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

2.1 Operation Description

An isolated buck converter is a synchronous buck converter with the inductor replaced by a coupled inductor or flyback-type transformer. The primary output is still regulated as in a sync buck. The secondary output is generated by a diode rectifying the secondary winding. Figure 2-1 shows an isolated buck converter with two outputs.

Figure 2-1 Isolated Buck Converter With Two Outputs

Figure 2-2 shows typical isolated buck operating waveforms, where VIN is the input voltage of the converter, VOUT1 is the primary output voltage and VOUT2 is the secondary output voltage. During TON, the high-side MOSFET Q1 is on and the voltage stress of the low side FET Q2 is the input voltage, VIN. The rectifier diode D1 is turned off, because the reflected voltage across the secondary winding is negative and its voltage stress is N×(VIN-VOUT1)+VOUT2. The isolated output capacitor COUT2 is discharged, supplying the load current, and the primary side behaves identical to a buck regulator. During TOFF, the low-side MOSFET is on, and the reflected voltage on the secondary winding turns positive, forcing the diode forward conducting. The current in the primary winding splits into two parts: one part continues to supply the primary output (the magnetizing current, im, similar to a buck converter inductor current), the other part starts to flow to the secondary output. The secondary current waveform is determined by the load, leakage inductance, and output capacitance.

Figure 2-2 Simplified Isolated Buck Operating Waveforms

The primary output voltage is the same as a buck converter and is given by Equation 1:

Equation 1. V O U T 1 = T o n T o n + T o f f V I N = D × V I N

The secondary output voltage is given by Equation 2:

Equation 2. V O U T 2 = V O U T 1 × N 2 N 1 - V F

where

  • N1 and N2 are the turns of the primary winding and secondary winding
  • VF is the forward voltage drop of the secondary rectifier diode