cupoftea
Advanced Member level 6

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)
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?
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...
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?