COMPONENT DESIGN


the inductor. Easily, we can calculate the inductance required using Equation 3.


Figure 4. A MAX17682 typical circuit in EE-Sim OASIS,


powered by SIMetrix/SIMPLIS.


 is the ripple current. As discussed previously, ripple current equals the DC output current 


 buck converter topology, however, there is a transformer and not an inductor. How do we deal with it when the component is a transformer rather than an inductor? As we know, the current ratio equals the inverse of 


TOFF is the secondary current that is converted to a primary current in TOFF time. We should add a transformer’s two-coil current as an equivalent inductor current.


  windings, then


Figure 5. A MAX17682 typical circuit simulated current waveform.


 based on the MAX17682. Figure 4 shows a MAX17682 circuit, which was drawn in EE-Sim  powered by . Current


 and , have been placed at both sides of the transformer. Figure 5 shows a transient simulation result from the two probes. The two current waveforms were added utilising Equation 6. The added current results (red) in a triangle wave and behaves just like the inductor in a non-isolated buck converter. So,   


Then the transformer’s primary inductance 


By using the turns ratio, primary inductance, output power, output current and isolation voltage, we can decide which inductor will be used or designed.   and apply the equation shown in the MAX17682 data sheet (see Figure 6).


  Then Equation 11 and the equation from Figure 6 are the same. The equation in the data sheet already selects the primary ripple 


OFF duty cycle equals the primary ripple current.


Conclusion


Usually, we assign a load ripple current that is 0.2 times the DC output current. So, K can  same time, the primary peak current should be designed less than the switch current 


 Figure 6, the user ensures a faster design with a primary ripple current that equals


Figure 6. A screenshot of the MAX17682 data sheet.


According to the previous discussion, Equation 10 can be rewritten to follow Equation 11 for TOFF time.


the TOFF primary ripple current or use another parameter, you can follow this tutorial.


FEBRUARY 2025 | ELECTRONICS FOR ENGINEERS


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