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Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
I'm needing help with a circuit which I cant understand. Using the FAN4800IN IC, id like to build a wall-line powered power supply. I cant figure out why this IC only has one PWM out pin, yet their example circuits show two transistors being driven, which appear to be push-pull. I'm looking for 1,000 watts -ish so I want to use a active PFC so as not to trash the power quality on the line side.
Here is the pic:
Here is the pic of the schematic, with the red outline showing the parts I don't understand. I don't know if those T1A and T1B coils are gate drive transformers, that would make this drive circuit more like Steve Wards circuit below.
Here are the waveforms id like to have, but a simple JK toggle gate didn't work.
Heres Steve Ward's half bridge, which would work similarly in my Push-Pull application, from a driving point of view. (the 555 only puts out a single pin PWM out.
]fan4800-187878.pdf[/file] Here is the full PDF from Fairchild, FAN4800IN.
The FAN4800 does limit the PWM signal to less than 50%, which if driving two transistors would resist shoot through, and only one on at a time.
Registered Member #2292
Joined: Fri Aug 14 2009, 05:33PM
Location: The Wild West AKA Arizona
Posts: 795
Well for starters that's not a push-pull converter that's a foreword converter. The PWM waveform you have drawn is not consistent with a foreword converters mode of operation (they don't run with interleaved PWM). Foreword converters operate in flyback mode. More or less this is a two switch flyback converter (foreword converter is just the fancy name).
As far as the PWM goes, T1B and T1A is a gate drive transformer for driving the high side gate of Q2.
Lastly this is not the PFC section. In their implementation that foreword convert is acting as a secondary PWM section down converting the high voltage produced by the PFC boost section (composed of Q1, L1, D1, and C4/C5).
Another thing to note is D3, C12, and the AUX winding of T2 form the parasitic power supply for the converter logic and gate drive.
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
just a side note that may or may not influence you, repairing / fault-finding a PFC/PWM smpsu is no fun ! equipment that has independent PFC and PWM stages is overall much more complex and expensive but may be more versatile in a hobby environment
P.S. independent opto-isolated status and control lines are useful.
Registered Member #2292
Joined: Fri Aug 14 2009, 05:33PM
Location: The Wild West AKA Arizona
Posts: 795
Sulaiman wrote ...
just a side note that may or may not influence you, repairing / fault-finding a PFC/PWM smpsu is no fun ! equipment that has independent PFC and PWM stages is overall much more complex and expensive but may be more versatile in a hobby environment
I agree with this completely. Modularity is a huge bonus when debugging. In addition it reduces complexity (per stage) and helps bread higher reliability as it builds some redundancy into the system.
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
I'm trying to avoid the forward converter and flyback mode. ive got a special planar transformer device I need to run, but it needs to be push-pull and PFC driven.
Ive been looking at steve wards active PFC, I always meant to keep the two stages separate.
Heres the common simplified circuit I have been studying.
Registered Member #2292
Joined: Fri Aug 14 2009, 05:33PM
Location: The Wild West AKA Arizona
Posts: 795
Don't get me wrong foreword converters are nice and simple with their low side only switches. The only down side is that it only utilizes half of the winding volume on the primary for any given switching half cycle. This has the tendency to drive the size of the magnetic up compared to other typologies that process the same amount of power.
It may not be a big deal for your application but it's worth mentioning. I would recommend a half/full bring drive if you plan on utilizing all of your transformer.
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
Goodchild wrote ...
It may not be a big deal for your application but it's worth mentioning. I would recommend a half/full bring drive if you plan on utilizing all of your transformer.
I'm planning to use a push-pull copper foil primary, I realize that half and full bridges are more often used at the high powers, but in this case I think the push pull starts to approach the packing efficiency of the single primary topologies.
Registered Member #11591
Joined: Wed Mar 20 2013, 08:20PM
Location: UK
Posts: 556
Patrick wrote ...
Goodchild wrote ...
It may not be a big deal for your application but it's worth mentioning. I would recommend a half/full bring drive if you plan on utilizing all of your transformer.
I'm planning to use a push-pull copper foil primary, I realize that half and full bridges are more often used at the high powers, but in this case I think the push pull starts to approach the packing efficiency of the single primary topologies.
this is the planar thread.
I like the push-pull topology. I've designed and (almost) made a 24 to 240V 3000W inverter using 4 MOSFETs (two in parallel) push/pulling a centre tapped transformer (ETD-59 core). Yes half of the primary isn't being used half of the time, however full-bridge you have the extra losses from the extra MOSFETs/IGBTs Really, full bridge and push pull are electrically very similar.
Be aware, when PWMing both full-bridge and push-pull, there needs to be something in place to prevent flux walking, where a DC component created by the uneven switching could get larger and larger until it saturates the transformer core, causing catastrophic overheating and component death. The easiest way to mitigate against this is to use current based control, in my inverter this was a problem because this requires either CTs or shunt resistors for current sensing, which would, with average primary currents up to 160A, cause unacceptable power wastage. My solution was to use a portion of the PCB track as a very low value shunt resistor and use a fast op-amp to get it to a level that the controller IC could use. With mains voltages this won't be a problem.
If you can acquire some GaN HEMTs (eg: TPH3205WS), a totem pole (bridgeless) topology will be more efficient as you don't have any diode drops. Switching losses will be too high with MOSFETs, may also work with IGBTs ( ).
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Need Help With a PFC and PWM Circuit.
