cupoftea
Advanced Member level 5
Hi,
We received a 750W PSU from a potential customer. They want to know if we can offer anything cheaper, but the same. Its offline 240VAC in, 24V out. Output is actually variable to a degree and uses a Buck converter (isolation is provided from a half bridge with vout = 42V, after the Boost PFC).
Looking at the output Buck converter alone, it has the schematic as shown in 1. We don’t think the gate drive is very good on this schem 1. We think it will see overcurrent on sudden startup, and on sudden shutdown, as well as ringing during times when load is changing etc.
We believe that the gate drive could be improved by making the changes seen in schem 2.
However, we believe that an even better gate drive would be produced by using schem 3.
Schem 3 uses a gate drive IC instead of totem poles. Schem 3 operates fine with Gate Drive Transformer (GDT) coupling factor (k) from 0.95 to 0.999 (at least it does in the sim). Schem 1 and 2 fall over when coupling goes much below k = 0.99. (Schem 1 falls over before Schem 2, that is schem 2 can handle wider GDT coupling range than schem 1) (....again, all in the sims). We dont yet have a unit in hand that we can probe.
Do you agree that the schem 1 Gate Drive is unsuitable?
…I mean… Yes it could work, but it seems a bit dodgy….i guess if a customer kind of switched it on once per month on a constant load than it might survive for some time though…..I have to be honest and admit the customer tells me that some of these PSU’s last longer than 10 years, though of course, we don’t know how they are used (ie how often on full load, etc etc etc).
PDF schems and LTspice sims of 1, 2 and 3 are attached.
We received a 750W PSU from a potential customer. They want to know if we can offer anything cheaper, but the same. Its offline 240VAC in, 24V out. Output is actually variable to a degree and uses a Buck converter (isolation is provided from a half bridge with vout = 42V, after the Boost PFC).
Looking at the output Buck converter alone, it has the schematic as shown in 1. We don’t think the gate drive is very good on this schem 1. We think it will see overcurrent on sudden startup, and on sudden shutdown, as well as ringing during times when load is changing etc.
We believe that the gate drive could be improved by making the changes seen in schem 2.
However, we believe that an even better gate drive would be produced by using schem 3.
Schem 3 uses a gate drive IC instead of totem poles. Schem 3 operates fine with Gate Drive Transformer (GDT) coupling factor (k) from 0.95 to 0.999 (at least it does in the sim). Schem 1 and 2 fall over when coupling goes much below k = 0.99. (Schem 1 falls over before Schem 2, that is schem 2 can handle wider GDT coupling range than schem 1) (....again, all in the sims). We dont yet have a unit in hand that we can probe.
Do you agree that the schem 1 Gate Drive is unsuitable?
…I mean… Yes it could work, but it seems a bit dodgy….i guess if a customer kind of switched it on once per month on a constant load than it might survive for some time though…..I have to be honest and admit the customer tells me that some of these PSU’s last longer than 10 years, though of course, we don’t know how they are used (ie how often on full load, etc etc etc).
PDF schems and LTspice sims of 1, 2 and 3 are attached.