Push-pull · topology: how it works

The battery connects to the primary's centre tap, and the MOSFETs at the two ends take turns, sweeping the core in both directions. Both MOSFET sources sit on ground, so they're easy to drive; that's why it's the classic way to get from 12/24/48 V to hundreds of volts. The price: the off MOSFET sees twice the battery voltage, and if the two pulses aren't equal the transformer walks into saturation.

⏱ Slow motion: what the driver IC outputs and what the circuit does (the highlighted slice is the moment shown on the schematic)

OUTA and OUTB are never HIGH together; the gap between them is the dead time. If feedback reduces the duty cycle, both pulses narrow equally. The two pulses must stay equal: even a small difference adds a little DC flux to the core every period (flux walking).

Tap the dashed parts for what they do, how to measure them and how they usually fail. Numbered points are scope measurements.

Phase 1/4: Q1 on

Q1 conducts: battery current enters the centre tap, flows through the UPPER half of the primary and returns to ground through Q1. The transformer steps it up to the secondary; D1 conducts and feeds the output inductor and capacitor. Meanwhile the off Q2 sees twice the battery voltage (≈48 V) on its drain.

SG3525 OUTA / OUTB: HIGH (≈12 V, switch on) / LOW (0 V, switch off)

1. Q1 drain

≈0 V while Q1 is on. When it turns off, a leakage spike (≈60 V here), ≈24 V during dead time and twice the battery, ≈48 V, while Q2 is on. That's why a 24 V system uses 100 V MOSFETs and a 12 V system 55–60 V ones.

On the scope

Multimeteruse the scope

🛠 Fault scenarios: pick a symptom and see what to measure, in order, on the schematic

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🔍 How to recognise it on a board

✓Thick copper on the battery side and two groups of parallel MOSFETs on a heatsink (2–6 per group)
✓Three heavy leads on the ferrite transformer's primary: the centre one goes straight to battery plus
✓An SG3525 or TL494 (16-pin PWM IC) nearby
✓Fast diodes and a big 400–450 V electrolytic after the transformer: an H-bridge sits behind it

Where it's used

Car inverters (12 V → 230 V), the battery → DC bus stage of UPSs and hybrid inverters, 12/24/48 V DC-DC boosters

Common ICs

SG3525 →TL494 / KA7500 →UC3846 (akım modlu)

Go further

How it works: UPS How it works: hybrid inverter Next stage: full bridge and SPWM Concept: dead time Concept: transformer saturation Concept: snubber

Other topologies

Flyback Half-bridge (LLC)PFC (power factor correction)Forward Buck (step-down)Boost (step-up)Full bridge (H-bridge) and SPWM MPPT solar charge controller