As global demands for consistent, high quality and cost effective power increase, so do the demands for uninterruptible power supply (UPS) systems, which support a vast array of today’s power-generation technologies. Wind, solar and conventional generative sources rely on UPS systems to condition power inputs. UPS designs that use the advances in today’s semiconductor technologies achieve higher voltage ratings, higher switching frequencies, and lower conduction losses. Incorporating laminated bus bar assemblies as a means of distributing power within the inverter gives designers the ability to take full advantage of these improvements.
The prime function of UPS systems is to improve power source quality as well as protect the loads against disturbances and, of course, power interruptions. Advanced UPS systems rely on their inverter’s performance to correct common power problems such as voltage fluctuations, noise, frequency instability, and harmonic distortion. In order to effectively address these issues, designers employ advanced electrical components such as insulated gate bipolar transistors (IGBTs), metal–oxide–semiconductor field-effect transistor (MOSFETS), and diodes.
The general categories of UPS systems today are standby, line-interactive, or on-line. In a standby UPS, the load derives power directly from the input power and backup power circuitry activates only when the utility power fails. A line-interactive UPS maintains the inverter in line and redirects the battery’s DC current path from the normal charging mode to supplying current when power is lost. Most UPS systems below 1 kVA are of the standby or line-interactive variety and are usually less expensive.
An on-line UPS system uses a double conversion method of accepting AC input, rectifying to DC for passing through the rechargeable battery, then inverting it back to 120V/230V AC for powering the protected equipment. An on-line UPS is much the same as a standby or line-interactive UPS, although usually more costly due to its greater AC-to-DC battery-charger/rectifier. Its rectifier and inverter operate continuously. One of the advantages of an on-line UPS is its ability to provide additional protection between the utility power and sensitive electronic equipment, as it allows control of output voltage and frequency regardless of the input.
Typically in switching applications, energy is stored in a circuit and stray inductance is realized in the form of an overshoot voltage spike added to the DC bus voltage and felt, for example, across an IGBT at the time of turn-off (see fig. 1). This overshoot voltage is proportional to the rate-of-change-of-current (di/dt) and the amount of stray inductance (L) in the circuit, supporting the formula V=L*di/dt. Therefore, designers must minimize stray inductance to fully utilize the semiconductor’s performance potential while avoiding component failure.
An effective way to minimize this inductance is to use a laminated bus bar instead of cables as a means of distributing power in the inverter. By properly shaping and routing the conductors so that the current flows equally and in opposite directions through each, their opposing magnetic fields effectively cancel each other, resulting in little added circuit inductance. The closer the conductors are together, the more this effect is realized. The dielectric material selected should be as thin as possible but with a dielectric strength appropriately exceeding the application voltage.
While not a new technology, a well-designed laminated bus bar offers electrical optimization and mechanical improvements within a UPS system. By utilizing flat copper conductors laminated together with a thin dielectric insulation in between, these bus bars minimize inductance. This assembly becomes a platform upon which electronic components can be directly mounted. Laminated bus bars offer numerous benefits, including reducing part count, virtually eliminating assembly errors, addressing component and personal safety issues, and improving overall system cost.