Where are all the parts?

[Th3_uN1Qu3]

Re: Where are all the parts?

It's interesting that HardwareSecrets always say the duty cycle is typically fixed at 30% or so. Obviously, they are wrong if the duty cycle is in fact varied.

They're not saying that. They've just settled for a typical value so they can run their math and post some theoretical numbers.

The truth is that in two transistor forward the maximum duty cycle could be 30%, could be 40%, could be 50%, could be over 50% depending on the design. Unlike half bridge, 2 transistor forward is single ended, so the duty cycle can go beyond 50%. This brings additional complications so it isn't widely used, but it can be done. Usually, the controllers used have max duty cycle limited to 50% tho.

Also, the output current has nothing to do with the duty cycle. The maximum output current is the diode's rated current, not some fraction of that like they keep saying.

[c_hegge]

Re: Where are all the parts?

Also, the output current has nothing to do with the duty cycle. The maximum output current is the diode's rated current, not some fraction of that like they keep saying.

I know

Hardware secrets also say that on newer designs, only one of the two diodes is used for rectification, while the other is used for discharging the coils (freewheeling). If that is the case, then on a forward design, the theoretical maximum is only half what the rectifier is rated for.

[Th3_uN1Qu3]

Re: Where are all the parts?

Hardware secrets also say that on newer designs, only one of the two diodes is used for rectification, while the other is used for discharging the coils (freewheeling). If that is the case, then on a forward design, the theoretical maximum is only half what the rectifier is rated for.

On two transistor forward this is true, it's half wave rectification, and the freewheeling diode only takes 1/3 of the load. Its job is to prevent reverse voltage spikes created by the inductor, and you will see that in a half-wave rectifier powered by normal sinewave mains, this diode is not needed.

So we can conclude that in a single-ended SMPS, the output current limit is given by the forward rectifier, and the freewheeling rectifier can be of lower current rating. That doesn't make HWS's math less misleading - if the duty cycle is different from 30%, all their calculations fall on their ass.

[PeteS in CA]

Re: Where are all the parts?

Also, the output current has nothing to do with the duty cycle. The maximum output current is the diode's rated current, not some fraction of that like they keep saying.

In theory, that is correct. The duty cycle, theoretically, is the product of the input voltage and the transformer ratio. With real world components, forward drops in the switch device(s) and rectifiers and voltage drops in the transformer windings, inductor windings and PCB traces all increase with increased load current, so the regulator increases the duty cycle to compensate.

Hardware secrets also say that on newer designs, only one of the two diodes is used for rectification, while the other is used for discharging the coils (freewheeling).

The forward converter topology is not especially new. It was around and in use when I got into power supplies bock in 1980. In a forward converter, during the switch "on" time, one diode delivers current to the load, also charging the inductor. During the switch "off" time, the inductor is discharging, supplying current to the load, with the other diode completing the current path to the other end of the inductor. IOW, during the "off" time, the inductor is a current source, and the freewheeling diode's function is to complete the current path. The "off" time is never long enough for the inductor to fully discharge. The output capacitors also charge during the "on" time and discharge some to supply current to the load during the "off" time.

[momaka]

Re: Where are all the parts?

The forward converter topology is not especially new. It was around and in use when I got into power supplies bock in 1980. In a forward converter, during the switch "on" time, one diode delivers current to the load, also charging the inductor. During the switch "off" time, the inductor is discharging, supplying current to the load, with the other diode completing the current path to the other end of the inductor. IOW, during the "off" time, the inductor is a current source, and the freewheeling diode's function is to complete the current path. The "off" time is never long enough for the inductor to fully discharge. The output capacitors also charge during the "on" time and discharge some to supply current to the load during the "off" time.

Cool
A few years back I was quite puzzled about why there would be diodes going between GND trace and each rail. Never really came back to that question. And now I don't have to.
Thanks for the explanation .

[PeteS in CA]

Re: Where are all the parts?

Glad to be helpful! Sometimes a seemingly simple circuit has a lot going on. I may do a design description post about a "simple" self-oscillating discontinuous flyback.