Uc3842 UC3843 Smps designer tool

⚡ UC3842 / UC3843 SMPS Designer

Select the variant you are using
Desired frequency in kHz (e.g., 100)
Max peak current through MOSFET in Amps
DC bus voltage (e.g., 320V for 220V AC)
ℹ️ Note: Calculations use standard datasheet formulas. Current sense threshold is 1.0V. Assumes a standard startup current of 0.5mA to rapidly charge the Vcc capacitor.

Component Values

UVLO Turn-On Voltage 0 V
UVLO Turn-Off Voltage 0 V
Timing Capacitor (Ct) 0 nF
Timing Resistor (Rt) 0 kΩ
Sense Resistor (Rsense) 0 Ω
Rsense Power Rating (Min) 0 W
Startup Resistor (Rstart) 0 kΩ
Rstart Power Rating (Min) 0 W

UC3842/UC3843 SMPS Designer

Current-Mode Mastery: The UC3842/UC3843 SMPS Designer

If the SG3525 is the grandfather of voltage-mode control, the UC384x family is the undisputed king of current-mode control. Introduced in the 1980s, the UC3842 and UC3843 remain among the most widely used PWM controllers on Earth, found in everything from cell phone chargers and laptop adapters to ATX power supplies and LED drivers. The UC3842/43 SMPS Designer is a targeted engineering tool that calculates the critical, failure-prone external components required to safely wake up and run these ICs in off-line flyback and forward converters.

The Shift to Current-Mode Control To understand the calculator, one must understand why current-mode control replaced voltage-mode control. In a voltage-mode controller (like the SG3525), the duty cycle is determined solely by comparing the output voltage error to a fixed oscillator ramp. This creates a double-pole feedback system that is notoriously difficult to stabilize and responds slowly to sudden input voltage changes.

The UC384x uses peak current-mode control. Instead of a fixed ramp, it compares the error voltage to a ramp generated by the actual current flowing through the primary winding of the transformer. When the primary current hits the limit set by the error amplifier, the IC shuts off the MOSFET. This transforms the feedback loop from a complex double-pole system into an easier-to-stabilize single-pole system. More importantly, it provides cycle-by-cycle current limiting. If the transformer saturates or the output shorts, the current spikes instantly, the IC detects it in microseconds, and shuts off the MOSFET before it can explode.

UVLO: Waking Up the IC The first calculation the tool performs is setting the Under-Voltage Lockout (UVLO) context. The UC384x has an internal zener diode clamping its VCC at 34V, but it requires a significant amount of current to drive its internal logic and the external MOSFET.

The tool asks the user to select the specific IC variant:

The tool automatically adjusts its internal logic and displays the correct UVLO thresholds, ensuring the engineer designs the startup circuitry for the correct voltage domain.

The Startup Resistor (

Rstart​) and Power Dissipation In an offline SMPS, the IC cannot initially be powered from the high-voltage 400V DC bus. To solve this, a startup resistor is connected from the 400V bus to the VCC pin. This resistor trickles a tiny current into the VCC capacitor, slowly charging it from 0V up to the UVLO turn-on threshold (16V for the UC3842).

The calculator determines the required

Rstart​ by targeting a 1mA startup current. The formula used is:

Rstart​=1mAVin(min)​−VUVLO(on)​​.

Crucially, the tool calculates the continuous power dissipation of this resistor using

P=Rstart​Vin2​​. In a 400V offline circuit, a 390kΩ startup resistor dissipates roughly 0.4 Watts continuously, 24 hours a day, even when the power supply is unloaded. This is a massive source of wasted energy (vampire power) and heat. The tool outputs the minimum power rating (e.g., telling the engineer to use a 1-Watt rated flameproof resistor) to prevent the component from burning up and causing a fire hazard. In modern, high-efficiency designs, this calculator highlights exactly why engineers eventually replace this resistor with an active high-voltage startup IC.

Oscillator Configuration:

Rt​ and

Ct​ The UC384x frequency is set by

Rt​ (connected to VREF) and

Ct​ (connected to GND). The internal oscillator charges

Ct​ through

Rt​ until it hits 2.8V, then discharges it to 1.2V. The datasheet formula is

f=Rt​×Ct​1.72​.

To keep the

Rt​ value in a range that guarantees stable oscillator operation (typically between 5kΩ and 50kΩ), the tool intelligently suggests a standard

Ct​ value based on the target frequency (e.g., 4.7nF for 50kHz, 2.2nF for 100kHz). It then calculates the exact

Rt​ required, ensuring the oscillator runs at the exact frequency needed to optimize the transformer size and minimize switching losses.

Current Sensing: The

Rsense​ Resistor The most critical protection feature of the UC384x is its current sense comparator. The IC monitors the voltage drop across a low-ohm sense resistor placed in series with the MOSFET source. The internal comparator triggers at exactly 1.0V.

The tool calculates

Rsense​=Ipk​1.0V​, where

Ipk​ is the maximum peak primary current. This ensures that if the current ever exceeds the safe limit (due to a short circuit or core saturation), the voltage across

Rsense​ hits 1.0V, and the IC instantly terminates the switching pulse.

The tool also calculates the power dissipation of

Rsense​. Because this resistor handles pulsed, high peak currents, its RMS heating is significant. The tool calculates the worst-case power using an estimated duty cycle, allowing the engineer to select a resistor with an adequate wattage rating (e.g., a 1W or 2W metal film resistor) so it does not desolder itself from the board under heavy load.

Conclusion The UC3842/43 SMPS Designer distills decades of power supply design best practices into a single, focused utility. By automatically handling the complex interplay between high-voltage startup circuitry, UVLO hysteresis, oscillator timing, and cycle-by-cycle current sensing, the tool ensures that the most critical protection and timing components are flawlessly calculated, resulting in a safe, robust, and reliable switch-mode power supply.