm thinking this as I'd likely be operating below 100KHz for the brick hence these would maybe need to be bigger than the 1uf caps in Kaizers's DRSSTC II that uses this configuration but at 320Khz so giving a 0.48ohm reactance. Maybe 4-5uf.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
If the bus voltage is below 400 volts, use one electrolytic capacitor (or more in parallel). Otherwise, use two electrolytic capacitors in series (or more of these pairs in parallel). Using two series caps at lower voltages results in unnecessary high parasitic inductance. It doesn't matter where you couple the tank capacitor's other end.
P.S. The HF peak current always flows through the electrolytic caps, you can not avoid this.
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Half Bridge capacitors
