SLUAAN0 December   2022 TPS62933F

 

  1.   Designing a Simple and Low-Cost Flybuck Solution With the TPS62933F
  2.   Trademarks
  3. 1Introduction
  4. 2Flybuck Converter Device Overview
    1. 2.1 Operation Description
    2. 2.2 Equations for Maximum Output Current
  5. 3Design Flybuck With TPS62933F
    1. 3.1 Primary Voltage and Turns Ratio
    2. 3.2 Feedback Resistor
    3. 3.3 Rectifier Diode
    4. 3.4 Primary Inductance
    5. 3.5 Primary Turns
    6. 3.6 Input and Output Capacitor
    7. 3.7 Pre-Load
    8. 3.8 Factors Affecting Voltage Regulation
    9. 3.9 Avoiding Low-Side Sink Current Limit
  6. 4Experimental Results
  7. 5Conclusion
  8. 6References

2.2 Equations for Maximum Output Current

In practice, the transformer has more or less leakage inductance, which determines the ramp rate of the current in the secondary winding to charge the output capacitor. The simplified current waveforms in Figure 2-3 shows its relationship with leakage inductance. With lower leakage inductance, the current ramps up quickly to a high value, charging the output capacitor quickly. With the leakage inductance increasing, the current rises slowly, which can result in less energy being supplied to the output capacitance and less output voltage. The higher leakage inductance is the higher peak charging current obtained in secondary winding under the same output power rate. A high negative current is reflected in the primary winding at the same time. Leakage inductance along with duty cycle impacts not only the output voltage regulation, but also limits the output power resulting from the minimum low-side sink current limit. Therefore, the leakage inductance must be minimized and the maximum duty cycle must be chosen carefully to mitigate their impacts.

Figure 2-3 Current Waveforms Affected by Leakage Inductance

The winding and output currents have a relationship as shown in Equation 3 and Equation 4 on one cycle average basis.

Equation 3. I p r i = I O U T 1
Equation 4. Isec=IOUT2

The magnetizing current in the transformer that combines the two windings current is identical to a buck converter. Therefore, the magnetizing current ripple can be derived as in Equation 5.

Equation 5. ∆ i m = V I N - V O U T 1 L p r i × f S W V O U T 1 V I N = V I N - V O U T 1 L p r i D f S W

The positive primary winding and switching peak current during TON is given by Equation 6.

Equation 6. i S W _ p o s p k = i p r i _ p o s p k = I O U T 1 + N 2 N 1 I O U T 2 + ∆ i m 2

Implementing the principle of charge balance to the secondary output capacitor, the secondary winding peak current can be approximately derived as Equation 7 for a higher leakage case.

Considering the worst case, the following equation is derived based on having higher leakage.

Equation 7. i s e c _ p k = 2 1 - D I O U T 2

Then the negative primary winding peak current can be derived as Equation 8.

Equation 8. i p r i _ n e g p k = - N 2 N 1 I s e c _ p k + i p r i _ p o s p k - ∆ i m = - N 2 N 1 I O U T 2 1 + D 1 - D - ∆ i m 2 + I O U T 1

In fact, we must ensure that during TON the positive primary winding peak current does not exceed the minimum high-side source current limit, ILIMHS(min), and that during TOFF, the negative primary winding peak current does not exceed the minimum low-side sink current limit, ILIMLSSOC(min). Therefore the positive and negative primary winding peak current should meet that of Equation 9.

Equation 9. i p r i _ p o s p k ≤ I H S _ L I M I T m i n i p r i _ n e g p k ≤ I L S _ N O C m i n