If you’ve ever built a DC-DC converter, a sine wave inverter, or a high-efficiency switching regulator, chances are you’ve encountered a family of PWM controller ICs. Among them, the SG3525 stands out as a reliable, flexible, and widely copied classic.
Designed by Silicon General (now part of Microsemi) and second-sourced by virtually every major semiconductor company, the SG3525 has been powering inverters, battery chargers, and power supplies since the 1980s. Even in the age of digital microcontrollers, this analog IC remains a favorite for hobbyists and professionals alike.
In this post, we’ll explore what the SG3525 does, why it matters, and how you can use it in your next power project.
What Is the SG3525?
The SG3525 is a pulse width modulation (PWM) controller IC specifically designed for switching power supplies. It generates the control signals needed to drive power transistors (MOSFETs or IGBTs) in topologies like push-pull, half-bridge, and full-bridge converters.
Unlike simpler PWM chips (like the TL494 or SG3524), the SG3525 includes several advanced features:
Totem-pole outputs (capable of 200mA source/sink) that can directly drive power MOSFET gates.
Under-voltage lockout (UVLO) to prevent erratic operation at low supply voltages.
Soft-start capability to limit inrush current during startup.
Wide operating frequency range (typically 100Hz to 500kHz).
Dual alternating outputs for push-pull topologies.
In short, it's a complete control system on a single 16-pin DIP or SOIC package.
Pinout and Key Functions
Let's look at the essential pins you'll use most often:
| Pin | Name | Function |
|---|---|---|
| 1 | Inv. Input | Inverting input of the error amplifier |
| 2 | Non‑Inv. Input | Non‑inverting input of the error amplifier |
| 5 | CT | Timing capacitor (sets oscillator frequency with RT) |
| 6 | RT | Timing resistor |
| 7 | Discharge | Discharge pin for the timing capacitor |
| 8 | Soft‑Start | Connect a capacitor to ground for soft‑start timing |
| 9 | Compensation | Error amplifier output / compensation network |
| 11 | Output A | PWM output A (active high) |
| 14 | Output B | PWM output B (active high) – 180° out of phase with A |
| 15 | VCC | Supply voltage (8V to 35V) |
| 16 | Vref | 5.1V reference output (up to 50mA) |
The two outputs (A and B) are designed to drive a push-pull transformer. They are never on simultaneously – there's built-in dead time to prevent cross-conduction.
How It Works (Simplified)
The SG3525 contains five main building blocks:
Oscillator – Set by external resistor (RT) and capacitor (CT). The oscillator generates a sawtooth waveform that determines the switching frequency. Frequency ≈ 1 / (RT × CT). A discharge pin (pin 7) allows fine control of dead time.
Error Amplifier – A high‑gain op‑amp that compares a feedback voltage (from the power supply output) to a reference. Its output controls the pulse width.
PWM Comparator – Compares the error amplifier output with the oscillator sawtooth. As the error voltage rises, the output pulse width increases (or decreases, depending on polarity).
Output Logic – Drives two alternating outputs with programmable dead time. For push-pull, Output A turns on, then both off (dead time), then Output B turns on.
Protection Features – Under‑voltage lockout shuts down the IC if VCC drops below ~7V. A shutdown pin (pin 10) allows external overcurrent or overtemperature protection.
A Simple Example: 12V to 220V Push-Pull Inverter
One of the most common beginner projects with the SG3525 is a DC-AC inverter. Here’s a conceptual design:
Input: 12V battery.
Transformer: Center-tapped high-frequency ferrite core (e.g., ETD39).
MOSFETs: Two IRFZ44N (or similar) driven by Output A and Output B.
Oscillator: RT=10kΩ, CT=0.1µF → frequency ≈ 50kHz (with appropriate dead time).
Feedback: None (open loop) – just a simple 50Hz modulation? Actually, for a true sine wave, you'd need a second stage. But for a square wave or modified sine wave inverter, the SG3525 directly drives the primary.
The outputs alternately switch the MOSFETs, creating a high-frequency AC voltage across the primary. The transformer steps this up to 220V, and a rectifier/smoothing filter yields DC – or you can run a low-frequency H‑bridge to get 50/60Hz AC.
Why Choose SG3525 Over a Microcontroller?
You might ask: “Why not just use an Arduino or a dedicated digital PWM chip?”
Simplicity – No programming, no clock drift, no firmware bugs. The SG3525 works instantly with passive components.
Robustness – Analog controllers are immune to electromagnetic interference (EMI) that can crash a microcontroller.
Speed – The comparator and logic react within nanoseconds. Microcontrollers have PWM resolution limits and interrupt latencies.
Cost – At around $1–2, the SG3525 is cheaper than most MCU+driver combinations.
That said, microcontrollers offer programmability (e.g., MPPT algorithms, soft-start profiles). Many modern designs use a hybrid: an MCU for setpoint control and an SG3525 for fast, reliable switching.
Common Pitfalls and Tips
Grounding – Use a star ground. The high‑current MOSFET returns must not share the same path as the sensitive analog ground (pins 1,2,9).
Gate Drive Resistors – Add 10–22Ω resistors in series with each gate to dampen ringing.
Dead Time – The SG3525 has fixed dead time (~0.5–2µs depending on version). For high‑frequency designs, you may need external gate‑drive transformers.
Soft‑Start Capacitor – Always include at least 1µF on pin 8. Without it, the duty cycle jumps to maximum at power‑up, potentially blowing FETs.
Shutdown Pin – Pull pin 10 high (>0.8V) to disable outputs. Use a voltage divider or optocoupler for overcurrent protection.
Where to Find the SG3525 Today
Original SG3525s are available from Texas Instruments, STMicroelectronics, Onsemi, and many Chinese manufacturers (e.g., KA3525, UC3525 – almost identical). You can buy them from:
Through‑hole: DIP‑16 – easy for breadboards and perfboards.
SMD: SOIC‑16 – for compact designs.
Be cautious of counterfeit chips from unknown sellers. Stick with reputable distributors (Mouser, DigiKey, LCSC) or well‑reviewed eBay/Amazon sellers.
Final Thoughts
The SG3525 is a testament to good analog design. It has survived for nearly four decades because it does one thing – PWM control with push-pull outputs – and does it exceptionally well. For hobbyist inverters, adjustable power supplies, battery chargers, and even Tesla coil drivers, the SG3525 remains a go‑to solution.
So next time you need to switch a lot of power efficiently, skip the complex microcontroller. Grab an SG3525, a handful of passives, and a couple of MOSFETs. You’ll have a robust, efficient controller running in minutes.
