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LM5017: breaking the 50% duty cycle recommendation for flybuck

Brian Antheunisse
Expert 5215 points
Part Number: LM5017

Hi guys,

My customer approached me with the below question:

We recently have a DC-DC isolated converter project in which we would like to consider the TI Buck regulator LM5017 to generate internal voltage source to power the PWM controller and drive MOSFETs. We refer to the schematic in LM5017 data sheet Figure 17, page 19 . The input range and output voltages are listed below for consideration:

Vin=7.5 – 18V

Vout1=6V

Vout2=11.5V

Total power less than 3W

 

The design note 8.2.2.2.1 states that “The primary output voltage is normally selected based on the input voltage range such that the duty cycle of the converter does not exceed 50% at the minimum input voltage. This condition is satisfied if VOUT1 < VIN_MIN / 2.” Why this buck regulator with isolated output requires 50% max duty ratio? (Single output shown in Figure 13 doesn’t have such limitation.)

In our case, if we need 6V at the primary. If we set this output as regulated output, the max. duty ratio would be 0.8. It seems we have to consider an artificial lower voltage output, saying 3V, as regulation target to satisfy such requirement. Is this the way to avoid duty ratio running into >50%? 

Now, I found an app note that mentions this requirement is because  "the isolated outputs only have the off-time window to transfer energy, for a duty cycle that is too high, the secondary winding current will have a huge spike, which leads to poor regulation." 

Is it ok to break the 50% rule if you don't need very good regulation on the isolated rail? Could you put an LDO on the output of that isolated rail to smooth it out if needed?

Thanks,

Brian

  • 0
    TI__Mastermind 25590 points

    Brian,

    The 50% recommendation for fly-bucks has to do with regulation, as you stated, but we must also look at leakage inductance of the transformer. Check out Figure 4 in this article. Check your leakage inductance - you may be fine with a higher duty cycle.

    But to your last question, yes, you can always use an LDO to clean up poor regulation at the expense of the cost of an LDO.

    -Sam

  • 0
    TI__Mastermind 49647 points
    Hi Brian,

    The 50% duty cycle spec isn't a hard and fast rule. You could push it to 60% (40% conduction time on the Fly-Buck output won't increase the secondary-side peak currents too much, particularly if it isn't heavily loaded).

    However, 6V/7.5V = 80% sounds really high, giving only a 20% Fly-Buck conduction interval on the 1 : 2 turns ratio step-up transformer. Our normal recommendation for low input voltages is to use an inverting buck-boost primary side that creates a negative primary-side output rail and then use a 1 : 1 transformer. However, that may not be an option here as you need +6V on the primary.

    So your propsed configuration depends on the secondary-side output power requirement. How much of the 3W is on the Fly-Buck output? And yes, you can use an LDO post reg if needed, just set the Fly-Buck output slightly higher to enable sufficient LDO headroom/dropout.

    Regards,
    Tim