🔌 SG3525 Buck/Boost SMPS Designer
SG3525 & Power Stage Specs
SG3525 SMPS Buck/Boost Converter Designer
Mastering Voltage-Mode Control: The SG3525 Buck/Boost Designer
The SG3525 is a legendary monolithic pulse-width modulation (PWM) controller IC that has anchored power supply design since the 1980s. While modern current-mode controllers have largely superseded it in isolated offline applications, the SG3525 remains a powerhouse for driving non-isolated DC-DC converters, particularly high-power Buck (step-down) and Boost (step-up) topologies. The SG3525 SMPS Designer is a comprehensive engineering tool that calculates both the IC’s internal timing components and the external LC power stage, bridging the gap between control theory and physical circuit implementation.
The Architecture of the SG3525 To utilize the calculator effectively, an engineer must understand the internal architecture of the SG3525. It is a voltage-mode controller. This means it regulates the output voltage by comparing a fraction of the output voltage (the feedback) to a highly stable internal 5.1V reference. The difference generates an error voltage, which is then compared to an internal sawtooth oscillator wave to determine the pulse width (duty cycle) of the output.
The tool features a topology selector. Because the SG3525 contains two internal totem-pole output transistors (A and B), it can be configured in two ways. In a Push-Pull topology, the outputs alternate, meaning the oscillator runs at twice the desired switching frequency. However, in a single-ended Buck or Boost converter, the two outputs are OR-ed together (tied to each other via diodes). In this mode, the oscillator frequency equals the switching frequency. The calculator dynamically adjusts its internal math based on this selection.
Oscillator Timing:
Rt and
Ct The heart of the SG3525 is its internal oscillator, whose frequency is set by an external timing resistor (
Rt) and timing capacitor (
Ct). The tool uses the exact formula from the SG3525 datasheet:
f=Ct×(0.7Rt+3Rd)1 (Assuming the dead-time resistor
Rd is 0 ohms, this simplifies to
Rt=0.7×f×Ct1).
To ensure the
Rt value stays within the IC’s optimal operational range (typically 1kΩ to 100kΩ), the tool intelligently suggests a standard
Ct value based on the user’s target frequency. For example, at 100 kHz, a 2.2nF capacitor is ideal. The tool then calculates the exact
Rt required to hit the user’s precise frequency target, eliminating the guesswork of reading standard logarithmic capacitor charts.
Power Stage Design: Buck and Boost Mathematics Once the IC is configured, the tool calculates the physical power stage components: the Inductor (
L) and the Output Capacitor (
Cout). The math differs significantly based on the chosen topology.
Buck Converter (Step-Down): In a Buck converter, the inductor is in series with the load. The tool calculates the required Duty Cycle (
D) as
D=Vout/Vin. To size the inductor, the user inputs a desired Inductor Current Ripple (typically 20-40% of the max load current). The tool uses the formula:
L=f×ΔIL(Vin−Vout)×D This ensures the inductor stores enough energy during the MOSFET ON-time to sustain the load during the OFF-time without entering Discontinuous Conduction Mode (DCM), which would cause severe output voltage ripple.
Boost Converter (Step-Up): In a Boost converter, the inductor is in series with the input, and the diode isolates the output. The Duty Cycle is calculated as
D=1−(Vin/Vout). The inductor formula changes because the inductor is charged directly by the input voltage:
L=f×ΔILVin×DCapacitance and ESR Constraints The output capacitor in a SMPS is critical for filtering the pulsed energy into a smooth DC voltage. The tool calculates the required capacitance using the voltage ripple formula. For a Buck converter:
C=8×f×ΔVoutΔIL For a Boost converter:
C=f×ΔVoutIout×D However, in real-world high-frequency design, the capacitance value is often secondary to the capacitor’s Equivalent Series Resistance (ESR). Every real capacitor has internal resistance. When the ripple current flows through this resistance, it generates a voltage drop (
V=I×ESR) that adds to the ripple. The tool calculates the maximum allowable ESR using
ESRmax=ΔVout/ΔIL. If the engineer selects a cheap capacitor with a higher ESR than this calculated limit, the output ripple will exceed specifications, regardless of how large the capacitance is.
Peak Inductor Current and Saturation A vital output of the calculator is the Peak Inductor Current. The tool calculates
Ipeak=Iavg+(ΔIL/2). This number is critical for component selection. If the engineer buys a 10µH inductor rated for 5 Amps, but the calculator shows the peak current will be 6 Amps, the inductor’s ferrite core will saturate. When an inductor saturates, its inductance drops to zero, and the only thing limiting current is the DC resistance of the wire. The MOSFET will short-circuit and explode. The tool provides this peak current so the engineer can select an inductor with a saturation rating safely above the calculated maximum.
Compensation and Stability While the tool calculates the main power stage, voltage-mode controllers like the SG3525 require a Type II compensation network (a resistor and capacitor network on the error amplifier pins) to stabilize the feedback loop. The tool provides the foundational math (DC gain and power stage poles) that an engineer needs to calculate the compensation network, ensuring the converter does not oscillate or ring under transient loads.
By unifying the IC oscillator configuration, the rigorous LC power stage math, and the critical ESR limits into a single, topology-aware interface, the SG3525 Designer transforms complex analog control theory into an actionable, build-ready design blueprint.