A High-Voltage Input Switching Power Supply using StackFET™

Industrial equipment operating from a three-phase AC supply often needs an auxiliary power supply stage that can provide a regulated low voltage DC supply for analog and digital circuitry. Examples of such applications include industrial drives, UPS systems, and energy meters.

The specification of such a power supply is much more demanding than that required by a standard off-the-shelf switcher. Not only is the input voltage higher in these applications, but also equipment designed for three-phase applications in an industrial environment typically must tolerate very wide voltage fluctuations—including extended sags, line surges and the occasional loss of one or more phases. Furthermore, the specified input voltage range of such an auxiliary power supply can be as wide as 57 V AC to 580 V AC.

Designing a switching power supply for such a wide range is a challenge primarily because of the high cost of high voltage MOSFETs and the limitations of conventional PWM control loops in terms of dynamic range. The StackFET technique enables use of a combination of an inexpensive low-voltage MOSFET rated for 600V and an integrated power supply controller from Power Integrations in order to design a simple and inexpensive switching power supply capable of operating over a wide input-voltage range.

Figure 1. Three-phase input 3W switching power supply using StackFET

The circuit operates as follows. Input to the circuit can be from a three-phase three-wire or four-wire system or even a single-phase system. The three-phase rectifier consists of diodes D1-D8. Resistors R1-R4 provide inrush current limiting. The use of fusible resistors enables these resistors to open safely during a fault condition and eliminates need for separate fuses. The pi filter—consisting of C5, C6, C7, C8 and L1— filters the rectified DC voltage.

Resistors R13 and R15 balance the voltage across the input filter capacitors.

When the MOSFET inside the integrated switcher (U1) turns ON, the source terminal of Q1 is pulled low, and gate current is supplied by R6, R7 and R8, and the junction capacitance of VR1 to VR3 turning ON Q1. Zener VR4 limits the gate source voltage applied to Q1. When the MOSFET inside U1 turns OFF, the maximum drain voltage of U1 is clamped by a 450 V clamping network, consisting of VR1, VR2, and VR3. This limits drain voltage of U1 close to 450 V. Any additional voltage at that end of the winding connected to Q1 will be impressed across Q1. This arrangement effectively distributes the sum total of rectified input DC voltage and flyback voltage across Q1 and U1. Resistor R9 limits high frequency ringing during switching transitions, and clamping network VR5, D9 and R10 limits the peak voltage across the primary due to leakage inductance during the flyback interval.

Output rectification is provided by D1. C2 is the output filter. L2 and C3 form the second stage filter to reduce switching ripple in the output.

VR6 conducts when the output voltage exceeds the total drop of the optocoupler diode and VR6. A change in output voltage causes a change in current through the optocoupler diode inside U2, which in turn changes current through the transistor inside U2B. When this current exceeds the FB pin threshold current of U1, the next cycle is inhibited. Output regulation is achieved by controlling the number of enabled and inhibited cycles. Once a switching cycle is enabled, the cycle is terminated when the current ramps to the internal current limit of U1. R11 limits current through the optocoupler during transient loading and also sets the gain of the feedback loop. Resistor R12 biases zener diode VR6.

This circuit is protected against loss of feedback, short circuit at output, and overload due to the built-in features of IC U1 (LNK 304). No additional bias winding is needed on the transformer since U1 is powered directly from its drain pin. C4 provides decoupling of the internal supply.