Product category:
Power Supply ICs and Controllers
News Release from: Allegro MicroSystems Europe | Subject: STR Series switching regulators
Edited by the Electronicstalk Editorial
Team on 30 August 2000
Integrated switching regulators cut PSU
cost
Allegro Sanken now offers a range of integrated high-voltage switching regulators which allow the designer to produce the lowest-cost switch-mode power supply possible
Allegro Sanken now offers a range of integrated high-voltage switching regulators, the STR Series, which allow the designer to produce the lowest-cost switch-mode power supply possible for ratings up to 300 W and beyond Typical switched mode PSUs use a discrete PWM (pulse-width modulated) controller with an external switching device (mosfet) combined with many discrete components
This article was originally published on Electronicstalk on 20 Sep 2000 at 8.00am (UK)
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The STR devices achieve their simplicity by combining the PWM circuitry, the power mosfet and all but one or two of the discrete components within a single package.
STR devices are available in 5-pin TO-220 and the lower- cost single-inline packages as well as the higher power rated TO-3P package.
The circuits are available for three different modes of operation: fixed-frequency, fixed off-time and quasi-resonant.
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The fixed-frequency devices are based on an internal 100 kHz oscillator, and are limited to power ratings of up to 75 W.
They find application in compact industrial-quality power supplies for chargers, auxiliary power units and motor drives.
The fixed off-time devices feature variable frequency and a 16 ęs off-time, and cover ratings from 15 W to 120 W.
In addition to industrial, telecom and charger applications, they are ideally suited to low-cost consumer applications in DVDs, CD players and VCRs.
Finally, variable-frequency quasi-resonant devices featuring self oscillation and soft switching cover ratings up to 300 W, and find application in noise-sensitive applications such as TV set-top boxes, instrumentation, high-end industrial and monitors.
This is the optimum mode for EMI performance and efficiency.
With a typical STR device, the only external components required are an external sense resistor and a start-up resistor.
Having these components outside the integrated section allows the user to select the optimum values to match the expected maximum output power.
Standard features of the devices include full protection against overcurrent , overtemperature and overvoltage, universal operation (85-265 V AC input), higher efficiency and higher reliability.
In some devices, the use of a 900 V avalanche rated mosfet means that the device is able to absorb the energy spikes from transients at start-up and overload, making the regulator more rugged and reliable.
The 900 V option effectively removes the primary protective snubbed requirement.
The use of a 900 V mosfet also means that no expensive snubbing devices are required for regulator protection.
Short-circuit protection is provided, since the pulse-by-pulse current limit is operative at all time including the beginning of each switching cycle.
There is no 'front edge blanking' so the regulator can protect against the high current spikes that occur when starting up at 276 V AC or into an overload or short circuit.
With front-edge blanking, the regulator might be destroyed under these conditions.
The programmable power limit means that no 'Sense-FET' is required.
An external sense resistor allows the output power to be limited more precisely.
Using a 'Sense-FET' can result in a very high power limit, much higher than the maximum load.
This means that the transformer and secondary components have to be rated for the much higher currents, increasing the cost and complexity of these components.
Typical reliability of these devices is better than 3 parts in ten billion failures per hour.
To summarise, these devices offer a protected, high-reliability solution in a choice of operating modes and topologies, covering ratings from 1 W to 300 W and beyond.
The multichip packaging allows the optimum controller/mosfet combination to be selected for each device.
Finally, the devices are designed with the lowest-cost solution in mind, and only use additional external components when there is no sense in integration for integration's sake.
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