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Electronic Design

Resettable Fuse Pair Promises High Voltage And Low Resistance

Two resettable fuses have joined the miniSMD line of PolySwitch polymetric positive temperature-coefficient de-vices. According to their manufacturer, Raychem Circuit Protection, the miniSMDM075/24 provides the industry's highest voltage rating while the miniSMDM260 boasts the lowest resistance.

With its 24-V and 40-A ratings, the miniSMDM075/24 is suitable for use in keyboards, mice, motherboards, monitors, and select 24-V IEEE-1394 (FireWire) peripheral designs. Along with a 0.75-A hold current, it features a 1.5-A trip current and a 200-mΩ typical resistance. UL, TUV, and CSA agency certification is pending.

Meanwhile, the miniSMDM260 fuse offers 8-V/40-A operation. With this, it can be employed in ganged USB port protection, battery-charger control modules, computer peripherals, motherboards, monitors, and other applications. This device also gives designers a 2.6-A hold current, 5.2-A trip current, and 50-mΩ resistance. Raychem claims that this figure is 10% lower than any resistance previously available in any device in this configuration. Additionally, the miniSMDM260 is recognized by UL, TUV, and CSA.

Samples of both models are available in EIA481-1 compliant tape-and-reel packages, at 3000 parts per 13-in. reel, for evaluation purposes. Prices start at $0.25 each in 100,000-unit lots. Contact the company for lead times.

Raychem Circuit Protection, 300 Constitution Dr., Menlo Park, CA 94025-1164; (800) 227-7040; fax (800) 611-2323;

See associated figure

Power-Factor Correction
In order to achieve good power-factor correction, many common solutions employ an additional power supply between the main power supply and the ac power line. This is mandatory in order to draw a sinusoidal-shaped current from the ac power line at frequencies of 50 or 60 Hz.

Another solution to meet the standards of EN 61000.3.2 or IEC 555 etc. is the addition of some components, like coils, capacitors, and diodes, to the original power supply just for the measurement point. Adding a dedicated power-factor-correction circuit is an alternative approach. Unfortunately, for normal picture conditions in a TV set (higher power consumption), the harmonics are significantly increased.

Although Rehm declined to be very specific in his description of the power-factor-correction performance of the Ecosphere, the designer concluded that "Ecosphere's power-factor-correction circuit takes advantage of the resonant mode and the high switching frequency, significantly decreases the harmonic consumption of the ac power line, even with high loads, and considerably reduces the size of the necessary additional components. The supply's efficiency remains at 85%. Furthermore, the additional power-factor-correction circuit is about 25 times smaller than that when using conventional solutions." (Fig. 2).

An important feature of the Ecosphere is its so-called Eco Standby Mode, accessible by remote control. In this mode, power consumption is reduced to between 100 and 200 mW. This exceptionally low value for a TV set is achieved by adding another standby power supply that switches Ecosphere's main power supply completely off.

To accomplish this, a startup circuit is required. But, this circuit shouldn't increase the ac power line's consumption in the Eco Standby mode while it still has to deliver enough energy to start the main supply. The Eco Standby supply powers the infrared receiver for remote-control operation and the remote control's LED in the forward mode as well. During flyback time, the secondary regulation is transmitted.

Using A Pulsed Mode
For those engineers who want to design their own green switched-mode power supply, rather than license an effective, already existing design, there's an alternative. ON Semiconductor, for example, promotes running power supplies in the pulsed mode.

This means that the design of a switched-mode power supply has to be changed in such a way that only the microprocessor is supplied with packets of energy that are injected into the system from time to time. Once the microprocessor detects that the system will become active, it reactivates the mode of normal operation.

As is well known, the efficiency of a switched-mode power supply sinks dramatically when the output power is significantly below the specified normal value for which the converter was designed. The pulsed-mode principle, however, allows the designer to fight against this decrease by delivering energy packets of relatively high power to the processor.

In most cases, the equipment is powered by a flyback converter with several output voltages to power different television functions. For example, a television set needs a relatively high voltage (e.g., 120 V) for the video function, a medium-range voltage (e.g., 24 V) for the sound function, and a relatively low voltage (e.g., 5 V) for the logic function and the microprocessor. Plus, a winding in the primary sensor circuit, which is used to feed the switched-mode power-supply controller, requires about 12 V for operation.

Operation in the pulsed mode is based on a modified secondary configuration where the microprocessor turns on a switch that changes the overall configuration of the switched-mode power supply. This forces the high-voltage winding to supply power to the processor.

The power supply works with a sequence of startup and operating phases, so energy packets are only supplied to the processor during the active times. By limiting the maximum transformer current, it's possible to keep the noise emission resulting from this configuration at a minimum, too.

The pulse duty cycle determines the amount of power needed for the microprocessor. ON Semiconductor has designed its MC44608 exactly for these applications, where the standby power needs to be below 1 W. The circuit offers power-factor correction as well as several protection circuits.

As mentioned earlier, Philips Semiconductors has introduced its second-generation GreenChip II switched-mode power-supply controller. This device, the TEA1507, is presently sampling. It operates directly from the rectified universal ac power line.

Special attention was paid to optimize the efficiency during the normal operation mode to around 90% while preserving the performance of the green standby mode in first-generation GreenChip ICs. "Due to the special built-in green functions, the efficiency is optimal at all power levels," explains Edwin Kluter, product marketing manager for power conversion ICs at Philips Semiconductors. "If burst-mode operation is applied, standby power can be reduced to below 1 W."

The TEA1507 is a multichip module (MCM) consisting of two ICs: a high-voltage DMOS chip for startup and high-voltage logic, and a biCMOS chip for low-voltage control. The high-voltage chip works directly from the rectified ac-line voltage. The second low-voltage biCMOS IC is used for protection functions and control.

This module is designed to be situated at the primary side of a flyback converter. That's where an auxiliary winding of the transformer is used for demagnetization protection and for powering the IC after startup. Compared to fixed-frequency "hard-switching" architectures, this device operates in a quasiresonant mode, yielding efficiencies between 80% and 90%. This is advantageous because the input power can often be kept below 75 W. An additional power-factor-correction circuit isn't necessary, then, reducing overall system costs.

The TEA1507 module has several operating modes. At high power levels, the next converter stroke is started only after demagnetization of the transformer current (zero current switching). Meanwhile, the drain voltage has reached the lowest voltage in order to reduce switching losses. The primary resonant circuit of the primary inductance and a drain capacitor ensure this quasiresonant operation. If required, the design can be optimized in a way that zero-voltage switching occurs across most parts of the ac power-line range.

To prevent operation at very high frequencies and at low loads, the quasiresonant operation changes smoothly in fixed-frequency PWM control with drain-voltage valley detection. At very low power levels (standby mode), the frequency is set to a minimum value of around 6 kHz via the voltage-controlled oscillator. An input power level below 3 W at an output power level of 100 mW is feasible for a typical power supply operating around 75 W.

Due to the device's high-voltage (650-V) startup circuit, there is no need for conventional startup bleeder circuitry. Furthermore, it enables a low-power standby mode.

The eight-pin module uses its "drain" pin to draw current from the high-voltage supply line. It also drives a current out of the IC to quickly charge the supply capacitor. Once the supply capacitor is charged, an auxiliary winding from the transformer takes over the charge. This reduces the IC's power dissipation close to zero. For good line regulation, the IC performs current-mode control.

In the flyback configuration, the IC can be used in a secondary regulation mode. Here, an optocoupler is utilized to feed back information about the ac power line's isolated output to the nonisolated primary side. This information is fed to the IC via the module's "control" pin.

The IC has a soft-start feature for reducing stress on components during system startup and burst/safe restart modes. The soft-start feature also prevents acoustic noise of the transformer, the so-called transformer-rattle. At the same time, the demagnetization feature lets the chip work in both fixed-frequency and quasiresonant modes while synchronizing the system externally.

To prevent the supply from starting at ac-line voltages that are too low, thereby preventing audible noise, a circuit disables the supply in this condition. (In conventional systems not using the GreenChip II module and operating from the ac power line, external components must be used. Or in integrated systems, a separate pin is used). By adding a single external resistor to the "drain" pin, the designer can adjust the ac-line voltage's enabling level. This function protects the switched-mode power supply when the ac line's electrolytic capacitor is disconnected.

In the quasiresonant mode, maximum power isn't just defined by the primary current. It's also defined by the ac line's input voltage, because the operating frequency depends on the ac line's voltage. When a certain amount of power is required with universal ac power lines, the GreenChip II module delivers that maximum power at a low line voltage. At the highest ac-line voltage, the maximum power delivered by the power supply can be more than twice as high as the maximum power delivered at low ac-line voltages. To prevent overdimensioning of secondary power components like transformer windings and secondary diodes, the TEA1507 incorporates a voltage compensation for the ac power line.

During the primary stroke, the rectified ac-line input voltage can be measured by sensing the current drawn from the "demag" pin. This current is used to adjust the peak drain current, which is measured via the "sense" pin.

"GreenChip II's internal compensation means that a nearly ac power-line-independent maximum output power can be realized," Kluter says. "Since the information is gathered via this existing pin and an existing inexpensive external component, there's no additional costs using this feature."

The green part of the MCM is the low-power standby function. For implementing the burst-mode standby function, the "control" pin is used together with the startup current source. Burst-mode standby operation is initiated by a microprocessor closing two switches on the secondary side, thereby shorting the output voltage to the microprocessor's supply voltage.

Now, the power is supplied exclusively to the microprocessor's supply, and the voltage on the microprocessor's capacitor rises. When this voltage exceeds a Zener-diode voltage, the optocoupler is activated and a signal is passed to the primary side. This signal is then detected by the TEA1507's "control" pin. After that, the device goes through a "hiccup" cycle, where it stops switching. The voltage on both the ICs and the microprocessor's supply capacitors drops.

Once the IC supply voltage dips below the so-called undervoltage-lockout level, the startup circuit is reactivated and subsequently recharges the IC supply capacitor back to its start voltages. So, the system goes through a burst of switching cycles, followed by a nonswitching period. Since the frequency of this operation is very low, a low-power and efficient standby mode is achieved.

These are only three examples of Earth-friendly designs for power supplies. Ultimately, it's up to the power-supply designers to introduce innovative green designs that will help the environment.

Companies Mentioned In This Report
IBR Ingenieur-Buero Rehm
+49 7721 737 17 (phone and fax)
e-mail: [email protected]

ON Semiconductor
(602) 244-6600
fax (602) 244-3345

Philips Semiconductors
+31 (40) 27 24825 (international)
(800) 234-7381 (U.S.)

Thomson Multimedia
+33 0 141 8650 00 (international phone)
+33 0 141 8661 00 (international fax)
(212) 319-0163 (U.S. phone)
(212) 319-0147 (U.S. fax)

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