4-Switch Synchronous Buck-Boost Calculator

Non-Inverting 4-Switch Synchronous Buck-Boost Topology

The 4-switch synchronous buck-boost converter consists of a standard buck half-bridge (Q1, Q2) cascaded into a boost half-bridge (Q3, Q4) sharing a single central inductor (L). It maintains constant positive output voltage whether (V_{\text{in}} > V_{\text{out}}), (V_{\text{in}} < V_{\text{out}}), or (V_{\text{in}} \approx V_{\text{out}}).

1. Three Operating Regions

• Pure Buck Mode ((V_{\text{in}} > 1.1 V_{\text{out}})): Q4 is held continuously ON (100% duty cycle) and Q3 is OFF. Only Q1 and Q2 switch as a standard synchronous buck converter.
• Pure Boost Mode ((V_{\text{in}} < 0.9 V_{\text{out}})): Q1 is held continuously ON (100% duty cycle) and Q2 is OFF. Only Q3 and Q4 switch as a synchronous boost converter.
• Mixed Buck-Boost Window ((V_{\text{in}} \approx V_{\text{out}})): All four switches operate cyclically to eliminate duty cycle instability and prevent subharmonic jitter.

2. The Pass-Through (100% Duty Cycle) Super-Power

When input voltage exactly matches the required output (e.g., 14V alternator input supplying a 14V auxiliary battery), modern controllers enter pass-through mode: Q1 and Q4 stay permanently ON while Q2 and Q3 remain OFF. Switching losses drop to absolute zero, delivering over 99.2% efficiency.

3. Inductor Sizing and Current Rating

Because average inductor current equals input current in boost mode and output current in buck mode, the inductor must be sized for the highest current condition:

$$I_{L,\text{avg}} = \frac{I_{\text{out}}}{1 - D_{\text{boost}}} = I_{\text{out}} \cdot \frac{V_{\text{out}}}{V_{\text{in}}}$$

Saturation current rating (I_{\text{sat}}) must exceed peak inductor current (I_{\text{pk}} = I_{L,\text{avg}} + \frac{\Delta I_L}{2}) by at least 20%.