Resonant converter at fixed frequency

[grizedale]

Hi,

Here is a resonant offline SMPS which runs at fixed frequency....

**broken link removed**


Do you believe then that this is a parallel resonant converter?

The pwm controller is an SG2525.

The SG2525 current input has been grounded, therefore.......
1. -There is absolutely NO primary side current sensing.
2. -This is obviously a voltage mode converter.

The two input electrolytics are 330uF, 200V.
-so when plugged into 240VAC there is just 115uF of input capacitance
-this does not seem much for the 360W maximum power that this SMPS claims to be able to produce.

So do you believe its a parallel resonant converter?

......incidentally , all the transformner windings are at most 2-stranded, and all strands are triple insulated wire.......as you know, this
increases leakage inductance , but for a resonant converter the leakage inductance is useful.

(the two rail-splitting resonant caps are not visible on the picture shown.
the toroid you can see is for gate drive))

Also, there is no PFC stage, and since it has passed CISPR22, this obviously means that
Audio SMPS's do not need PFC.

(the board actually contains the SMPS and the class d amplifier on the secondary side)

 

[Orson Cart]

the mains psu appears to be using a phase shift bridge so there are no switching losses on the mosfets - it appears not to be a parallel resonant converter - rather standard phase shift - Regards Orson Cart.

 

[FvM]

the mains psu appears to be using a phase shift bridge

3525 won't be able to drive it, I think.

@grizedale: As you apparently have the device at your fingertips - why don't you extract the full schematic?

 

[atripathi]

This does not look like resonant controller. SG2525 is simple PWM controller, with 84% efficiency, it could be any topology? Why do you say it is resonant converter?

 

[Orson Cart]

THe SG2525 could be used to drive an LLC resonant converter, with the absense of ouput inductors it way well be an LLC, by the way we have used the UC3856/SG2525 ouputs to create phase shift drive for higher power converters - its not too difficult to do this - Regards, Orson Cart

 

[FvM]

we have used the UC3856/SG2525 ouputs to create phase shift drive for higher power converters - its not too difficult to do this

I assume, it isn't, but it doesn't work by making a 2525 directly drive the output transistors?

 

[grizedale]

Hi,

I cannot extract the whole schematic because my boss wont allow me to power it up just yet.......

also its a four layer board, and very hard to see where tracks go off to.

The main output diodes empty directly into the output capacitors, so its obviously a resonant converter.

...the two inductors you can see in the picture are the class D amplififiers filter inductors.

-The transformer is a simple ETD29.....it is not a sectioned bobbin type, and there is no discrete resonant inductor.....so they have used the leakage in the transformer as a resonant inductor......sounds a bit dodgy to me.(?)

Also, the resonant capacitor is comprised of two capacitors splitting the input rail.
...each of these are 330nF......this is a very high capacitance for a resonant capacitor, and probably must mean that the resonant inductor is small value.

All the windings on the transformer appear to be of triple-insulated-wire.

I cannot see how the SG2525 can do phase modulated transistor driving.

 

[mtwieg]

The main output diodes empty directly into the output capacitors, so its obviously a resonant converter.

Not necessarily. Could be a two switch flyback. Or the output inductance could be comprised of leakage in the transformer.

Also I've seen forward, half bridge, and push pull converters where they neglect the output inductors entirely. Shoddy design does happen...

I cannot see how the SG2525 can do phase modulated transistor driving.

Me neither, and it's not suitable for variable frequency, even with external modifications. So I think it may be non resonant.

 

[grizedale]

hi
its not a two switch flyback as there are no primary diodes as there would be with that.

.........

 

[Orson Cart]

its not a two switch flyback as there are no primary diodes as there would be with that

A two switch flyback does not require primary diodes - generally a forward converter does though.

 

[grizedale]

sorry but a 2 sw flyback does require diodes.you cannot rely on fet internal diodes

 

[Orson Cart]

A single switch flyback has no diode, and a two switch is no different - the diodes are not needed in a proper 2-switch flyback design, a lot of lazy apps engineers have included primary diodes in examples of 2-sw flybacks - but then they don't understand the limitations of this approach (<50% duty cycle - limitation on o/p volts, etc...) the usual RCD catch snubber is sufficient to limit the peak volts (on the mosfets) for the single or two switch version... In either case the internal mosfet diodes are never used...

 

[FvM]

a lot of lazy apps engineers have included primary diodes in examples of 2-sw flybacks

I think the statement misjudges an important motivation for using the 2-switch topology (either in forward or flyback mode). Besides reducing the switched voltage of a single transistor, it's mainly the lossless recovery of energy stored in leakage inductance. It's in fact contradicting a wide voltage range flyback concept. Without primary diodes, a 2-switch flyback design would be effectively useless for 230 VAC input voltage, and I'm not aware of any commercial applications.

 

[Orson Cart]

An interesting comment that perhaps overlooks the primary concept of the flyback, if primary side diodes are used and they conduct each cycle then some of the energy stored in the transformer is put back onto the input HVDC bus (minus losses) a wasteful cycling of energy, and the output voltage range is limited and the lowest input voltage is very limited too, 2 switch flybacks are used extensively in small 3 phase AC drives and operate from 400-800VDC input to provide all the auxilliary supplies - all using the same RCD peak catch snubber that you see on a single switch flyback, typically 2 x 700/800V fets are used giving plenty of headroom for spikes etc and allowing duty cycles higher than 50% for low input voltages - which would be impossible if primary diodes were used. For narrow applications with well defined input and output voltages primary diodes can be used to good effect, Regards, Orson Cart.

 

[FvM]

Mostly agreeing with your point of view, I only see a minor controverse if primary diodes can serve any purpose in two-switch flyback designs, and I didn't finally make up my mind about it. I already mentioned, that the diodes contradict the wide supply range feature of the flyback concept. Obviously they won't work with the auxilary power supplies you mentioned. But are there reasonable applications at all?

The two-switch flyback point was introduced by grizedale in a previous thread. He also referred to an LM5015 example circuit, that uses primary diodes. https://www.edaboard.com/threads/216062/#post918560 In this circuit, the diodes are not just showing the application engineers laziness, they are the only realiable way to operate the device with up to 72 V input voltage without exeeding it's maximum ratings. In so far, the diode clamp serves a purpose I think. The restrictions implied by this circuit aren't discussed however. We'll possibly come to the conclusion, that it isn't more than good looking.

But if the diode clamp feature doesn't bring much benefit, then a reasonable application of the two-switch topology would be restricted to those cases, where we need to divide down the switch transistor's voltage rating, which usually isn't the case for 230 VAC input.

 

[Orson Cart]

A good reply FVM, for 230VAC input flybacks - it is usual to use a 800/900V fet (single sw) but one could use 2 x 400/500/600V fets if the power level justified the extra gate drive (as these fets can be had with very low Ron and they are cheaper). clamping for the LM5015 is noted and serves a good purpose there. Regards, Orson Cart.