Transformer Polarity & Phasing: Why Direction Matters in Flyback Converters
Understanding transformer dot notation and why connecting a flyback transformer backwards will reverse your output voltage polarity!
The Question
When designing the LM2586 flyback converter circuit for voltage inversion (+15V → -15V), a critical question came up:
"About this transformer, is the direction important? Is it common knowledge of transformers?"
Short answer: Yes! Transformer polarity is absolutely critical for flyback converters. Connect it backwards and you'll get positive voltage instead of negative. This is fundamental knowledge for all switching power supply design.
What is Dot Notation?
Transformer windings have polarity marks (dots) that show which terminals are in-phase with each other.
MSD1514-473MED Coupled Transformer (4-pin SMD):
● Pin 1 (Dot marking - visible on PCB footprint)
┌─────────┐
│1 • 4│● Pins 1 & 4 have dots (in-phase)
│ T1 │
│2 3│ Pins 2 & 3 (opposite phase)
└─────────┘
Pin Assignment:
- Pins 1-2: Primary winding (47µH)
- Pins 3-4: Secondary winding (47µH)
- Dot on Pin 1 and Pin 4 = same magnetic polarity
What "in-phase" means: When Pin 1 is positive, Pin 4 is also positive at the same instant. When Pin 1 goes negative, Pin 4 also goes negative.
Physical Meaning of Dots
The dot indicates the start of the winding (where the wire begins). Transformers with dots in the same relative position have windings wound in the same direction.
Winding Direction (Conceptual):
Primary: Secondary:
Start • Start •
↓ 1 ↓ 4
│ │
│ 47µH │ 47µH
│ │
↓ 2 ↓ 3
End End
Same winding direction = In-phase
Why Polarity Matters for Flyback Converters
Flyback converters use transformer polarity to invert voltage. The secondary must be wound opposite to the primary's connection to create negative output.
Correct Connection (Voltage Inversion)
Flyback Topology (Correct Orientation):
+15V IN ──┤1 •T1 4│●──── GND
│ │
SW ─┤2 3├───── D4 Cathode → -15V OUT
Energy Flow:
1. SW closes: Current flows 1→2 in primary, storing magnetic energy
2. SW opens: Magnetic field collapses, inducing voltage in secondary
3. Due to winding polarity: Pin 3 swings NEGATIVE, Pin 4 stays at GND
4. D4 rectifies negative pulses → -15V DC output
Result: Voltage INVERTED (+15V → -15V) ✅
Wrong Connection (No Inversion!)
Flyback Topology (Incorrect - Rotated 180°):
+15V IN ──┤2 T1 3│──── GND
│ │
SW ─┤1 • 4│●──── D4 Cathode → +15V?? WRONG!
Energy Flow:
1. SW closes: Current flows 2→1 in primary
2. SW opens: Magnetic field collapses
3. Due to REVERSED polarity: Pin 4 swings POSITIVE, Pin 3 stays at GND
4. D4 blocks positive pulses (wrong direction!)
Result: Circuit doesn't work - no voltage inversion! ❌
What happens: The output will either be:
- Wrong polarity (+15V instead of -15V)
- No output (diode blocking everything)
- Damaged components (if diode conducts in reverse)
How Flyback Energy Transfer Works
Unlike normal transformers (which transfer energy continuously), flyback transformers work in two phases:
Phase 1: Energy Storage (Switch ON)
Switch Closed (SW pin connects to GND internally):
+15V ──┤1 •T1 │
│ │ Secondary side:
│ Primary │ No current flows
│ current │ (D4 blocks)
SW│ flows │
↓ │
GND GND
Magnetic field builds up in transformer core
Phase 2: Energy Release (Switch OFF)
Switch Open (SW pin floating):
│ │●4── GND
│ Primary │
│ voltage │ Secondary
│ spikes │ voltage
SW│ high │ appears
│ │ NEGATIVE
X (open) 3├─→ D4 → -15V OUT
Magnetic field collapses, energy transfers to secondary
Critical Point: The polarity of the secondary voltage during collapse depends on how the windings are connected relative to their dots!
Dot Convention Rules
Rule 1: Dots Show Positive Voltage at Same Time
When current enters a dotted terminal on the primary, voltage will be positive at the dotted terminal on the secondary.
Primary current enters Pin 1 (dot)
→ Secondary Pin 4 (dot) goes positive
→ Secondary Pin 3 (no dot) goes negative
Rule 2: For Voltage Inversion, Connect Opposite Polarity
For flyback inversion, we intentionally connect the secondary backwards relative to how we'd connect a normal step-down transformer:
Normal Step-Down Transformer:
Input+ → Primary dot Secondary dot → Output+
Input- → Primary (no dot) Secondary (no dot) → Output-
(Same polarity in/out)
Flyback Inverter:
Input+ → Primary dot Secondary dot → GND
Input- → Primary (no dot) Secondary (no dot) → Output-
(Opposite polarity = voltage inversion!)
PCB Assembly Considerations
Identifying Pin 1 on the Footprint
When placing T1 on the PCB, always verify the pin 1 marking:
MSD1514 Footprint Markings:
- Pin 1 dot (filled circle) on top-left or top-right corner
- Sometimes has beveled corner or notch
- Check component datasheet for orientation
Critical: If you rotate the component 180° during assembly, the transformer will be backwards!
Verification Checklist
Before PCB assembly:
- ✅ Pin 1 marking on footprint matches schematic
- ✅ Primary pins (1-2) connect to +15V and SW
- ✅ Secondary pins (3-4) connect to D4 cathode and GND
- ✅ Dot on Pin 4 connects to GND (for inversion)
- ✅ No-dot on Pin 3 connects to D4 cathode (becomes -15V)
Common Assembly Mistakes
| Mistake | Symptom | Fix |
|---|---|---|
| Component rotated 180° | No output or wrong polarity | Check pin 1 orientation, rotate component |
| Primary/secondary swapped | Extremely low output voltage | Verify pins 1-2 are primary, 3-4 are secondary |
| Dot connected to output instead of GND | Positive output instead of negative | Swap secondary connections (pins 3 and 4) |
Is This Common Knowledge?
For power supply designers: Yes, transformer phasing is fundamental.
For beginners: Often confusing and overlooked!
Where Polarity is Critical
- Flyback converters (like our LM2586 circuit) - polarity determines output sign
- Push-pull converters - wrong phasing causes shoot-through and damage
- Forward converters - affects reset winding operation
- Current transformers - reversed polarity gives wrong current direction
Where Polarity is Less Critical
- Isolation transformers - only affects which terminal is "hot" vs "neutral"
- Signal transformers - only affects signal phase (180° shift)
- Audio transformers - might invert audio signal (usually not critical)
Bottom line: In switching power supplies, transformer polarity is always critical. Never assume "transformer is just a transformer."
Practical Example: LM2586 Flyback Circuit (Historical Reference)
This section describes the old LM2586 flyback converter design that has been replaced with a simpler LM2596S inverting buck-boost topology. This information is kept for educational purposes only.
Current design: See Diagram4 (+15V → -13.5V inverting buck-boost using LM2596S U4)
In the old USB-PD power supply design, the LM2586 flyback converter generated -15V from +15V:
Complete Flyback Circuit (OLD Diagram4 - Not Used):
+15V IN ────┬────────────────────────────────┐
│ │
┌───┴────┐ │
│ C13 │ 100µF bulk │
│ C16 │ 100nF ceramic │
└───┬────┘ │
│ │
┌────┴──────────┐ │
│7 VIN SW 5├──┐ │
│ │ │ │
│ LM2586SX-ADJ │ │ ┌──────────┐ │
│ │ └──┤1 •T1 4│●───┘ (Pin 4 to +15V)
│ ON/OFF 1 ○ │ │ │
│ │ │ 47µH │
│ FB 3 │ │ :47µH │
│ ├────────────┼─────┤2 3├───┐
│ │ COMP 2 │ └──────────┘ │
└──┴──┬─────────┘ │
│ │ │ D4 (SS34)
R7 └─── R9 ─── C15 ▼ Cathode
10kΩ 3kΩ 47nF │
│ │ │ ├──→ -15V OUT
R8 GND GND │
910Ω │
│ C14
GND 100µF
│
GND
Component Values:
- T1: MSD1514-473MED (47µH:47µH, 1:1 ratio)
- Primary (Pins 1-2): Connected to +15V and SW
- Secondary (Pins 3-4): Pin 4 to GND, Pin 3 to D4 cathode
- D4: Schottky diode (SS34 or MBRS340)
- Output: -15V (inverted from +15V input)
Key Connection: Secondary Pin 4 (dot) → GND, Pin 3 (no dot) → D4 cathode
This configuration ensures:
- When SW closes: Primary stores energy
- When SW opens: Secondary Pin 3 swings negative
- D4 rectifies negative pulses → -15V output
If we reversed the transformer (rotated 180°):
- Primary would still work (just connected to pins 2-1 instead of 1-2)
- But secondary Pin 3 (no dot) would be at GND
- And secondary Pin 4 (dot) would swing positive
- D4 would block or conduct backwards → circuit failure!
Summary
Transformer polarity is critical for flyback converters because:
- Dots show in-phase terminals - when primary dot is positive, secondary dot is positive
- Flyback uses polarity to invert voltage - we intentionally connect secondary backwards
- Wrong polarity = wrong output - reversed transformer gives wrong output polarity or no output
- This is fundamental power supply knowledge - not optional, not something to guess
Remember: Always check the datasheet for dot notation, verify pin assignments, and never rotate a transformer component without understanding the polarity consequences!
References
- LM2586 Datasheet: Figure 16 (Flyback Regulator Application, page 14)
- Coilcraft MSD1514 Series: Coupled Inductors Datasheet
- Texas Instruments Application Note: "Flyback Transformer Design"
- Old Circuit (Not Used): Diagram4 used LM2586 Flyback (replaced with LM2596S inverting buck-boost)