FAQ
TL;DR: If your 5→15 V flyback only gives ~6 V, check transformer polarity, duty cycle, and snubbing; "The 555 is running at around 126kHz?" [Elektroda, Steve Lawson, post #21661335]
Why it matters: This FAQ helps beginners fix low-output flyback simulations and builds by tuning polarity, duty cycle, drive, and protection, fast.
Quick Facts
- Duty-cycle target (no regulation): Vo ≈ Vin·(Ns/Np)·D/(1−D), include ~0.7 V diode drop. A 5→15 V, Np=Ns case needs ≈76% D. [Elektroda, Earl Albin, post #21661341]
- Use a push‑pull BJT gate driver and ~10 Ω gate resistor to cap drive current ≲500 mA and prevent HF oscillation. [Elektroda, DAVID CUTHBERT, post #21661338]
- Add an RCD snubber (diode+capacitor series, resistor across C; optional Zener clamp) across the primary to absorb leakage energy. [Elektroda, DAVID CUTHBERT, post #21661338]
- Correct transformer polarity: dot on primary bottom, dot on secondary top; secondary must invert when the MOSFET turns off. [Elektroda, DAVID CUTHBERT, post #21661334]
- Flybacks are load‑dependent; use voltage (and often current) feedback to avoid runaway or sag. [Elektroda, DAVID CUTHBERT, post #21661338]
Why am I only getting ~6 V instead of 15 V from my 5→15 V flyback?
Common causes are reversed secondary polarity, insufficient duty cycle, weak MOSFET drive, or missing snubber. Verify transformer dots, then set duty cycle using Vo ≈ Vin·(Ns/Np)·D/(1−D). With Np=Ns, 5→15 V needs about 76% duty. Ensure a proper gate driver and add an RCD snubber to control leakage spikes. Without feedback, output varies with load. "Use voltage feedback on flybacks." [Elektroda, Earl Albin, post #21661341]
How do I check if the transformer polarity is wrong in my simulation or build?
Scope the secondary before the rectifier. The upper end should swing negative when the MOSFET turns on, then positive on turn‑off. If your rectified output is low, swap secondary leads and retest. Add polarity dots: bottom on primary, top on secondary. This fixes the classic inverted secondary issue. [Elektroda, DAVID CUTHBERT, post #21661334]
Should I put a diode directly across the primary winding?
No. Do not clamp the primary like a buck inductor. A flyback must “fly back” to a higher voltage so energy transfers to the secondary. Use a proper RCD snubber to handle leakage energy and protect the MOSFET instead. "NO, DO NOT connect a diode across the primary winding." [Elektroda, DAVID CUTHBERT, post #21661338]
Can a 555 timer drive the MOSFET directly at ~100–130 kHz?
A 555 struggles with MOSFET gate charge at these speeds. Use a small complementary BJT pair (e.g., 2N2222/2N2907) as a totem‑pole driver and add ~10 Ω in series with the gate to limit peak current and tame ringing. The 555 alone suits ≈20 kHz. [Elektroda, DAVID CUTHBERT, post #21661338]
What frequency was the example in the thread running at? Does it matter?
One build ran near 126 kHz. Frequency matters because your transformer must be designed for it. Core choice, air gap, and turns depend on operating frequency. Mismatch raises losses and hurts regulation. Confirm your magnetics suit your chosen fSW. "The 555 is running at around 126kHz?" [Elektroda, Steve Lawson, post #21661335]
How do I set duty cycle for a simple, open‑loop flyback?
Use Vo ≈ Vin·(Ns/Np)·D/(1−D), including ~0.7 V for the diode. For 5→15 V with Np=Ns, D ≈ 15.7/(5+15.7) ≈ 0.76. Adjust the 555’s RC network to reach that duty. Expect voltage to vary with load without feedback. [Elektroda, Earl Albin, post #21661341]
What’s a quick 3‑step checklist to diagnose low output in a flyback?
- Verify transformer polarity with dots and, if needed, reverse the secondary. 2. Set duty cycle using the open‑loop equation and your turns ratio. 3. Add proper gate drive and an RCD snubber to control leakage spikes. [Elektroda, DAVID CUTHBERT, post #21661334]
What is an RCD snubber and where does it go?
An RCD snubber is a diode in series with a capacitor across the primary, with a resistor across the capacitor. It absorbs leakage inductance energy and limits drain voltage. Add an optional Zener across the resistor for a hard clamp during startup. [Elektroda, DAVID CUTHBERT, post #21661338]
Why does my flyback’s output rise with light load or sag under heavy load?
Flybacks transfer stored magnetizing energy per cycle. If the load does not draw that energy, the output climbs; heavy loads draw more and drop voltage. Implement voltage feedback (and often inner current loop) to stabilize regulation across loads. [Elektroda, DAVID CUTHBERT, post #21661338]
What is discontinuous‑mode (DCM) operation in a flyback?
In DCM, primary current falls to zero before the next cycle. Output then depends on duty cycle, inductance, and load. Without regulation, Vo/Vi ≈ D·√(Rload/(2·L)). Provide magnetic parameters if you need precise design guidance. [Elektroda, Earl Albin, post #21661342]
Do I need an air gap in the flyback transformer?
Yes. Flybacks store energy in the magnetic path. Designers introduce an air gap so magnetizing energy resides in air, not the ferrite. This prevents core saturation and enables predictable energy transfer each cycle. [Elektroda, DAVID CUTHBERT, post #21661338]
Is my circuit accidentally an autotransformer instead of an isolated flyback?
If the primary and secondary share a node, you created an autotransformer. That removes galvanic isolation. For isolation, keep primary and secondary windings separate and couple only magnetically through the core. Consider a true boost converter if isolation is not needed. [Elektroda, Steve Lawson, post #21661337]
What happens at startup and why can MOSFETs fail there?
At startup the leakage energy and drain spikes peak. Without a proper snubber or clamp, the MOSFET can avalanche and fail. Use an RCD snubber and consider a Zener clamp to cap the drain voltage during this stress condition. [Elektroda, DAVID CUTHBERT, post #21661338]
Which rectifier diode type should I use on the secondary?
Use a fast diode suited to your frequency and current. Schottky diodes are typical for low‑voltage secondaries due to low Vf and fast recovery, reducing losses and ripple at ~100 kHz switching. Size it to expected current. [Elektroda, Steve Lawson, post #21661335]
What is a 555 timer in this context?
A 555 timer is a simple oscillator used to set switching frequency and duty cycle. In open‑loop flybacks, it provides the gate drive waveform but no regulation. Add a gate driver stage for higher frequencies and MOSFET gate charge. [Elektroda, Steve Lawson, post #21661335]
Edge case: Why did reversing the secondary instantly fix my low output?
Your original polarity opposed energy transfer after MOSFET turn‑off. Reversing the secondary aligned the rectifier with the positive flyback pulse, restoring correct charging of the output capacitor and full voltage. Always mark and verify dots. [Elektroda, DAVID CUTHBERT, post #21661334]
