For a push pull converter operating with a 12V DC input, a 1:4 transformer turns ratio (Ns/Np), and a 40% (0.40) per-switch duty cycle, the theoretical output voltage is exactly 38.4V. The governing formula for Continuous Conduction Mode (CCM) is Vout = Vin × (Ns/Np) × 2D. Substituting your exact query values: Vout = 12V × 4 × (2 × 0.40) = 38.4V. This calculation assumes ideal diodes, zero winding resistance, and continuous inductor current.
The Core Conversion Formula and Neighboring Values
The push-pull topology uses two switches (typically MOSFETs) driving a center-tapped primary winding. Because each switch conducts for a duty cycle D (where D must remain below 0.50 to prevent shoot-through), the total primary utilization is 2D. The transformer steps this up by the turns ratio n (Ns/Np), and a full-wave rectifier on the secondary converts it to DC.
The assumption that fixes this answer is Continuous Conduction Mode (CCM). In CCM, the output inductor current never falls to zero during the switching cycle, meaning the output voltage is strictly a function of the input voltage, turns ratio, and duty cycle, independent of the load current.
Below is a reference table showing how the output voltage shifts across a ±20% range of your target 40% duty cycle (from 32% to 48%), holding the 12V input and 1:4 ratio constant.
| Per-Switch Duty Cycle (D) | Total Primary Duty (2D) | Ideal Vout (12V In, 1:4 Ratio) | Real-World Vout (Approx. 88% Eff.) |
|---|---|---|---|
| 32% (0.32) | 64% | 30.72V | 26.5V |
| 36% (0.36) | 72% | 34.56V | 29.8V |
| 40% (0.40) | 80% | 38.40V | 33.2V |
| 44% (0.44) | 88% | 42.24V | 36.5V |
| 48% (0.48) | 96% | 46.08V | 39.8V |
Note: Real-world values account for an estimated 88% system efficiency, including MOSFET RDS(on) losses, transformer copper/core losses, and a 0.5V Schottky diode forward voltage drop.
How Input Voltage and Topology Assumptions Shift the Output
A common mistake in power supply design is treating DC-DC conversion formulas as universal across all input voltages. Because the push-pull topology is strictly a DC-DC converter, feeding it directly from 120V AC, 230V AC, or 3-phase AC is physically impossible without a front-end rectifier and Power Factor Correction (PFC) stage.
If your system includes an AC-DC front-end, the DC bus voltage drastically shifts the push-pull input. A 120V AC line yields a ~160V to 390V DC bus (depending on PFC), while a 230V AC line yields a ~320V to 400V DC bus. At these high DC bus voltages, the push-pull topology is abandoned. The push-pull circuit subjects its MOSFETs to a voltage stress of at least 2 × Vin, plus leakage inductance spikes. On a 320V DC bus, the MOSFETs would need to withstand >700V, making the topology highly inefficient and prone to failure. For 120V/230V/3-phase derived DC buses, engineers switch to full-bridge or phase-shifted topologies. Consequently, Power Factor (PF) is a metric of the AC-DC front-end and is entirely meaningless to the isolated push-pull DC-DC stage itself.
Here is how the math and component selection shift across standard DC battery/bus inputs, assuming a fixed 1:3 turns ratio and a 40% duty cycle:
| Nominal Input | DC Bus Origin | Ideal Vout (1:3 Ratio, D=0.4) | Real-World Vout (85% Eff, 0.5V Drop) | Min. MOSFET VDS Rating Required |
|---|---|---|---|---|
| 12V DC | Battery / Solar | 28.8V | 24.0V | 40V (Use 60V MOSFETs) |
| 24V DC | Truck / Marine Battery | 57.6V | 48.4V | 80V (Use 100V MOSFETs) |
| 36V DC | E-Bike / Telecom | 86.4V | 72.9V | 120V (Use 150V MOSFETs) |
| 160V DC | 120V AC Rectified (No PFC) | 384.0V | 325.9V | 400V (Topology Invalid / Use Full-Bridge) |
For deep-dive topology selection and snubber design, refer to the Microchip AN1273 application note on push-pull converters or the Texas Instruments Isolated Power Supply design guides.
When This Conversion Becomes Meaningless
The Vout = Vin × n × 2D formula is a powerful design tool, but it breaks down completely under three specific bench and field conditions:
If your load current drops low enough that the output inductor current reaches zero before the next switching cycle begins, the converter enters DCM. In DCM, the output voltage is no longer fixed by the duty cycle; it becomes highly dependent on the load current and the inductor value. A 40% duty cycle might yield 38.4V at a 5A load, but spike to 55V at a 0.1A load if the feedback loop is open.
The formula mathematically allows for D = 0.60 (yielding 57.6V in our 12V example). In physical reality, a per-switch duty cycle above 50% means both MOSFETs will eventually conduct simultaneously. This creates a direct short circuit across the primary winding (shoot-through), instantly destroying the MOSFETs. Practical controllers enforce a maximum duty cycle of 45% to 48% to guarantee dead-time.
Real transformers have leakage inductance. When the MOSFETs switch off, this inductance generates massive voltage spikes (V = L × di/dt). If you do not implement an RCD snubber or active clamp, these spikes not only threaten the MOSFETs but also effectively reduce the volt-seconds transferred to the secondary, causing the real-world output voltage to fall significantly below the theoretical calculation.
Push Pull Converter Design FAQs
Why do we use a push-pull topology instead of a flyback?
Push-pull converters are preferred over flyback converters for medium-power applications (typically 100W to 500W) because they utilize the transformer core in both directions (first and third quadrants of the B-H curve). This prevents core saturation, allows for a smaller transformer, and results in lower output ripple current compared to the flyback's single-quadrant operation.
How much dead-time is required between the two switches?
Dead-time is the period where both MOSFETs are strictly OFF. For modern MOSFETs with fast gate charge characteristics, a dead-time of 100ns to 300ns is usually sufficient. However, you must account for the controller's propagation delay and the MOSFET's turn-off tail current. Always verify dead-time on the bench using a high-bandwidth oscilloscope and current probes on the primary winding.
Can I use a standard center-tapped transformer for a push-pull?
Yes, the primary must be center-tapped to allow the two switches to alternately drive current in opposite directions through the core. However, the secondary winding configuration depends on your output voltage. For low-voltage, high-current outputs (like 5V or 12V), a center-tapped secondary with two Schottky diodes is standard. For higher voltage outputs, a full-bridge secondary rectifier is more efficient as it reduces the reverse voltage stress on the diodes.
