In SMPS, the circuit clamps the input AC voltage to the safe level of power MOSFET operation.

Most isolated, off-line SMPS (Switch Mode Power Supplies), including flyback, forward, and resonance, must operate at input voltages ranging from 90 to 260V rms. In some cases, a line voltage with an effective value of 400V & plusmn; 10% is even used, which leads to an increase in the voltage level of the device and increases the overall design cost. Using the input limiting circuit, the input voltage can be increased to 440V rms without damaging the power supply device.

The circuit in Figure 1 limits or clamps the input AC voltage greater than 260V rms to the safe level of power MOSFET operation. The circuit uses a MOSFET Q1, such as a 100Hz switching shunt regulator IC1 TL431CZ, to set the clamped high voltage level through the voltage divider R2 and R4. The circuit uses the device values shown in the figure. The clamped output voltage is DC 360V, the input voltage is 260V RMS, and the maximum input voltage is 440V RMS. The tested circuit consumes 5 to 10W of power.

When the input voltage is less than the effective value of 260V, and the point C is less than 2.5V, IC1 is closed, and the cathode current in the minimum off state decreases. Zener diode D2 collapses to 15V, ensuring stable state of Q1. This operation is the normal state of Q1 when the input voltage is less than 260V rms. Thus, at these voltage levels, the circuit operates as a standard full-bridge rectifier under capacitive load C3.

valid value

For input voltages of 260V or greater, point C is greater than 2.5V, IC1 is turned on, and D2 current is commutated and reduced. The gate-source voltage Q1 drops to about 2V and Q1 turns off. Now, even though the D1 bridge rectifier diode is forward biased, no current flows to the charge pump capacitor C3. The rectified input AC voltage is greater than the voltage through C3, but Q1 is turned off, the loop is interrupted, and no current flows. Therefore, the output DC voltage on C3 is limited because there is no suitable discharge current.

When the rectified AC input voltage begins to decrease, it eventually reaches the 2.5V limit at point C, and Q1 turns on again. But since the rectifier bridge diodes are reverse biased, no current flows; the rectified input AC voltage is less than the voltage across C3. This voltage decreases at a rate determined by the output power level. Eventually, when the rectifier bridge diodes become forward biased, the voltage is at the same level as the rectified input AC voltage. Q1 is still on; therefore, the discharge current begins to flow. Follows instantly, leading through Q1 and D1. Short discharge pulses supplement the energy loss, increasing the voltage to the limit level. When the input voltage is higher than the rms value of 260V, Q1 is closed again and the whole process is repeated.

Q1 consumes less energy. Each switching cycle, the MOSFET is only closed for 450 µs, resulting in high efficiency for this high voltage limiting circuit. It can be used as a MOSFET switch with STMicroelectronics SuperMesh MOSFET STP4NK50Z. It is available in a TO-220 package, but can also use a Dpak package to save space because the MOSFET is not a drain voltage limiter. When the 50/60Hz rectifier diode is pre-biased, the current through Q1 is interrupted. The current interruption causes the drain to source voltage. The clamp circuit passes the managed EMI (Electromagnetic Interference) test according to EN 55022 Class B using peak and average detection. 1-mH, 0.2A blocking, L1 and L2 suppress EMI. Through the rectifier tube of the D1 bridge, the 220-nF, 440V AC capacitor C1 is a simple buffer element.

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