Wide input, Multi output SMPS dsesign

Balaji

Sorry for the delay, I was waiting for some answers for you.

1. TL2844 vs. UC2844:
-----------------------------
UC2844 is the original device developed by Unitrode. TL2844 is a second source device developed by TI before Unitrode was acquired by TI.
UC2844 is a superior device, and TL2844 close second source device. UC2844 has premium pricing due to the superior performance.

2. UCC28740 vs TL2844:
-------------------------------
The UCC28740 is specifically targeted at high-efficiency flyback converters, whereas TL2844 is a general purpose single-ended controller that can be used a variety of topologies such as Flyback, forward, buck.
The UCC28740 employs valley-switching for better heavy-load eff, and modulates the freq lower at lighter load for better light-load eff. It has other features/advantages, such as integrated HV startup (no need for external resistor string for startup, saves a lot of standby power - esp when the TL2844 bias current is so high), much lower bias power consumption, internal low power modes for low bias power at low Fsw, output CC-mode for constant current limit under overload conditions, input brown-out/line UV protection, output OVP, etc.

3. Startup current:
-------------------------
The startup resistor value depends on the size of the VDD cap and the allowable startup delay. But with the TL2844 startup current of 1 mA, this means the startup resistor will need to be smaller value, which then means it will have very high dissipation at high line. The only way to help is to allow for a longer startup delay, and minimise the VDD cap value.

Alternatively, the UCC28704 has very low startup current (30 uA max) and has integrated HV startup, which eliminates the external resistors, and the HV startup is turned off after startup to get almost zero power loss in the startup path.

4. Snubber:
---------------
Whether you use RCD snubber or TVS clamp, this depends on your system requirements. This app note gives good background on the pros and cons of each type:
www.ti.com/lit/an/snva744/snva744.pdf

5. Transformer tool:
---------------------------
As I mentioned above the Webench tool includes a transformer design & optimisation tool for some parts. This is available for UCC28740 (depending on the design parameters), and should be available later for UC2844 and TL2844. There are also design calculators available for UCC28740 that help you to choose the inductance and turns ratio, although not the detailed transformer spec/construction.

6. Regulation:
-------------------
If you regulate the DC bias rail, the regulation of the isolated rails will not be so good. In this case, I assume that the cross-loading on the isolated rails won't be an issue, i.e. they should all see similar loading? Cross-reg for any flyback will be poor, even with opto feedback, if some outputs see heavy load while others see no or very light load.

If you want to avoid opto feedback, there are other versions of UCC28740 that use PSR (primary-side-regulation) where the bias winding is sensed directly, before the aux diode, and they give much better regulation than regulating the DC bias rail. Typically better than +/-5% is achievable for single output. For multi output it would be worse, esp if cross-loading can occur. But with similar loading on all outputs, the regulation should be pretty good. There are versions available with built-in HV startup (UCC28710, UCC28730), and with no HV (UCC28704, UCC2870x) - since the startup current is so low, the versions with no HV can still use large value startup resistors, with quite low dissipation in the resistors.

I hope this helps answer your questions. If so, please click the "verify answer" button.

Thanks,
Bernard