A flyback converter does not transfer energy through its primary and secondary windings at the same time. It first stores magnetic energy during the primary switch ON interval and then delivers that stored energy to the secondary during the switch OFF interval. This chapter explains those current paths, voltage polarities, waveforms, equations, transitions, and measurement points in practical engineering terms.
By the end of this chapter, you should be able to identify the current path in each switching state, calculate the primary current ramp and stored energy, relate primary and secondary current through the turns ratio, estimate demagnetization time, distinguish physical winding current from primary-referred magnetizing current, and recognize the nonideal effects that shape real converter waveforms.
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DESIGN ASSUMPTION — IDEALIZED POWER-STAGE MODEL
The first sections use an idealized flyback model to make the energy-transfer sequence clear. Unless otherwise stated, the model assumes an ideal switch, ideal transformer coupling, negligible winding resistance, negligible semiconductor voltage drop, no leakage inductance, no parasitic capacitance, and input and output voltages that remain approximately constant during one switching period.
Real leakage inductance, winding capacitance, MOSFET output capacitance, diode behavior, resistance, core loss, and switching-transition time are added later in this chapter. The idealized model establishes the physical mechanism; it is not a complete production model.
1. Flyback Energy Transfer Is a Two-Interval Process
The simplified flyback power stage contains a primary switch, a coupled magnetic component, a secondary rectifier, an output capacitor, and the load. The transformer provides isolation and voltage scaling, but its magnetizing inductance also acts as the intentional energy-storage element.
The switching period is divided into two principal energy-processing intervals:
- Switch ON — energy storage: the input voltage is applied to the primary winding, primary current rises, and magnetic-field energy increases. The secondary rectifier is reverse biased in the ideal model.
- Switch OFF — energy delivery: the winding voltage polarities reverse, the secondary rectifier conducts, and the stored magnetizing energy is delivered to the output.
A third interval can appear in discontinuous conduction mode after the transformer has fully demagnetized. During that zero-current interval, neither primary nor secondary winding carries magnetizing current. Chapter 3 examines DCM, BCM, and CCM in detail.
| Power-Stage Element | Role During Energy Transfer |
|---|---|
| Primary MOSFET | Connects the input source to the primary winding during the energy-storage interval. |
| Flyback transformer magnetizing inductance | Stores energy while primary current rises and establishes the current waveform. |
| Turns ratio | Relates the secondary output voltage to the primary reflected voltage and scales winding currents. |
| Secondary rectifier | Blocks during primary energy storage and conducts during secondary energy delivery. |
| Output capacitor | Supplies the load while the secondary is off and absorbs pulsed secondary current while the transformer delivers energy. |
| Clamp or snubber | Manages leakage-inductance energy and limits primary-switch voltage stress in the real converter. |
SolidMag Engineering Insight
A Flyback Converter Separates Energy Storage from Energy Delivery
In the ideal flyback mechanism, energy is not transferred continuously from primary to secondary. The primary winding stores energy in the magnetizing inductance during switch ON time; the secondary winding delivers that energy during switch OFF time.
This time-separated behavior is why magnetizing inductance, peak current, turns ratio, demagnetization time, leakage inductance, output capacitance, and control mode are tightly connected design variables.
2. Winding Polarity and the Two Switching States
The dot convention on a transformer schematic identifies the relative instantaneous polarity of the windings. When voltage makes the dotted end of one winding positive, the dotted ends of the other windings become positive relative to their undotted ends according to the indicated turns ratio.
The flyback rectifier is oriented so that the secondary is reverse biased while the primary switch is ON. When the MOSFET turns OFF, the magnetizing current forces the winding voltages to reverse polarity. The secondary rectifier then becomes forward biased and provides a path for the current required to continue the magnetic state.
The winding phasing determines whether the secondary diode blocks during energy storage and conducts during energy delivery. Reversing one winding or misinterpreting the dot convention can short the output path, prevent energy transfer, or produce destructive voltage stress.
Figure 2-1. Idealized flyback current paths and winding polarities during the MOSFET ON and OFF intervals.
ENGINEERING CAUTION — VERIFY WINDING PHASE BEFORE POWER-UP
Do not rely only on physical winding direction or pin order when verifying polarity. Confirm the dot convention, start and finish leads, rectifier orientation, and measured winding polarity before applying full converter voltage.
A low-voltage pulse test or approved transformer test method can verify relative winding polarity before the transformer is installed in a high-energy converter.
3. MOSFET ON Interval — Energy Storage
When the MOSFET turns ON, the input source applies voltage across the primary winding. In the ideal model, the secondary diode is reverse biased, so physical secondary winding current is zero. The load is supplied by the output capacitor during this interval.
Primary Current Ramp
The primary current slope is determined by the voltage applied to the primary winding and the primary-referred magnetizing inductance Lm:
For approximately constant primary voltage during the ON interval:
Using duty cycle D and switching frequency fs:
In DCM and BCM, the magnetizing current begins near zero, so the current change is approximately the peak primary current. In CCM, the current begins at a nonzero minimum value and rises by ΔIP to the peak value.
Stored Magnetic Energy
Within the approximately linear operating region, the energy associated with the magnetizing inductance is:
During the idealized ON interval, the primary winding current equals the primary-referred magnetizing current. The stored energy therefore rises with the square of current. A modest increase in peak current can create a much larger increase in required energy storage and magnetic stress.
For DCM or BCM starting near zero current, the peak stored energy is:
SolidMag Engineering Insight
The Core Does Not “Consume” the Energy
The flyback transformer stores magnetic-field energy associated with its magnetizing inductance. In a deliberately gapped ferrite design, much of the incremental field energy is concentrated in the air-gap region, while the ferrite provides the low-reluctance flux path.
The core, gap, turns, and current waveform must therefore be designed together. Saying that the energy is stored “in the core” is a useful shorthand, but it is not a complete physical description of a gapped flyback magnetic circuit.
4. Turn-Off Transition — Current Commutation and Leakage Energy
When the MOSFET turns OFF, current in an inductance cannot change instantaneously. The magnetizing current therefore forces the transformer winding voltages to reverse polarity until a secondary conduction path is established.
In the ideal model, physical primary winding current falls to zero after the switching transition, while secondary winding current begins. The magnetic state is continuous, but the current is now carried by the secondary winding. This distinction is important when interpreting simulated or measured waveforms.
Primary-Referred Magnetizing Current vs. Physical Primary Current
A primary-referred magnetizing-current waveform is a mathematical representation of the magnetic state referred to the primary side. It can be drawn continuously through both ON and OFF intervals. Physical primary winding current, however, flows only while the primary switch or another primary-side path conducts.
ENGINEERING CAUTION — LABEL WAVEFORMS PRECISELY
Do not label a continuous primary-referred magnetizing-current trace simply as “primary current” during the OFF interval. That wording can imply that current continues through the open MOSFET.
Use “physical primary winding current” for the measured primary current and “magnetizing current, primary-referred” for the continuous state-variable representation.
Leakage-Inductance Current
Not all primary leakage flux couples to the secondary winding. Energy stored in primary-referred leakage inductance cannot transfer through ideal transformer action and must flow into the clamp, snubber, MOSFET capacitance, transformer capacitance, or other parasitic paths during turn-off.
A simplified primary-switch peak-voltage relationship is:
The ideal reflected voltage VR is a normal part of flyback operation. The additional spike and ringing arise from leakage inductance, circuit inductance, parasitic capacitance, switching speed, and clamp behavior.
Figure 2-2. Real turn-off transition showing magnetizing-current commutation to the secondary and leakage-energy flow into the primary clamp and parasitic capacitances.
5. MOSFET OFF Interval — Energy Delivery
After the winding voltages reverse, the secondary rectifier conducts. The stored magnetizing energy is then delivered to the output capacitor and load. The secondary current begins at its highest value and decreases as the magnetizing energy is removed.
Reflected Output Voltage
Define the primary-to-secondary turns ratio as n = NP/NS. The output voltage and rectifier drop are reflected to the primary as:
During the idealized OFF interval, the primary winding voltage is approximately negative VR. That negative voltage causes the primary-referred magnetizing current and core flux to decrease.
Initial Secondary Current
Immediately after ideal current commutation, ampere-turn balance gives the approximate initial secondary current:
A low-voltage secondary with relatively few turns can therefore carry a much larger peak current than the primary winding. This is one reason secondary conductor, rectifier, termination, and PCB current paths require separate analysis.
Secondary Current Decay
The primary magnetizing inductance referred to the secondary is:
With an approximately constant output voltage, the idealized secondary current slope is:
The current decreases until the next switching event or until the transformer demagnetizes completely. The exact endpoint determines whether the converter operates in CCM, BCM, or DCM.
6. Volt-Second Balance and Demagnetization Time
In steady-state operation, the positive primary volt-seconds applied during switch ON time must be balanced by the negative reflected volt-seconds applied during secondary conduction. Otherwise, the magnetic operating point would drift from cycle to cycle.
Using switching-period fractions:
Therefore, the secondary conduction fraction is:
In DCM, a positive zero-current fraction remains after demagnetization:
| Operating Mode | End of Secondary Conduction | Energy-Transfer Consequence |
|---|---|---|
| DCM | Secondary current reaches zero before the next switching cycle. | A positive zero-current interval remains. All idealized magnetizing energy is delivered before the next ON interval. |
| BCM / CrCM | Secondary current reaches zero at approximately the next turn-on instant. | The zero-current interval approaches zero. |
| CCM | Secondary current does not reach zero before the next switching cycle. | Residual magnetizing current and stored energy remain for the next cycle. |
Chapter 3 develops these operating modes, current relationships, control implications, and magnetic-design consequences in greater detail.
7. Worked Example — One Complete DCM Energy-Transfer Cycle
Worked Example: 48 V Input, 12 V Output, 100 kHz DCM Cycle
This simplified example calculates the primary current ramp, peak stored energy, reflected voltage, secondary peak current, demagnetization time, and zero-current interval for one idealized switching condition.
| Parameter | Assumed Value |
|---|---|
| Primary ON voltage, VP,ON | 48 V |
| Magnetizing inductance, Lm | 120 µH |
| Switching frequency, fs | 100 kHz |
| Duty cycle, D | 0.35 |
| Primary-to-secondary turns ratio, NP/NS | 4.0 |
| Output voltage, VO | 12 V |
| Secondary rectifier drop, VD | 0.5 V |
Step 1 — Calculate ON Time
Step 2 — Calculate Peak Primary Current
Step 3 — Calculate Peak Stored Energy
If this energy were transferred once per cycle without loss, the corresponding idealized input power would be approximately 11.76 W.
Step 4 — Calculate Reflected Voltage
Step 5 — Calculate Initial Secondary Current
Step 6 — Calculate Secondary Conduction Time
Step 7 — Verify the Zero-Current Interval
The switching period is 10 µs. The remaining zero-current time is:
Because the zero-current interval is positive, this operating point is in DCM under the stated ideal assumptions.
EXAMPLE LIMITATIONS
The example neglects leakage inductance, winding resistance, core loss, semiconductor voltage drops other than the stated rectifier drop, clamp behavior, switching-transition time, output-voltage ripple, and control tolerances.
A production design must replace the assumed values with worst-case converter conditions and verify the completed transformer and switching waveforms in hardware.
8. The Output Capacitor’s Role
The output capacitor makes the pulsed flyback energy delivery appear as a relatively continuous output voltage and load current. It performs two different functions during the switching cycle.
During Primary Energy Storage
While the MOSFET is ON and the secondary rectifier is blocking, the output capacitor supplies the load. The ideal capacitor current is approximately:
During Secondary Energy Delivery
When the secondary rectifier conducts, the secondary current splits between the load and the output capacitor:
When secondary current exceeds load current, the capacitor charges. Later in the interval, the secondary current may fall below load current and the capacitor again supplies part of the load.
Output-capacitor selection must therefore consider capacitance, ESR, RMS ripple current, temperature, control-loop behavior, transient response, lifetime, and the pulsed nature of secondary current. The capacitor is an active participant in flyback energy transfer, not merely an output filter added after the transformer is designed.
9. Core Flux and Stored Energy Through the Cycle
The primary and secondary voltage waveforms establish the rate of change of core flux. Faraday’s law gives:
During the ON interval, positive primary voltage causes flux and magnetizing current to move in one direction. During secondary conduction, the reflected voltage across the primary reverses sign and drives the flux back toward its next-cycle value.
In DCM and BCM, magnetizing current returns to approximately zero, but the ferrite can retain remanent flux. It is more precise to say that the incremental flux associated with the magnetizing current returns toward its starting point than to claim that absolute core flux becomes exactly zero.
In CCM, current and stored energy remain nonzero at the beginning of the next cycle. The AC flux excursion can be smaller than the absolute peak flux, so both ripple and bias must be considered when checking core loss and saturation margin.
SolidMag Engineering Insight
Current, Flux, and Energy Are Different Views of the Same Magnetic State
The applied winding voltage determines the flux slope, magnetizing inductance relates current to flux linkage, and the current determines stored magnetic energy. These are not separate design problems.
A self-consistent flyback design should produce compatible results when the same operating point is checked from volt-seconds, current ramp, stored energy, turns, core area, and reflected-voltage relationships.
10. Reading Flyback Current and Voltage Waveforms
A clear flyback waveform diagram should distinguish physical winding currents from referred magnetic variables and should identify each switching interval. The most useful traces are summarized below.
| Waveform | Idealized Behavior | What It Reveals |
|---|---|---|
| MOSFET gate voltage | High during primary energy storage; low during secondary energy delivery. | Switching timing, duty cycle, dead time, and controller behavior. |
| Physical primary winding current | Rises during the ON interval and falls to zero after turn-off commutation. | Peak current, current-limit margin, RMS current, and possible saturation behavior. |
| Secondary winding current | Zero during the idealized ON interval; begins at a peak and decays during OFF time. | Energy-delivery interval, rectifier stress, RMS current, and operating mode. |
| Magnetizing current, primary-referred | Represents the continuous magnetic state referred to the primary side. | Residual current, ripple, DCM/BCM/CCM classification, and stored energy. |
| MOSFET drain voltage | Low while ON; rises to input plus reflected voltage and nonideal spike while OFF. | Reflected voltage, clamp behavior, ringing, and voltage margin. |
| Core flux density | Rises during primary excitation and falls during secondary demagnetization. | Volt-second balance, bias, saturation margin, and core-loss waveform. |
| Stored magnetic energy | Rises with current during ON time and falls as energy is delivered during OFF time. | Energy per cycle and operating-mode behavior. |
| Output voltage | Small ripple around the regulated value. | Capacitor performance, regulation, load transient behavior, and control-loop response. |
Figure 2-3. One complete flyback switching cycle showing gate drive, physical primary current, secondary current, primary-referred magnetizing current, drain voltage, flux density, and stored magnetic energy.
11. Nonideal Effects That Shape Real Waveforms
The ideal two-interval model explains the principal energy-transfer mechanism, but real converter waveforms include additional transitions, losses, resonances, and delays.
| Nonideal Effect | Waveform or Energy-Transfer Consequence |
|---|---|
| Leakage inductance | Creates turn-off voltage spike, clamp or snubber loss, and ringing because leakage energy does not couple ideally to the secondary. |
| MOSFET output capacitance | Absorbs and releases switching-node energy; participates in drain ringing and valley switching. |
| Transformer winding capacitance | Transfers displacement current, affects common-mode EMI, and participates in resonances. |
| Winding resistance and AC resistance | Reduce delivered energy and generate winding heat. |
| Core loss | Converts part of the cyclic magnetic energy into heat. |
| Secondary rectifier drop and recovery behavior | Change reflected voltage, secondary current, loss, ringing, and reverse stress. |
| Finite switching transition time | Creates overlap loss and alters the exact commutation interval. |
| Clamp or snubber behavior | Limits switch voltage but can dissipate or recover leakage energy depending on topology. |
| PCB loop inductance | Adds voltage overshoot and ringing that are not part of transformer leakage alone. |
These effects do not invalidate the ideal model. They explain why a production flyback converter must be analyzed as a transformer, semiconductor, clamp, capacitor, and PCB system rather than as isolated components.
SolidMag Engineering Insight
The Switching Transition Is Part of the Energy Path
At turn-off, magnetizing energy should transfer to the secondary, while leakage energy follows a different path through the clamp and parasitic capacitances. The measured drain waveform is therefore a direct window into both intended and unintended energy flow.
A transformer with acceptable magnetizing behavior can still produce poor efficiency, excessive voltage stress, or EMI if leakage, capacitance, clamp design, and PCB layout are not controlled.
12. Measuring and Verifying Energy Transfer
Bench measurements should confirm the switching sequence, current slopes, reflected voltage, demagnetization interval, clamp behavior, and output-capacitor response predicted by the design.
HIGH-VOLTAGE MEASUREMENT CAUTION
Offline flyback converters can contain lethal voltages and energy. Use appropriately rated isolated or differential probes, current probes, protective equipment, discharge procedures, creepage, and laboratory practices. Never connect an earth-referenced oscilloscope ground lead directly to a high-side switching node unless the measurement arrangement is explicitly designed for it.
Only qualified personnel should probe energized mains-referenced converters.
Useful measurements include:
- Gate-to-source voltage: confirms switching timing, duty cycle, burst behavior, and dead time.
- Primary current: confirms current slope, peak current, current limit, operating mode, and possible saturation behavior.
- MOSFET drain voltage: confirms reflected voltage, leakage spike, clamp level, and ringing.
- Secondary or rectifier current: confirms current commutation, peak current, demagnetization time, and DCM/BCM/CCM behavior.
- Output voltage ripple: confirms capacitor charge balance, ESR contribution, and regulation.
- Transformer temperatures: confirms that the intended energy path is not producing excessive core, winding, gap-fringing, or termination loss.
Correlate the waveform timing. The primary current should rise while the MOSFET is ON. Secondary current should begin after turn-off and decay according to the reflected output voltage. The drain voltage should settle near input plus reflected voltage after the leakage transition. Deviations point toward incorrect phasing, unexpected leakage, clamp behavior, parasitics, control timing, saturation, or measurement error.
13. Chapter Summary and Technical References
The essential flyback energy-transfer sequence can be summarized as follows:
- The primary MOSFET applies voltage to the primary winding during the ON interval.
- Primary magnetizing current and stored magnetic energy increase during ON time.
- The secondary rectifier blocks in the ideal model while the output capacitor supplies the load.
- Turn-off reverses winding polarity and commutates the magnetic state to the secondary winding.
- Secondary current begins at a peak value determined by primary peak current and turns ratio, then decays as energy is delivered.
- Volt-second balance links primary ON time, reflected voltage, and secondary demagnetization time.
- DCM, BCM, and CCM are distinguished by the magnetizing current remaining at the next switching cycle.
- Leakage inductance, capacitance, resistance, semiconductor behavior, and PCB inductance shape real waveforms and losses.
- Measured current and voltage waveforms should confirm the intended energy path before the design is released.
SolidMag Engineering Insight
A Flyback Transformer Is Designed Around the Complete Switching Cycle
The primary winding cannot be designed from the ON interval alone, and the secondary cannot be designed from the OFF interval alone. The same stored energy, magnetizing current, flux trajectory, turns ratio, losses, and timing connect both intervals.
A robust design treats one complete switching cycle as a closed electrical and magnetic energy process, then verifies that process across input voltage, load, frequency, temperature, tolerance, startup, and transient conditions.
Technical References for This Chapter
- Texas Instruments — Under the Hood of Flyback SMPS Designs (SLUP254)
- Texas Instruments — Designing a DCM Flyback Converter
- Texas Instruments — How to Design a Flyback Converter with the LM3481 Boost Controller
- Infineon — 40 W Flyback Evaluation-Board Engineering Report
Related SolidMagnetics Resources
Complete Flyback Transformer Design Guide
Review the full flagship guide for turns, cores, air gaps, windings, leakage, loss, thermal design, and the worked design example.
Chapter 3 — Operating Modes
Continue to DCM, BCM, and CCM Flyback Operation to examine current waveforms, mode boundaries, design tradeoffs, and control behavior.
