SG3525AN

The SG3525AN from onsemi is a classic pulse width modulation (PWM) control circuit that has been a workhorse in switch-mode power supply design since its introduction. The device integrates all the functions needed for push-pull, half-bridge, and forward converter topologies into a single 16-pin DIP package. The internal 5.1V +/-1% voltage reference provides a stable reference for the error amplifier and oscillator. The oscillator frequency is programmable from 100Hz to 400kHz using an external timing resistor (RT) and capacitor (CT), with the oscillator output available for synchronization of multiple converters. The dual totem-pole output stages can source and sink up to 200mA each, capable of directly driving power MOSFET gates in low-power applications or gate driver transformers for higher power. In push-pull mode, the two outputs alternate at half the oscillator frequency with programmable dead time (via the RD pin) to prevent cross-conduction. The internal error amplifier with access to its inverting input, non-inverting input, and output (compensation pin) allows flexible feedback loop design. The soft-start function (external capacitor on pin 8) gradually increases the duty cycle at power-up, reducing inrush current. The shutdown pin (pin 10) provides an immediate output disable for overcurrent protection when driven high. Undervoltage lockout (UVLO) prevents operation below approximately 8V supply. The SG3525AN is one of the most widely used and well-understood PWM controllers, with extensive application literature and proven reliability in industrial and consumer power supplies.

The SG3525AN operates as a voltage-mode PWM controller with dual alternating outputs. Key subsystems include: (1) Oscillator – an external RT and CT set the oscillator frequency; the CT charges through RT and discharges through an internal transistor; the sawtooth waveform on CT sets the PWM ramp frequency; in push-pull mode, the internal flip-flop divides the oscillator frequency by 2, so each output switches at fOSC/2; (2) PWM Comparator – the error amplifier output (COMP pin) is compared with the oscillator ramp (CT pin); when the ramp exceeds the error amplifier voltage, the output pulse is terminated; higher error amplifier voltage means wider pulse width (higher duty cycle); maximum duty cycle per output is approximately 49% in push-pull mode; (3) Error Amplifier – an op-amp with accessible inputs (INV, NI) and output (COMP); the inverting input typically receives a fraction of the output voltage via a resistor divider; the non-inverting input typically connects to the 5.1V reference or a setpoint voltage; a compensation network (R-C from COMP to INV) sets the feedback loop bandwidth and stability; (4) Dead Time Control – the RD (dead time resistor) pin adds a fixed offset to the PWM ramp, ensuring a minimum off-time between the two outputs; this prevents both switches from being on simultaneously (cross-conduction) in push-pull and half-bridge topologies; (5) Soft-Start – a capacitor on the SS pin (pin 8) is charged by an internal 50uA current source; the slowly rising voltage on the SS pin limits the maximum duty cycle during startup, reducing inrush current; (6) Shutdown – a high level on pin 10 immediately sets the PWM latch, terminating both output pulses; this is used for overcurrent protection when combined with a current sensing circuit.

• Push-pull DC-DC: 24V to +/-15V 100W converter at 100kHz with dual outputs driving MOSFET gates
• Half-bridge inverter: 12V to 220VAC 200W inverter using two SG3525 outputs at 50Hz/60Hz
• Battery charger: voltage-mode buck charger with soft-start and current-limit shutdown
• Forward converter: single-ended output for 100W telecom supply with external gate driver