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Integrating circuit-protection functions reduces power source costs. (Special Report: Power Sources & Circuit Protection).

In order to stay competitive, most manufacturers look for ways to lower their production costs. Ideally, this should be done without sacrificing product quality and reliability. Power supplies often need expensive circuit protection components to keep them from failing during abnormal conditions, but many circuit protection measures used in power supplies are not very obvious, and their costs are not readily apparent.

These obscure protection measures involve the over-sizing of power train and related components such as the switch device, its heat sink and clamp circuits, the transformer core size and output rectifiers and capacitors. Eliminating external circuit protection devices and hidden protection measures can significantly reduce power supply costs. But, simply doing away with circuit protection is not feasible since that sacrifices reliability, robustness and/or supply safety. However, moving circuit protection functions onto a highly integrated power conversion IC can safely eliminate some expensive discrete parts and allow power train components to be sized for the load they will normally deliver.

Scope

This article describes circuit protection components that can be eliminated, by using a chip with protection functions integrated and the reasonable cost savings that could result from doing so. All proposed savings are based on high-volume manufacturing component costs (> 100K pieces), meaning that better savings should be attainable at lower volumes. The functions that have been integrated onto the newest power conversion chips are line over-voltage (OV) shutdown, switch current sensing and limiting, output overload and open-loop protection (OP), over-temperature shutdown, and parametric temperature compensation and tolerance control.

Line Over-Voltage [OV] Shutdown

Line OV shutdown halts normal operation during line surges and swells, and can eliminate the need for MOVs which can save from $0.04 to $0.12. For example, Power Integrations (PI) has incorporated OV protection into many of its off-line switcher IC families. When these devices detect a line OV condition, they cease switching until the rectified line voltage drops below the function's deactivation threshold. This allows the IC to withstand line surges and swells up to the maximum rating of its internal MOSFET without requiring an MOV to protect it. Figure 1 shows a line surge causing the rectified DC rail to reach a peak of 592V. By halting switching, the voltage on the part's DRAIN pin stays below 700V. Countries with poor power quality can experience line voltage swells of over 100 msec in duration, so this OV shutdown function can be more valuable than a discrete MOSFET's millijoule avalanche rating, or even an MOV's transient power dissipation rating, both of which would be inadequate to survive line swell s of that duration.

Switch Current Sensing and Limiting

Most PWM ICs sense switch current so that a switching cycle can be terminated if the current through the device becomes excessive. This typically requires an external current sense resistor and low pass RC filter. Most P1 device families use the integrated MOSFET's on-resistance as the sense resistor. This, in conjunction with built-in Leading Edge Blanking (LEB) of the current sense comparator, eliminates all external current sensing and conditioning components which can save from $0.04 to $0.10. Eliminating the sense resistor and its conduction losses also improves efficiency. In a DC-DC converter that may use a current sense transformer, the savings could reach $0.50. Additionally, LEB eliminates the need to filter the current sense signal which enables faster and more accurate switch cycle termination. This provides added protection for the MOSFET at no additional cost.

Output Overload and Open-loop Protection [OP]

Most PWM ICs depend on the voltage which powers the chip falling below the device under-voltage lockout threshold to interrupt normal operation during abnormal loading. Many of P1's device families use feedback from the output to gate supply current into the chip (see Figure 2). In this configuration, loss of feedback from the output or loss of output regulation puts the chip into an auto-restart mode in which only a small fraction of full output power is delivered to the load (see Figure 3). This greatly improves the robustness of the design while eliminating the need for secondary supervisory circuits fold-back or crowbar protection circuitry, saving from $0.05 to $0.20. This also allows the supply's output diodes and capacitors to be sized for normal full load output power, further reducing cost.

Over Temperature Shutdown

The integration of this function eliminates thermistors, their associated circuitry and installation labor costs for an initial savings of $0.07 to $0.13. Since the switch is often the highest temperature component in a power supply, on-chip temperature sensing provides robust thermal protection for the entire supply. Additionally, if this function has a wide thermal hysteresis window, it can keep PCB temperatures below 100[degrees]C under fault conditions, allowing the use of less expensive PCB material for additional savings of $0.05 to $0.15.

Parametric Temperature Compensation and Tolerance Control

Typically, the looser a part's tolerances are, the less it costs. But when that part is the control chip, the over-sizing of power train components necessary to compensate for its loose tolerances can cost more than was saved by using the low-cost chip. For example, the variance of switching frequency and the switch's current limit trip-point determine how much overload power can be demanded from the supply. The wider the tolerance is on these two parameters, the more overload power the supply can deliver. The output diodes, transformer core, switch heat sink and clamp circuits have to be sized for the potential overload power, not just the rated full load power. The cost savings realized when these components are sized for full load only can range from $0.10 to $0.25. Therefore, a power chip must be evaluated on the tolerances and temperature compensation of its critical performance parameters.

Conclusion

Designing a power supply around a highly integrated, power conversion IC can reduce its BOM cost from $0.35 to $0.95 by eliminating some circuit protection components and obscure protection measures without losing the protection they provided.

John Jovalusky is Technical Marketing Engineer for Power Integrations, Inc., San Jose, CA. He can be reached at (408) 414-9694; john.jovalusky@powerint.com

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Author:Jovalusky, John
Publication:ECN-Electronic Component News
Date:Jun 1, 2003
Words:1031
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