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UCC256404: Optimum VCC generation and value for UCC256404

Part Number: UCC256404
Other Parts Discussed in Thread: PMP22087, , TLV760

In reference to the PMP22087 reference design, I notice that the UCC256404 VCC is powered from a zener controlled linear regulator at ~20VDC. In the data sheet, most of the typical plots have VCC listed at 15VDC.

1. Why was a 24VDC winding used to create a ~20V supply to power VCC on the UCC256404 instead of providing it with a 15VDC supply?

2. Is a regulated supply required the VCC net? Why not use a Aux winding at 16V followed by a diode or a low cost bridge rectifier followed by a capacitor (or RC Network) instead?

3. If regulation is required, why not use a TLV760 with fixed 15V output following the rectifier stage?

Thanks in advance,

  • In looking at the EVM, I notice that a simple diode + capacitor  was used to generate VCC, shown below:

    I guess I can assume from this that the zener controlled regulator was an arbitrary choice? What should I make of the voltage selected for VCC (i.e. 20V vs. 15V)?

  • HI Cody,

    Thanks for reaching out. I don't have a copy of the PMP22087 LLC transformer datasheet at the moment but with LLC designs using integrated leakage as the resonant inductor, the coupling is not great between the primary and secondary winding windows. You can see considerable variation in the AUX winding voltage depending on the loading of the LLC as a result. In the EVM, the AUX winding is wound on the secondary winding window using triple insulated wire and fed back to the primary to improve the coupling from AUX to SEC and minimize the variation on the AUX winding voltage. I would imagine the AUX winding in the PMP22087 design is wound on the primary winding window and the designer used the LDO structure to ensure the VCC pin rating was not exceeded. If a lower AUX winding voltage was selected, the LDO structure would likely not be needed. The designer probably chose 20V to give ample headroom on VCC in case he needed to push the burst mode threshold higher to improve standby power. I would also like to mention the concerns over AUX winding voltage variation with converter loading are generally associated with LLC designs using integrated resonant inductance. Designs with an external resonant inductance typically do not have the AUX winding voltage variance. Using an LDO IC such as TLV760 instead of the zener-clamped emitter follower is perfectly valid as well.

    I would say selecting the AUX:SEC winding ratio depends on what are the key careabouts for the design. If very low standby power is needed, then a higher VCC voltage is beneficial as it will allow for more headroom on VCC to push the burst threshold higher. The amount of variation in the AUX winding voltage will depend on the coupling between AUX and SEC. Winding the AUX winding on the secondary winding window with triple insulated wire certainly improves the coupling but whether this is feasible or not will depend on the transformer manufacturing process being used. Both 15V and 20V are acceptable for the VCC voltage, I would just recommend keeping VCC within the recommended max of 26V.

    Best Regards,

    Ben Lough