Inductor Voltage Reversal and Negative Voltage Generation
This document explains how inductors work in switching converters and how voltage polarity reversal enables negative voltage generation from positive input voltage.
Overview
Understanding inductor behavior is crucial for comprehending inverting buck-boost converters like Diagram4 (+15V → -13.5V). The key concept is voltage polarity reversal - when the switch opens, the inductor's voltage flips to maintain current flow, creating negative voltage.
Table of Contents
- Inductor Fundamentals
- Voltage Polarity Reversal Mechanism
- How Negative Voltage is Created
- Component Roles in the Circuit
- Common Misconceptions
- Analogies for Understanding
Inductor Fundamentals
What is an Inductor?
An inductor stores energy in a magnetic field when current flows through it.
╔═══════╗
────╢ Inductor╟────
╚═══════╝
Current flows → Magnetic field forms → Energy stored
The Fundamental Law
The relationship between voltage and current in an inductor:
V = L × di/dt
V : Voltage across inductor [V]
L : Inductance [H]
di/dt : Rate of change of current [A/s]
Critical Points:
- Voltage depends on rate of change of current, NOT the current itself
- When current increases (di/dt > 0): Voltage has one polarity
- When current decreases (di/dt < 0): Voltage reverses polarity
Lenz's Law
Inductors resist changes in current:
- Trying to increase current → Inductor opposes with counter-voltage
- Trying to decrease current → Inductor opposes with reversed voltage
This is Lenz's Law: the induced voltage always opposes the change that created it.
Voltage Polarity Reversal Mechanism
Why Does Polarity Reverse?
The inductor acts differently depending on whether it's being driven or is driving:
Phase 1 - Being Driven (Switch ON, current increasing):
+15V (External source)
│
[Switching Node]
│ ╔═══════╗
└──╢ L3 ╟──┐
+ ╚═══════╝ │ - ← Voltage polarity
│
GND (0V)
• Current flows DOWN ↓
• Current is increasing (di/dt > 0)
• External source (VIN) pushes current through inductor
• Inductor resists with back-EMF (top +, bottom -)
Phase 2 - Driving Itself (Switch OFF, current tries to decrease):
[Switching Node] ~-14V!
│ ╔═══════╗
└──╢ L3 ╟──┐
- ╚═══════╝ │ + ← Polarity REVERSED!
│
GND (0V)
• Current STILL flows DOWN ↓ (same direction)
• But switch is open, so current wants to decrease (di/dt < 0)
• Inductor fights this by REVERSING voltage polarity
• Switching node drops BELOW GND to maintain current
The Key Insight: Source vs Load
Switch ON: External source → Inductor acts as load (absorbs energy)
Voltage: + on top, - on bottom
Switch OFF: Inductor → Acts as source (releases energy)
Voltage: - on top, + on bottom (REVERSED!)
This is NOT special component design - it's fundamental physics that ALL inductors exhibit!
How Negative Voltage is Created
Circuit Configuration (Diagram4: +15V → -13.5V)
Inverting Buck-Boost Circuit (LM2596S-ADJ, U4):
┌──────────────────┐
│ LM2596S-ADJ │
+15V IN ─────────────────────┤1 VIN │
│ (U4) │
│ │
│ ON/OFF 5 ──┼──○ (Float = ON)
│ │
│ FB 4──┼─── (Feedback)
│ │
│ OUT 2──┼─── (Switching Node)
│ │
│3 IC GND ─────────┼──── -13.5V OUT
└──────────────────┘ ⚠️ IC GND at -13.5V!
Switching Section:
┌─────────────────────────┐
│ U4 pin 2 (OUT) │
└──────┬──────────────────┘
│
Switching Node
│
┌────────────┴────────────┐
│ │
[ L3 ] D3 Cathode
Inductor │
100µH Schottky
│ SS34
│ 3A 40V
│ │
│ D3 Anode
│ │
System GND (0V) -13.5V OUT
(= IC GND pin 3)
Output Filter Capacitor:
System GND (0V) ──── C11 (470µF) ──── -13.5V OUT
(+ terminal) (- terminal)
Bootstrapped Ground Concept
Most Important Point:
IC GND pin (pin 3) is NOT connected to System GND (0V)!
Normal Buck Converter (Diagram2/3):
IC GND pin → System GND (0V) ✅ Standard reference
Inverting Buck-Boost (Diagram4):
IC GND pin → -13.5V OUT ⚠️ NOT at system ground!
This is called "bootstrapped ground" - the IC operates with its ground reference at -13.5V.
Step-by-Step Operation
Step 1: Energy Storage (Switch ON, ~3µs)
+15V ──[Switch CLOSED]── OUT pin ≈ +15V
│
[L3]
│
GND
Operation:
1. OUT pin at +15V (connected to VIN through internal switch)
2. Current flows DOWN through L3 to GND
3. Current increases (di/dt > 0)
4. Magnetic energy accumulates in L3
5. D3 is reverse-biased (cathode +15V > anode -13.5V)
→ D3 blocks current ✅
Step 2: Voltage Reversal (Switch OFF, ~3.7µs)
+15V ──[Switch OPEN]─X─ OUT pin ≈ -13.8V!
│
[L3]
│
GND
Operation:
1. Switch opens
2. Current tries to decrease (di/dt < 0)
3. L3 reverses voltage polarity to maintain current
4. OUT pin drops to ~-13.8V (BELOW System GND!)
5. D3 becomes forward-biased (cathode -13.8V < anode -13.5V)
→ D3 conducts current ✅
6. Current flows through D3 to -13.5V output
7. C11 charges negatively
Current Path During Switch OFF:
System GND (0V) ──→ L3 (UP!) ──→ OUT pin ──→ D3 ──→ -13.5V OUT
↑ (-13.8V) ↓ ↓
│ Cathode Anode
Inductor maintains (-13.8V) (-13.5V)
current by reversing ↓
voltage polarity C11 (- terminal)
↓
C11 (+ terminal)
↓
System GND (0V)
Loop: GND → L3 → D3 → C11 → GND
This charges C11 more negative each cycle
Step 3: Steady State (After Many Cycles)
Startup sequence:
Cycle 1: C11 = 0V → -0.5V
Cycle 10: C11 = -8V → -9V
Cycle 100: C11 = -13.3V → -13.5V ✅ Stable
Feedback loop maintains -13.5V regulation
PWM Operation
The circuit operates like PWM at 150kHz:
OUT pin (Switching Node) voltage waveform:
+15V ────┐ ┐ ┐ ┐
│ │ │ │ ← Switch ON periods
│ │ │ │
0V ─────┤ │ │ │
│ │ │ │
-13.8V └─────┘ └─────┘ ← Switch OFF periods
├─ON─┤├OFF┤├─ON─┤├OFF┤
Time →→→→→→→→→→→→→→→→→→→→→
Period = 1/150kHz ≈ 6.7µs
Duty cycle ≈ 50% (varies with load)
The output capacitor C11 filters this switching waveform into stable DC:
Switching Node (OUT pin):
+15V ─┐ ┐ ┐ ┐ ← Fast switching (150kHz)
│ │ │ │
-13.8V └───┘ └───┘
│
↓ (through D3 and L3)
C11 filters to:
-13.5V ─────────────── ← Steady DC (small ripple ~50mV)
Component Roles in the Circuit
1. L3 (Inductor): Voltage Generator ⚡
Role: Creates voltage reversal to generate negative voltage
Switch ON: VIN (+15V) forces current through L3
L3 stores energy in magnetic field
Switch OFF: L3 reverses voltage to maintain current
← This creates the negative voltage!
Switching node drops to -13.8V
Without L3: No voltage reversal → No negative voltage → Circuit doesn't work
2. D3 (Schottky Diode): One-Way Gate 🚪
Role: Controls current direction (does NOT generate voltage!)
Switch ON: D3 blocks current (reverse-biased)
Cathode (+15V) > Anode (-13.5V) → OFF
Switch OFF: D3 allows current (forward-biased)
Cathode (-13.8V) < Anode (-13.5V) → ON
Without D3: No current path during switch-OFF → Voltage spikes → IC damage
Why Schottky Diode?
| Feature | Regular Diode | Schottky Diode |
|---|---|---|
| Forward voltage drop | ~0.7V | ~0.3V ⚡ Less loss |
| Switching speed | Slow (~µs) | Fast (~ns) ⚡ Better for 150kHz |
| Reverse recovery | Slow | Fast ⚡ Less noise |
| Basic function | ✅ One-way valve | ✅ One-way valve (same!) |
Schottky is more efficient, but doesn't have special negative voltage generation capability - it's still just a one-way valve!
3. C11 (Electrolytic Capacitor): Voltage Storage 🔋
Role: Stores charge and filters switching into stable DC
Switching waveform (OUT pin):
+15V ─┐ ┐ ┐ ┐ ← High-frequency switching (150kHz)
│ │ │ │
-13.8V └───┘ └───┘
↓ (via D3 and L3)
C11 filters to:
-13.5V ─────────────── ← Stable DC (ripple ~50mV)
Without C11: Output voltage oscillates ±15V → No stable -13.5V DC
Component Cooperation
Voltage Creator Current Director Voltage Storage
↓ ↓ ↓
╔═══════╗ ┌───┴───┐ ┌────────┐
─────╢ L3 ╟───┬────────┤ D3 ├───────────┤ C11 ├──── GND
╚═══════╝ │ └───────┘ └────────┘
│ (one-way (stores
(creates │ valve) negative
voltage │ voltage)
reversal) │
│
OUT pin
(switches ±15V)
Common Misconceptions
Misconception 1: "The diode creates negative voltage"
❌ Wrong: Diode generates negative voltage ✅ Correct: Inductor (L3) creates voltage reversal; diode just routes current at the right time
Misconception 2: "FB pin current makes GND go below 0V"
❌ Wrong: FB pin accepts too much current, pulling GND negative ✅ Correct: Switching action and inductor voltage reversal charge C11 negatively
Misconception 3: "Schottky diode has special voltage inversion feature"
❌ Wrong: Schottky has special negative voltage generation capability ✅ Correct: Schottky is more efficient (lower drop, faster switching) but still just a one-way valve
Misconception 4: "Inductor just smooths the output like a capacitor"
❌ Wrong: Inductor only smooths output voltage ✅ Correct: Inductor CREATES the negative voltage through voltage reversal; capacitor SMOOTHS it
True Roles Summary
| Component | Misconception | Actual Role |
|---|---|---|
| L3 | Stores current | ✅ Creates voltage reversal (negative voltage source!) |
| D3 | Generates negative voltage | ❌ Just a one-way valve (efficient timing control) |
| C11 | Just filtering | ✅ Filtering + stores negative charge (battery-like) |
| IC GND | At 0V reference | ❌ At -13.5V (bootstrapped ground) |
Analogies for Understanding
Why the "Water Height" Analogy Fails
Traditional voltage analogy:
Voltage = Height:
High voltage = Mountain top
Low voltage = Valley
Current flows downhill ✅
But for inductors:
Voltage suddenly reverses (+15V → -14V)
"The mountain flips upside down!" 🤔
This doesn't make sense in water analogy ❌
The problem: Water-height assumes voltage is a fixed reference (like gravity). Inductors generate voltage dynamically, so the "mountain" can flip!
Better Analogy 1: Water Balloon 🎈
Capturing the "push back" concept:
Filling balloon (Switch ON):
Water flows in → Balloon expands → Stores pressure
(Current flows → Magnetic field builds → Stores energy)
Stop filling (Switch OFF):
Balloon pushes water back! → Water flows out
(Magnetic field collapses → Voltage reverses → Current continues)
This is good! The inductor "pushes back" to maintain current.
Better Analogy 2: Flywheel (Best!) 🎡
The flywheel (spinning wheel) perfectly captures voltage reversal:
Pushing flywheel (Switch ON):
You push → Flywheel spins faster → Stores rotational energy
(Voltage applied → Current increases → Stores magnetic energy)
Let go (Switch OFF):
Flywheel keeps spinning → Now it pushes YOU! → Force reverses
(Switch opens → Current continues → Voltage reverses!)
Why this is the best analogy:
- ✅ Explains why "force" (voltage) reverses direction
- ✅ Shows continuous motion (current) even when you stop pushing
- ✅ The "world flip" feeling makes sense - flywheel becomes the driver
- ✅ Demonstrates "current inertia" - resistance to changes
Inductor vs Capacitor: Duality
They're complementary opposites:
| Property | Capacitor | Inductor |
|---|---|---|
| Stores | Voltage (electric field) | Current (magnetic field) |
| Resists changes in | Voltage | Current |
| Energy formula | E = ½CV² | E = ½LI² |
| V-I relationship | I = C × dV/dt | V = L × di/dt |
| DC steady state | Acts like open circuit (blocks DC) | Acts like short circuit (passes DC) |
Key difference:
Capacitor:
──┤├── BLOCKS DC current after charging
Inductor:
──╫╫── PASSES DC current after stabilizing
They're not the same - they're dual to each other!
Summary
The Three Core Principles
- Inductor Voltage Reversal
- V = L × di/dt fundamental law
- When current decreases, voltage polarity reverses
- This is basic physics of ALL inductors (not special components)
- Switching Creates Negative Voltage
- Switch ON: L3 stores energy (+15V → GND)
- Switch OFF: L3 reverses voltage (OUT pin → -13.8V!)
- Repeating at 150kHz continuously generates negative voltage
- Bootstrapped Ground
- IC GND pin connected to -13.5V output
- IC operates with -13.5V as its ground reference
- FB pin at -13.5V + 1.23V = -12.27V (system GND reference)
- But IC sees +1.23V (normal operation)
Complete Operation Flow
PWM Switching (IC)
↓
Inductor Creates Negative Voltage (L3)
↓
Diode Controls Timing (D3)
↓
Capacitor Smooths Output (C11)
↓
Stable -13.5V DC Output ✅
↓
Feedback Regulates Voltage (R5/R6)
Applications
This inverting buck-boost technique is widely used in:
- Modular synthesizer ±12V power supplies
- Op-amp negative supply generation
- Camera flash charging circuits
- Automotive ignition coils
- LCD panel backlight drivers
Related Documentation
- Circuit Diagrams - Complete circuit schematics
- Buck Converter Feedback - How feedback regulation works
- LM2596S-ADJ Component - IC datasheet information
Document created: 2026-01-04 Circuit reference: Diagram4 Inverting Buck-Boost (+15V → -13.5V) IC used: LM2596S-ADJ (U4)