High voltage input switching power supply using StackFET.

Industrial equipment that operates from three-phase AC often requires an auxiliary power stage that can provide stable low-voltage DC power to analog and digital circuits. Examples of such applications include industrial drives, UPS systems, and energy meters.

The specifications for such power supplies are much tighter than those required for standard off-the-shelf switches. Not only are the input voltages in these applications higher, but equipment designed for three-phase applications in industrial environments must also tolerate very wide fluctuations—including extended sags, surges, and the occasional loss of one or more phases. Furthermore, the specified input voltage range for such auxiliary power supplies can be as wide as 57 VAC to 580 VAC.

Designing a switching power supply with such a wide range can be a challenge, mainly due to the high cost of high-voltage MOSFETs and the limited dynamic range of traditional PWM control loops. StackFET technology allows the use of less expensive, 600V rated low-voltage MOSFETs and integrated power controllers from Power Integrations, which can design a simple and inexpensive switching power supply that can operate over a wide input voltage range.

Figure 4: Three-phase input 3W switching power supply using StackFET technology

[page]The circuit works as follows: The input current to the circuit can come from a three-phase three-wire or four-wire system, or even from a single-phase system. The three-phase rectifier is formed by diodes D1-D8. Resistors R1-R4 provide inrush current limiting. If fusible resistors are used, these can be disconnected safely during a fault, eliminating the need for separate fuses. The pi filter, formed by C5, C6, C7, C8 and L1, filters the rectified DC voltage.

Resistors R13 and R15 are used to balance the voltage between the input filter capacitors.

When the MOSFET in the integrated switch (U1) turns on, the source of Q1 is pulled low, R6, R7, and R8 provide the gate current, and the junction capacitance from VR1 to VR3 turns Q1 on. Zener diode VR4 is used to limit the gate-source voltage applied to Q1. When the MOSFET in U1 turns off, the maximum drain voltage of U1 is clamped by a 450 V clamp network formed by VR1, VR2, and VR3. This limits the drain voltage of U1 to nearly 450 V.

Any additional voltage at the end of the winding connected to Q1 is applied to Q1. This design effectively divides the total rectified input DC voltage and flyback voltage between Q1 and U1. Resistor R9 is used to limit high frequency ringing during switching, and the clamp network VR5, D9 and R10 are used to limit the peak voltage on 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 a secondary filter to reduce the switching ripple at the output.

When the output voltage exceeds the total voltage drop across the optocoupler diode and VR6, VR6 will turn on. The change in output voltage causes a change in the current through the optocoupler diode in U2, which in turn changes the current through the transistor in U2B. When this current exceeds the FB pin threshold current of U1, the next cycle is inhibited. Output regulation can be achieved by controlling the number of enable and inhibit cycles. Once a switching cycle is initiated, the cycle ends when the current rises to the internal current limit of U1. R11 is used to limit the current through the optocoupler during transient loads and to adjust the gain of the feedback loop. Resistor R12 is used to bias Zener diode VR6.

IC U1 (LNK 304) has built-in features so that the circuit is protected from loss of feedback signal, short circuit at the output, and overload. Since U1 is powered directly from its drain pin, there is no need to add an additional bias winding on the transformer. C4 is used to provide internal power supply decoupling.