Offline current mode Half Bridge converter

[cupoftea]

Finally we have a realistic solution for 200-3kW Offline SMPS’s. It’s the attached Current Mode Half Bridge converter, which switches off the post-PFC HVDC Bus. Its in Current Mode courtesy of the two little low-power Flybacks which keep the voltage on the split capacitors equal. (LTspice and PNG attached)

Being in Current Mode, its much more amenable to customer connection of large capacitive loads which would make the old-fashioned voltage Mode Half Bridge converter’s go unstable.

Also, it requires only simple type 2 compensation. If wanted, it can even be run in Constant Off Time mode and then doesn’t even need slope compensation.

Only uses Two primary transistors and only switches half the input bus, so transformer can be smaller than eg Two Tran forward. Also, the half bridge is more amenable to bootstrap high side drive than the two tran forward.

Being a hard switched half bridge, it doesn’t suffer the numerous ills from which the half bridge LLC suffers. The PSFB is also known to suffer similar failure modes as the LLC. Their resonant type behaviour comes with the cost that they are generally not as robust as a hard switched converter (as long as the hard switched converter is thermally well handled).

Specially in the presence of multiple output short circuits occurring etc due to customer having access to the output terminals, which the customer so often does.
The equalisation flybacks only need to be small because in a current mode half bridge, the rate at which the split caps start going unbalanced is relatively slow.

LLC failure rate (and PSFB failure rate)

High failure rate of LLC converter

Hi, The attached LLC LED driver was once due to be the future of offline LED Drivers. The old Fluorescent ballasts used PFC stages followed by Soft Switched Half bridge Fluorescent ballast Drivers using simple bootstrap high side FET drive. The "equivalent" to that is the attached Half Bridge...

www.edaboard.com

Why are we not seeing the silicon venders coming out with chips for this?...why instead are they concentrating this power level toward the fragile half bridge LLC?

 

[Easy peasy]

Respectfully - rather than 2 extra small flybacks - how about a full bridge with a CT for current mode, and two small GD transformers to fire diagonal pairs of power fets ( i.e. both on then off, same for other diagonal pair ) - this has been a tried and true converter topology for well over 3 decades

benefit - the switches see half the current, both approaches see the same Vds

switching half the current allows the turn on to be softer without incurring high sw losses per device - turn off losses can be made very small with good turn off GD.

less total bits = higher reliability - esp when the currents are less - food for thought. ( 3kW, 400V bus = 7.5A in a fet + losses and ripple )

 

[cupoftea]

Thanks, that sounds great, the reason for the emphasis to the half bridge was the assembly hassle of heatsinking 4 full bridge FETs.
Also, GDTs seem always low stocked, and if winding oneself..then why not just do two small flyback transformers instead.

 

[cupoftea]

As you know, heatsinking is a major assembly process issue...someone has to be payed to screw all those fets on and even then you dont know if they did it tightly enough....and they have to fiddle with the insulation pad thing, and the TO220 screw and washer kit......then solder all those heatsink/fet assemblies. Even if you use those clamp-clip type heatsinks, someone still has to sit there all day tighting them, and may not have done it properly.....i even knew one company that were so fed up with fet heatsinking in prod, that they re-designed a 150W streetlight to have an all SMD board, which just got screwed to a large earthed heatsink which was also the entire head of the streetlight....so one process and no messing with power fet screwing/fiddling as they were all just machine SMD soldered to the board on pads with plated over thermal vias.